Expanding functional beta cell mass remains a prime goal of diabetes research. We previously demonstrated that IRS2 deficient mice have reduced islet expression of cyclin D2, and that overexpressing cyclin D2 rescues proliferation in IRS2 deficient beta cells in vitro. Since cyclin D2 partners with CDK4 to drive cell cycle progression, we hypothesized that an activated form of CDK4, CDK4-R24C (resistant to inhibition by Ink4A cell cycle inhibitors) might rescue the in vivo proliferation defect in IRS2 deficient mice. Mice heterozygous for both whole body Irs2 deletion and whole body Cdk4-R24C knock-in were mated, yielding 6 genotypes of interest: Irs2+/+; Cdk4-WT, Irs2+/+; Cdk4-R24C/WT, Irs2+/+; Cdk4-R24C/R24C, Irs2-/-; Cdk4-WT, Irs2-/-; Cdk4-R24C/WT, Irs2-/-; Cdk4-R24C/R24C. Two copies, but not one, of Cdk4-R24C completely rescued fasting and non-fasting blood glucose, glucose intolerance, and beta cell mass and proliferation, without rescuing peripheral insulin resistance. Interestingly, Irs2-/-; Cdk4-WT islets contained dedifferentiated (reduced PDX1, nuclear FOXO1, ALDH1A3+) beta cells, and this was completely reversed in Irs2-/-; Cdk4-R24C/R24C islets. An in vitro primary beta cell model of increased nuclear FOXO1 activity (short-term starvation) resulted in mRNA changes consistent with nuclear FOXO1 activity (including reduced Pdx1) and mimicked IRS2 deficiency in vivo. Intriguingly, overexpressing cell cycle activators CDK4 and cyclin D2 partially rescued the mRNA changes indicative of nuclear FOXO1 activity, while expressing an inhibitor (CDKN2A/p16) did not. Overexpression of CDK4 increased FOXO1 phosphorylation. We conclude that CDK4 may play additional roles outside of the cell cycle in beta cells, by activating the insulin signaling pathway downstream of IRS2 and exerting antidiabetic action by maintaining beta cell differentiation.
R.E. Stamateris: None. R.B. Sharma: None. B. Gablaski: None. S.G. Rane: None. L.C. Alonso: Consultant; Self; Fairbanks Pharmaceuticals Inc. Research Support; Self; American Diabetes Association.
American Diabetes Association (1-18-IBS-233 to L.C.A.)