p62/SQSTM1 upregulation constitutes a survival mechanism that occurs during granulocytic differentiation of acute myeloid leukemia cells

The p62/SQSTM1 adapter protein has an important role in the regulation of several key signaling pathways and helps transport ubiquitinated proteins to the autophagosomes and proteasome for degradation. Here, we investigate the regulation and roles of p62/SQSTM1 during acute myeloid leukemia (AML) cell maturation into granulocytes. Levels of p62/SQSTM1 mRNA and protein were both significantly increased during all-trans retinoic acid (ATRA)-induced differentiation of AML cells through a mechanism that depends on NF-κB activation. We show that this response constitutes a survival mechanism that prolongs the life span of mature AML cells and mitigates the effects of accumulation of aggregated proteins that occurs during granulocytic differentiation. Interestingly, ATRA-induced p62/SQSTM1 upregulation was impaired in maturation-resistant AML cells but was reactivated when differentiation was restored in these cells. Primary blast cells of AML patients and CD34⁺ progenitors exhibited significantly lower p62/SQSTM1 mRNA levels than did mature granulocytes from healthy donors. Our results demonstrate that p62/SQSTM1 expression is upregulated in mature compared with immature myeloid cells and reveal a pro-survival function of the NF-κB/SQSTM1 signaling axis during granulocytic differentiation of AML cells. These findings may help our understanding of neutrophil/granulocyte development and will guide the development of novel therapeutic strategies for refractory and relapsed AML patients with previous exposure to ATRA.p62 or sequestosome 1 (p62/SQSTM1) is a scaffold protein, implicated in a variety of biological processes including those that control cell death, inflammation, and metabolism.^{1, 2} Through its multi-domain structure, p62/SQSTM1 interacts specifically with key signaling proteins, including atypical PKC family members, NF-κB, and mTOR to control cellular responses.^{3, 4, 5, 6, 7} p62/SQSTM1 functions also as a key mediator of autophagy. Through its interaction with LC3, an essential protein involved in autophagy, p62/SQSTM1 selectively directs ubiquitinated substrates to autophagosomes leading to their subsequent degradation in lysosomes.^{8, 9} At the molecular level, p62/SQSTM1 acts as a pro-tumoral molecule by ensuring efficient and selective activation of cell signaling axes involved in cell survival, proliferation, and metabolism (i.e., NF-κB, mTOR, and Nrf-2 pathways).^{3, 5, 6, 7, 10, 11, 12, 13} p62/SQSTM1 can also signal anti-tumoral responses either by inactivating the pro-oncogenic signaling through BCR-ABL¹⁴ and Wnt pathways^{15, 16} or by inducing the activation of caspase 8, a pro-death protein.^{17, 18} Interestingly, in response to stress, autophagy promotes the degradation of p62, thus limits the activation of p62-regulatory pathways that control tumorigenesis.¹⁰ In addition, p62/SQSTM1 controls pathways that modulate differentiation of normal and cancerous cells. For example, p62/SQSTM1 has been shown to antagonize basal ERK activity and adipocyte differentiation.¹⁹ In contrast, p62/SQSTM1 favors differentiation of osteoclasts,²⁰ osteoblasts,²¹ neurons,²² megakaryocytes²³ and macrophages.²⁴ The role and regulation of p62/SQSTM1 during leukemia cell differentiation has been poorly documented.Acute myeloid leukemia (AML) is a hematological disease characterized by multiple deregulated pathways resulting in a blockade of myeloid precursors at different stages of maturation.^{25, 26} Acute promyelocyte leukemia (APL) is the M3 type of AML characterized by an arrest of the terminal differentiation of promyelocytes into granulocytes and frequently associated with the expression of the oncogenic PML-RAR alpha fusion gene.^{27, 28} All-trans retinoic acid (ATRA), a potent activator of cellular growth arrest, differentiation, and death of APL cells, has been shown to effectively promote complete clinical remission of APL when combined with chemotherapy.^{29, 30, 31} Despite the success of this treatment, some APL patients are refractory to ATRA treatment or relapse owing to the development of resistance to ATRA in leukemia cells.^{32, 33, 34}Our previous results revealed that autophagy flux is activated during granulocyte differentiation of myeloid leukemia cell lines induced by ATRA.³⁵ In the present study, we observed that p62/SQSTM1, an autophagic substrate, is markedly upregulated at both mRNA and protein levels during the granulocytic differentiation process. Here, we investigated the regulation and the function of p62/SQSTM1 during AML cells differentiation into neutrophils/granulocytes.     

IGFBP7 induces apoptosis of acute myeloid leukemia cells and synergizes with chemotherapy in suppression of leukemia cell survival

Despite high remission rates after chemotherapy, only 30–40% of acute myeloid leukemia (AML) patients survive 5 years after diagnosis. This extremely poor prognosis of AML is mainly caused by treatment failure due to chemotherapy resistance. Chemotherapy resistance can be caused by various features including activation of alternative signaling pathways, evasion of cell death or activation of receptor tyrosine kinases such as the insulin growth factor-1 receptor (IGF-1R). Here we have studied the role of the insulin-like growth factor-binding protein-7 (IGFBP7), a tumor suppressor and part of the IGF-1R axis, in AML. We report that IGFBP7 sensitizes AML cells to chemotherapy-induced cell death. Moreover, overexpression of IGFBP7 as well as addition of recombinant human IGFBP7 is able to reduce the survival of AML cells by the induction of a G2 cell cycle arrest and apoptosis. This effect is mainly independent from IGF-1R activation, activated Akt and activated Erk. Importantly, AML patients with high IGFBP7 expression have a better outcome than patients with low IGFBP7 expression, indicating a positive role for IGFBP7 in treatment and outcome of AML. Together, this suggests that the combination of IGFBP7 and chemotherapy might potentially overcome conventional AML drug resistance and thus might improve AML patient survival.Only 30–40% of acute myeloid leukemia (AML) patients survive 5 years after diagnosis.¹ This extremely poor prognosis is mainly caused by treatment failure due to chemotherapy resistance. This resistance is often a multifactorial phenomenon that can include enhanced expression or activation of receptor tyrosine kinases such as the insulin growth factor-1 receptor (IGF-1R).^{2, 3} The IGF-1R stimulates proliferation, protects cells from apoptosis and has been implicated in the development and maintenance of various cancers.^{4, 5} Several oncogenes require an intact IGF-1R pathway for their transforming activity⁶ and moreover, disruption or inhibition of IGF-1R activity has been shown to inhibit the growth and motility of a wide range of cancer cells in vitro and in mouse models.^{4, 5} IGF-1Rs are membrane receptors and binding of their ligand, the insulin-like growth factor-1 (IGF-1), results in receptor phosphorylation and activation of MAPK and PI3K/Akt signaling.⁴ Importantly, IGF-1, normally produced by the liver and bone marrow stromal cells, can stimulate the proliferation of cancer cells in vitro and genetic manipulations that reduce IGF-1 signaling can lead to decreased tumor growth.^{7, 8}In hematological malignancies, a role for IGF-1 signaling has been demonstrated in multiple myeloma (MM) where it stimulates growth and potently mediates survival.⁹ Several anti-IGF-1R strategies have been shown to inhibit MM growth.^{10, 11} In AML, expression of the IGF-1R and IGF-1 was detected in AML cell lines and primary AML blasts and stimulation with IGF-1 can promote the growth of AML cells.^{12, 13, 14} In addition, neutralizing IGF-1R antibodies and the tyrosine kinase inhibitors (TKIs) NVP-AEW541 and NVP-ADW742, have been shown to inhibit proliferation and to induce apoptosis.^{15, 16}In addition to its mitogenic and anti-apoptotic roles, directly influencing tumor development, IGF-1R appears to be a critical determinant of response to numerous anti-cancer therapies, including TKIs and chemotherapy.^{2, 3, 17, 18, 19, 20, 21, 22} In AML, activated IGF-1R signaling has been linked to cytarabine resistance, a drug included in every AML treatment schedule.¹⁷ Notably, in several cancer cell lines, a small subpopulation of drug-tolerant cancer cells exists that maintains their viability, after treatment with a lethal drug dose, via engagement of the IGF-1R.¹⁸The activity of the IGF-1R is tightly controlled at multiple levels, including their processing, endocytosis, trafficking and availability of its ligands.⁴ Ligand bioavailability is partly controlled by the family of secreted insulin-like growth factor-binding protein (IGFBP1 to IGFBP6), which can bind to IGFs therewith regulating the interaction of these ligands to their receptors. However, as IGFBPs are able to induce IGF-dependent and IGF-independent effects, the results of several studies on their role in cancer cell survival appeared to be controversial and complex.^{23, 24} In addition to IGFBPs, various IGFBP-related proteins have been identified.^{23, 25} One of these is the IGFB-related protein 1, also known as insulin-like growth factor-binding protein-7 (IGFBP7). IGFBP7 has 30% homology to IGFBP1 to IGFBP6 in its N-terminal domain and functions predominantly as a tumor suppressor.^{23, 24, 25, 26} In contrast to IGFBP1 to IGFBP6, which bind to the IGFs,²³ IGFBP7 is a secreted protein that can directly bind to the IGF-1R and thereby inhibits its activity.²⁷ The abundance of IGFBP7 is inversely correlated with tumor progression in hepatocellular carcinoma.²⁸ Importantly, decreased expression of IGFBP7 has been associated with therapy resistance^{29, 30} and increasing IGFBP7 levels can inhibit melanoma and breast cancer growth.^{31, 32} IGFBP7 was originally identified as being involved in Raf-mediated apoptosis and senescence³³ and also has been shown to induce senescence in mesenchymal stromal cells.³⁴We established that IGFBP7 induces a cell cycle block and apoptosis in AML cells and cooperates with chemotherapy in the induction of leukemia cell death. AML patients with low IGFBP7 expression have a worse outcome than patients with high IGFBP7 expression, indicating that AML patients might benefit from a combination therapy consisting of chemotherapy and IGFBP7. Our results define IGFBP7 as a focus to enhance chemotherapy efficacy and improve AML patient survival.     

The mechanisms of IDH mutations in tumorigenesis

Tumor-associated mutations in the isocitrate dehydrogenase 1 and 2 (IDH1 and IDH2) genes result in the loss of normal catalytic activity, the production of α-ketoglutarate (α-KG), and gain of a new activity, the production of an oncometabolite, R-2-hydroxylglutarate (R-2-HG). New evidence supports previous findings that R-2-HG acts as an antagonist of α-KG to competitively inhibit the activity of multiple α-KG-dependent dioxygenases, including both histones and DNA demethylases involved in epigenetic control of gene expression and cell differentiation, and also reveals an intriguing new facet of R-2-HG in tumorigenesis.The NADP⁺-dependent isocitrate dehydrogenase IDH1 and IDH2 catalyze the oxidative decarboxylation of isocitrate to α-ketoglutarate (α-KG). IDH1 and IDH2 are localized in the cytoplasm and mitochondria, respectively, and represent by far the most frequently mutated metabolic enzymes in human cancer¹. The tumor-derived mutants of both IDH1 and IDH2 lose their activity in producing α-KG^2,3, and gain a surprising new catalytic activity, the production of R-2-hydroxyglutarate (R-2-HG) by reduction of α-KG⁴. Previous studies have shown that R-2-HG acts as an antagonist of α-KG to competitively inhibit a number of α-KG-dependent dioxygenases, including the JmjC domain-containing histone demethylases (KDMs) and the TET (ten-eleven translocation) family of DNA hydroxylases that catalyze the sequential oxidation of 5-methlycytosine (5mC) to 5-hydroxymethylcytosine (5hmC), 5-formylcytosine (5fC), and 5-carboxylcytosine (5caC), leading to eventual DNA demethylation (Figure 1)^5,6. Three papers recently published in Nature provide additional evidence that α-KG-dependent dioxygenases are the pathophysiological targets of mutant IDH1/2, and further underscore the presumptive role of R-2-HG as the first oncometabolite in contributing to tumorigenesis after IDH1/2 mutations.Open in a separate windowFigure 1Summarization of reported mechanisms linking IDH mutation to tumorigensis. Regulation of α-KG-dependent dioxygenases by R-2-HG is likely to play a major role in the pathophysiology of tumors with IDH mutation.A subset of glioblastoma, known as the proneural subgroup, has previously found to display hypermethylation at a large number of loci and is enriched with IDH1 mutations⁷. In one of the three Nature papers, Turcan et al.⁸ determined whether IDH1 mutation alone is sufficient to cause the hypermethylation phenotype by ectopic expression of IDH1^R132H mutant in immortalized primary human astrocytes, a cell type from which glioblastoma is believed to develop. The authors found that introduction of mutant IDH1 induced extensive DNA hypermethylation, altered the methylation of specific histones, and reshaped the methylome in a fashion that mirrors the changes observed in IDH1-mutated low-grade gliomas. The observed hypermethylation of DNA and histones can be explained by the direct inhibition of TET methylcytosine hydroxylases and JmjC family histone demethylases by R-2-HG, respectively. In keeping with the notion that TET hydroxylases directly regulate genomic DNA methylation levels and can be inhibited by the R-2-HG accumulated in IDH1/2-mutated cells, Turcan et al. also showed that ectopic expression of TET2 in cultured astrocytes decreased 5mC and increased 5hmC, and that both changes were inhibited by the co-expression of TET2 with mutant IDH1. These results are consistent with the findings made in acute myeloid leukemia (AML) in which IDH1/2 and TET2 genes are mutated in a mutually exclusive manner⁹. Moreover, Turcan et al. found that expression of wild-type IDH1 decreased the average DNA methylation level in the genome, supporting the notion that the concentration of α-KG may be a rate-limiting factor of TET-catalyzed DNA demethylation⁵.In the second paper, Lu et al.¹⁰ reported that ectopic expression of tumor-derived mutant IDH1/2 or feeding cells with cell-permeable R-2-HG increases histone demethylation and results in blockade of the differentiation of 3T3-L1 adipoblasts to adipocytes. These results indicate that mutation of IDH1/2 and accumulation of R-2-HG can broadly impair cell differentiation beyond the cell types in which IDH1/2 mutations are found to associate with tumorigenesis. The authors further confirmed that IDH1-mutated gliomas have elevated levels of histone methylation compared with gliomas retaining the wild-type IDH1^5,6. As previously reported^5,6, multiple KDMs that are inhibited by 2-HG, including KDM4C/JMJD2C, which causes repressive histone H3K9 di- and trimethylation and, when suppressed by RNA interference, blocks the 3T3-L1 adipogenesis. It remains to be determined whether collective inhibition of multiple KDMs or a few individual ones, such as KDM4C, is responsible for altering cell differentiation in IDH1/2-mutated cells. The authors also noted that expression of mutant IDH1 increased histone methylation prior to the increase of DNA methylation, raising an intriguing possibility that histone methylation status may affect DNA methylation.In the third paper, Koivunen et al.¹¹ proposed an enantiomer-specific mechanism of 2-HG in tumorigenesis. The authors reported two surprising findings. They showed first that immortalized human astrocytes stably expressing tumor-derived IDH1^R132H mutant proliferate faster during late passages than those expressing either wild-type IDH1 or IDH1^R132H/3DN mutant that lacks 2-HG-producing activity. Ectopic expression of R132H mutant IDH1 has previously been reported to decrease the growth of D54 glioblastoma cells¹², raising an intriguing possibility that the mutation of IDH1/2 may exhibit different effects on cell growth in a cell context-dependent manner. More surprisingly, they found that R-2-HG, but not its enantiomer S-2-HG, substitutes for α-KG as a co-substrate, as opposed to an inhibitor, of EGLN, an α-KG-dependent prolylhydroxylase responsible for promoting the degradation of hypoxia inducible factor 1α (HIF-1α) (Figure 1). As the result of stimulating EGLN, accumulation of R-2-HG was found to associate with diminished, instead of increased, HIF-1α levels in cells expressing mutant IDH1/2. At first glance, these observations appear to be at odds with the generally accepted role of both enantiomers of 2-HG as inhibitors of α-KG-dependent dioxygenases, and HIF-1α as an oncogene in tumorigenesis, but may at least in part explain the apparent selection for IDH mutations to produce R-, but not S-2-HG in cancer. This data, also for the first time, reveals a qualitatively different property of two 2-HG enantiomers with respect to α-KG-dependent dioxygenases. It will be interesting to determine the strutural basis of this enantimoer-specific effect of 2-HG toward different α-KG-dependent dixoygenases. The observation that ectopic increase of R-2-HG reduces HIF-1α suggests that endogenous α-KG is limiting for HIF-1α hydroxylation by EGLN. The study by Koivunen et al. also suggests the complexity of EGLN regulation by R-2-HG and subsequent downregulation of HIF-1α. It remains to be determined genetically whether a reduction or fluctuation of HIF-1α levels contributes to gliomagenesis in IDH1/2-mutated cells, because elevated HIF-1α generally contributes to cancer development. The only piece of genetic evidece—IDH1/2 mutation occurs in a mutually exclusive manner with TET2 mutation in AML—supports the notion that epigenetic alteration plays a direct and perhaps a key role in IDH1/2 mutation-associated tumorigenesis.IDH1/2 mutation has rapidly emerged as a favorable diagnostic and prognostic marker for certain tumors, such as low-grade gliomas and benign cartilaginous tumors. While the full mechanism linking IDH mutation to tumorigenesis is incompletely understood, regulation of α-KG-dependent dioxygenases by 2-HG is likely to play a major role in the pathophysiology of tumors with IDH mutation. These recent reports also highlight the impact of altered metabolism and metabolites on the epigenetic modification of cell differentiation and tumorigenesis.     

D-2-Hydroxyglutarate does not mimic all the IDH mutation effects,in particular the reduced etoposide-triggered apoptosis mediated by an alteration in mitochondrial NADH

Somatic mutations in isocitrate dehydrogenase (IDH)-1 and -2 have recently been described in glioma. This mutation leads to a neomorphic enzymatic activity as the conversion of isocitrate to alpha ketoglutarate (αKG) is replaced by the conversion of αKG to D-2-hydroxyglutarate (D-2HG) with NADPH oxidation. It has been suggested that this oncometabolite D-2HG via inhibition of αKG-dioxygenases is involved in multiple functions such as epigenetic modifications or hypoxia responses. The present study is aimed at deciphering how the mutant IDH can affect cancer pathogenesis, in particular with respect to its associated oncometabolite D-2HG. We show that the overexpression of mutant IDH in glioma cells or treatment with D-2HG triggered an increase in cell proliferation. However, although mutant IDH reduced cell sensitivity to the apoptotic inducer etoposide, D-2HG exhibited no effect on apoptosis. Instead, we found that the apoptotic effect was mediated through the mitochondrial NADH pool reduction and could be inhibited by oxamate. These data show that besides D-2HG production, mutant IDH affects other crucial metabolite pools. These observations lead to a better understanding of the biology of IDH mutations in gliomas and their response to therapy.Gliomas are the most common type of human brain tumors and can be classified based on clinical and pathological criteria in four grades. The grade IV glioma, commonly known as glioblastoma multiforme (GBM), is the most invasive form and has a dismal prognosis with <5% patient survival at 5 years. These GBM can develop de novo (primary GBM) or through the progression from low-grade tumors (secondary GBM). Although these two types of GBM are histologically similar, primary and secondary GBM exhibit distinct genetic patterns. A recent integrated genome analysis of human GBM shows that 12% of these tumors have a mutation in the gene encoding isocitrate dehydrogenase 1 (IDH1) and to a lesser extent in IDH2 gene.¹ This mutation is present in >90% secondary GBMs, whereas it is present in <5% primary GBMs.² Mutations in IDH1 and IDH2 have also been identified in acute myeloid leukemia (AML)³ and chondrosarcomas.⁴ The occurrence of IDH mutations predicts a significantly longer survival for patients affected by GBM or grade III gliomas.^{1, 2} Whether this difference is driven by IDH mutations or reflects other fundamental biological differences between primary and secondary GBM is, as yet, unclear. For example, the prognostic significance of IDH mutations may be secondary to their prevalence among younger patients, as age is a well-known prognostic factor in gliomas.⁵ In AML, the prognostic significance of IDH mutations is more ambiguous. Several studies have reported that IDH mutations do not affect the prognosis in AML, whereas other studies have found that IDH mutations are associated with an increased or decreased risk of relapse when compared with IDH wild-type patients.^{6, 7}The human genome has five IDH genes coding for three different IDH isoforms, the activities of which depend on either nicotinamide adenine dinucleotide (NAD+) for IDH3 or nicotinamide adenine dinucleotide phosphate (NADP+) for IDH1 and IDH2. Both IDH2 and IDH3 are located in the mitochondria where they participate in the TCA cycle, whereas IDH1 is mostly cytosolic.⁸ To date, all reported mutations are located in the IDH1 and IDH2 genes and result in an amino-acid substitution at residues located in the enzymatic active site, respectively, R132 for IDH1 and R140 or R172 for IDH2. This mutation disrupts the normal enzymatic function of IDH, that is, the conversion of isocitrate to alpha ketoglutarate (αKG) with the concomitant production of NADPH. Instead, mutant IDH displays a neomorphic activity converting αKG into D-2-hydroxyglutarate (D-2HG), although reducing NADPH.⁹ As a result, mutant IDH may alter the redox state of cells, modulate the activity of metabolic and epigenetic tumor suppressor enzymes that use αKG as a co-substrate.¹⁰ Loss of IDH function may also alter normal mitochondrial function and promote a metabolic switch in cancer cells to glycolysis.^{11, 12}Mutant IDH is widely believed to have the ability to transform cells by modulating αKG-dependent enzymes. D-2HG and αKG are structurally similar suggesting that D-2HG may act as a competitive inhibitor of αKG-dioxygenases including prolyl hydroxylase involved in HIF-1α stability, histone demethylases and the Ten-Eleven Translocation (TET) family of 5-methylcytosine hydroxylases involved in epigenetic modifications of DNA.^{13, 14} In fact, IDH mutations lead to numerous metabolic abnormalities besides D-2HG production. Deciphering the relative importance of either D-2HG production, αKG or NADPH reduction in cancer pathogenesis remains to be determined. In this paper, we show that mutant IDH increases cell proliferation and reduces etoposide (ETO)-induced cell death through different metabolic pathways. Although cell proliferation changes are mediated through D-2HG, alteration in the mitochondrial NADH pool is involved in the response to apoptosis.     

The Fcp1-Wee1-Cdk1 axis affects spindle assembly checkpoint robustness and sensitivity to antimicrotubule cancer drugs

To grant faithful chromosome segregation, the spindle assembly checkpoint (SAC) delays mitosis exit until mitotic spindle assembly. An exceedingly prolonged mitosis, however, promotes cell death and by this means antimicrotubule cancer drugs (AMCDs), that impair spindle assembly, are believed to kill cancer cells. Despite malformed spindles, cancer cells can, however, slip through SAC, exit mitosis prematurely and resist killing. We show here that the Fcp1 phosphatase and Wee1, the cyclin B-dependent kinase (cdk) 1 inhibitory kinase, play a role for this slippage/resistance mechanism. During AMCD-induced prolonged mitosis, Fcp1-dependent Wee1 reactivation lowered cdk1 activity, weakening SAC-dependent mitotic arrest and leading to mitosis exit and survival. Conversely, genetic or chemical Wee1 inhibition strengthened the SAC, further extended mitosis, reduced antiapoptotic protein Mcl-1 to a minimum and potentiated killing in several, AMCD-treated cancer cell lines and primary human adult lymphoblastic leukemia cells. Thus, the Fcp1-Wee1-Cdk1 (FWC) axis affects SAC robustness and AMCDs sensitivity.The spindle assembly checkpoint (SAC) delays mitosis exit to coordinate anaphase onset with spindle assembly. To this end, SAC inhibits the ubiquitin ligase Anaphase-Promoting Complex/Cyclosome (APC/C) to prevent degradation of the anaphase inhibitor securin and cyclin B, the major mitotic cyclin B-dependent kinase 1 (cdk1) activator, until spindle assembly.¹ However, by yet poorly understood mechanisms, exceedingly prolonging mitosis translates into cell death induction.^{2, 3, 4, 5, 6, 7} Although mechanistic details are still missing on how activation of cell death pathways is linked to mitosis duration, prolongation of mitosis appears crucial for the ability of antimicrotubule cancer drugs (AMCDs) to kill cancer cells.^{2, 3, 4, 5, 6, 7} These drugs, targeting microtubules, impede mitotic spindle assembly and delay mitosis exit by chronically activating the SAC. Use of these drugs is limited, however, by toxicity and resistance. A major mechanism for resistance is believed to reside in the ability of cancer cells to slip through the SAC and exit mitosis prematurely despite malformed spindles, thus resisting killing by limiting mitosis duration.^{2, 3, 4, 5, 6, 7} Under the AMCD treatment, cells either die in mitosis or exit mitosis, slipping through the SAC, without or abnormally dividing.^{2, 3, 4} Cells that exit mitosis either die at later stages or survive and stop dividing or proliferate, giving rise to resistance.^{2, 3, 4} Apart from a role for p53, what dictates cell fate is still unknown; however, it appears that the longer mitosis is protracted, the higher the chances for cell death pathway activation are.^{2, 3, 4, 5, 6, 7}Although SAC is not required per se for killing,⁶ preventing SAC adaptation should improve the efficacy of AMCD by increasing mitosis duration.^{2, 3, 4, 5, 6, 7} Therefore, further understanding of the mechanisms by which cells override SAC may help to improve the current AMCD therapy. Several kinases are known to activate and sustain SAC, and cdk1 itself appears to be of primary relevance.^{1, 8, 9} By studying mitosis exit and SAC resolution, we recently reported a role for the Fcp1 phosphatase to bring about cdk1 inactivation.^{10, 11} Among Fcp1 targets, we identified cyclin degradation pathway components, such as Cdc20, an APC/C co-activator, USP44, a deubiquitinating enzyme, and Wee1.^{10, 11} Wee1 is a crucial kinase that controls the G2 phase by performing inhibitory phosphorylation of cdk1 at tyr-15 (Y15-cdk1). Wee1 is also in a feedback relationship with cdk1 itself that, in turn, can phosphorylate and inhibit Wee1 in an autoamplification loop to promote the G2-to-M phase transition.¹² At mitosis exit, Fcp1 dephosphorylated Wee1 at threonine 239, a cdk1-dependent inhibitory phosphorylation, to dampen down the cdk1 autoamplification loop, and Cdc20 and USP44, to promote APC/C-dependent cyclin B degradation.^{10, 11, 12} In this study we analysed the Fcp1 relevance in SAC adaptation and AMCD sensitivity.     

Induction of COX-2-PGE2 synthesis by activation of the MAPK/ERK pathway contributes to neuronal death triggered by TDP-43-depleted microglia

Neuroinflammation is a striking hallmark of amyotrophic lateral sclerosis (ALS) and other neurodegenerative disorders. Previous studies have shown the contribution of glial cells such as astrocytes in TDP-43-linked ALS. However, the role of microglia in TDP-43-mediated motor neuron degeneration remains poorly understood. In this study, we show that depletion of TDP-43 in microglia, but not in astrocytes, strikingly upregulates cyclooxygenase-2 (COX-2) expression and prostaglandin E2 (PGE2) production through the activation of MAPK/ERK signaling and initiates neurotoxicity. Moreover, we find that administration of celecoxib, a specific COX-2 inhibitor, greatly diminishes the neurotoxicity triggered by TDP-43-depleted microglia. Taken together, our results reveal a previously unrecognized non-cell-autonomous mechanism in TDP-43-mediated neurodegeneration, identifying COX-2-PGE2 as the molecular events of microglia- but not astrocyte-initiated neurotoxicity and identifying celecoxib as a novel potential therapy for TDP-43-linked ALS and possibly other types of ALS.Amyotrophic lateral sclerosis (ALS) is an adult-onset neurodegenerative disease characterized by the degeneration of motor neurons in the brain and spinal cord.¹ Most cases of ALS are sporadic, but 10% are familial. Familial ALS cases are associated with mutations in genes such as Cu/Zn superoxide dismutase 1 (SOD1), TAR DNA-binding protein 43 (TARDBP) and, most recently discovered, C9orf72. Currently, most available information obtained from ALS research is based on the study of SOD1, but new studies focusing on TARDBP and C9orf72 have come to the forefront of ALS research.^{1, 2} The discovery of the central role of the protein TDP-43, encoded by TARDBP, in ALS was a breakthrough in ALS research.^{3, 4, 5} Although pathogenic mutations of TDP-43 are genetically rare, abnormal TDP-43 function is thought to be associated with the majority of ALS cases.¹ TDP-43 was identified as a key component of the ubiquitin-positive inclusions in most ALS patients and also in other neurodegenerative diseases such as frontotemporal lobar degeneration,^{6, 7} Alzheimer''s disease (AD)^{8, 9} and Parkinson''s disease (PD).^{10, 11} TDP-43 is a multifunctional RNA binding protein, and loss-of-function of TDP-43 has been increasingly recognized as a key contributor in TDP-43-mediated pathogenesis.^{5, 12, 13, 14}Neuroinflammation, a striking and common hallmark involved in many neurodegenerative diseases, including ALS, is characterized by extensive activation of glial cells including microglia, astrocytes and oligodendrocytes.^{15, 16} Although numerous studies have focused on the intrinsic properties of motor neurons in ALS, a large amount of evidence showed that glial cells, such as astrocytes and microglia, could have critical roles in SOD1-mediated motor neuron degeneration and ALS progression,^{17, 18, 19, 20, 21, 22} indicating the importance of non-cell-autonomous toxicity in SOD1-mediated ALS pathogenesis.Very interestingly, a vital insight of neuroinflammation research in ALS was generated by the evidence that both the mRNA and protein levels of the pro-inflammatory enzyme cyclooxygenase-2 (COX-2) are upregulated in both transgenic mouse models and in human postmortem brain and spinal cord.^{23, 24, 25, 26, 27, 28, 29} The role of COX-2 neurotoxicity in ALS and other neurodegenerative disorders has been well explored.^{30, 31, 32} One of the key downstream products of COX-2, prostaglandin E2 (PGE2), can directly mediate COX-2 neurotoxicity both in vitro and in vivo.^{33, 34, 35, 36, 37} The levels of COX-2 expression and PGE2 production are controlled by multiple cell signaling pathways, including the mitogen-activated protein kinase (MAPK)/ERK pathway,^{38, 39, 40} and they have been found to be increased in neurodegenerative diseases including AD, PD and ALS.^{25, 28, 32, 41, 42, 43, 44, 45, 46} Importantly, COX-2 inhibitors such as celecoxib exhibited significant neuroprotective effects and prolonged survival or delayed disease onset in a SOD1-ALS transgenic mouse model through the downregulation of PGE2 release.²⁸Most recent studies have tried to elucidate the role of glial cells in neurotoxicity using TDP-43-ALS models, which are considered to be helpful for better understanding the disease mechanisms.^{47, 48, 49, 50, 51} Although the contribution of glial cells to TDP-43-mediated motor neuron degeneration is now well supported, this model does not fully suggest an astrocyte-based non-cell autonomous mechanism. For example, recent studies have shown that TDP-43-mutant astrocytes do not affect the survival of motor neurons,^{50, 51} indicating a previously unrecognized non-cell autonomous TDP-43 proteinopathy that associates with cell types other than astrocytes.Given that the role of glial cell types other than astrocytes in TDP-43-mediated neuroinflammation is still not fully understood, we aim to compare the contribution of microglia and astrocytes to neurotoxicity in a TDP-43 loss-of-function model. Here, we show that TDP-43 has a dominant role in promoting COX-2-PGE2 production through the MAPK/ERK pathway in primary cultured microglia, but not in primary cultured astrocytes. Our study suggests that overproduction of PGE2 in microglia is a novel molecular mechanism underlying neurotoxicity in TDP-43-linked ALS. Moreover, our data identify celecoxib as a new potential effective treatment of TDP-43-linked ALS and possibly other types of ALS.     

A cellular screen identifies ponatinib and pazopanib as inhibitors of necroptosis

Necroptosis is a form of regulated necrotic cell death mediated by receptor-interacting serine/threonine-protein kinase 1 (RIPK1) and RIPK3. Necroptotic cell death contributes to the pathophysiology of several disorders involving tissue damage, including myocardial infarction, stroke and ischemia-reperfusion injury. However, no inhibitors of necroptosis are currently in clinical use. Here we performed a phenotypic screen for small-molecule inhibitors of tumor necrosis factor-alpha (TNF-α)-induced necroptosis in Fas-associated protein with death domain (FADD)-deficient Jurkat cells using a representative panel of Food and Drug Administration (FDA)-approved drugs. We identified two anti-cancer agents, ponatinib and pazopanib, as submicromolar inhibitors of necroptosis. Both compounds inhibited necroptotic cell death induced by various cell death receptor ligands in human cells, while not protecting from apoptosis. Ponatinib and pazopanib abrogated phosphorylation of mixed lineage kinase domain-like protein (MLKL) upon TNF-α-induced necroptosis, indicating that both agents target a component upstream of MLKL. An unbiased chemical proteomic approach determined the cellular target spectrum of ponatinib, revealing key members of the necroptosis signaling pathway. We validated RIPK1, RIPK3 and transforming growth factor-β-activated kinase 1 (TAK1) as novel, direct targets of ponatinib by using competitive binding, cellular thermal shift and recombinant kinase assays. Ponatinib inhibited both RIPK1 and RIPK3, while pazopanib preferentially targeted RIPK1. The identification of the FDA-approved drugs ponatinib and pazopanib as cellular inhibitors of necroptosis highlights them as potentially interesting for the treatment of pathologies caused or aggravated by necroptotic cell death.Programmed cell death has a crucial role in a variety of biological processes ranging from normal tissue development to diverse pathological conditions.^{1, 2} Necroptosis is a form of regulated cell death that has been shown to occur during pathogen infection or sterile injury-induced inflammation in conditions where apoptosis signaling is compromised.^{3, 4, 5, 6} Given that many viruses have developed strategies to circumvent apoptotic cell death, necroptosis constitutes an important, pro-inflammatory back-up mechanism that limits viral spread in vivo.^{7, 8, 9} In contrast, in the context of sterile inflammation, necroptotic cell death contributes to disease pathology, outlining potential benefits of therapeutic intervention.¹⁰ Necroptosis can be initiated by death receptors of the tumor necrosis factor (TNF) superfamily,¹¹ Toll-like receptor 3 (TLR3),¹² TLR4,¹³ DNA-dependent activator of IFN-regulatory factors¹⁴ or interferon receptors.¹⁵ Downstream signaling is subsequently conveyed via RIPK1¹⁶ or TIR-domain-containing adapter-inducing interferon-β,^{8, 17} and converges on RIPK3-mediated^{13, 18, 19, 20} activation of MLKL.²¹ Phosphorylated MLKL triggers membrane rupture,^{22, 23, 24, 25, 26} releasing pro-inflammatory cellular contents to the extracellular space.²⁷ Studies using the RIPK1 inhibitor necrostatin-1 (Nec-1) ²⁸ or RIPK3-deficient mice have established a role for necroptosis in the pathophysiology of pancreatitis,¹⁹ artherosclerosis,²⁹ retinal cell death,³⁰ ischemic organ damage and ischemia-reperfusion injury in both the kidney³¹ and the heart.³² Moreover, allografts from RIPK3-deficient mice are better protected from rejection, suggesting necroptosis inhibition as a therapeutic option to improve transplant outcome.³³ Besides Nec-1, several tool compounds inhibiting different pathway members have been described,^{12, 16, 21, 34, 35} however, no inhibitors of necroptosis are available for clinical use so far.^{2, 10} In this study we screened a library of FDA approved drugs for the precise purpose of identifying already existing and generally safe chemical agents that could be used as necroptosis inhibitors. We identified the two structurally distinct kinase inhibitors pazopanib and ponatinib as potent blockers of necroptosis targeting the key enzymes RIPK1/3.     

Stress-induced ceramide generation and apoptosis via the phosphorylation and activation of nSMase1 by JNK signaling

Neutral sphingomyelinase (nSMase) activation in response to environmental stress or inflammatory cytokine stimuli generates the second messenger ceramide, which mediates the stress-induced apoptosis. However, the signaling pathways and activation mechanism underlying this process have yet to be elucidated. Here we show that the phosphorylation of nSMase1 (sphingomyelin phosphodiesterase 2, SMPD2) by c-Jun N-terminal kinase (JNK) signaling stimulates ceramide generation and apoptosis and provide evidence for a signaling mechanism that integrates stress- and cytokine-activated apoptosis in vertebrate cells. An nSMase1 was identified as a JNK substrate, and the phosphorylation site responsible for its effects on stress and cytokine induction was Ser-270. In zebrafish cells, the substitution of Ser-270 for alanine blocked the phosphorylation and activation of nSMase1, whereas the substitution of Ser-270 for negatively charged glutamic acid mimicked the effect of phosphorylation. The JNK inhibitor SP600125 blocked the phosphorylation and activation of nSMase1, which in turn blocked ceramide signaling and apoptosis. A variety of stress conditions, including heat shock, UV exposure, hydrogen peroxide treatment, and anti-Fas antibody stimulation, led to the phosphorylation of nSMase1, activated nSMase1, and induced ceramide generation and apoptosis in zebrafish embryonic ZE and human Jurkat T cells. In addition, the depletion of MAPK8/9 or SMPD2 by RNAi knockdown decreased ceramide generation and stress- and cytokine-induced apoptosis in Jurkat cells. Therefore the phosphorylation of nSMase1 is a pivotal step in JNK signaling, which leads to ceramide generation and apoptosis under stress conditions and in response to cytokine stimulation. nSMase1 has a common central role in ceramide signaling during the stress and cytokine responses and apoptosis.The sphingomyelin pathway is initiated by the hydrolysis of sphingomyelin to generate the second messenger ceramide.¹ Sphingomyelin hydrolysis is a major pathway for stress-induced ceramide generation. Neutral sphingomyelinase (nSMase) is activated by a variety of environmental stress conditions, such as heat shock,^{1, 2, 3} oxidative stress (hydrogen peroxide (H₂O₂), oxidized lipoproteins),¹ ultraviolet (UV) radiation,¹ chemotherapeutic agents,⁴ and β-amyloid peptides.^{5, 6} Cytokines, including tumor necrosis factor (TNF)-α,^{7, 8, 9} interleukin (IL)-1β,¹⁰ Fas ligand,¹¹ and their associated proteins, also trigger the activation of nSMase.¹² Membrane-bound Mg²⁺-dependent nSMase is considered to be a strong candidate for mediating the effects of stress and inflammatory cytokines on ceramide.³Among the four vertebrate nSMases, nSMase1 (SMPD2) was the first to be cloned and is localized in the endoplasmic reticulum (ER) and Golgi apparatus.¹³ Several studies have focused on the potential signaling roles of nSMase1, and some reports have suggested that nSMase1 is important for ceramide generation in response to stress.^{5, 6, 14, 15} In addition, nSMase1 is responsible for heat-induced apoptosis in zebrafish embryonic cultured (ZE) cells, and a loss-of-function study showed a reduction in ceramide generation, caspase-3 activation, and apoptosis in zebrafish embryos.¹⁶ However, nSMase1-knockout mice showed no lipid storage diseases or abnormalities in sphingomyelin metabolism.¹⁷ Therefore, the molecular mechanisms by which nSMase1 is activated have yet to be elucidated.Environmental stress and inflammatory cytokines^{1, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27} stimulate stress-activated protein kinase (SAPK)/c-Jun N-terminal kinase (JNK) signaling, which involves the sequential activation of members of the mitogen-activated protein kinase (MAPK) family, including MAPK/ERK kinase kinase (MEKK)1/MAPK kinase (MKK)4, and/or SAPK/ERK kinase (SEK)1/MKK7, JNK, and c-jun. Both the JNK and sphingomyelin signaling pathways coordinately mediate the induction of apoptosis.¹ However, possible crosstalk between the JNK and sphingomyelin signaling pathways has not yet been characterized. Previously, we used SDS-PAGE to determine that nSMase1 polypeptides migrated at higher molecular masses,¹⁶ suggesting that the sphingomyelin signaling pathway might cause the production of a chemically modified phosphorylated nSMase1, which is stimulated under stressed conditions in ZE cells.¹⁶ Here, we demonstrate that JNK signaling results in the phosphorylation of Ser-270 of nSMase1, which initiates ceramide generation and apoptosis. We also provide evidence for a signaling mechanism that integrates cytokine- and stress-activated apoptosis in vertebrate cells. We studied stress-induced ceramide generation in two cell types: ZE cells and human leukemia Jurkat T-lymphoid cells. Stress-induced apoptosis has been investigated in these systems previously.^{16, 28}     

TRAF2 is a biologically important necroptosis suppressor

Tumor necrosis factor α (TNFα) triggers necroptotic cell death through an intracellular signaling complex containing receptor-interacting protein kinase (RIPK) 1 and RIPK3, called the necrosome. RIPK1 phosphorylates RIPK3, which phosphorylates the pseudokinase mixed lineage kinase-domain-like (MLKL)—driving its oligomerization and membrane-disrupting necroptotic activity. Here, we show that TNF receptor-associated factor 2 (TRAF2)—previously implicated in apoptosis suppression—also inhibits necroptotic signaling by TNFα. TRAF2 disruption in mouse fibroblasts augmented TNFα–driven necrosome formation and RIPK3-MLKL association, promoting necroptosis. TRAF2 constitutively associated with MLKL, whereas TNFα reversed this via cylindromatosis-dependent TRAF2 deubiquitination. Ectopic interaction of TRAF2 and MLKL required the C-terminal portion but not the N-terminal, RING, or CIM region of TRAF2. Induced TRAF2 knockout (KO) in adult mice caused rapid lethality, in conjunction with increased hepatic necrosome assembly. By contrast, TRAF2 KO on a RIPK3 KO background caused delayed mortality, in concert with elevated intestinal caspase-8 protein and activity. Combined injection of TNFR1-Fc, Fas-Fc and DR5-Fc decoys prevented death upon TRAF2 KO. However, Fas-Fc and DR5-Fc were ineffective, whereas TNFR1-Fc and interferon α receptor (IFNAR1)-Fc were partially protective against lethality upon combined TRAF2 and RIPK3 KO. These results identify TRAF2 as an important biological suppressor of necroptosis in vitro and in vivo.Apoptotic cell death is mediated by caspases and has distinct morphological features, including membrane blebbing, cell shrinkage and nuclear fragmentation.^{1, 2, 3, 4} In contrast, necroptotic cell death is caspase-independent and is characterized by loss of membrane integrity, cell swelling and implosion.^{1, 2, 5} Nevertheless, necroptosis is a highly regulated process, requiring activation of RIPK1 and RIPK3, which form the core necrosome complex.^{1, 2, 5} Necrosome assembly can be induced via specific death receptors or toll-like receptors, among other modules.^{6, 7, 8, 9} The activated necrosome engages MLKL by RIPK3-mediated phosphorylation.^{6, 10, 11} MLKL then oligomerizes and binds to membrane phospholipids, forming pores that cause necroptotic cell death.^{10, 12, 13, 14, 15} Unchecked necroptosis disrupts embryonic development in mice and contributes to several human diseases.^{7, 8, 16, 17, 18, 19, 20, 21, 22}The apoptotic mediators FADD, caspase-8 and cFLIP suppress necroptosis.^{19, 20, 21, 23, 24} Elimination of any of these genes in mice causes embryonic lethality, subverted by additional deletion of RIPK3 or MLKL.^{19, 20, 21, 25} Necroptosis is also regulated at the level of RIPK1. Whereas TNFα engagement of TNFR1 leads to K63-linked ubiquitination of RIPK1 by cellular inhibitor of apoptosis proteins (cIAPs) to promote nuclear factor (NF)-κB activation,²⁶ necroptosis requires suppression or reversal of this modification to allow RIPK1 autophosphorylation and consequent RIPK3 activation.^{2, 23, 27, 28} CYLD promotes necroptotic signaling by deubiquitinating RIPK1, augmenting its interaction with RIPK3.²⁹ Conversely, caspase-8-mediated CYLD cleavage inhibits necroptosis.²⁴TRAF2 recruits cIAPs to the TNFα-TNFR1 signaling complex, facilitating NF-κB activation.^{30, 31, 32, 33} TRAF2 also supports K48-linked ubiquitination and proteasomal degradation of death-receptor-activated caspase-8, curbing apoptosis.³⁴ TRAF2 KO mice display embryonic lethality; some survive through birth but have severe developmental and immune deficiencies and die prematurely.^{35, 36} Conditional TRAF2 KO leads to rapid intestinal inflammation and mortality.³⁷ Furthermore, hepatic TRAF2 depletion augments apoptosis activation via Fas/CD95.³⁴ TRAF2 attenuates necroptosis induction in vitro by the death ligands Apo2L/TRAIL and Fas/CD95L.³⁸ However, it remains unclear whether TRAF2 regulates TNFα-induced necroptosis—and if so—how. Our present findings reveal that TRAF2 inhibits TNFα necroptotic signaling. Furthermore, our results establish TRAF2 as a biologically important necroptosis suppressor in vitro and in vivo and provide initial insight into the mechanisms underlying this function.     

Adiponectin receptor-mediated signaling ameliorates cerebral cell damage and regulates the neurogenesis of neural stem cells at high glucose concentrations: an in vivo and in vitro study

In the central nervous system (CNS), hyperglycemia leads to neuronal damage and cognitive decline. Recent research has focused on revealing alterations in the brain in hyperglycemia and finding therapeutic solutions for alleviating the hyperglycemia-induced cognitive dysfunction. Adiponectin is a protein hormone with a major regulatory role in diabetes and obesity; however, its role in the CNS has not been studied yet. Although the presence of adiponectin receptors has been reported in the CNS, adiponectin receptor-mediated signaling in the CNS has not been investigated. In the present study, we investigated adiponectin receptor (AdipoR)-mediated signaling in vivo using a high-fat diet and in vitro using neural stem cells (NSCs). We showed that AdipoR1 protects cell damage and synaptic dysfunction in the mouse brain in hyperglycemia. At high glucose concentrations in vitro, AdipoR1 regulated the survival of NSCs through the p53/p21 pathway and the proliferation- and differentiation-related factors of NSCs via tailless (TLX). Hence, we suggest that further investigations are necessary to understand the cerebral AdipoR1-mediated signaling in hyperglycemic conditions, because the modulation of AdipoR1 might alleviate hyperglycemia-induced neuropathogenesis.Adiponectin secreted by the adipose tissue^{1, 2} exists in either a full-length or globular form.^{3, 4, 5, 6} Adiponectin can cross the blood–brain barrier, and various forms of adiponectin are found in the cerebrospinal fluid.^{7, 8, 9, 10, 11} Adiponectin exerts its effect by binding to the adiponectin receptor 1 (AdipoR1) and adiponectin receptor 2 (AdipoR2)^{12, 13} that have different affinities for the various circulating adiponectins.^{12, 14, 15, 16, 17} Several studies reported that both receptor subtypes are expressed in the central nervous system (CNS).^{7, 12, 18} As adiponectin modulates insulin sensitivity and inflammation,¹⁹ its deficiency induces insulin resistance and glucose intolerance in animals fed a high-fat diet (HFD).^{19, 20, 21} In addition, adiponectin can ameliorate the glucose homeostasis and increase insulin sensitivity.^{22, 23, 24} Adiponectin, which is the most well-known adipokine, acts mainly as an anti-inflammatory regulator,^{25, 26} and is associated with the onset of neurological disorders.²⁷ In addition, a recent study reported that adiponectin promotes the proliferation of hippocampal neural stem cells (NSCs).²⁸ Considering that adiponectin acts by binding to the adiponectin receptors, investigation of the adiponectin receptor-mediated signaling in the brain is crucial to understand the cerebral effects of adiponectin and the underlying cellular mechanisms.The prevalence of type II diabetes mellitus (DM2) and Alzheimer''s disease increases with aging.²⁹ According to a cross-sectional study, in people with DM2, the risk of dementia is 2.5 times higher than that in the normal population.^{30, 31} A study performed between 1980 and 2002 suggested that an elevated blood glucose level is associated with a greater risk for dementia in elderly patients with DM2.³² In addition, according to a 9-year-long longitudinal cohort study, the risk of developing Alzheimer''s disease was 65% higher in people with diabetes than in control subjects.³³ A community-based cohort study also reported that higher plasma glucose concentrations are associated with an increased risk for dementia, because the higher glucose level has detrimental effects on the brain.³¹ High blood glucose level causes mitochondria-dependent apoptosis,^{34, 35, 36} and aggravates diverse neurological functions.^{37, 38} Inflammation and oxidative stress, which are commonly observed in people with diabetes, inhibit neurogenesis.^{39, 40, 41} Similarly, neurogenesis is decreased in mice and rats with genetically induced type I diabetes.^{42, 43} In addition, diabetic rodents have a decreased proliferation rate of neural progenitors.^{43, 44} Furthermore, several studies suggested that an HFD leads to neuroinflammation, the impairment of synaptic plasticity, and cognitive decline.^{45, 46}Here, we investigated whether AdipoR1-mediated signaling is associated with cell death in the brain of mice on a HFD, and whether high glucose level modifies the proliferation and differentiation capacity of NSCs in vitro. Our study provides novel findings about the role of AdipoR1-mediated signaling in hyperglycemia-induced neuropathogenesis.     

Cell cycle-dependent Cdc25C phosphatase determines cell survival by regulating apoptosis signal-regulating kinase 1

Cdc25C (cell division cycle 25C) phosphatase triggers entry into mitosis in the cell cycle by dephosphorylating cyclin B-Cdk1. Cdc25C exhibits basal phosphatase activity during interphase and then becomes activated at the G₂/M transition after hyperphosphorylation on multiple sites and dissociation from 14-3-3. Although the role of Cdc25C in mitosis has been extensively studied, its function in interphase remains elusive. Here, we show that during interphase Cdc25C suppresses apoptosis signal-regulating kinase 1 (ASK1), a member of mitogen-activated protein (MAP) kinase kinase kinase family that mediates apoptosis. Cdc25C phosphatase dephosphorylates phospho-Thr-838 in the activation loop of ASK1 in vitro and in interphase cells. In addition, knockdown of Cdc25C increases the activity of ASK1 and ASK1 downstream targets in interphase cells, and overexpression of Cdc25C inhibits ASK1-mediated apoptosis, suggesting that Cdc25C binds to and negatively regulates ASK1. Furthermore, we showed that ASK1 kinase activity correlated with Cdc25C activation during mitotic arrest and enhanced ASK1 activity in the presence of activated Cdc25C resulted from the weak association between ASK1 and Cdc25C. In cells synchronized in mitosis following nocodazole treatment, phosphorylation of Thr-838 in the activation loop of ASK1 increased. Compared with hypophosphorylated Cdc25C, which exhibited basal phosphatase activity in interphase, hyperphosphorylated Cdc25C exhibited enhanced phosphatase activity during mitotic arrest, but had significantly reduced affinity to ASK1, suggesting that enhanced ASK1 activity in mitosis was due to reduced binding of hyperphosphorylated Cdc25C to ASK1. These findings suggest that Cdc25C negatively regulates proapoptotic ASK1 in a cell cycle-dependent manner and may play a role in G₂/M checkpoint-mediated apoptosis.Cell division cycle 25 (Cdc25) phosphatases are dual-specificity phosphatases involved in cell cycle regulation. By removing inhibitory phosphate groups from phospho-Thr and phospho-Tyr residues of cyclin-dependent kinases (CDKs),¹ Cdc25 proteins regulate cell cycle progression in S phase and mitosis. In mammals, three isoforms of Cdc25 phosphatases have been reported: Cdc25A, which controls the G₁/S transition;^{2, 3} Cdc25B, which is a mitotic starter;⁴ and Cdc25C, which controls the G₂/M phase.⁵ Overexpression of Cdc25 phosphatases is frequently associated with various cancers.⁶ Upon exposure to DNA-damaging reagents like UV radiation or free oxygen radicals, Cdc25 phosphatases are key targets of the checkpoint machinery, resulting in cell cycle arrest and apoptosis. The 14-3-3 proteins bind to phosphorylated Ser-216 of Cdc25C and induce Cdc25C export from the nucleus during interphase in response to DNA damage,^{7, 8} but they have no apparent effect on Cdc25C phosphatase activity.^{9, 10} In addition, hyperphosphorylation of Cdc25C correlates to its enhanced phosphatase activity.¹¹ Most studies with Cdc25C have focused on its role in mitotic progression. However, the role of Cdc25C is not clear when it is sequestered in the cytoplasm by binding to 14-3-3.Apoptosis signal-regulating kinase 1 (ASK1), also known as mitogen-activated protein kinase kinase kinase 5 (MAPKKK5), is a ubiquitously expressed enzyme with a molecular weight of 170 kDa. The kinase activity of ASK1 is stimulated by various cellular stresses, such as H₂O₂,^{12, 13} tumor necrosis factor-α (TNF-α),¹⁴ Fas ligand,¹⁵ serum withdrawal,¹³ and ER stress.¹⁶ Stimulated ASK1 phosphorylates and activates downstream MAP kinase kinases (MKKs) involved in c-Jun N-terminal kinase (JNK) and p38 pathways.^{17, 18, 19} Phosphorylation and activation of ASK1 can induce apoptosis, differentiation, or other cellular responses, depending on the cell type. ASK1 is regulated either positively or negatively depending on its binding proteins.^{12, 13, 15, 18, 19, 20, 21, 22, 23, 24, 25}ASK1 is regulated by phosphorylation at several Ser/Thr/Tyr residues. Phosphorylation at Thr-838 leads to activation of ASK1, whereas phosphorylation at Ser-83, Ser-967, or Ser-1034 inactivates ASK1.^{24, 26, 27, 28} ASK1 is basally phosphorylated at Ser-967 by an unidentified kinase, and 14-3-3 binds to this site to inhibit ASK1.²⁴ Phosphorylation at Ser-83 is known to be catalyzed by Akt or PIM1.^{27, 29} Oligomerization-dependent autophosphorylation at Thr-838, which is located in the activation loop of the kinase domain, is essential for ASK1 activation.^{14, 18, 30} Phosphorylation at Tyr-718 by JAK2 induces ASK1 degradation.³¹ Several phosphatases that dephosphorylate some of these sites have been identified. Serine/threonine protein phosphatase type 5 (PP5) and PP2C dephosphorylate phosphorylated (p)-Thr-838,^{28, 32} whereas PP2A and SHP2 dephosphorylate p-Ser-967 and p-Tyr-718, respectively.^{31, 33} Little is known about the kinase or phosphatase that regulates phosphorylation at Ser-1034. Although ASK1 phosphorylation is known to be involved in the regulation of apoptosis, only a few reports show that ASK1 phosphorylation or activity is dependent on the cell cycle.^{21, 34}In this study, we examined the functional relationship between Cdc25C and ASK1 and identified a novel function of Cdc25C phosphatase that can dephosphorylate and inhibit ASK1 in interphase but not in mitosis. Furthermore, we demonstrated that Cdc25C phosphorylation status plays a critical role in the interaction with and the activity of ASK1. These results reveal a novel regulatory function of Cdc25C in the ASK1-mediated apoptosis signaling pathway.     

New-Onset Diabetes Mellitus After Transplantation in a Cynomolgus Macaque (Macaca fasicularis)

A 5.5-y-old intact male cynomolgus macaque (Macaca fasicularis) presented with inappetence and weight loss 57 d after heterotopic heart and thymus transplantation while receiving an immunosuppressant regimen consisting of tacrolimus, mycophenolate mofetil, and methylprednisolone to prevent graft rejection. A serum chemistry panel, a glycated hemoglobin test, and urinalysis performed at presentation revealed elevated blood glucose and glycated hemoglobin (HbA1c) levels (727 mg/dL and 10.1%, respectively), glucosuria, and ketonuria. Diabetes mellitus was diagnosed, and insulin therapy was initiated immediately. The macaque was weaned off the immunosuppressive therapy as his clinical condition improved and stabilized. Approximately 74 d after discontinuation of the immunosuppressants, the blood glucose normalized, and the insulin therapy was stopped. The animal''s blood glucose and HbA1c values have remained within normal limits since this time. We suspect that our macaque experienced new-onset diabetes mellitus after transplantation, a condition that is commonly observed in human transplant patients but not well described in NHP. To our knowledge, this report represents the first documented case of new-onset diabetes mellitus after transplantation in a cynomolgus macaque.Abbreviations: NODAT, new-onset diabetes mellitus after transplantationNew-onset diabetes mellitus after transplantation (NODAT, formerly known as posttransplantation diabetes mellitus) is an important consequence of solid-organ transplantation in humans.^{7-10,15,17,19,21,25-28,31,33,34,37,38,42} A variety of risk factors have been identified including increased age, sex (male prevalence), elevated pretransplant fasting plasma glucose levels, and immunosuppressive therapy.^{7-10,15,17,19,21,25-28,31,33,34,37,38,42} The relationship between calcineurin inhibitors, such as tacrolimus and cyclosporin, and the development of NODAT is widely recognized in human medicine.^{7-10,15,17,19,21,25-28,31,33,34,37,38,42} Cynomolgus macaques (Macaca fasicularis) are a commonly used NHP model in organ transplantation research. Cases of natural and induced diabetes of cynomolgus monkeys have been described in the literature;^14,43,45 however, NODAT in a macaque model of solid-organ transplantation has not been reported previously to our knowledge.