The physiological mechanisms underlying the compensatory growth of ␤-cell mass in insulin-resistant states are poorly understood. Using the insulin-resistant Zucker fatty (fa/fa) (ZF) rat and the corresponding Zucker lean control (ZLC) rat, we investigated the factors contributing to the age-/obesity-related enhancement of ␤-cell mass. A 3.8-fold ␤-cell mass increase was observed in ZF rats as early as 5 weeks of age, an age that precedes severe insulin resistance by several weeks. Closer investigation showed that ZF rat pups were not born with heightened ␤-cell mass but developed a modest increase over ZLC rats by 20 days that preceded weight gain or hyperinsulinemia that first developed at 24 days of age. In these ZF pups, an augmented survival potential of ␤-cells of ZF pups was observed by enhanced activated (phospho-) Akt, phospho-BAD, and Bcl-2 immunoreactivity in the postweaning period. However, increased ␤-cell proliferation in the ZF rats was only detected at 31 days of age, a period preceding massive ␤-cell growth. During this phase, we also detected an increase in the numbers of small ␤-cell clusters among ducts and acini, increased duct pancreatic/duodenal homeobox-1 (PDX-1) immunoreactivity, and an increase in islet number in the ZF rats suggesting duct-and acini-mediated heightened ␤-cell neogenesis. Interestingly, in young ZF rats, specific cells associated with ducts, acini, and islets exhibited an increased frequency of PDX-1 ؉ /phospho-Akt ؉ staining, indicating a potential role for Akt in ␤-cell differentiation. Thus, several adaptive mechanisms account for the compensatory growth of ␤-cells in ZF rats, a combination of enhanced survival and neogenesis with a transient rise in proliferation before 5 weeks of age, with Akt serving as a potential mediator in these processes. Diabetes 54: 2294 -2304, 2005 T he pancreatic islet ␤-cell regulates cellular fuel metabolism and glucose homeostasis through its secretion of insulin. Numerous studies in rodents and humans have shown a tremendous capacity for ␤-cell expansion in response to physiological and pathophysiological changes in tissue insulin requirements, i.e., insulin resistance (1-3). Over the last several years, much has been learned about the molecular regulation of ␤-cell development from mice with targeted genetic mutations of transcription factors and cell signaling intermediates. These studies have identified a key role for the insulin/IGF-1 signaling pathway in the adaptive response to insulin resistance, particularly the intermediates insulin receptor substrate-2 (IRS-2) (4 -9), Akt/protein kinase B (PKB) (10 -13), and the forkhead transcription factor Foxo1 (14) as putative regulators of the gene encoding pancreatic-duodenal homeobox-1 (PDX-1) (15). Although the details are not known about how the ␤-cell mass is regulated in response to progressive insulin resistance, a recent study implicates PDX-1 with a key role (16). Although the relative contribution of hyperplasia of existing ␤-cells versus new cell development (neogenesis) from...

␤-Cell mass is determined by a dynamic balance of proliferation, neogenesis, and apoptosis. The precise mechanisms underlying compensatory ␤-cell mass (BCM) homeostasis are not fully understood. To evaluate the processes that maintain normoglycemia and regulate BCM during pancreatic regeneration, C57BL/6 mice were analyzed for 15 days following 60% partial pancreatectomy (Px). BCM increased in Px mice from 2 days onwards and was ϳ68% of the shams by 15 days, partly due to enhanced ␤-cell proliferation. A transient ϳ2.8-fold increase in the prevalence of ␤-cell clusters/small islets at 2 days post-Px contributed substantially to BCM augmentation, followed by an increase in the number of larger islets at 15 days. To evaluate the signaling mechanisms that may regulate this compensatory growth, we examined key intermediates of the insulin signaling pathway. We found insulin receptor substrate (IRS)2 and enhanced-activated Akt immunoreactivity in islets and ducts that correlated with increased pancreatic duodenal homeobox (PDX)1 expression. In contrast, forkhead box O1 expression was decreased in islets but increased in ducts, suggesting distinct PDX1 regulatory mechanisms in these tissues. Px animals acutely administered insulin exhibited further enhancement in insulin signaling activity. These data suggest that the IRS2-Akt pathway mediates compensatory ␤-cell growth by activating ␤-cell proliferation with an increase in the number of ␤-cell clusters/small islets. Diabetes 55: 3289 -3298, 2006

The onset of type 2 diabetes is characterized by transition from successful to failed insulin secretory compensation to obesity-related insulin resistance and dysmetabolism. Energy-rich diets in rodents are commonly studied models of compensatory increases in both insulin secretion and β cell mass. However, the mechanisms of these adaptive responses are incompletely understood, and it is also unclear why these responses eventually fail. We measured the temporal trends of glucose homeostasis, insulin secretion, β cell morphometry, and islet gene expression in C57BL/6NTac mice fed a 60% high-fat diet (HFD) or control diet for up to 16 weeks. A 2-fold increased hyperinsulinemia was maintained for the first 4 weeks of HFD feeding and then further increased through 16 weeks. β cell mass increased progressively starting at 4 weeks, principally through nonproliferative growth. Insulin sensitivity was not significantly perturbed until 11 weeks of HFD feeding. Over the first 8 weeks, we observed two distinct waves of increased expression of β cell functional and prodifferentiation genes. This was followed by activation of the unfolded protein response at 8 weeks and overt β cell endoplasmic reticulum stress at 12-16 weeks. In summary, β cell adaptation to an HFD in C57BL/6NTac mice entails early insulin hypersecretion and a robust growth phase along with hyperexpression of related genes that begin well before the onset of observed insulin resistance. However, continued HFD exposure results in cessation of gene hyperexpression, β cell functional failure, and endoplasmic reticulum stress. These data point to a complex but not sustainable integration of β cell-adaptive responses to nutrient overabundance, obesity development, and insulin resistance.

The physiological mechanisms that preserve pancreatic ␤-cell mass (BCM) are not fully understood. Although the regulation of islet function by the autonomic nervous system (ANS) is well established, its potential roles in BCM homeostasis and compensatory growth have not been adequately explored. The parasympathetic vagal branch of the ANS serves to facilitate gastrointestinal function, metabolism, and pancreatic islet regulation of glucose homeostasis, including insulin secretion. Given the functional importance of the vagus nerve and its branches to the liver, gut, and pancreas in control of digestion, motility, feeding behavior, and glucose metabolism, it may also play a role in BCM regulation. We have begun to examine the potential roles of the parasympathetic nervous system in short-term BCM maintenance by performing a selective bilateral celiac branch-vagus nerve transection (CVX) in normal Sprague-Dawley rats. CVX resulted in no detectable effects on basic metabolic parameters or food intake through 1 wk postsurgery. Although there were no differences in BCM or apoptosis in this 1-wk time frame, ␤-cell proliferation was reduced 50% in the CVX rats, correlating with a marked reduction in activated protein kinase B/Akt. Unexpectedly, acinar proliferation was increased 50% in these rats. These data suggest that the ANS, via the vagus nerve, contributes to the regulation of BCM maintenance at the level of cell proliferation and may also mediate the drive for enhanced growth under physiological conditions when insulin requirements have increased. Furthermore, the disparate effects of CVX on ␤-cell and acinar cells suggest that the endocrine and exocrine pancreas respond to different neural signals in regard to mass homeostasis.␤-cell mass; celiac vagotomy THE PANCREATIC ␤-CELL HAS A TREMENDOUS CAPACITY to functionally compensate in response to physiological and pathophysiological changes in tissue insulin demands. A key feature of this adaptive response is the ability of the ␤-cell mass (BCM) to be dynamically regulated. For instance, BCM is normally maintained proportionally to body size. It increases during obesity and pregnancy (5,14,20) and regresses postpartum (42) to adjust to physiological insulin requirements. The steady-state BCM is determined by new cell recruitment by hyperplasia of existing cells and neogenesis from pancreatic epithelial progenitors, apoptotic death and clearance, and hypertrophy (5). The relative importance of each of these different processes is largely unknown and depends on the model under study; nonetheless, several recent reports suggest that proliferation from preexisting ␤-cells predominates in mice (10,11,29,30).The interplay of nutrients, growth factors, and hormones is known to impact ␤-cell growth; however, the influence of the nervous system on ␤-cell growth processes has been understudied. There are a handful of reports highlighting the importance of the vagus efferents in ␤-cell proliferation. A transient decrease in ␤-cell proliferation was observed in ob/ob mice, bu...

The physiological mechanisms underlying pancreatic ␤-cell mass (BCM) homeostasis are complex and not fully resolved. Here we examined the factors contributing to the increased BCM following a mild glucose infusion (GI) whereby normoglycemia was maintained through 96 h. We used morphometric and immunochemical methods to investigate enhanced ␤-cell growth and survival in Sprague-Dawley rats. BCM was elevated Ͼ2.5-fold over saline-infused control rats by 48 h and increased modestly thereafter. Unexpectedly, increases in ␤-cell proliferation were not observed at any time point through 4 days. Instead, enhanced numbers of insulin ϩ cell clusters and small islets (400 -12,000 m 2 ; ϳ23-to 124-m diameter), mostly scattered among the acini, were observed in the GI rats by 48 h despite no difference in the numbers of medium to large islets. We previously showed that increased ␤-cell growth in rodent models of insulin resistance and pancreatic regeneration involves increased activated Akt/PKB, a key ␤-cell signaling intermediate, in both islets and endocrine cell clusters. GI in normal rats also leads to increased Akt activation in islet ␤-cells, as well as in insulin ϩ and insulin Ϫ cells in the common duct epithelium and endocrine clusters. This correlated with strong Pdx1 expression in these same cells. These results suggest that mechanisms other than proliferation underlie the rapid ␤-cell growth response following a mild GI in the normal rat and involve Akt-regulated enhanced ␤-cell survival potential and neogenesis from epithelial precursors.