Novel insights into nutrient-dependent regulation of beta cell proliferation

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A decline in functional β-cell mass and subsequent inability to maintain adequate glycemic control are hallmarks of both type 1 and type 2 diabetes. Innovative therapeutic approaches are aimed at preserving and restoring functional β-cell mass in diabetes; however, strategies to safely expand β-cell mass remain to be identified. The predominant mechanism for adapting β-cell mass to states of increased insulin demand is through modulation of β-cell replication. Therefore, there has been considerable interest in understanding the mechanisms that regulate β-cell replication with the goal of discovering new therapeutic targets to promote β-cell regeneration. Preliminary unpublished evidence from our laboratory suggests that the NAD+-dependent cytoplasmic deacetylase Sirtuin 2 (SIRT2) acts as a nutrient-dependent regulator of mitogenic signaling in rodent and human β-cells. Using mouse genetic and inhibitor approaches in human islets, we found that loss of SIRT2 activity stimulates β-cell proliferation and β-cell mass expansion under hyperglycemic conditions. We have also obtained evidence that mimicking nutrient state changes by manipulating NAD+ availability regulates β-cell proliferation in a manner consistent with SIRT2-dependent responses. Since intracellular NAD+ levels fluctuate with glucose availability, we hypothesize that SIRT2 couples β-cell proliferation to glucose metabolism. Furthermore, we have found that SIRT2 inhibits β-cell proliferation by dampening MAPK signaling and that SIRT2 inhibition in systemic hyperglycemia promotes β-cell proliferation, while protecting β-cells from activating pro-apoptotic signaling downstream of the endoplasmic reticulum (ER) stress response. In this proposal, we will explore how SIRT2 regulates mitogenic signaling as well as ER stress responses in β-cells. To accomplish this, we will pursue three Aims. In Aim 1 we will employ mouse genetic approaches and experiments in human islets to determine how glucose and nutrient state affect SIRT2-dependent regulation of β-cell proliferation. Here, we will investigate links between NAD metabolism, activity of the master regulator of cellular energy homeostasis AMPK, SIRT2 activity, and β-cell proliferation to gain mechanistic insight into the signaling cascades that couple nutrient availability to proliferation in β-cells. To understand how SIRT2 modulates intracellular signaling to affect glucose-induced proliferative and apoptotic responses in β-cells, in Aim 2, we will identify the downstream effectors of SIRT2 in the regulation of β-cell proliferation, employing proteomic as well as in vitro and in vivo approaches. Finally, in Aim 3, we will examine the effects of SIRT2 inhibition on human β-cell proliferation and function in vivo and explore whether combinatorial targeting of different mitogenic signaling pathways can augment pro-proliferative effects of SIRT2 inhibition. Together, experiments under this proposal will uncover how β-cells translate nutrient cues into mitogenic signals as well as pave the way for developing pharmacological strategies to safely increase β-cell mass in humans with diabetes.