Hepatic JAK2 protects against atherosclerosis through circulating IGF-1.

Atherosclerosis is considered both a metabolic and inflammatory disease; however, the specific tissue and signaling molecules that instigate and propagate this disease remain unclear. The liver is a central site of inflammation and lipid metabolism that is critical for atherosclerosis, and JAK2 is a key mediator of inflammation and, more recently, of hepatic lipid metabolism. However, precise effects of hepatic Jak2 on atherosclerosis remain unknown. We show here that hepatic Jak2 deficiency in atherosclerosis-prone mouse models exhibited accelerated atherosclerosis with increased plaque macrophages and decreased plaque smooth muscle cell content. JAK2's essential role in growth hormone signalling in liver that resulted in reduced IGF-1 with hepatic Jak2 deficiency played a causal role in exacerbating atherosclerosis. As such, restoring IGF-1 either pharmacologically or genetically attenuated atherosclerotic burden. Together, our data show hepatic Jak2 to play a protective role in atherogenesis through actions mediated by circulating IGF-1 and, to our knowledge, provide a novel liver-centric mechanism in atheroprotection.

FAK signalling controls insulin sensitivity through regulation of adipocyte survival.

Focal adhesion kinase (FAK) plays a central role in integrin signalling, which regulates growth and survival of tumours. Here we show that FAK protein levels are increased in adipose tissue of insulin-resistant obese mice and humans. Disruption of adipocyte FAK in mice or in 3T3 L1 cells decreases adipocyte survival. Adipocyte-specific FAK knockout mice display impaired adipose tissue expansion and insulin resistance on prolonged metabolic stress from a high-fat diet or when crossed on an obese db/db or ob/ob genetic background. Treatment of these mice with a PPARγ agonist does not restore adiposity or improve insulin sensitivity. In contrast, inhibition of apoptosis, either genetically or pharmacologically, attenuates adipocyte death, restores normal adiposity and improves insulin sensitivity. Together, these results demonstrate that FAK is required for adipocyte survival and maintenance of insulin sensitivity, particularly in the context of adipose tissue expansion as a result of caloric excess.

JAK2 promotes brown adipose tissue function and is required for diet- and cold-induced thermogenesis in mice.

AIMS/HYPOTHESIS: Non-shivering thermogenesis in adipose tissue can be activated by excessive energy intake or following cold exposure. The molecular mechanisms regulating this activation have not been fully elucidated. The Janus kinase (JAK) - signal transducer and activator of transcription (STAT) pathway mediates the signal transduction of numerous hormones and growth factors that regulate adipose tissue development and function, and may play a role in adaptive thermogenesis. METHODS: We analysed mRNA and protein levels of uncoupling protein 1 (UCP1) and JAK2 in different adipose depots in response to metabolic and thermal stress. The in vivo role of JAK2 in adaptive thermogenesis was examined using mice with adipocyte-specific Jak2 deficiency (A-Jak2 KO). RESULTS: We show in murine brown adipose tissue (BAT) that JAK2 is upregulated together with UCP1 in response to high-fat diet (HFD) feeding and cold exposure. In contrast to white adipose tissue, where JAK2 was dispensable for UCP1 induction, we identified an essential role for BAT JAK2 in diet- and cold-induced thermogenesis via mediating the thermogenic response to ß-adrenergic stimulation. Accordingly, A-Jak2 KO mice were unable to upregulate BAT UCP1 following a HFD or after cold exposure. Therefore, A-Jak2 KO mice were cold intolerant and susceptible to HFD-induced obesity and diabetes. CONCLUSIONS/INTERPRETATION: Taken together, our results suggest that JAK2 plays a critical role in BAT function and adaptive thermogenesis. Targeting the JAK-STAT pathway may be a novel therapeutic approach for the treatment of obesity and related metabolic disorders.

Rb and p107 are required for alpha cell survival, beta cell cycle control and glucagon-like peptide-1 action.

AIMS/HYPOTHESIS: Diabetes mellitus is characterised by beta cell loss and alpha cell expansion. Analogues of glucagon-like peptide-1 (GLP-1) are used therapeutically to antagonise these processes; thus, we hypothesised that the related cell cycle regulators retinoblastoma protein (Rb) and p107 were involved in GLP-1 action. METHODS: We used small interfering RNA and adenoviruses to manipulate Rb and p107 expression in insulinoma and alpha-TC cell lines. In vivo we examined pancreas-specific Rb knockout, whole-body p107 knockout and Rb/p107 double-knockout mice. RESULTS: Rb, but not p107, was downregulated in response to the GLP-1 analogue, exendin-4, in both alpha and beta cells. Intriguingly, this resulted in opposite outcomes of cell cycle arrest in alpha cells but proliferation in beta cells. Overexpression of Rb in alpha and beta cells abolished or attenuated the effects of exendin-4 supporting the important role of Rb in GLP-1 modulation of cell cycling. Similarly, in vivo, Rb, but not p107, deficiency was required for the beta cell proliferative response to exendin-4. Consistent with this finding, Rb, but not p107, was suppressed in islets from humans with diabetes, suggesting the importance of Rb regulation for the compensatory proliferation that occurs under insulin resistant conditions. Finally, while p107 alone did not have an essential role in islet homeostasis, when combined with Rb deletion, its absence potentiated apoptosis of both alpha and beta cells resulting in glucose intolerance and diminished islet mass with ageing. CONCLUSIONS/INTERPRETATION: We found a central role of Rb in the dual effects of GLP-1 in alpha and beta cells. Our findings highlight unique contributions of individual Rb family members to islet cell proliferation and survival.

Adipocyte-specific deficiency of Janus kinase (JAK) 2 in mice impairs lipolysis and increases body weight, and leads to insulin resistance with ageing.

AIMS/HYPOTHESIS: The growing obesity epidemic necessitates a better understanding of adipocyte biology and its role in metabolism. The Janus kinase (JAK)-signal transducer and activator of transcription (STAT) pathway mediates signalling by numerous cytokines and hormones that regulate adipocyte function, illustrating the physiological importance of adipose JAK-STAT. The aim of this study was to investigate potential roles of adipocyte JAK2, an essential player in the JAK-STAT pathway, in adipocyte biology and metabolism. METHODS: We generated adipocyte-specific Jak2 knockout (A-Jak2 KO) mice using the Cre-loxP system with Cre expression driven by the Ap2 (also known as Fabp4) promoter. RESULTS: Starting at 2-3 months of age, male and female A-Jak2 KO mice gradually gained more body weight than control littermates primarily due to increased adiposity. This was associated with reduced energy expenditure in A-Jak2 KO mice. In perigonadal adipose tissue, the expression of numerous genes involved in lipid metabolism was differentially regulated. In addition, adipose tissue from A-Jak2 KO mice displayed impaired lipolysis in response to isoprenaline, growth hormone and leptin stimulation, suggesting that adipose JAK2 directly modulates the lipolytic program. Impaired lipid homeostasis was also associated with disrupted adipokine secretion. Accordingly, while glucose metabolism was normal at 2 months of age, by 5-6 months of age, A-Jak2 KO mice had whole-body insulin resistance. CONCLUSIONS/INTERPRETATION: Our results suggest that adipocyte JAK2 plays a critical role in the regulation of adipocyte biology and whole-body metabolism. Targeting of the JAK-STAT pathway could be a novel therapeutic option for the treatment of obesity and type 2 diabetes.

Overexpression of HIF-2α in pancreatic ß cells does not alter glucose homeostasis.

Both type 1 and type 2 diabetes are associated with insufficient functional ß-cell mass. Understanding intracellular signaling pathways associated with this decline is important in broadening our understanding of the disease and potential therapeutic strategies. The hypoxia inducible factor pathway (HIF) plays a critical role in cellular adaptation to hypoxic conditions. Activation of this pathway increases expression of numerous genes involved in multiple cellular processes and has been shown to impact the regulation of ß-cell function. Previously, deletion of HIF-1α or HIF-1ß in pancreatic ß-cells, as well as constitutive activation of the HIF pathway in ß-cells, was shown to result in glucose intolerance and impaired insulin secretion. The objective of this study was to delineate roles of HIF-2α overexpression in pancreatic ß-cells in vivo. We overexpressed HIF-2α in pancreatic ß-cells by employing the Cre-loxP system driven by the Pdx1 promoter to delete a stop codon. Our study revealed that pancreatic HIF-2α overexpression does not result in significant differences in glucose tolerance, insulin sensitivity or ß-cell area compared to wild-type littermates under basal conditions or after high fat diet. Together, our study shows excess HIF-2α in the pancreatic ß-cells does not play a significant role in ß-cell function and glucose homeostasis.

Sodium arsenite and cadmium chloride induction of proteasomal inhibition and HSP accumulation in Xenopus laevis A6 kidney epithelial cells.

Sodium arsenite (NA) and cadmium chloride (CdCl(2)) are relatively abundant environmental toxicants that have multiple toxic effects including carcinogenesis, dysfunction of gene regulation and DNA and protein damage. In the present study, treatment of Xenopus laevis A6 kidney epithelial cells with concentrations of NA (20-30 µM) or CdCl(2) (100-200 µM) that induced HSP30 and HSP70 accumulation also produced an increase in the relative levels of ubiquitinated protein. Actin protein levels were unchanged in these experiments. In time course experiments, the levels of ubiquitinated protein and HSPs increased over a 24h exposure to NA or CdCl(2). Furthermore, treatment of cells with NA or CdCl(2) reduced the relative levels of proteasome chymotrypsin (CT)-like activity compared to control. Interestingly, pretreatment of cells with the HSP accumulation inhibitor, KNK437, prior to NA or CdCl(2) exposure decreased the relative levels of ubiquitinated protein as well as HSP30 and HSP70. A similar finding was made with ubiquitinated protein induced by proteasomal inhibitors, MG132 and celastrol, known to induce HSP accumulation in A6 cells. However, the NA- or CdCl(2)-induced decrease in proteasome CT-like activity was not altered by KNK437 pretreatment. This study has shown for the first time in poikilothermic vertebrates that NA and CdCl(2) can inhibit proteasomal activity and that there is a possible association between HSP accumulation and the mechanism of protein ubiquitination.