aPKC and DOC2b in glucose transport

Aims/introduction: Double C2 domain protein b (DOC2b), one of the synaptotagmins, has been shown to translocate to the plasma membrane, and to initiate membrane-fusion processes of vesicles containing glucose transporter 4 proteins on insulin stimulation. However, the mechanism by which DOC2b is regulated remains unclear. Herein, we identified the upstream regulatory factors of DOC2b in insulin signal transduction. We also examined the role of DOC2b on systemic homeostasis using DOC2b knockout (KO) mice. Materials and Methods: We first identified DOC2b binding proteins by immunoprecipitation and mutagenesis experiments. Then, DOC2b KO mice were generated by disrupting the first exon of the DOC2b gene. In addition to the histological examination, glucose metabolism was assessed by measuring parameters on glucose/insulin tolerance tests. Insulin-stimulated glucose uptake was also measured using isolated soleus muscle and epididymal adipose tissue. Results: We identified an isoform of atypical protein kinase C (protein kinase C iota) that can bind to DOC2b and phosphorylates one of the serine residues of DOC2b (S34). This phosphorylation is essential for DOC2b translocation. DOC2b KO mice showed insulin resistance and impaired oral glucose tolerance on insulin and glucose tolerance tests, respectively. Insulin-stimulated glucose uptake was impaired in isolated soleus muscle and epididymal adipose tissues from DOC2b KO mice. Conclusions: We propose a novel insulin signaling mechanism by which protein kinase C iota phosphorylates DOC2b, leading to glucose transporter 4 vesicle translocation, fusion and facilitation of glucose uptake in response to insulin. The present results also showed DOC2b to play important roles in systemic glucose homeostasis

Myosin motor Myo1c and its receptor NEMO/IKK-γ promote TNF-α–induced serine307 phosphorylation of IRS-1

Tumor necrosis factor-α (TNF-α) signaling through the IκB kinase (IKK) complex attenuates insulin action via the phosphorylation of insulin receptor substrate 1 (IRS-1) at Ser307. However, the precise molecular mechanism by which the IKK complex phosphorylates IRS-1 is unknown. In this study, we report nuclear factor κB essential modulator (NEMO)/IKK-γ subunit accumulation in membrane ruffles followed by an interaction with IRS-1. This intracellular trafficking of NEMO requires insulin, an intact actin cytoskeletal network, and the motor protein Myo1c. Increased Myo1c expression enhanced the NEMO–IRS-1 interaction, which is essential for TNF-α– induced phosphorylation of Ser307–IRS-1. In contrast, dominant inhibitory Myo1c cargo domain expression diminished this interaction and inhibited IRS-1 phosphorylation. NEMO expression also enhanced TNF-α–induced Ser307–IRS-1 phosphorylation and inhibited glucose uptake. In contrast, a deletion mutant of NEMO lacking the IKK-β–binding domain or silencing NEMO blocked the TNF-α signal. Thus, motor protein Myo1c and its receptor protein NEMO act cooperatively to form the IKK–IRS-1 complex and function in TNF-α–induced insulin resistance

Chronic high fat feeding restricts islet mRNA translation initiation independently of ER stress via DNA damage and p53 activation

Under conditions of high fat diet (HFD) consumption, glucose dyshomeostasis develops when β-cells are unable to adapt to peripheral insulin demands. Few studies have interrogated the molecular mechanisms of β-cell dysfunction at the level of mRNA translation under such conditions. We sought to address this issue through polyribosome profile analysis of islets from mice fed 16-weeks of 42% HFD. HFD-islet analysis revealed clear trends toward global reductions in mRNA translation with a significant reduction in the polyribosome/monoribosome ratio for Pdx1 mRNA. Transcriptional and translational analyses revealed endoplasmic reticulum stress was not the etiology of our findings. HFD-islets demonstrated evidence of oxidative stress and DNA damage, as well as activation of p53. Experiments in MIN-6 β-cells revealed that treatment with doxorubicin to directly induce DNA damage mimicked our observed effects in islets. Islets from animals treated with pioglitazone concurrently with HFD demonstrated a reversal of effects observed from HFD alone. Finally, HFD-islets demonstrated reduced expression of multiple ribosome biogenesis genes and the key translation initiation factor eIF4E. We propose a heretofore unappreciated effect of chronic HFD on β-cells, wherein continued DNA damage owing to persistent oxidative stress results in p53 activation and a resultant inhibition of mRNA translation

Self-organized network design by link survivals and shortcuts

One of the challenges for future infrastructures is how to design a network with high efficiency and strong connectivity at low cost. We propose self-organized geographical networks beyond the vulnerable scale-free structure found in many real systems. The networks with spatially concentrated nodes emerge through link survival and path reinforcement on routing flows in a wireless environment with a constant transmission range of a node. In particular, we show that adding some shortcuts induces both the small-world effect and a significant improvement of the robustness to the same level as in the optimal bimodal networks. Such a simple universal mechanism will open prospective ways for several applications in wide-area ad hoc networks, smart grids, and urban planning.Comment: 15 pages, 8 figures, 2 table

Evolutionary Construction of Geographical Networks with Nearly Optimal Robustness and Efficient Routing Properties

Robust and efficient design of networks on a realistic geographical space is one of the important issues for the realization of dependable communication systems. In this paper, based on a percolation theory and a geometric graph property, we investigate such a design from the following viewpoints: 1) network evolution according to a spatially heterogeneous population, 2) trimodal low degrees for the tolerant connectivity against both failures and attacks, and 3) decentralized routing within short paths. Furthermore, we point out the weakened tolerance by geographical constraints on local cycles, and propose a practical strategy by adding a small fraction of shortcut links between randomly chosen nodes in order to improve the robustness to a similar level to that of the optimal bimodal networks with a larger degree $O(\sqrt{N})$ for the network size $N$. These properties will be useful for constructing future ad-hoc networks in wide-area communications.Comment: 14 pages, 10 figures, 1 tabl

Glucose and Fatty Acids Synergize to Promote B-Cell Apoptosis through Activation of Glycogen Synthase Kinase 3β Independent of JNK Activation

The combination of elevated glucose and free-fatty acids (FFA), prevalent in diabetes, has been suggested to be a major contributor to pancreatic β-cell death. This study examines the synergistic effects of glucose and FFA on β-cell apoptosis and the molecular mechanisms involved. Mouse insulinoma cells and primary islets were treated with palmitate at increasing glucose and effects on apoptosis, endoplasmic reticulum (ER) stress and insulin receptor substrate (IRS) signaling were examined.Increasing glucose (5-25 mM) with palmitate (400 µM) had synergistic effects on apoptosis. Jun NH2-terminal kinase (JNK) activation peaked at the lowest glucose concentration, in contrast to a progressive reduction in IRS2 protein and impairment of insulin receptor substrate signaling. A synergistic effect was observed on activation of ER stress markers, along with recruitment of SREBP1 to the nucleus. These findings were confirmed in primary islets. The above effects associated with an increase in glycogen synthase kinase 3β (Gsk3β) activity and were reversed along with apoptosis by an adenovirus expressing a kinase dead Gsk3β.Glucose in the presence of FFA results in synergistic effects on ER stress, impaired insulin receptor substrate signaling and Gsk3β activation. The data support the importance of controlling both hyperglycemia and hyperlipidemia in the management of Type 2 diabetes, and identify pancreatic islet β-cell Gsk3β as a potential therapeutic target

Interorgan Crosstalk Contributing to β-Cell Dysfunction

Type 2 diabetes mellitus (T2DM) results from pancreatic β-cell failure in the setting of insulin resistance. In the early stages of this disease, pancreatic β-cells meet increased insulin demand by both enhancing insulin-secretory capacity and increasing β-cell mass. As the disease progresses, β-cells fail to maintain these compensatory responses. This involves both extrinsic signals and mediators intrinsic to β-cells, which adversely affect β-cells by impairing insulin secretion, decreasing proliferative capacities, and ultimately causing apoptosis. In recent years, it has increasingly been recognized that changes in circulating levels of various factors from other organs play roles in β-cell dysfunction and cellular loss. In this review, we discuss current knowledge of interorgan communications underlying β-cell failure during the progression of T2DM