2120-P: The Protein Phosphatase M1K Connects Glucose Sensing to Protein Translation in Pancreatic Beta Cells

Abstract

The protein phosphatase m1K (PPm1K) dephosphorylates and activates the rate limiting enzyme in the branched-chain amino acid (BCAA) catabolism pathway, branched chain a-keto acid dehydrogenase (BCKDH). A common single nucleotide polymorphism upstream of the gene for PPm1K is associated with elevated circulating levels of BCAA and increased risk for development of type 2 diabetes; however, the function of PPm1K in the pancreatic beta cell has not been assessed. In line with our previous observations in liver, we found that increasing glucose concentrations in INS1 cells results in elevated expression of the ChREBP β isoform and lower PPM1K mRNA expression. SiRNA-mediated knockdown (KD) of PPm1K to levels achieved by glucose in INS1 cells lead to robust phosphorylation of the ribosomal protein S6 on its regulatory phosphosites ser235/236 and ser240/s244. Notably, this occurred absent any effect on the phosphorylation of the upstream proteins mammalian target of rapamycin (mTOR) or p70 S6 Kinase (S6K) and could not be reversed by inhibiting BCKDH phosphorylation with the small molecule, BT2. Thus Ppm1K appears to regulate S6 independently of mTOR, S6K, and BCKDH. Surface sensing of translation (SUnSET) studies revealed that the increase in S6 phosphorylation in PPm1K KD INS1 cells is associated with a 2-fold increase in protein translation rate. Accordingly, PPm1K KD increased both insulin content and glucose stimulated insulin secretion in INS1 cells. In line with the higher translation rate, PPm1K KD cells displayed signs of endoplasmic reticulum stress including increased expression of spliced xbp1 and Bip mRNA. In conclusion, our data highlight a new role for PPm1K in pancreatic beta cells, as a mechanism connecting glucose sensing to protein translation via modulation of S6 phosphorylation. Disclosure Y. Deleye: None. J. Herring: None. K.H. Hess: None. B. Leininger: None. R.W. McGarrah: None. J.S. Tessem: None. P.J. White: None. Funding American Diabetes Association/Pathway to Stop Diabetes (1-16-INI-17 to P.J.W.)