Adaptive beta-cell replication occurs in response to increased metabolic demand during insulin resistance. The intracellular mediators of this compensatory response are poorly defined and their identification could provide significant targets for beta-cell regeneration therapies. Here we show that glucose and insulin in vitro and insulin resistance in vivo activate protein kinase C zeta (PKCzeta) in pancreatic islets and beta-cells. PKCzeta is required for glucose- and glucokinase activator-induced proliferation of rodent and human beta-cells in vitro. Furthermore, either kinase-dead PKCzeta expression (KD-PKCzeta) or disruption of PKCzeta in mouse beta-cells blocks compensatory beta-cell replication when acute hyperglycemia/hyperinsulinemia is induced. Importantly, KD-PKCzeta inhibits insulin resistance-mediated mammalian target of rapamycin (mTOR) activation and cyclin-D2 upregulation independent of Akt activation. In summary, PKCzeta activation is key for early compensatory beta-cell replication in insulin resistance by regulating the downstream signals mTOR and cyclin-D2. This suggests that alterations in PKCzeta expression or activity might contribute to inadequate beta-cell mass expansion and beta-cell failure leading to type 2 diabetes.

An important goal in diabetes research is to understand the processes that trigger endogenous beta-cell proliferation. Hyperglycemia induces beta-cell replication, but the mechanism remains debated. A prime candidate is insulin, which acts locally through the insulin receptor. Having previously developed an in vivo mouse hyperglycemia model, we tested whether glucose induces beta-cell proliferation through insulin signaling. By using mice lacking insulin signaling intermediate insulin receptor substrate 2 (IRS2), we confirmed that hyperglycemia-induced beta-cell proliferation requires IRS2 both in vivo and ex vivo. Of note, insulin receptor activation was not required for glucose-induced proliferation, and insulin itself was not sufficient to drive replication. Glucose and insulin caused similar acute signaling in mouse islets, but chronic signaling differed markedly, with mammalian target of rapamycin (MTOR) and extracellular signal-related kinase (ERK) activation by glucose and AKT activation by insulin. MTOR but not ERK activation was required for glucose-induced proliferation. Cyclin D2 was necessary for glucose-induced beta-cell proliferation. Cyclin D2 expression was reduced when either IRS2 or MTOR signaling was lost, and restoring cyclin D2 expression rescued the proliferation defect. Human islets shared many of these regulatory pathways. Taken together, these results support a model in which IRS2, MTOR, and cyclin D2, but not the insulin receptor, mediate glucose-induced proliferation.