Regulationof TDH In Somatic Cell Reprogramming And Study On ER Stress Caused Cell Cycle Arrest

Increasing evidence suggests that metabolic remodeling plays an important role in the regulation of somatic cell reprogramming. Threonine catabolism mediated byL-threonine dehydrogenase (TDH) has been recognized as a specific metabolic trait of mouse ES cells. TDH-catalyzed threonine hydrolyzation is required for rapid growth of mouse ES cells. However, it remains unknown whether TDH-mediated threonine catabolism could regulate reprogramming. Here we report TDH as a novel regulator of somatic cell reprogramming. Knockdown of TDH greatly inhibits, whereas induction of TDH markedly enhances, reprogramming efficiency. Moreover, miR-9post-transcriptionally regulates the expression of TDH and thereby inhibits reprogramming efficiency. Furthermore, protein arginine methytransferase5(PRMT5) interacts with TDH and mediates its post-translational arginine methylation. PRMT5appears to regulate TDH enzyme activity through both methytransferase-dependent and-independent mechanisms. Functionally, TDH-mediated reprogramming efficiency is enhanced by PRMT5. These results suggest that TDH-mediated threonine catabolism controls somatic cell reprogramming, and indicate theimportance of post-transcriptional and post-translational regulation of TDHThe accumulation of unfolded proteins in the endoplasmic reticulum(ER) triggers the unfolded protein response (UPR), a stress signaling pathway. The UPR coordinates the induction of ER chaperones with decreased protein synthesis and growth arrest in G1phase of the cell cycle. However, the molecular mechanism underlying UPR-induced G1cell cycle arrest remains largely unknown. Here we report that activation of the UPR response by tunicamycin (TM), an ER stress inducer, leads to accumulation of p27and G1cell cycle arrest in melanoma cells. This accumulation of p27is due to the inhibition on its polyubiquitination and subsequent degradation upon TM treatment. Correlated with p27stabilization, the levels of Skp2, an E3ligase for p27, are decreased in response to TM treatment. More importantly, knockdown of p27greatly reduces TM-induced G1cell cycle arrest. Taken together, these data implicate p27as a critical mediator of ER stress-induced growth arrest.