Salt-Inducible Kinase 1 Terminates cAMP Signaling by an Evolutionarily Conserved Negative-Feedback Loop in ß-Cells.

Pancreatic ß-cells are critical in the regulation of glucose homeostasis by controlled secretion of insulin in mammals. Activation of protein kinase A by cAMP is shown to be responsible for enhancing this pathway, which is countered by phosphodiesterase (PDE) that converts cAMP to AMP and turns off the signal. Salt-inducible kinases (SIKs) were also known to inhibit cAMP signaling, mostly by promoting inhibitory phosphorylation on CREB-regulated transcription coactivators. Here, we showed that SIK1 regulates insulin secretion in ß-cells by modulating PDE4D and cAMP concentrations. Haploinsufficiency of SIK1 led to the improved glucose tolerance due to the increased glucose-stimulated insulin secretion. Depletion of SIK1 promoted higher cAMP concentration and increased insulin secretion from primary islets, suggesting that SIK1 controls insulin secretion through the regulation of cAMP signaling. By using a consensus phosphorylation site of SIK1, we identified PDE4D as a new substrate for this kinase family. In vitro kinase assay as well as mass spectrometry analysis revealed that the predicted Ser(136) and the adjacent Ser(141) of PDE4D are critical in SIK1-mediated phosphorylation. We found that overexpression of either SIK1 or PDE4D in ß-cells reduced insulin secretion, while inhibition of PDE4 activity by rolipram or knockdown of PDE4D restored it, showing indeed that SIK1-dependent phosphorylation of PDE4D is critical in reducing cAMP concentration and insulin secretion from ß-cells. Taken together, we propose that SIK1 serves as a part of a self-regulatory circuit to modulate insulin secretion from pancreatic ß-cells by controlling cAMP concentration through modulation of PDE4D activity.

Arginine methylation of CRTC2 is critical in the transcriptional control of hepatic glucose metabolism.

Fasting glucose homeostasis is maintained in part through cAMP (adenosine 3',5'-monophosphate)-dependent transcriptional control of hepatic gluconeogenesis by the transcription factor CREB (cAMP response element-binding protein) and its coactivator CRTC2 (CREB-regulated transcriptional coactivator 2). We showed that PRMT6 (protein arginine methyltransferase 6) promotes fasting-induced transcriptional activation of the gluconeogenic program involving CRTC2. Mass spectrometric analysis indicated that PRMT6 associated with CRTC2. In cells, PRMT6 mediated asymmetric dimethylation of multiple arginine residues of CRTC2, which enhanced the association of CRTC2 with CREB on the promoters of gluconeogenic enzyme-encoding genes. In mice, ectopic expression of PRMT6 promoted higher blood glucose concentrations, which were associated with increased expression of genes encoding gluconeogenic factors, whereas knockdown of hepatic PRMT6 decreased fasting glycemia and improved pyruvate tolerance. The abundance of hepatic PRMT6 was increased in mouse models of obesity and insulin resistance, and adenovirus-mediated depletion of PRMT6 restored euglycemia in these mice. We propose that PRMT6 is involved in the regulation of hepatic glucose metabolism in a CRTC2-dependent manner.

Protein arginine methyltransferase 1 regulates hepatic glucose production in a FoxO1-dependent manner.

UNLABELLED: Postprandial insulin plays a critical role in suppressing hepatic glucose production to maintain euglycemia in mammals. Insulin-dependent activation of protein kinase B (Akt) regulates this process, in part, by inhibiting FoxO1-dependent hepatic gluconeogenesis by direct phosphorylation and subsequent cytoplasmic exclusion. Previously, it was demonstrated that protein arginine methyltransferase 1 (PRMT1)-dependent arginine modification of FoxO1 interferes with Akt-dependent phosphorylation, both in cancer cells and in the Caenorhabditis elegans model, suggesting that this additional modification of FoxO1 might be critical in its transcriptional activity. In this study, we attempted to directly test the effect of arginine methylation of FoxO1 on hepatic glucose metabolism. The ectopic expression of PRMT1 enhanced messenger RNA levels of FoxO1 target genes in gluconeogenesis, resulting in increased glucose production from primary hepatocytes. Phosphorylation of FoxO1 at serine 253 was reduced with PRMT1 expression, without affecting the serine 473 phosphorylation of Akt. Conversely, knockdown of PRMT1 promoted an inhibition of FoxO1 activity and hepatic gluconeogenesis by enhancing the phosphorylation of FoxO1. In addition, genetic haploinsufficiency of Prmt1 reduced hepatic gluconeogenesis and blood-glucose levels in mouse models, underscoring the importance of this factor in hepatic glucose metabolism in vivo. Finally, we were able to observe an amelioration of the hyperglycemic phenotype of db/db mice with PRMT1 knockdown, showing a potential importance of this protein as a therapeutic target for the treatment of diabetes. CONCLUSION: Our data strongly suggest that the PRMT1-dependent regulation of FoxO1 is critical in hepatic glucose metabolism in vivo.