Glycogen Synthase Kinase-3 (GSK-3) Signaling to Mammalian Target of Rapamycin (mTOR) Regulates DNA Synthesis and Cell Cycle Progression in Rodent and Human Islets.

Pappan, Kirk L.; Liu, Hui; Remedi, Maria S.; Marshall, Connie A.; Kwon, Guim; McDaniel, Michael L.

Stimulating islet β-cells to proliferate is a clinical goal that could have a beneficial impact on both T1 and T2DM. The nutrient- and energy-sensing protein kinase, mammalian Target of Rapamycin (mTOR), plays a prominent role in regulating protein and DNA synthesis and entry into the cell cycle from the quiescent state, and we have searched for ways to promote the activation of this centrally positioned kinase in hopes of inducing adult β-cell regeneration. Although glucose, leucine, free fatty acids, and the K[sub ATP] channel inhibitor glyburide showed great potential for activating mTOR in rodent islets, none of these agents translated into consistent activators of human islet mTOR. A recent report linked the inactivation of glycogen synthase kinase-3 (GSK3) to inactivation of the mTOR repressor protein Tuberous Sclerosis 2 in non-islet cells, and we examined the existent of this pathway in islets. When isolated rat islets are cultured for 4d at 8 mM glucose with an inhibitor of GSK-3, lithium chloride (LiCl), we observe a 2-fold increase in DNA synthesis, over that seen with 8 mM glucose alone, that is completely reversed by the mTOR inhibitor rapamycin (25 nM). Treatment of rat islets for 4d with a more specific inhibitor of GSK-3, SB-216763 (5 µM), also yields a significant increase in DNA synthesis that is blocked by rapamycin. Acute (2h) LiCl treatment of rat islets increases the phosphorylation of GSK-3 on serine residues that results in its inactivation. Steady state levels β-catenin, which is normally targeted for rapid proteolytic degradation when GSK-3 is active, also increase in rat islets upon acute LiCl treatment. LiCl in the presence of basal (5 mM) or intermediate (8 mM) glucose concentrations promotes a significant entry of rot islet cells into the cell cycle as indicated by flow cytometry. Most significantly, with cadaver-derived human islets we observe that 4d LiCl treatment consistently promotes a greater than 2-fold stimulation of rapamycin-sensitive DNA synthesis and 2h LiCl exposure increases phospho-Ser-GSK-3 levels. Collectively, these studies identify a novel GSK-3/mTOR signaling pathway that is operative in both rodent and human islets and which has the potential to be modulated to promote β-cell proliferation. ADA-Funded Research

GLYCOGEN synthase kinase-3; RAPAMYCIN; DNA synthesis; CELL cycle; PANCREATIC beta cells; PROTEIN kinases; LITHIUM chloride; GLUCOSE

Diabetes, 2007, Vol 56, pA444

0012-1797

Academic Journal