p16INK4a induces an age-dependent decline in islet regenerative potential

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Abstract
In this issue, three separate labs report the discovery of a protein that regulates ageing specifically in stem cells. This helps answer a fundamental question: why do mammalian progenitor cells gradually lose their ability to divide and generate new cells as they grow old? Norman Sharpless and colleagues generated a knockout mouse lacking tumour suppressor p16INK4a, a protein involved in cell cycle control and known to be expressed in an age-dependent manner. Studying its role in regeneration of the blood, pancreas and brain, the three groups separately found that p16INK4a is not only a biomarker, but an effector of ageing. By comparing the effect of elevated or reduced p16INK4a expression in mice, they found that p16INK4a halts proliferation of stem cells, but only in older mice. Taken together, the work suggests that p16INK4a reduces cancer incidence via its tumour suppressor action, at the same time contributing to ageing by reducing stem cell function. The work also suggests that type 2 diabetes might be linked to the failure of the pancreatic islets to renew, and that blocking this protein in certain tissues might combat some effects of ageing. Three separate labs report that p16INK4a, a protein known to be expressed in an age-dependent manner regulates ageing specifically in stem cells. Studying its role in regeneration of three different tissues, the blood, pancreas, and brain, the three groups separately found that p16INK4a is not only a biomarker, but an effector of ageing. The p16INK4a tumour suppressor accumulates in many tissues as a function of advancing age1,2,3. p16INK4a is an effector of senescence4,5 and a potent inhibitor of the proliferative kinase Cdk4 (ref. 6), which is essential for pancreatic β-cell proliferation in adult mammals7,8. Here we show that p16INK4a constrains islet proliferation and regeneration in an age-dependent manner. Expression of the p16INK4a transcript is enriched in purified islets compared with the exocrine pancreas, and islet-specific expression of p16INK4a, but not other cyclin-dependent kinase inhibitors, increases markedly with ageing. To determine the physiological significance of p16INK4a accumulation on islet function, we assessed the impact of p16INK4a deficiency and overexpression with increasing age and in the regenerative response after exposure to a specific β-cell toxin. Transgenic mice that overexpress p16INK4a to a degree seen with ageing demonstrated decreased islet proliferation. Similarly, islet proliferation was unaffected by p16INK4a deficiency in young mice, but was relatively increased in p16INK4a-deficient old mice. Survival after toxin-mediated ablation of β-cells, which requires islet proliferation, declined with advancing age; however, mice lacking p16INK4a demonstrated enhanced islet proliferation and survival after β-cell ablation. These genetic data support the view that an age-induced increase of p16INK4a expression limits the regenerative capacity of β-cells with ageing.