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

BACKGROUND: Over 400 million people are diabetic. Type 1 and type 2 diabetes are characterized by decreased functional ß-cell mass and, consequently, decreased glucose-stimulated insulin secretion. A potential intervention is transplantation of ß-cell containing islets from cadaveric donors. A major impediment to greater application of this treatment is the scarcity of transplant-ready ß-cells. Therefore, inducing ß-cell proliferation ex vivo could be used to expand functional ß-cell mass prior to transplantation. Various molecular pathways are sufficient to induce proliferation of young ß-cells; however, aged ß-cells are refractory to these proliferative signals. Given that the majority of cadaveric donors fit an aged demographic, defining the mechanisms that impede aged ß-cell proliferation is imperative. RESULTS: We demonstrate that aged rat (5-month-old) ß-cells are refractory to mitogenic stimuli that otherwise induce young rat (5-week-old) ß-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 ß-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. SIGNIFICANCE: As the majority of transplant-ready ß-cells come from aged donors, it is imperative that we understand why aged ß-cells are refractory to mitogenic stimuli. Our findings demonstrate that increased p27KIP1 expression occurs early in ß-cell aging, which corresponds with impaired ß-cell proliferation. Furthermore, the correlation between HDAC1 and p27 levels suggests that pathways that activate HDAC1 in aged ß-cells could be leveraged to decrease p27KIP1 levels and enhance ß-cell proliferation.

HDAC1 overexpression enhances ß-cell proliferation by down-regulating Cdkn1b/p27.

The homeobox transcription factor Nkx6.1 is sufficient to increase functional ß-cell mass, where functional ß-cell mass refers to the combination of ß-cell proliferation, glucose-stimulated insulin secretion (GSIS) and ß-cell survival. Here, we demonstrate that the histone deacetylase 1 (HDAC1), which is an early target of Nkx6.1, is sufficient to increase functional ß-cell mass. We show that HDAC activity is necessary for Nkx6.1-mediated proliferation, and that HDAC1 is sufficient to increase ß-cell proliferation in primary rat islets and the INS-1 832/13 ß-cell line. The increase in HDAC1-mediated proliferation occurs while maintaining GSIS and increasing ß-cell survival in response to apoptotic stimuli. We demonstrate that HDAC1 overexpression results in decreased expression of the cell cycle inhibitor Cdkn1b/p27 which is essential for inhibiting the G1 to S phase transition of the cell cycle. This corresponds with increased expression of key cell cycle activators, such as Cyclin A2, Cyclin B1 and E2F1, which are activated by activation of the Cdk4/Cdk6/Cyclin D holoenzymes due to down-regulation of Cdkn1b/p27. Finally, we demonstrate that overexpression of Cdkn1b/p27 inhibits HDAC1-mediated ß-cell proliferation. Our data suggest that HDAC1 is critical for the Nkx6.1-mediated pathway that enhances functional ß-cell mass.