Hypophosphorylated pRb knock-in mice exhibit hallmarks of aging and vitamin C-preventable diabetes

Despite extensive in vitro analyses of retinoblastoma protein (pRB) phosphorylation, it is still unclear how this modification influences development and homeostasis in vivo. Here, we show that homozygous Rb∆K4 and Rb∆K7 knock-in mice, in which either four or all seven phosphorylation sites in the C-terminal region of pRb, respectively, have been abolished by Ser/Thr-to-Ala substitutions, undergo normal embryogenesis and early development, notwithstanding suppressed phosphorylation of additional upstream sites. Whereas Rb∆K4 mice exhibit telomere attrition but no other abnormalities, Rb∆K7 mice are smaller and display additional hallmarks of premature aging including infertility, kyphosis and diabetes, indicating a cumulative effect of blocking multiple pRb phosphorylation events. Diabetes in Rb∆K7 mice is insulin-sensitive and associated with failure of quiescent pancreatic -cells to re-enter the cell cycle in response to mitogens, resulting in induction of DNA damage response (DDR), senescence-associated secretory phenotype (SASP), and reductions in pancreatic islet mass and circulating insulin levels. Pre-treatment with the epigenetic regulator vitamin C reduces the DDR, increases cell cycle re-entry, improves islet morphology, and attenuates diabetes. These results have direct implications for cell-cycle regulation, CDK-inhibitor therapeutics, diabetes and longevity.