l-Asparaginase regulates mTORC1 activity via a TSC2-dependent pathway in pancreatic beta cells.

Eif2ak4, a susceptibility gene for type 2 diabetes, encodes GCN2, a molecule activated by amino acid deficiency. Mutations or deletions in GCN2 in pancreatic ß-cells increase mTORC1 activity by decreasing Sestrin2 expression in a TSC2-independent manner. In this study, we searched for molecules downstream of GCN2 that suppress mTORC1 activity in a TSC2-dependent manner. To do so, we used a pull-down assay to identify molecules that competitively inhibit the binding of the T1462 phosphorylation site of TSC2 to 14-3-3. l-asparaginase was identified. Although l-asparaginase is frequently used as an anticancer drug for acute lymphoblastic leukemia, little is known about endogenous l-asparaginase. l-Asparaginase, which is expressed downstream of GCN2, was found to bind 14-3-3 and thereby to inhibit its binding to the T1462 phosphorylation site of TSC2 and contribute to TSC2 activation and mTORC1 inactivation upon TSC2 dephosphorylation. Further investigation of the regulation of mTORC1 activity in pancreatic ß-cells by l-asparaginase should help to elucidate the mechanism of diabetes and insulin secretion failure during anticancer drug use.

GCN2 regulates pancreatic ß cell mass by sensing intracellular amino acid levels.

EIF2AK4, which encodes the amino acid deficiency-sensing protein GCN2, has been implicated as a susceptibility gene for type 2 diabetes in the Japanese population. However, the mechanism by which GCN2 affects glucose homeostasis is unclear. Here, we show that insulin secretion is reduced in individuals harboring the risk allele of EIF2AK4 and that maintenance of GCN2-deficient mice on a high-fat diet results in a loss of pancreatic ß cell mass. Our data suggest that GCN2 senses amino acid deficiency in ß cells and limits signaling by mechanistic target of rapamycin complex 1 to prevent ß cell failure during the consumption of a high-fat diet.

PHD3 regulates glucose metabolism by suppressing stress-induced signalling and optimising gluconeogenesis and insulin signalling in hepatocytes.

Glucagon-mediated gene transcription in the liver is critical for maintaining glucose homeostasis. Promoting the induction of gluconeogenic genes and blocking that of insulin receptor substrate (Irs)2 in hepatocytes contributes to the pathogenesis of type 2 diabetes. However, the molecular mechanism by which glucagon signalling regulates hepatocyte metabolism is not fully understood. We previously showed that a fasting-inducible signalling module consisting of general control non-repressed protein 5, co-regulator cAMP response element-binding protein binding protein/p300-interacting transactivator with Glu/Asp-rich carboxy-terminal domain 2, and protein kinase A is required for glucagon-induced transcription of gluconeogenic genes. The present study aimed to identify the downstream effectors of this module in hepatocytes by examining glucagon-induced potential target genes. One of these genes was prolyl hydroxylase domain (PHD)3, which suppressed stress signalling through inhibition of the IκB kinase-nuclear factor-κB pathway in a proline hydroxylase-independent manner to maintain insulin signalling. PHD3 was also required for peroxisome proliferator-activated receptor γ coactivator 1α-induced gluconeogenesis, which was dependent on proline hydroxylase activity, suggesting that PHD3 regulates metabolism in response to glucagon as well as insulin. These findings demonstrate that glucagon-inducible PHD3 regulates glucose metabolism by suppressing stress signalling and optimising gluconeogenesis and insulin signalling in hepatocytes.

MTORC1 Regulates both General Autophagy and Mitophagy Induction after Oxidative Phosphorylation Uncoupling.

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-/- cells), 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 putative kinase 1 (PINK1) and PARK2 translocation to uncoupled mitochondria, all of which were recovered by MTORC1 inhibition or expression of constitutively active forkhead box protein O1 (FoxO1). These data prove the necessity of intact MTORC1 signaling to regulate two synergistic processes required for clearance of damaged mitochondria: (i) general autophagy initiation and (ii) PINK1/PARK2-mediated selective targeting of uncoupled mitochondria to the autophagic machinery.

Histone deacetylase regulates insulin signaling via two pathways in pancreatic ß cells.

Recent studies demonstrated that insulin signaling plays important roles in the regulation of pancreatic ß cell mass, the reduction of which is known to be involved in the development of diabetes. However, the mechanism underlying the alteration of insulin signaling in pancreatic ß cells remains unclear. The involvement of epigenetic control in the onset of diabetes has also been reported. Thus, we analyzed the epigenetic control of insulin receptor substrate 2 (IRS2) expression in the MIN6 mouse insulinoma cell line. We found concomitant IRS2 up-regulation and enhanced insulin signaling in MIN6 cells, which resulted in an increase in cell proliferation. The H3K9 acetylation status of the Irs2 promoter was positively associated with IRS2 expression. Treatment of MIN6 cells with histone deacetylase inhibitors led to increased IRS2 expression, but this occurred in concert with low insulin signaling. We observed increased IRS2 lysine acetylation as a consequence of histone deacetylase inhibition, a modification that was coupled with a decrease in IRS2 tyrosine phosphorylation. These results suggest that insulin signaling in pancreatic ß cells is regulated by histone deacetylases through two novel pathways affecting IRS2: the epigenetic control of IRS2 expression by H3K9 promoter acetylation, and the regulation of IRS2 activity through protein modification. The identification of the histone deacetylase isoform(s) involved in these mechanisms would be a valuable approach for the treatment of type 2 diabetes.

The GCN5-CITED2-PKA signalling module controls hepatic glucose metabolism through a cAMP-induced substrate switch.

Hepatic gluconeogenesis during fasting results from gluconeogenic gene activation via the glucagon-cAMP-protein kinase A (PKA) pathway, a process whose dysregulation underlies fasting hyperglycemia in diabetes. Such transcriptional activation requires epigenetic changes at promoters by mechanisms that have remained unclear. Here we show that GCN5 functions both as a histone acetyltransferase (HAT) to activate fasting gluconeogenesis and as an acetyltransferase for the transcriptional co-activator PGC-1α to inhibit gluconeogenesis in the fed state. During fasting, PKA phosphorylates GCN5 in a manner dependent on the transcriptional coregulator CITED2, thereby increasing its acetyltransferase activity for histone and attenuating that for PGC-1α. This substrate switch concomitantly promotes both epigenetic changes associated with transcriptional activation and PGC-1α-mediated coactivation, thereby triggering gluconeogenesis. The GCN5-CITED2-PKA signalling module and associated GCN5 substrate switch thus serve as a key driver of gluconeogenesis. Disruption of this module ameliorates hyperglycemia in obese diabetic animals, offering a potential therapeutic strategy for such conditions.

Central Insulin Action Activates Kupffer Cells by Suppressing Hepatic Vagal Activation via the Nicotinic Alpha 7 Acetylcholine Receptor.

Central insulin action activates hepatic IL-6/STAT3 signaling, which suppresses the gene expression of hepatic gluconeogenic enzymes. The vagus nerve plays an important role in this centrally mediated hepatic response; however, the precise mechanism underlying this brain-liver interaction is unclear. Here, we present our findings that the vagus nerve suppresses hepatic IL-6/STAT3 signaling via α7-nicotinic acetylcholine receptors (α7-nAchR) on Kupffer cells, and that central insulin action activates hepatic IL-6/STAT3 signaling by suppressing vagal activity. Indeed, central insulin-mediated hepatic IL-6/STAT3 activation and gluconeogenic gene suppression were impeded in mice with hepatic vagotomy, pharmacological cholinergic blockade, or α7-nAchR deficiency. In high-fat diet-induced obese and insulin-resistant mice, control of the vagus nerve by central insulin action was disturbed, inducing a persistent increase of inflammatory cytokines. These findings suggest that dysregulation of the α7-nAchR-mediated control of Kupffer cells by central insulin action may affect the pathogenesis of chronic hepatic inflammation in obesity.

Pathogenesis of selective insulin resistance in isolated hepatocytes.

The development of insulin resistance (IR) in the liver is a key pathophysiologic event in the development of type 2 diabetes. Although insulin loses its ability to suppress glucose production, it largely retains its capacity to drive lipogenesis. This selective IR results in the characteristic hyperglycemia and dyslipidemia of type 2 diabetes. The delineation of two branched pathways of insulin receptor (InsR) signaling to glucose versus triglyceride production, one through FoxO and the other through SREBP-1c, provides a mechanism to account for this pathophysiological abnormality. We tested the complementary hypothesis that selective IR arises due to different intrinsic sensitivities of glucose production versus de novo lipogenesis to insulin as a result of cell-autonomous down-regulation of InsR number in response to chronic hyperinsulinemia. We demonstrate in mouse primary hepatocytes that chronic hyperinsulinemia abrogates insulin's inhibition of glucose production, but not its stimulation of de novo lipogenesis. Using a competitive inhibitor of InsR, we show that there is a 4-fold difference between levels of InsR inhibition required to cause resistance of glucose production versus lipogenesis to the actions of insulin. Our data support a parsimonious model in which differential InsR activation underlies the selective IR of glucose production relative to lipogenesis, but both processes require signaling through Akt1/2.

Expression of AKR1C3 and CNN3 as markers for detection of lymph node metastases in colorectal cancer.

The aim of the study was to identify a set of discriminating genes that could be used for the prediction of Lymph node (LN) metastasis in human colorectal cancer (CRC), and for this, we compared the whole genome profiles of two CRC cell lines (the primary cell line SW480 and its LN metastatic variant, SW620) and identified eight genes [S100 calcium-binding protein P; aldo-keto reductase family 1(AKR1), member B1 (aldose reductase; AKR1B1); AKR1, member C3 (AKR1C3); calponin 3, acidic; metastasis associated in colon cancer 1; hemoglobin, epsilon 1; trefoil factor 3; and FGGY carbohydrate kinase domain containing]. These genes were examined by quantitative RT-PCR in tissues and LNs in 14 CRC patients and 11 control patients. The level of AKR1C3 mRNA expression was significantly different between the Dukes' stage A, B, and C groups and the control group (p < 0.05, p < 0.001, and p < 0.001) and was also significantly different between Dukes' stage C and A or B groups (p < 0.05 and p < 0.001, respectively). The expression of CNN3 was significantly different between the Dukes' stage C and B or control groups (p < 0.001 and p < 0.01, respectively). There were significant correlations between the expression levels of AKR1C3 and CNN3. AKR1C3 and CNN3 expressions are more accurate and suitable markers for the diagnosis of LN metastasis than the other six genes examined in this study.

Pancreatic ß-cell failure mediated by mTORC1 hyperactivity and autophagic impairment.

Hyperactivation of the mammalian target of rapamycin complex 1 (mTORC1) in ß-cells is usually found as a consequence of increased metabolic load. Although it plays an essential role in ß-cell compensatory mechanisms, mTORC1 negatively regulates autophagy. Using a mouse model with ß-cell-specific deletion of Tsc2 (ßTsc2(-/-)) and, consequently, mTORC1 hyperactivation, we focused on the role that chronic mTORC1 hyperactivation might have on ß-cell failure. mTORC1 hyperactivation drove an early increase in ß-cell mass that later declined, triggering hyperglycemia. Apoptosis and endoplasmic reticulum stress markers were found in islets of older ßTsc2(-/-) mice as well as accumulation of p62/SQSTM1 and an impaired autophagic response. Mitochondrial mass was increased in ß-cells of ßTsc2(-/-) mice, but mitophagy was also impaired under these circumstances. We provide evidence of ß-cell autophagy impairment as a link between mTORC1 hyperactivation and mitochondrial dysfunction that probably contributes to ß-cell failure.

Significance of ELF3 mRNA expression for detection of lymph node metastases of colorectal cancer.

BACKGROUND: Lymph node (LN) evaluation is an important factor for the prognosis of colorectal cancer (CRC). The purpose of our study was to investigate the effectiveness of E74-like factor 3 (ELF3) and carcinoembryonic antigen (CEA) as useful markers to detect LN metastases in CRC. MATERIALS AND METHODS: We examined the mRNA expression of ELF3 and CEA in LNs and tissues from 22 patients with CRC and in controls with ulcerative colitis (UC) by real-time quantitative reverse transcription polymerase chain reaction, as well as by hematoxylin-eosin staining. RESULTS: ELF3 and CEA expression showed statistically significant differences among four LN groups: LNs from patients with CRC categorized into three Dukes' stages and LNs from patients with UC (p<0.001 and p<0.001, respectively). We found a statistical correlation between the expression levels of both markers in patients with CRC compared with each Dukes' stage. CONCLUSION: ELF3, as a gene marker, may be sufficiently practical to detect LN metastases of CRC, rather than CEA.

Association between polymorphisms in UDP-glucuronosyltransferase 1A6 and 1A7 and colorectal cancer risk.

Genetic polymorphisms of uridine diphosphate-glucuronosyltransferases 1A6 (UGT1A6) and 1A7 (UGT1A7) may lead to genetic instability and colorectal cancer carcinogenesis. Our objective was to measure the interaction between polymorphisms of these repair genes and tobacco smoking in colorectal cancer (CRC). A total of 68 individuals with CRC and 112 non-cancer controls were divided into non-smoker and smoker groups according to pack-years of smoking. Genetic polymorphisms of UGT1A6 and UGT1A7 were examined using polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP). We found a weak association of UGT1A6 polymorphisms with CRC risk (crude odds ratio [OR], 1.65; 95% confidence interval [95%CI], 0.9-3.1, P=0.107; adjusted OR 1.95, 95%CI 1.0-3.8, P=0.051). The ORs for the UGT1A7 polymorphisms were statistically significant (crude OR: 26.40, 95%CI: 3.5-198.4, P=0.001; adjusted OR: 21.52, 95%CI: 2.8-164.1, P=0.003). The joint effect of tobacco exposure and UGT1A6 polymorphisms was significantly associated with colorectal cancer risk in non-smokers (crude OR, 2.11; 95%CI, 0.9-5.0, P=0.092; adjusted OR 2.63, 95%CI 1.0-6.7, P=0.042). In conclusion, our findings suggest that UGT1A6 and UGT1A7 gene polymorphisms are associated with CRC risk in the Japanese population. In particular, UGT1A6 polymorphisms may strongly increase CRC risk through the formation of carcinogens not associated with smoking.

DPP4 inhibitor vildagliptin preserves ß-cell mass through amelioration of endoplasmic reticulum stress in C/EBPB transgenic mice.

The development of type 2 diabetes is accompanied by a progressive decline in ß-cell mass and function. Vildagliptin, a dipeptidyl peptidase 4 inhibitor, is representative of a new class of antidiabetic agents that act through increasing the expression of glucagon-like peptide-1. The protective effect of this agent on ß cells was studied in diabetic mice. Diabetic pancreatic ß cell-specific C/EBPB transgenic (TG) mice exhibit decreased ß-cell mass associated with increased apoptosis, decreased proliferation, and aggravated endoplasmic reticulum (ER) stress. Vildagliptin was orally administered to the TG mice for a period of 24 weeks, and the protective effects of this agent on ß cells were examined, along with the potential molecular mechanism of protection. Vildagliptin ameliorated hyperglycemia in TG mice by increasing the serum concentration of insulin and decreasing the serum concentration of glucagon. This agent also markedly increased ß-cell mass, improved aggravated ER stress, and restored attenuated insulin/IGF1 signaling. A decrease in pancreatic and duodenal homeobox 1 expression was also observed in ß cells isolated from our mouse model, but this was also restored by vildagliptin treatment. The expression of C/EBPB protein, but not mRNA, was unexpectedly downregulated in vildagliptin-treated TG mice and in exenatide-treated MIN6 cells. Activation of the GLP1 pathway induced proteasome-dependent C/EBPB degradation in ß cells as the proteasome inhibitor MG132 restored the downregulation of C/EBPB protein by exenatide. Vildagliptin elicits protective effects on pancreatic ß cells, possibly through C/EBPB degradation, and has potential for preventing the progression of type 2 diabetes.

CITED2 links hormonal signaling to PGC-1α acetylation in the regulation of gluconeogenesis.

During fasting, induction of hepatic gluconeogenesis is crucial to ensure proper energy homeostasis. Such induction is dysregulated in type 2 diabetes, resulting in the development of fasting hyperglycemia. Hormonal and nutrient regulation of metabolic adaptation during fasting is mediated predominantly by the transcriptional coactivator peroxisome proliferative activated receptor γ coactivator 1α (PGC-1α) in concert with various other transcriptional regulators. Although CITED2 (CBP- and p300-interacting transactivator with glutamic acid- and aspartic acid-rich COOH-terminal domain 2) interacts with many of these molecules, the role of this protein in the regulation of hepatic gluconeogenesis was previously unknown. Here we show that CITED2 is required for the regulation of hepatic gluconeogenesis through PGC-1α. The abundance of CITED2 was increased in the livers of mice by fasting and in cultured hepatocytes by glucagon-cAMP-protein kinase A (PKA) signaling, and the amount of CITED2 in liver was higher in mice with type 2 diabetes than in non-diabetic mice. CITED2 inhibited the acetylation of PGC-1α by blocking its interaction with the acetyltransferase general control of amino acid synthesis 5-like 2 (GCN5). The consequent downregulation of PGC-1α acetylation resulted in an increase in its transcriptional coactivation activity and an increased expression of gluconeogenic genes. The interaction of CITED2 with GCN5 was disrupted by insulin in a manner that was dependent on phosphoinositide 3-kinase (PI3K)-thymoma viral proto-oncogene (Akt) signaling. Our results show that CITED2 functions as a transducer of glucagon and insulin signaling in the regulation of PGC-1α activity that is associated with the transcriptional control of gluconeogenesis and that this function is mediated through the modulation of GCN5-dependent PGC-1α acetylation. We also found that loss of hepatic CITED2 function suppresses gluconeogenesis in diabetic mice, suggesting it as a therapeutic target for hyperglycemia.

XRCC3 gene polymorphism is associated with survival in Japanese lung cancer patients.

We focused on OGG1 Ser326Cys, MUTYH Gln324His, APEX1 Asp148Glu, XRCC1 Arg399Gln, and XRCC3 Thr241Met and examined the relationship between the different genotypes and survival of Japanese lung cancer patients. A total of 99 Japanese lung cancer patients were recruited into our study. Clinical data were collected, and genotypes of the target genes were identified by polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP). Survival analysis to verify the impact of these gene polymorphisms on the clinical outcome of lung cancer showed that lung squamous cell carcinoma patients with the Thr/Met genotype at XRCC3 had a significantly shorter survival time than those with the Thr/Thr genotype (13 months versus 48 months; log-rank test, p < 0.0001). Cox regression analysis showed that the carriers of XRCC3 genotypes were at a significantly higher risk [adjusted hazard ratio (HR) = 9.35, 95% confidence interval (CI) = 2.52-34.68, p = 0.001; adjusted HR = 9.05, 95% CI = 1.89-44.39, p = 0.006]. Our results suggest that XRCC3 Thr241Met may act as a favorable prognostic indicator for lung squamous cell carcinoma patients.

Ablation of TSC2 enhances insulin secretion by increasing the number of mitochondria through activation of mTORC1.

AIM: We previously found that chronic tuberous sclerosis protein 2 (TSC2) deletion induces activation of mammalian target of rapamycin Complex 1 (mTORC1) and leads to hypertrophy of pancreatic beta cells from pancreatic beta cell-specific TSC2 knockout (ßTSC2(-/-)) mice. The present study examines the effects of TSC2 ablation on insulin secretion from pancreatic beta cells. METHODS: Isolated islets from ßTSC2(-/-) mice and TSC2 knockdown insulin 1 (INS-1) insulinoma cells treated with small interfering ribonucleic acid were used to investigate insulin secretion, ATP content and the expression of mitochondrial genes. RESULTS: Activation of mTORC1 increased mitochondrial DNA expression, mitochondrial density and ATP production in pancreatic beta cells of ßTSC2(-/-) mice. In TSC2 knockdown INS-1 cells, mitochondrial DNA expression, mitochondrial density and ATP production were increased compared with those in control INS-1 cells, consistent with the phenotype of ßTSC2(-/-) mice. TSC2 knockdown INS-1 cells also exhibited augmented insulin secretory response to glucose. Rapamycin inhibited mitochondrial DNA expression and ATP production as well as insulin secretion in response to glucose. Thus, ßTSC2(-/-) mice exhibit hyperinsulinemia due to an increase in the number of mitochondria as well as enlargement of individual beta cells via activation of mTORC1. CONCLUSION: Activation of mTORC1 by TSC2 ablation increases mitochondrial biogenesis and enhances insulin secretion from pancreatic beta cells.

APEX1 Asp148Glu gene polymorphism is a risk factor for lung cancer in relation to smoking in Japanese.

DNA repair enzymes play an important role in the development of various kinds of cancer. We here analyzed associations of XPD Lys751Gln, APEX1 Asp148Glu, XRCC1 Arg399Gln, and XRCC3 Thr241Met gene polymorphisms in DNA repair pathways in relation to the risk of lung cancer using PCR-RFLP. The study involved 104 lung cancer patients and 120 non-cancer controls divided into non-smokers and smokers. We found a statistically significant interaction between APEX1 Asp148Glu and the risk for lung cancer (adjusted OR 2.78, 95% CI 1.58-4.90, p=0.0004), of both adenocarcinoma (adjusted OR 2.24, 95%CI 1.18-4.25, p=0.014) and squamous cell carcinoma (adjusted OR 4.75, 95%CI 1.79-12.6, p=0.002) types. XRCC1 Arg399Gln showed a borderline significant association with adenocarcinoma (adjusted OR 1.89, 95%CI 1.00-3.57, p=0.051). The combined effect of smoking and presence of the APEX1 Asp148Glu demonstrated a significant association with risk of lung cancer (adjusted OR 3.61, 95% CI 1.74-7.50, p=0.001). The XPD Lys751Gln and XRCC3 Thr241Met genotypes displayed no statistically significant risk. Our findings suggest that the APEX1 Asp148Glu is associated with increased risk for primary lung cancer in Japanese individuals partaking in smoking.

Reduced insulin signaling and endoplasmic reticulum stress act synergistically to deteriorate pancreatic beta cell function.

The total pancreatic beta cell mass is reduced in individuals with type 2 diabetes. We analyzed the islets of leptin receptor-deficient (Lepr-/-) mice, a model animal for type 2 diabetes with obesity. The plasma insulin levels in Lepr-/- mice peaked at approximately 7 weeks, an age at which the animals manifest normoglycemia to moderate hyperglycemia. Consistent with this, the beta cell mass was enlarged as compared with Lepr+/- mice, and it decreased thereafter. Thus, we focused on the islets of Lepr-/- mice at 7 weeks to elucidate the mechanism underlying beta cell failure. Endoplasmic reticulum (ER) stress was enhanced in beta cells of Lepr-/- mice at 7 weeks, as indicated by the increase in c-Jun and eIF2 alpha phosphorylation. Lepr-/- mice also exhibited a reduction in insulin signaling in beta cells at 7 weeks, as indicated by the decrease in Akt phosphorylation. These results indicate that both augmented ER stress and reduced insulin signaling occur before the onset of frank diabetes. Next, to examine the mutual effect of ER stress and insulin signaling in beta cells in vitro, we used MIN6 insulinoma cells. Tunicamycin induced ER stress as well as inhibited insulin signaling. Conversely, the PI-3 kinase inhibitor, LY294002, enhanced ER stress. Furthermore, the reduction in insulin signaling by LY294002 facilitated the induction of ER stress with tunicamycin. Taken together, we concluded that both ER stress and reduced insulin signaling might synergistically affect pancreatic beta cell dysfunction.

Biphasic response of pancreatic beta-cell mass to ablation of tuberous sclerosis complex 2 in mice.

Recent studies have demonstrated the importance of insulin or insulin-like growth factor 1 (IGF-1) for regulation of pancreatic beta-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 beta-cell function. Here, we show that mice deficient in TSC2, specifically in pancreatic beta cells (betaTSC2(-/-) 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 beta cells. These mice also exhibit hypoglycemia and hyperinsulinemia at young ages (4 to 28 weeks). After 40 weeks of age, however, the betaTSC2(-/-) mice develop progressive hyperglycemia and hypoinsulinemia accompanied by a reduction in islet mass due predominantly to a decrease in the number of beta cells. These results thus indicate that TSC2 regulates pancreatic beta-cell mass in a biphasic manner.

Unique mechanisms of growth regulation and tumor suppression upon Apc inactivation in the pancreas.

beta-catenin signaling is heavily involved in organogenesis. Here, we investigated how pancreas differentiation, growth and homeostasis are affected following inactivation of an endogenous inhibitor of beta-catenin, adenomatous polyposis coli (Apc). In adult mice, Apc-deficient pancreata were enlarged, solely as a result of hyperplasia of acinar cells, which accumulated beta-catenin, with the sparing of islets. Expression of a target of beta-catenin, the proto-oncogene c-myc (Myc), was increased in acinar cells lacking Apc, suggesting that c-myc expression is essential for hyperplasia. In support of this hypothesis, we found that conditional inactivation of c-myc in pancreata lacking Apc completely reversed the acinar hyperplasia. Apc loss in organs such as the liver, colon and kidney, as well as experimental misexpression of c-myc in pancreatic acinar cells, led to tumor formation with high penetrance. Surprisingly, pancreas tumors failed to develop following conditional pancreas Apc inactivation. In Apc-deficient acini of aged mice, our studies revealed a cessation of their exaggerated proliferation and a reduced expression of c-myc, in spite of the persistent accumulation of beta-catenin. In conclusion, our work shows that beta-catenin modulation of c-myc is an essential regulator of acinar growth control, and unveils an unprecedented example of Apc requirement in the pancreas that is both temporally restricted and cell-specific. This provides new insights into the mechanisms of tumor pathogenesis and tumor suppression in the pancreas.