Recent studies have demonstrated the importance of insulin or insulin-like growth factor 1 (IGF-1) for regulation of pancreatic ␤-cell mass. Given the role of tuberous sclerosis complex 2 (TSC2) as an upstream molecule of mTOR (mammalian target of rapamycin), we examined the effect of TSC2 deficiency on ␤-cell function. Here, we show that mice deficient in TSC2, specifically in pancreatic ␤ cells (␤TSC2 ؊/؊ mice), manifest increased IGF-1-dependent phosphorylation of p70 S6 kinase and 4E-BP1 in islets as well as an initial increased islet mass attributable in large part to increases in the sizes of individual ␤ cells. These mice also exhibit hypoglycemia and hyperinsulinemia at young ages (4 to 28 weeks). After 40 weeks of age, however, the ␤TSC2 ؊/؊ mice develop progressive hyperglycemia and hypoinsulinemia accompanied by a reduction in islet mass due predominantly to a decrease in the number of ␤ cells. These results thus indicate that TSC2 regulates pancreatic ␤-cell mass in a biphasic manner.

The total mass of islets of Langerhans is reduced in individuals with type 2 diabetes, possibly contributing to the pathogenesis of this condition. Although the regulation of islet mass is complex, recent studies have suggested the importance of a signaling pathway that includes the insulin or insulin-like growth factor-1 receptors, insulin receptor substrate and phosphatidylinositol (PI) 3-kinase. 3-Phosphoinositide-dependent protein kinase 1 (PDK1) is a serine-threonine kinase that mediates signaling downstream of PI 3-kinase. Here we show that mice that lack PDK1 specifically in pancreatic beta cells (betaPdk1-/- mice) develop progressive hyperglycemia as a result of a loss of islet mass. The mice show reductions in islet density as well as in the number and size of cells. Haploinsufficiency of the gene for the transcription factor Foxo1 resulted in a marked increase in the number, but not the size, of cells and resulted in the restoration of glucose homeostasis in betaPdk1-/- mice. These results suggest that PDK1 is important in maintenance of pancreatic cell mass and glucose homeostasis.

Pancreatic β cell failure is thought to underlie the progression from glucose intolerance to overt diabetes, and ER stress is implicated in such β cell dysfunction. We have now shown that the transcription factor CCAAT/ enhancer-binding protein β (C/EBPβ) accumulated in the islets of diabetic animal models as a result of ER stress before the onset of hyperglycemia. Transgenic overexpression of C/EBPβ specifically in β cells of mice reduced β cell mass and lowered plasma insulin levels, resulting in the development of diabetes. Conversely, genetic ablation of C/EBPβ in the β cells of mouse models of diabetes, including Akita mice, which harbor a heterozygous mutation in Ins2 (Ins2 WT/C96Y ), and leptin receptor-deficient (Lepr -/-) mice, resulted in an increase in β cell mass and ameliorated hyperglycemia. The accumulation of C/EBPβ in pancreatic β cells reduced the abundance of the molecular chaperone glucose-regulated protein of 78 kDa (GRP78) as a result of suppression of the transactivation activity of the transcription factor ATF6α, thereby increasing the vulnerability of these cells to excess ER stress. Our results thus indicate that the accumulation of C/EBPβ in pancreatic β cells contributes to β cell failure in mice by enhancing susceptibility to ER stress.

In the liver, signal transducer and activator of transcription 3 (STAT3) plays an important role in the suppression of gluconeogenic enzyme expression. While obesity-associated endoplasmic reticulum (ER) stress has been shown to increase hepatic gluconeogenic enzyme expression, the role of ER stress in STAT3-dependent regulation of such expression is unclear. The current study aimed to elucidate the effect of ER stress on the STAT3-dependent regulation of hepatic gluconeogenic enzyme expression. Genetically obese/diabetic db/db mice and db/db mouse–derived isolated hepatocytes were used as ER stress models. A tyrosine phosphatase inhibitor, a deacetylation inhibitor, and an acetylated mutant of STAT3 were used to examine the effect of ER stress on hepatic STAT3 action. ER stress inhibited STAT3-dependent suppression of gluconeogenic enzyme gene expression by suppressing hepatic Janus kinase (JAK)2 and STAT3 phosphorylation. A tyrosine phosphatase inhibitor restored ER stress–induced suppression of JAK2 phosphorylation but exhibited no improving effect on suppressed STAT3 phosphorylation. STAT3 acetylation is known to correlate with its phosphorylation. ER stress also decreased STAT3 acetylation. An acetylated mutant of STAT3 was resistant to ER stress–induced inhibition of STAT3-phosphorylation and STAT3-dependent suppression of hepatic gluconeogenic enzyme gene expression in vitro and in vivo. Trichostatin A, a histone deacetylase (HDAC) inhibitor, ameliorated ER stress–induced inhibition of STAT3 acetylation and phosphorylation. The current study revealed that ER stress inhibits STAT3-dependent suppression of hepatic gluconeogenic enzymes via JAK2 dephosphorylation and HDAC-dependent STAT3 deacetylation, playing an important role in the increase of hepatic glucose production in obesity and diabetes.

The mechanistic target of rapamycin complex 1 (MTORC1) is a critical negative regulator of general autophagy. We hypothesized that MTORC1 may specifically regulate autophagic clearance of damaged mitochondria. To test this, we used cells lacking tuberous sclerosis complex 2 (TSC2 -/-), which show constitutive MTORC1 activation. TSC2 -/- cells show MTORC1-dependent impaired autophagic flux after chemical uncoupling of mitochondria, increased mitochondrial protein aging and accumulation of p62/SQSTM1 positive mitochondria. Mitochondrial autophagy (mitophagy) was also deficient in cells lacking TSC2, associated with altered expression of PTEN-induced kinase 1 (PINK1) and PARK2 translocation to uncoupled mitochondria, all of which were recovered by MTORC1 inhibition or expression of constitutively active FoxO1. These data prove the necessity of intact MTORC1 signaling to regulate two synergistic processes required for clearance of damaged mitochondria: 1) general autophagy initiation, and 2) PINK1/PARK2-mediated selective targeting of uncoupled mitochondria to the autophagic machinery.