Protein kinase A‐mediated phosphorylation of naked cuticle homolog 2 stimulates cell‐surface delivery of transforming growth factor‐α for epidermal growth factor receptor transactivation

The classic mode of G protein‐coupled receptor (GPCR)‐mediated transactivation of the receptor tyrosine kinase epidermal growth factor receptor (EGFR) transactivation occurs via matrix metalloprotease (MMP)‐mediated cleavage of plasma membrane‐anchored EGFR ligands. Herein, we show that the Gαs‐activating GPCR ligands vasoactive intestinal peptide (VIP) and prostaglandin E₂ (PGE₂) transactivate EGFR through increased cell‐surface delivery of the EGFR ligand transforming growth factor‐α (TGFα) in polarizing madin‐darby canine kidney (MDCK) and Caco‐2 cells. This is achieved by PKA‐mediated phosphorylation of naked cuticle homolog 2 (NKD2), previously shown to bind TGFα and direct delivery of TGFα‐containing vesicles to the basolateral surface of polarized epithelial cells. VIP and PGE₂ rapidly activate protein kinase A (PKA) that then phosphorylates NKD2 at Ser‐223, a process that is facilitated by the molecular scaffold A‐kinase anchoring protein 12 (AKAP12). This phosphorylation stabilized NKD2, ensuring efficient cell‐surface delivery of TGFα and increased EGFR activation. Thus, GPCR‐triggered, PKA/AKAP12/NKD2‐regulated targeting of TGFα to the cell surface represents a new mode of EGFR transactivation that occurs proximal to ligand cleavage by MMPs.      

Mutual regulation between butyrate and hypoxia‐inducible factor‐1α in epithelial cell promotes expression of tight junction proteins

Inflammatory bowel disease is a kind of multi‐aetiological chronic disease that is driven by multidimensional factors. Hypoxia‐inducible factor‐1α (HIF‐1α) plays an important role in anti‐inflammatory and cellular responses to hypoxia. Previous studies have found that B or T‐cell‐specific HIF‐1α knock out mice exhibit severe colonic inflammation. However, we know very little about other functions of HIF‐1α in intestinal epithelial cells (IECs). In our study, HIF‐1α^ΔIEC mice were used to study the function of HIF‐1α in IECs. HIF‐1α was knocked down in Caco‐2 cells by transfection with a small interfering (si) RNA. Immunohistochemical staining and western blotting were used to detect the expression of zonula occluden‐1 (ZO‐1) and Occludin. The content of colon was harvested for high‐performance liquid chromatography analysis to examine the levels of butyrate in the gut. Our research found that HIF‐1α played a protective role in dextran sulphate sodium‐induced colitis, which was partly due to its regulation of tight junction (TJ) protein expression. Further study revealed that HIF‐1α mediated TJ proteins levels by moderating the content of butyrate. Moreover, we found that butyrate regulated TJ protein expression, which is dependent on HIF‐1α. These results indicated that there is a mutual regulatory mechanism between butyrate and HIF‐1α, which has an important role in the maintenance of barrier function of the gastrointestinal tract.     

Aquaporin‐4 deficiency reduces TGF‐β1 in mouse midbrains and exacerbates pathology in experimental Parkinson's disease

Aquaporin‐4 (AQP4), the main water‐selective membrane transport protein in the brain, is localized to the astrocyte plasma membrane. Following the establishment of a 1‐methyl‐4‐phenyl‐1,2,3,6‐tetrahydropyridine (MPTP)‐induced Parkinson's disease (PD) model, AQP4‐deficient (AQP4^?/?) mice displayed significantly stronger microglial inflammatory responses and remarkably greater losses of tyrosine hydroxylase (TH⁺)‐positive neurons than did wild‐type AQP4 (AQP4^+/+) controls. Microglia are the most important immune cells that mediate immune inflammation in PD. However, recently, few studies have reported why AQP4 deficiency results in more severe hypermicrogliosis and neuronal damage after MPTP treatment. In this study, transforming growth factor‐β1 (TGF‐β1), a key suppressive cytokine in PD onset and development, failed to increase in the midbrain and peripheral blood of AQP4^?/? mice after MPTP treatment. Furthermore, the lower level of TGF‐β1 in AQP4^?/? mice partially resulted from impairment of its generation by astrocytes; reduced TGF‐β1 may partially contribute to the uncontrolled microglial inflammatory responses and subsequent severe loss of TH⁺ neurons in AQP4^?/? mice after MPTP treatment. Our study provides not only a better understanding of both aetiological and pathogenical factors implicated in the neurodegenerative mechanism of PD but also a possible approach to developing new treatments for PD via intervention in AQP4‐mediated immune regulation.     

Nucleotide diversity of vernalization and flowering‐time‐related genes in a germplasm collection of meadow fescue (Festuca pratensis Huds. syn. Lolium pratense (Huds.) Darbysh.)

In plant species, control of flowering time is an important factor for adaptation to local natural environments. The Vrn1, CO, FT1 and CK2α genes are key components in the flowering‐specific signaling pathway of grass species. Meadow fescue is an agronomically important forage grass species, which is naturally distributed across Europe and Western Asia. In this study, meadow fescue flowering‐time‐related genes were resequenced to assess nucleotide diversity in European and Western Asian subpopulations. Identified sequence polymorphisms were then converted into PCR‐based molecular genetic markers, and a meadow fescue germplasm collection was genotyped to investigate global allelic variation. Lower nucleotide diversities were observed for the Vrn1 and CO orthologs, while relatively higher values were observed for the FT1 and casein kinase II α‐subunit (CK2α) orthologs. The nucleotide diversity for FT1 orthologs in the Western Asian subpopulation was significantly higher than those of the European subpopulation. Similarly, significant differences in nucleotide diversity for the remaining genes were observed between several combinations of subpopulation. The global allele distribution pattern was consistent with observed level of nucleotide diversity. These results suggested that the degree of purifying selection acting on the genes differs according to geographical location. As previously shown for model plant species, functional specificities of flowering‐time‐related genes may also vary according to environmental conditions.     

c‐FLIP is involved in erythropoietin‐mediated protection of erythroid‐differentiated cells from TNF‐α‐induced apoptosis

The TNF‐α (tumour necrosis factor) affects a wide range of biological activities, such as cell proliferation and apoptosis. Cell life or death responses to this cytokine might depend on cell conditions. This study focused on the modulation of factors that would affect the sensitivity of erythroid‐differentiated cells to TNF‐α. Hemin‐differentiated K562 cells showed higher sensitivity to TNF‐induced apoptosis than undifferentiated cells. At the same time, hemin‐induced erythroid differentiation reduced c‐FLIP (cellular FLICE‐inhibitory protein) expression. However, this negative effect was prevented by prior treatment with Epo (erythropoietin), which allowed the cell line to maintain c‐FLIP levels. On the other hand, erythroid‐differentiated UT‐7 cells – dependent on Epo for survival – showed resistance to TNF‐α pro‐apoptotic action. Only after the inhibition of PI3K (phosphatidylinositol‐3 kinase)‐mediated pathways, which was accompanied by negative c‐FLIP modulation and increased erythroid differentiation, were UT‐7 cells sensitive to TNF‐α‐triggered apoptosis. In summary, erythroid differentiation might deregulate the balance between growth promotion and death signals induced by TNF‐α, depending on cell type and environmental conditions. The role of c‐FLIP seemed to be critical in the protection of erythroid‐differentiated cells from apoptosis or in the determination of their sensitivity to TNF‐mediated programmed cell death. Epo, which for the first time was found to be involved in the prevention of c‐FLIP down‐regulation, proved to have an anti‐apoptotic effect against the pro‐inflammatory factor. The identification of signals related to cell life/death switching would have significant implications in the control of proliferative diseases and would contribute to the understanding of mechanisms underlying the anaemia associated with inflammatory processes.     

Phosphatidylinositol‐3‐OH kinase regulatory subunits are differentially expressed during development of the rat cerebellum

Recent evidence implicates a central role for PI3K signalling in mediating cell survival during the process of neuronal differentiation. Although PI3K activity is stimulated by a wide range of growth factors and cytokines in different cell lines and tissues, activation of this pathway by insulin‐like growth factor I (IGF‐I) most likely represents the main survival signal during neuronal differentiation. IGF‐I is highly expressed during development of the central nervous system, and thus is a critical factor for the development and maturation of the cerebellum. Upon ligand binding, the IGF‐I receptor phosphorylates tyrosine residues in SHC and insulin receptor substrates (IRSs) initiating two main signalling cascades, the MAP kinase and the phosphatidylinositol 3‐kinase (PI3K) pathways. Activated PI3K is composed of a catalytic subunit (p110α or β) associated with one of a large family of regulatory subunits (p85α, p85β, p55γ, p55α, and p50α). To evaluate the contributions of these various regulatory subunits to neuronal differentiation, we have used antibodies specific for each of the PI3K subunits. Using these antisera, we now demonstrate that PI3K subunits are differentially regulated in cerebellar development, and that the expression level of the p55γ regulatory subunit reaches a maximum during postnatal development, decreasing thereafter to low levels in the adult cerebellum. Furthermore, our studies reveal that the distribution of the various PI3K regulatory subunits varies during development of the cerebellum. Interestingly, p55γ is expressed in both glial and neuronal cells; moreover, in Purkinje neurones, this subunit colocalises with the IGF‐IR. © 2001 John Wiley & Sons, Inc. J Neurobiol 47: 39–50, 2001     

The Mood‐Stabilizing Agent Valproate Inhibits the Activity of Glycogen Synthase Kinase‐3

Abstract : Valproic acid (VPA) is a potent broad‐spectrum anti‐epileptic with demonstrated efficacy in the treatment of bipolar affective disorder. It has previously been demonstrated that both VPA and lithium increase activator protein‐1 (AP‐1) DNA binding activity, but the mechanisms underlying these effects have not been elucidated. However, it is known that phosphorylation of c‐jun by glycogen synthase kinase (GSK)‐3β inhibits AP‐1 DNA binding activity, and lithium has recently been demonstrated to inhibit GSK‐3β. These results suggest that lithium may increase AP‐1 DNA binding activity by inhibiting GSK‐3β. In the present study, we sought to determine if VPA, like lithium, regulates GSK‐3. We have found that VPA concentration‐dependently inhibits both GSK‐3α and ‐3β, with significant effects observed at concentrations of VPA similar to those attained clinically. Incubation of intact human neuroblastoma SH‐SY5Y cells with VPA results in an increase in the subsequent in vitro recombinant GSK‐3β‐mediated ³²P incorporation into two putative GSK‐3 substrates (~85 and 200 kDa), compatible with inhibition of endogenous GSK‐3β by VPA. Consistent with GSK‐3β inhibition, incubation of SH‐SY5Y cells with VPA results in a significant time‐dependent increase in both cytosolic and nuclear β‐catenin levels. GSK‐3β plays a critical role in the CNS by regulating various cytoskeletal processes as well as long‐term nuclear events and is a common target for both lithium and VPA ; inhibition of GSK‐3β in the CNS may thus underlie some of the long‐term therapeutic effects of mood‐stabilizing agents.     

IFN‐γ inhibits basal and α‐MSH‐induced melanogenesis

Inflammatory cytokines are closely related to pigmentary changes. In this study, the effects of IFN‐γ on melanogenesis were investigated. IFN‐γ inhibits basal and α‐MSH‐induced melanogenesis in B16 melanoma cells and normal human melanocytes. MITF mRNA and protein expressions were significantly inhibited in response to IFN‐γ. IFN‐γ inhibited CREB binding to the MITF promoter but did not affect CREB phosphorylation. Instead, IFN‐γ inhibited the association of CBP and CREB through the increased association between CREB binding protein (CBP) and STAT1. These findings suggest that IFN‐γ inhibits both basal and α‐MSH‐induced melanogenesis by inhibiting MITF expression. The inhibitory action of IFN‐γ in α‐MSH‐induced melanogenesis is likely to be associated with the sequestration of CBP via the association between CBP and STAT1. These data suggest that IFN‐γ plays a role in controlling inflammation‐ or UV‐induced pigmentary changes.     

Prostaglandin I2 upregulates the expression of anterior pharynx‐defective‐1α and anterior pharynx‐defective‐1β in amyloid precursor protein/presenilin 1 transgenic mice

Cyclooxygenase‐2 (COX‐2) has been recently identified to be involved in the pathogenesis of Alzheimer's disease (AD). Yet, the role of an important COX‐2 metabolic product, prostaglandin (PG) I₂, in the pathogenesis of AD remains unknown. Using human‐ and mouse‐derived neuronal cells as well as amyloid precursor protein/presenilin 1 (APP/PS1) transgenic mice as model systems, we elucidated the mechanism of anterior pharynx‐defective (APH)‐1α and pharynx‐defective‐1β induction. In particular, we found that PGI₂ production increased during the course of AD development. Then, PGI₂ accumulation in neuronal cells activates PKA/CREB and JNK/c‐Jun signaling pathways by phosphorylation, which results in APH‐1α/1β expression. As PGI₂ is an important metabolic by‐product of COX‐2, its suppression by NS398 treatment decreases the expression of APH‐1α/1β in neuronal cells and APP/PS1 mice. More importantly, β‐amyloid protein (Aβ) oligomers in the cerebrospinal fluid (CSF) of APP/PS1 mice are critical for stimulating the expression of APH‐1α/1β, which was blocked by NS398 incubation. Finally, the induction of APH‐1α/1β was confirmed in the brains of patients with AD. Thus, these findings not only provide novel insights into the mechanism of PGI₂‐induced AD progression but also are instrumental for improving clinical therapies to combat AD.