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␤-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

OBJECTIVE—Prolonged exposure of isolated islets of Langerhans to elevated levels of fatty acids, in the presence of high glucose, impairs insulin gene expression via a transcriptional mechanism involving nuclear exclusion of pancreas-duodenum homeobox-1 (Pdx-1) and loss of MafA expression. Whether such a phenomenon also occurs in vivo is unknown. Our objective was therefore to ascertain whether chronic nutrient oversupply inhibits insulin gene expression in vivo. RESEARCH DESIGN AND METHODS—Wistar rats received alternating 4-h infusions of glucose and Intralipid for a total of 72 h. Control groups received alternating infusions of glucose and saline, saline and Intralipid, or saline only. Insulin and C-peptide secretion were measured under hyperglycemic clamps. Insulin secretion and gene expression were assessed in isolated islets, and β-cell mass was quantified by morphometric analysis. RESULTS—Neither C-peptide secretion nor insulin sensitivity was different among infusion regimens. Insulin content and insulin mRNA levels were lower in islets isolated from rats infused with glucose plus Intralipid. This was associated with reduced Pdx-1 binding to the endogenous insulin promoter, and an increased proportion of Pdx-1 localized in the cytoplasm versus the nucleus. In contrast, MafA mRNA and protein levels and β-cell mass and proliferation were unchanged. CONCLUSIONS—Cyclical and alternating infusions of glucose and Intralipid in normal rats inhibit insulin gene expression without affecting insulin secretion or β-cell mass. We conclude that fatty acid inhibition of insulin gene expression, in the presence of high glucose, is an early functional defect that may contribute to β-cell failure in type 2 diabetes.

Aims/hypothesis Prolonged exposure of pancreatic beta cells to excessive levels of glucose and fatty acids, referred to as glucolipotoxicity, is postulated to contribute to impaired glucose homeostasis in patients with type 2 diabetes. However, the relative contribution of defective beta cell function vs diminished beta cell mass under glucolipotoxic conditions in vivo remains a subject of debate. We therefore sought to determine whether glucolipotoxicity in rats is due to impaired beta cell function and/ or reduced beta cell mass, and whether older animals are more susceptible to glucolipotoxic condition. Methods Wistar rats (2 and 6 months old) received a 72 h infusion of glucose + intravenous fat emulsion or saline control. In vivo insulin secretion and sensitivity were assessed by hyperglycaemic clamps. Ex vivo insulin secretion, insulin biosynthesis and gene expression were measured in isolated islets. Beta cell mass and proliferation were examined by immunohistochemistry. Results A 72 h infusion of glucose + intravenous fat emulsion in 2-month-old Wistar rats did not affect insulin sensitivity, insulin secretion or beta cell mass. In 6-month-old rats by contrast it led to insulin resistance and reduced insulin secretion in vivo, despite an increase in beta cell mass and proliferation. This was associated with: (1) diminished glucose-stimulated second-phase insulin secretion and proinsulin biosynthesis; (2) lower insulin content; and (3) reduced expression of beta cell genes in isolated islets. Conclusions/interpretation In this in vivo model, glucolipotoxicity is characterised by an age-dependent impairment of glucose-regulated beta cell function despite a marked increase in beta cell mass.

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...