Regulation of pancreatic ␤ cell mass and function is a major determinant for the development of diabetes. Growth factors and nutrients are important regulators of ␤ cell mass and function. The signaling pathways by which these growth signals modulate these processes have not been completely elucidated. Tsc2 is an attractive candidate to modulate these processes, because it is a converging point for growth factor and nutrient signals. In these experiments, we generated mice with conditional deletion of Tsc2 in ␤ cells (␤Tsc2 ؊/؊ ). These mice exhibited decreased glucose levels and hyperinsulinemia in the fasting and fed state. Improved glucose tolerance in these mice was observed as early as 4 weeks of age and was still present in 52-week-old mice. Deletion of Tsc2 in ␤ cells induced expansion of ␤ cell mass by increased proliferation and cell size. Rapamycin treatment reversed the metabolic changes in ␤Tsc2 ؊/؊ mice by induction of insulin resistance and reduction of ␤ cell mass. The reduction of ␤ cell mass in ␤Tsc2 ؊/؊ mice by inhibition of the mTOR/Raptor (TORC1) complex with rapamycin treatment suggests that TORC1 mediates proliferative and growth signals induced by deletion of Tsc2 in ␤ cells. These studies uncover a critical role for the Tsc2/mTOR pathway in regulation of ␤ cell mass and carbohydrate metabolism in vivo.mTOR ͉ pancreas ͉ islets

Growth factors, insulin signaling, and nutrients are important regulators of ␤-cell mass and function. The events linking these signals to the regulation of ␤-cell mass are not completely understood. The mTOR pathway integrates signals from growth factors and nutrients. Here, we evaluated the role of the mTOR/ raptor (mTORC1) signaling in proliferative conditions induced by controlled activation of Akt signaling. These experiments show that the mTORC1 is a major regulator of ␤-cell cycle progression by modulation of cyclin D2, D3, and Cdk4 activity. The regulation of cell cycle progression by mTORC1 signaling resulted from modulation of the synthesis and stability of cyclin D2, a critical regulator of ␤-cell cycle, proliferation, and mass. These studies provide novel insights into the regulation of cell cycle by the mTORC1, provide a mechanism for the antiproliferative effects of rapamycin, and imply that the use of rapamycin could negatively impact the success of islet transplantation and the adaptation of ␤-cells to insulin resistance.The defects that result in diabetes are diverse, but the loss of pancreatic ␤-cell mass is a critical determinant for the development of this disease (1, 2). The capacity for ␤-cells to expand in response to insulin resistance is required to maintain glucose homeostasis and prevent type 2 diabetes. Pancreatic ␤-cell mass is regulated by a dynamic balance of neogenesis, proliferation, hypertrophy, and apoptosis (3). In particular, ␤-cell proliferation (determined by the number of mature ␤-cells entering the cell cycle) has a major role in the maintenance of ␤-cell mass in adult life and after proliferative stimuli (4). Although there has been much research showing the role of ␤-cell mass in diabetes, there is a lack of knowledge pertaining to how ␤-cells enter the cell cycle, proliferate, and increase mass.In pancreatic ␤-cells, glucose, amino acids, and growth factors have been shown to induce G 1 -S progression (5-7). Recent studies have demonstrated that mTOR integrates growth factors and nutrient signals and is essential for cell growth and proliferation (8). One of the major mechanisms by which nutrient and growth factors regulate mTOR activity involves the tuberous sclerosis complex 2 (TSC2) 2 gene product (tuberin) as well as TSC1 (hamartin) and the small G protein Ras homolog enriched in brain. Phosphorylation of TSC2 by the serine-threonine kinase AKT induces mTOR signaling by derepressing the TSC2 GTPase-activating protein activity toward Ras homolog enriched in brain, (9 -13). Recent findings indicate that mTOR is a part of two distinct complexes: mTORC1 and mTORC2 (14,15). The mammalian mTORC1 contains Raptor and the G protein ␤-subunit-like protein (G␤L). mTORC1 activates key regulators of protein translation; ribosomal S6 kinase (S6K), eukaryote initiation factor 4E-binding protein 1, and eukaryote initiation factor 4E (16). The mTORC2 complex includes mTOR and rictor and is insensitive to rapamycin (14, 15). This complex is potentially important for the regulation o...

Dendritic cells are ideally suited to orchestrate the innate and adaptive immune responses to infection, but we know little about how these cells respond to infection with common respiratory viruses. Paramyxoviral infections are the most frequent cause of serious respiratory illness in childhood and are associated with an increased risk of asthma. We therefore used a high-fidelity mouse model of paramyxoviral respiratory infection triggered by Sendai virus to examine the response of conventional and plasmacytoid dendritic cells (cDCs and pDCs, respectively) in the lung. We found that pDCs are scarce at baseline but become the predominant population of lung dendritic cells during infection. This recruitment allows for a source of IFN-α locally at the site of infection. In contrast, cDCs rapidly differentiate into myeloid cDCs and begin to migrate from the lung to draining lymph nodes within 2 h after viral inoculation. These events cause the number of lung cDCs to decrease rapidly and remain decreased at the site of viral infection. Maturation and migration of lung cDCs depends on Ccl5 and Ccr5 signals because these events are significantly impaired in Ccl5−/− and Ccr5−/− mice. cDCs failure to migrate to draining lymph nodes in Ccl5−/− or Ccr5−/− mice is associated with impaired up-regulation of CCR7 that would normally direct this process. Our results indicate that pDCs and cDCs respond distinctly to respiratory paramyxoviral infection with patterns of movement that should serve to coordinate the innate and adaptive immune responses, respectively.

T he capacity of β cells to expand in response to insulin resistance is a critical factor in the development of type 2 diabetes. Proliferation of β cells is a major component for these adaptive responses in animal models. The extracellular signals responsible for β-cell expansion include growth factors, such as insulin, and nutrients, such as glucose and amino acids. AKT activation is one of the important components linking growth signals to the regulation of β-cell expansion. Downstream of AKT, tuberous sclerosis complex 1 and 2 (TSC1/2) and mechanistic target of rapamycin complex 1 (mTORC1) signaling have emerged as prime candidates in this process, because they integrate signals from growth factors and nutrients. Recent studies demonstrate the importance of mTORC1 signaling in β cells. This review will discuss recent advances in the understanding of how this pathway regulates β-cell mass and present data on the role of TSC1 in modulation of β-cell mass. Herein, we also demonstrate that deletion of Tsc1 in pancreatic β cells results in improved glucose tolerance, hyperinsulinemia and expansion of β-cell mass that persists with aging.

OBJECTIVEThe purpose of this study was to evaluate the role of the S6K arm of mammalian target of rapamycin complex 1 (mTORC1) signaling in regulation of β-cell mass and function. Additionally, we aimed to delineate the importance of in vivo S6K activation in the regulation of insulin signaling and the extent to which alteration of insulin receptor substrate (IRS) signaling modulates β-cell mass and function.RESEARCH DESIGN AND METHODSThe current experiments describe the phenotype of transgenic mice overexpressing a constitutively active form of S6K under the control of the rat insulin promoter.RESULTSActivation of S6K signaling in these mice improved insulin secretion in the absence of changes in β-cell mass. The lack of β-cell mass expansion resulted from decreased G1-S progression and increased apoptosis. This phenotype was associated with increased p16 and p27 and decreased Cdk2 levels. The changes in cell cycle were accompanied by diminished survival signals because of impaired IRS/Akt signaling.CONCLUSIONSThis work defines the importance of S6K in regulation of β-cell cycle, cell size, function, and survival. These experiments also demonstrate that in vivo downregulation of IRS signaling by TORC1/S6K induces β-cell insulin resistance, and that this mechanism could explain some of the abnormalities that ultimately result in β-cell failure and diabetes in conditions of nutrient overload.