Evidence for involvement of protein kinase C (PKC)-zeta and noninvolvement of diacylglycerol-sensitive PKCs in insulin-stimulated glucose transport in L6 myotubes

We examined the question of whether insulin activates protein kinase C (PKC)-zeta in L6 myotubes, and the dependence of this activation on phosphatidylinositol (PI) 3-kinase. We also evaluated a number of issues that are relevant to the question of whether diacylglycerol (DAG)-dependent PKCs or DAG-insensitive PKCs, such as PKC-zeta, are more likely to play a role in insulin-stimulated glucose transport in L6 myotubes and other insulin-sensitive cell types. We found that insulin increased the enzyme activity of immunoprecipitable PKC-zeta in L6 myotubes, and this effect was blocked by PI 3-kinase inhibitors, wortmannin and LY294002; this suggested that PKC-zeta operates downstream of PI 3-kinase during insulin action. We also found that treatment of L6 myotubes with 5 microM tetradecanoyl phorbol-13-acetate (TPA) for 24 h led to 80-100% losses of all DAG-dependent PKCs (alpha, beta1, beta2, delta, epsilon) and TPA-stimulated glucose transport (2-deoxyglucose uptake); in contrast, there was full retention of PKC-zeta, as well as insulin-stimulated glucose transport and translocation of GLUT4 and GLUT1 to the plasma membrane. Unlike what has been reported in BC3H-1 myocytes, TPA treatment did not elicit increases in PKCbeta2 messenger RNA or protein in L6 myotubes, and selective retention of this PKC isoform could not explain the retention of insulin effects on glucose transport after prolonged TPA treatment. Of further interest, TPA acutely activated membrane-associated PI 3-kinase in L6 myotubes, and acute effects of TPA on glucose transport were inhibited, not only by the PKC inhibitor, LY379196, but also by both wortmannin and LY294002; this suggested that DAG-sensitive PKCs activate glucose transport through cross-talk with phosphatidylinositol (PI) 3-kinase, rather than directly through PKC. Also, the cell-permeable, myristoylated PKC-zeta pseudosubstrate inhibited insulin-stimulated glucose transport both in non-down-regulated and PKC-depleted (TPA-treated) L6 myotubes; thus, the PKC-zeta pseudosubstrate appeared to inhibit a protein kinase that is required for insulin-stimulated glucose transport but is distinct from DAG-sensitive PKCs. In keeping with the latter dissociation of DAG-sensitive PKCs and insulin-stimulated glucose transport, LY379196, which inhibits PKC-beta (preferentially) and other DAG-sensitive PKCs at relatively low concentrations, inhibited insulin-stimulated glucose transport only at much higher concentrations, not only in L6 myotubes, but also in rat adipocytes, BC3H-1 myocytes, 3T3/L1 adipocytes and rat soleus muscles. Finally, stable and transient expression of a kinase-inactive PKC-zeta inhibited basal and insulin-stimulated glucose transport in L6 myotubes. Collectively, our findings suggest that, whereas PKC-zeta is a reasonable candidate to participate in insulin stimulation of glucose transport, DAG-sensitive PKCs are unlikely participants.     

Ligation of the T cell co-stimulatory receptor CD28 activates the serine-threonine protein kinase protein kinase B

The intracellular signaling pathways activated upon ligation of the co-stimulatory receptor CD28 remain relatively ill-defined, although CD28 ligation does result in the strong association with, and activation of, phosphatidylinositol (PI) 3-kinase. The downstream effector targets of the CD28-activated PI 3-kinase-dependent signaling pathway remain poorly defined, but recent evidence from other systems has shown that Akt/protein kinase B (PKB) is a major target of PI 3-kinase and have indicated that a major function of PKB is the regulation of cell survival events. Given the strong coupling of CD28 to PI 3-kinase and the known protective effects of both CD28 and PI 3-kinase against apoptosis in different cell models, we investigated the effects of CD28 on PKB activation. We demonstrate that ligation of CD28 by either anti-CD28 monoclonal antibodies or the natural ligand B7.1, results in the marked activation of PKB in both the leukemic T cell line Jurkat and freshly isolated human peripheral blood-derived normal T lymphocytes. Our data suggest therefore, that PKB may be an important intracellular signal involved in CD28 signal transduction and demonstrate CD28 coupling to downstream elements of a signaling cascade known to promote cell survival.     

Protein kinase C-zeta as a downstream effector of phosphatidylinositol 3-kinase during insulin stimulation in rat adipocytes. Potential role in glucose transport

Insulin provoked rapid increases in enzyme activity of immunoprecipitable protein kinase C-zeta (PKC-zeta) in rat adipocytes. Concomitantly, insulin provoked increases in 32P labeling of PKC-zeta both in intact adipocytes and during in vitro assay of immunoprecipitated PKC-zeta; the latter probably reflected autophosphorylation, as it was inhibited by the PKC-zeta pseudosubstrate. Insulin-induced activation of immunoprecipitable PKC-zeta was inhibited by LY294002 and wortmannin; this suggested dependence upon phosphatidylinositol (PI) 3-kinase. Accordingly, activation of PI 3-kinase by a pYXXM-containing peptide in vitro resulted in a wortmannin-inhibitable increase in immunoprecipitable PKC-zeta enzyme activity. Also, PI-3,4-(PO4)2, PI-3,4,5-(PO4)3, and PI-4,5-(PO4)2 directly stimulated enzyme activity and autophosphoralytion in control PKC-zeta immunoprecipitates to levels observed in insulin-treated PKC-zeta immunoprecipitates. In studies of glucose transport, inhibition of immunoprecipitated PKC-zeta enzyme activity in vitro by both the PKC-zeta pseudosubstrate and RO 31-8220 correlated well with inhibition of insulin-stimulated glucose transport in intact adipocytes. Also, in adipocytes transiently expressing hemagglutinin antigen-tagged GLUT4, co-transfection of wild-type or constitutive PKC-zeta stimulated hemagglutinin antigen-GLUT4 translocation, whereas dominant-negative PKC-zeta partially inhibited it. Our findings suggest that insulin activates PKC-zeta through PI 3-kinase, and PKC-zeta may act as a downstream effector of PI 3-kinase and contribute to the activation of GLUT4 translocation.     

Mutations of CLCN5 in Japanese children with idiopathic low molecular weight proteinuria, hypercalciuria and nephrocalcinosis

We designed the present study to examine the cross-sectional relation between age-related lens opacities and vitamin C supplement use over a 10-12-y period before assessment of lens status in women without diagnosed cataract or diabetes. This design avoids biased measurement of nutrient intake that results when knowledge of lens opacities influences nutrition-related behavior or its reporting. The participants were 247 Boston-area women aged 56-71 y selected from the Nurses' Health Study cohort with oversampling of women with high or low vitamin C intakes. Lens opacities were graded with the Lens Opacification Classification System II. Use of vitamin C supplements for > or = 10 y (n = 26) was associated with a 77% lower prevalence of early lens opacities (odds ratio: 0.23; 95% CI: 0.09, 0.60) at any lens site and a 83% lower prevalence of moderate lens opacities (odds ratio: 0.17; 95% CI: 0.03, 0.85) at any lens site compared with women who did not use vitamin C supplements (n = 141) after adjustment for age and other potentially confounding variables. Women who consumed vitamin C supplements for < 10 y showed no evidence of a reduced prevalence of early opacities. These data, together with data from earlier experimental and epidemiologic studies, suggest that long-term consumption of vitamin C supplements may substantially reduce the development of age-related lens opacities.