HuR stabilizes lnc-Sox5 mRNA to promote tongue carcinogenesis

Long noncoding RNAs (lncRNAs) have been recently regarded as systemic regulators in multiple biological processes including tumorigenesis. In this study, we report an ultra-highly expressed lncRNA, lnc-Sox5, in tongue tumor tissues. The results imply that lnc-Sox5 may play vital role in tongue carcinoma progression. We observed that the growth of Tca8113 cells was suppressed by lnc-Sox5 downregulation. Additionally, lnc-Sox5 knockdown simultaneously increased Tca8113 cell apoptosis, but the cell cycle was arrested. RNA immunoprecipitation suggested that HuR directly bound to and stabilized lnc-Sox5 RNA. Consistently, HuR knockdown reduced the level of lnc-Sox5 in Tca8113 cells. However, overexpression of HuR induced more lnc-Sox5 in Tca8113 cells. Both lnc-Sox5 knockdown and HuR knockdown suppressed Tca8113 cell tumorigenesis in xenograft models. These results suggest that lnc-Sox5, which was stabilized by HuR, could regulate carcinogenesis of tongue cancer and may serve as a predicted target for tongue carcinoma therapies.     

Mek1 stabilizes Hop1-Thr318 phosphorylation to promote interhomolog recombination and checkpoint responses during yeast meiosis

Red1, Hop1 and Mek1 are three yeast meiosis-specific chromosomal proteins that uphold the interhomolog (IH) bias of meiotic recombination. Mek1 is also an effector protein kinase in a checkpoint that responds to aberrant DNA and/or axis structure. The activation of Mek1 requires Red1-dependent Hop1-Thr(T)318 phosphorylation, which is mediated by Mec1 and Tel1, the yeast homologs of the mammalian DNA damage sensor kinases ATR and ATM. As the ectopic expression of Mek1-glutathione S-transferase (GST) was shown to promote IH recombination in the absence of Mec1/Tel1-dependent checkpoint function, it was proposed that Mek1 might play dual roles during meiosis by directly phosphorylating targets that are involved in the recombination checkpoint. Here, we report that Mek1 has a positive feedback activity in the stabilization of Mec1/Tel1-mediated Hop1-T318 phosphorylation against the dephosphorylation mediated by protein phosphatase 4. Our results also reveal that GST-Mek1 or Mek1-GST further increases Hop1-T318 phosphorylation. This positive feedback function of Mek1 is independent of Mek1’s kinase activity, but dependent on Mek1’s forkhead-associated (FHA) domain and its arginine 51 residue. Arginine 51 directly mediates the interaction of Mek1-FHA and phosphorylated Hop1-T318. We suggest that the Hop1–Mek1 interaction is similar to the Rad53-Dun1 signaling pathway, which is mediated through the interaction of phosphorylated Rad53 and Dun1-FHA.     

MST1 promotes apoptosis through regulating Sirt1-dependent p53 deacetylation

Mammalian Sterile 20-like kinase 1 (MST1) protein kinase plays an important role in the apoptosis induced by a variety of stresses. The MST1 is a serine/threonine kinase that is activated upon apoptotic stimulation, which in turn activates its downstream targets, JNK/p38, histone H2B and FOXO. It has been reported that overexpression of MST1 initiates apoptosis by activating p53. However, the molecular mechanisms underlying MST1-p53 signaling during apoptosis are unclear. Here, we report that MST1 promotes genotoxic agent-induced apoptosis in a p53-dependent manner. We found that MST1 increases p53 acetylation and transactivation by inhibiting the deacetylation of Sirtuin 1 (Sirt1) and its interaction with p53 and that Sirt1 can be phosphorylated by MST1 leading to the inhibition of Sirt1 activity. Collectively, these findings define a novel regulatory mechanism involving the phosphorylation of Sirt1 by MST1 kinase which leads to p53 activation, with implications for our understanding of signaling mechanisms during DNA damage-induced apoptosis.     

Human SIR2 deacetylates p53 and antagonizes PML/p53-induced cellular senescence

The yeast Sir2 protein mediates chromatin silencing through an intrinsic NAD-dependent histone deacetylase activity. Sir2 is a conserved protein and was recently shown to regulate lifespan extension both in budding yeast and worms. Here, we show that SIRT1, the human Sir2 homolog, is recruited to the promyelocytic leukemia protein (PML) nuclear bodies of mammalian cells upon overexpression of either PML or oncogenic Ras (Ha-rasV12). SIRT1 binds and deacetylates p53, a component of PML nuclear bodies, and it can repress p53-mediated transactivation. Moreover, we show that SIRT1 and p53 co-localize in nuclear bodies upon PML upregulation. When overexpressed in primary mouse embryo fibroblasts (MEFs), SIRT1 antagonizes PML-induced acetylation of p53 and rescues PML-mediated premature cellular senescence. Taken together, our data establish the SIRT1 deacetylase as a novel negative regulator of p53 function capable of modulating cellular senescence.     

Sequestosome 1/p62: across diseases

Sequestosome 1/p62 is a signal modulator or adaptor protein involved in receptor-mediated signal transduction. Sequestosome 1/p62 is gaining attention as it is involved in several diseases including Parkinson disease, Alzheimer disease, liver and breast cancer, Paget's disease of bone, obesity and insulin resistance. In this review, we will focus on the most recent advances on the physiological function of p62 relevant to human diseases.     

Sirt1 and the Mitochondria

Sirt1 is the most prominent and extensively studied member of sirtuins, the family of mammalian class III histone deacetylases heavily implicated in health span and longevity. Although primarily a nuclear protein, Sirt1’s deacetylation of Peroxisome proliferator-activated receptor Gamma Coactivator-1α (PGC-1α) has been extensively implicated in metabolic control and mitochondrial biogenesis, which was proposed to partially underlie Sirt1’s role in caloric restriction and impacts on longevity. The notion of Sirt1’s regulation of PGC-1α activity and its role in mitochondrial biogenesis has, however, been controversial. Interestingly, Sirt1 also appears to be important for the turnover of defective mitochondria by mitophagy. I discuss here evidences for Sirt1’s regulation of mitochondrial biogenesis and turnover, in relation to PGC-1α deacetylation and various aspects of cellular physiology and disease.     

Lysine 63-linked polyubiquitin potentially partners with p62 to promote the clearance of protein inclusions by autophagy

Although protein inclusions associated with neurodegenerative diseases are typically enriched with ubiquitin, it is currently unclear whether the topology of ubiquitin linkage plays a role in their biogenesis. In an attempt to clarify this, our recent work identified K63-linked polyubiquitin as a key regulator of inclusion dynamics. We found in the setting of ectopic overexpression of different ubiquitin species in cultured cells that K63-linked ubiquitination promotes the formation and autophagic clearance of protein inclusions linked to several major neurodegenerative diseases. Further supporting this, we report here a similar phenomenon in cells co-expressing Ubc13 and Uev1a but not those expressing UbcH7 or UbcH8. Notably, Ubc13 in association with Uev1a is known to promote K63-linked ubiquitination. In exploring how K63-linked ubiquitination could promote the clearance of inclusions by autophagy, we also found in our current study that K63-linked polyubiquitin interacts with p62, a ubiquitin-binding protein previously demonstrated by others to facilitate autophagy-mediated clearance of inclusions. Further, K63 ubiquitin-positive inclusions were found to be enriched with p62. Given the observed intimate relationship between p62 and K63 polyubiquitin, our results suggest that p62 and K63-linked polyubiquitin may function as key partners involved in directing clearance of protein inclusions by autophagy.

Addendum to: Tan JMM, Wong ESP, Kirkpatrick DS, Pletnikova O, Ko HS, Tay S-P, Ho M.W.L., Troncoso J, Gygi SP, Lee MK, Dawson VL, Dawson TM, Lim K-L. Lysine 63-linked ubiquitination promotes the formation and autophagic clearance of protein inclusions associated with neurodegenerative diseases. Human Mol Genet; In press.     

Autophagic receptors Nbr1 and p62 coregulate skeletal remodelling

Skeletal remodelling is an ongoing process requiring the coordinated action of different cell types to maintain homeostatic control of bone synthesis and degradation. Mutations in p62/SQSTM1 are associated with sporadic and 5q35-linked Paget’s Disease of Bone (PDB), characterized by focal increased bone turnover. These mutations cluster in the ubiquitin associated (UBA) domain and are thought to lead to enhancement of NFκB pathway activation involved in osteoclastogenesis and hyper-responsiveness to receptor activator of nuclear factor-κB ligand (RANKL). The structurally similar selective autophagic receptor, Nbr1, binds to LC3 and p62, and is sequestered into autophagosomes, whereas it accumulates in autophagic-deficient tissues. We have shown that truncation of Nbr1 in a murine model, where it can still interact with p62 but not LC3, leads to increased osteoblast differentiation and activity in vivo. This results in an age-dependent increase in bone mass and bone mineral density. This is a molecular consequence of loss of autophagy receptor function via deletion of its C-terminal UBA domain, and/or modulation of the p38 MAPK cellular signalling pathway.