Decreased proliferation of aged rat beta cells corresponds with enhanced expression of the cell cycle inhibitor p27KIP1.

Abstract

Over 400 million people are diabetic. Type 1 and type 2 diabetes are characterized by decreased functional β-cell mass and, consequently, decreased glucose-stimulated insulin secretion. A potential intervention is transplantation of β-cell containing islets from cadaveric donors. A major impediment to greater application of this treatment is the scarcity of transplant-ready β-cells. Therefore, inducing β-cell proliferation ex vivo could be used to expand functional β-cell mass prior to transplantation. Various molecular pathways are sufficient to induce proliferation of young β-cells; however, aged β-cells are refractory to these proliferative signals. Given that the majority of cadaveric donors fit an aged demographic, defining the mechanisms that impede aged β-cell proliferation is imperative. We demonstrate that aged rat (5-month-old) β-cells are refractory to mitogenic stimuli that otherwise induce young rat (5-week-old) β-cell proliferation. We hypothesized that this change in proliferative capacity could be due to differences in cyclin-dependent kinase inhibitor expression. We measured levels of p16INK4a , p15INK4b , p18INK4c , p19INK4d , p21CIP1 , p27KIP1 and p57KIP2 by immunofluorescence analysis. Our data demonstrates an age-dependent increase of p27KIP1 in rat β-cells by immunofluorescence and was validated by increased p27KIP1 protein levels by western blot analysis. Interestingly, HDAC1, which modulates the p27KIP1 promoter acetylation state, is downregulated in aged rat islets. These data demonstrate increased p27KIP1 protein levels at 5 months of age, which may be due to decreased HDAC1 mediated repression of p27KIP1 expression. As the majority of transplant-ready β-cells come from aged donors, it is imperative that we understand why aged β-cells are refractory to mitogenic stimuli. Our findings demonstrate that increased p27KIP1 expression occurs early in β-cell aging, which corresponds with impaired β-cell proliferation. Furthermore, the correlation between HDAC1 and p27 levels suggests that pathways that activate HDAC1 in aged β-cells could be leveraged to decrease p27KIP1 levels and enhance β-cell proliferation.