Lysophosphatidylcholine as an effector of fatty acid-induced insulin resistance.

The mechanism of FFA-induced insulin resistance is not fully understood. We have searched for effector molecules(s) in FFA-induced insulin resistance. Palmitic acid (PA) but not oleic acid (OA) induced insulin resistance in L6 myotubes through C-Jun N-terminal kinase (JNK) and insulin receptor substrate 1 (IRS-1) Ser307 phosphorylation. Inhibitors of ceramide synthesis did not block insulin resistance by PA. However, inhibition of the conversion of PA to lysophosphatidylcholine (LPC) by calcium-independent phospholipase A2 (iPLA2) inhibitors, such as bromoenol lactone (BEL) or palmitoyl trifluoromethyl ketone (PACOCF3), prevented insulin resistance by PA. iPLA2 inhibitors or iPLA2 small interfering RNA (siRNA) attenuated JNK or IRS-1 Ser307 phosphorylation by PA. PA treatment increased LPC content, which was reversed by iPLA2 inhibitors or iPLA2 siRNA. The intracellular DAG level was increased by iPLA2 inhibitors, despite ameliorated insulin resistance. Pertussis toxin (PTX), which inhibits LPC action through the G-protein coupled receptor (GPCR)/Gα(i), reversed insulin resistance by PA. BEL administration ameliorated insulin resistance and diabetes in db/db mice. JNK and IRS-1Ser307 phosphorylation in the liver and muscle of db/db mice was attenuated by BEL. LPC content was increased in the liver and muscle of db/db mice, which was suppressed by BEL. These findings implicate LPC as an important lipid intermediate that links saturated fatty acids to insulin resistance.

Regulation of adiponectin receptor 2 expression via PPAR-alpha in NIT-1 cells.

Adiponectin receptors mediate the antidiabetic effects of adiponectin. Although suggested to be mainly expressed in muscle, liver, and adipocyte cells, the expression of adiponectin receptors in beta cells is unclear. Given the primary involvement of this cell type in diabetes mellitus, we presently examined the expression level of adiponectin receptor 2 (AdiR2) in beta cells. Expression was significantly increased under acute hyperlipidemic conditions but impaired under chronic conditions. The impaired AdiR2 expression may play a role in worsened beta cell function. Clofibrate, an agonist of peroxisome proliferator-activated receptor-alpha (PPAR-alpha) delayed the palmitate-induced impairment of AdiR2 expression and PPAR-alpha; this delay was abolished by PPAR-alpha targeted small interfering RNA. The results suggest that AdiR2 expression is regulated by palmitate via PPAR-alpha.

Imatinib mesylate reduces endoplasmic reticulum stress and induces remission of diabetes in db/db mice.

OBJECTIVE: Imatinib has been reported to induce regression of type 2 diabetes in chronic leukemia patients. However, the mechanism of diabetes amelioration by imatinib is unknown, and it is uncertain whether imatinib has effects on type 2 diabetes itself without other confounding diseases like leukemia. We studied the effect of imatinib on diabetes in db/db mice and investigated possible mechanism's underlying improved glycemic control by imatinib. RESEARCH DESIGN AND METHODS: Glucose tolerance and insulin tolerance tests were done after daily intraperitoneal injection of 25 mg/kg imatinib into db/db and C57BL/6 mice for 4 weeks. Insulin signaling and endoplasmic reticulum stress responses were studied by Western blotting. beta-Cell mass and apoptotic beta-cell number were determined by combined terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling (TUNEL) staining and insulin immunohistochemistry. The in vitro effect of imatinib was studied using HepG2 cells. RESULTS: Imatinib induced remission of diabetes in db/db mice and amelioration of insulin resistance. Expression of endoplasmic reticulum stress markers in the liver and adipose tissues of db/db mice, such as phospho-PERK, phospho-eIF2alpha, TRB3, CHOP, and phospho-c-Jun NH(2)-terminal kinase, was reduced by imatinib. Insulin receptor substrate-1 tyrosine phosphorylation and Akt phosphorylation after insulin administration were improved by imatinib. Serum aminotransferase levels and hepatic triglyceride contents were decreased by imatinib. Pancreatic beta-cell mass was increased by imatinib, accompanied by decreased TUNEL(+) beta-cell and increased BrdU(+) beta-cell numbers. Imatinib attenuated endoplasmic reticulum stress in hepatoma cells in vitro. CONCLUSIONS: Imatinib ameliorated endoplasmic reticulum stress and induced remission of diabetes in db/db mice. Imatinib or related compounds could be used as therapeutic agents against type 2 diabetes and metabolic syndrome.

Loss of autophagy diminishes pancreatic beta cell mass and function with resultant hyperglycemia.

Autophagy is a cellular degradation-recycling system for aggregated proteins and damaged organelles. Although dysregulated autophagy is implicated in various diseases including neurodegeneration, its role in pancreatic beta cells and glucose homeostasis has not been described. We produced mice with beta cell-specific deletion of Atg7 (autophagy-related 7). Atg7 mutant mice showed impaired glucose tolerance and decreased serum insulin level. beta cell mass and pancreatic insulin content were reduced because of increased apoptosis and decreased proliferation of beta cells. Physiological studies showed reduced basal and glucose-stimulated insulin secretion and impaired glucose-induced cytosolic Ca2+ transients in autophagy-deficient beta cells. Morphologic analysis revealed accumulation of ubiquitinated protein aggregates colocalized with p62, which was accompanied by mitochondrial swelling, endoplasmic reticulum distension, and vacuolar changes in beta cells. These results suggest that autophagy is necessary to maintain structure, mass and function of pancreatic beta cells, and its impairment causes insulin deficiency and hyperglycemia because of abnormal turnover and function of cellular organelles.

Lysophosphatidylcholine as a death effector in the lipoapoptosis of hepatocytes.

The pathogenesis of nonalcoholic steatohepatitis (NASH) is unclear, despite epidemiological data implicating FFAs. We studied the pathogenesis of NASH using lipoapoptosis models. Palmitic acid (PA) induced classical apoptosis of hepatocytes. PA-induced lipoapoptosis was inhibited by acyl-CoA synthetase inhibitor but not by ceramide synthesis inhibitors, suggesting that conversion products other than ceramide are involved. Phospholipase A(2) (PLA(2)) inhibitors blocked PA-induced hepatocyte death, suggesting an important role for PLA(2) and its product lysophosphatidylcholine (LPC). Small interfering RNA for Ca(2+)-independent phospholipase A(2) (iPLA(2)) inhibited the lipoapoptosis of hepatocytes. PA increased LPC content, which was reversed by iPLA(2) inhibitors. Pertussis toxin or dominant-negative Galpha(i) mutant inhibited hepatocyte death by PA or LPC acting through G-protein-coupled receptor (GPCR)/Galpha(i). PA decreased cardiolipin content and induced mitochondrial potential loss and cytochrome c translocation. Oleic acid inhibited PA-induced hepatocyte death by diverting PA to triglyceride and decreasing LPC content, suggesting that FFAs lead to steatosis or lipoapoptosis according to the abundance of saturated/unsaturated FFAs. LPC administration induced hepatitis in vivo. LPC content was increased in the liver specimens from NASH patients. These results demonstrate that LPC is a death effector in the lipoapoptosis of hepatocytes and suggest potential therapeutic values of PLA(2) inhibitors or GPCR/Galpha(i) inhibitors in NASH.

Toll-like receptor 2 senses beta-cell death and contributes to the initiation of autoimmune diabetes.

Although it is established that defective clearance and, hence, increased accumulation of apoptotic cells can lead to autoimmunity, the mechanism by which this occurs remains elusive. Here, we observed that apoptotic cells undergoing secondary necrosis but not intact apoptotic cells provoked substantial immune responses, which were mediated through the toll-like receptor 2 (TLR2) pathway. The development of autoimmune diabetes was markedly inhibited in Tlr2(-/-) mice but not in Tlr4(-/-) mice, showing that TLR2 plays an important role in the initiation of the disease. Apoptotic beta-cell injury could stimulate the priming of diabetogenic T cells through a TLR2-dependent, but TLR4-independent, activation of antigen-presenting cells. These findings suggest that beta-cell death and its sensing via TLR2 may be an initial event for the stimulation of antigen-presenting cells and development of autoimmune diabetes.

Essential role for signal transducer and activator of transcription-1 in pancreatic beta-cell death and autoimmune type 1 diabetes of nonobese diabetic mice.

OBJECTIVE: We have reported important roles for signal transducer and activator of transcription-1 (STAT1) in pancreatic beta-cell death by cytokines in vitro. However, in vivo evidence supporting the role for STAT1 in natural type 1 diabetes has not been reported. We studied whether STAT1 plays an important role in the development of natural type 1 diabetes. RESEARCH DESIGN AND METHODS: We produced nonobese diabetic (NOD)/STAT1(-/-) mice by backcrossing and studied the in vivo role of STAT1 in beta-cell death and type 1 diabetes. RESULTS: STAT1(-/-) islet cells were resistant to death by interferon (IFN)-gamma/tumor necrosis factor (TNF)-alpha or IFN-gamma/interleukin (IL)-1 beta combination. Cytochrome c translocation by IFN-gamma/TNF-alpha was abrogated in STAT1(-/-) islet cells. The induction of X-linked inhibitor of apoptosis protein by TNF-alpha was inhibited by IFN-gamma in STAT1(+/-) islet cells but not in STAT1(-/-) islet cells. Inducible nitric oxide (NO) synthase induction and NO production by IFN-gamma/IL-1 beta were impaired in STAT1(-/-) islet cells. Strikingly, diabetes and insulitis were completely abrogated in NOD/STAT1(-/-) mice. Development of diabetes after CD4(+) diabetogenic T-cell transfer was inhibited in those mice. STAT1(-/-) neonatal pancreata were not destroyed when grafted into diabetic NOD/BDC2.5 mice that developed CD4(+) T-cell-dependent islet cell death. In NOD/STAT1(-/-) mice, autoreactive T-cell priming was not impaired, but Th1 differentiation was impaired. A janus kinase (JAK) 2 inhibitor upstream of STAT1 attenuated islet cell death by IFN-gamma/TNF-alpha or IFN-gamma/IL-1 beta and delayed diabetes onset in NOD/BDC2.5-SCID mice. CONCLUSIONS: These data demonstrate a critical role for STAT1 in beta-cell death, T-cell immunoregulation, and type 1 diabetes in vivo and suggest potential therapeutic values of STAT1 or JAK inhibitors in the treatment/prevention of type 1 diabetes.

p38 MAPK plays a role in IL-4 synthesis in jacalin plus CD28-stimulated CD4+ T cells--II.

We have previously shown that jacalin, a CD4+ T cell lectin, induces phosphorylation of intracellular events, moderate levels of interleukin (IL)-2 secretion. We have also shown that in the presence of CD28 costimulation, jacalin induces IL-4 secretion. In the present study, we showed that stimulation of normal CD4+ T cells with jacalin plus CD28 cross-linking (CD28XL) resulted in phosphorylation of signal transducer and activator of transcription (STAT)-6 and expression of Bcl-2 and Bcl-xL, which were inhibited significantly when cells were cultured in the presence of the p38 mitogen-activated protein kinase (MAPK) inhibitor SB203580. We further generated jacalin-induced CD4+ T cell blasts, examined the effects of CD28XL, and observed enhanced up-regulation of p38 and activation of STAT-6, Bcl-2, and Bcl-xL. Engagement of CD28 alone induced a marked degree of phosphorylation of p38 MAPK and IL-4 secretion in memory T cells (jacalin blasts), whereas in naïve T cells, jacalin plus CD28XL was required to induce these molecules. Incubation of cells with p38 inhibitor prior to CD28XL resulted in down-modulation of all these molecules. Further treatment with IL-4 has not reversed this trend. Our studies imply that p38 MAPK may play an important role in induction of these molecules and a putative role in protecting cells from undergoing apoptosis.

Analysis of a widely tunable long-period grating by use of an ultrathin cladding layer and higher-order cladding mode coupling.

A widely tunable long-period grating in single-mode fiber is analyzed by use of an ultrathin cladding layer and higher-order cladding mode coupling. The numerical simulation shows that a 225-nm tuning range in the newly designed ultrathin long-period grating (cladding thickness, 35 microm) with third-order cladding mode coupling can be obtained. The analyzed tuning range is seven times wider than those of the other known long-period gratings. We believe that the proposed highly sensitive long-period grating will be widely used as a gain-flattening filter for ultrawideband optical amplifiers and fast tunable filters in dynamic optical communication systems.

Design of a highly nonlinear dispersion-shifted fiber with a small effective area by use of the beam propagation method with the Gaussian approximation method.

A new design of a highly nonlinear dispersion-shifted fiber (HNDSF) with an effective area of 9.3 microm2 is presented. The three-dimensional beam propagation method combined with the Gaussian approximation method is used to analyze the new HNDSF. This innovative HNDSF has a unique triple-cladding structure that can offer not only a large nonlinear coefficient but also low attenuation, low splicing, and bending losses. It is a suitable candidate to implement an all-fiber wavelength converter by four-wave mixing.