Acute effects of phorbol ester and insulin on insulin-induced glucose uptake and protein kinase C activation in rat adipocytes

Specific protein kinase C isoforms as transducers and modulators of insulin signaling

Recent studies implicate specific PKC isoforms in the insulin-signaling cascade. Insulin activates PKCs alpha, betaII, delta and zeta in several cell types. In addition, as will be documented in this review, certain members of the PKC family may also be activated and act upstream of PI3 and MAP kinases. Each of these isoforms has been shown one way or another either to mimic or to modify insulin-stimulated effects in one or all of the insulin-responsive tissues. Moreover, each of the isoforms has been shown to be activated by insulin stimulation or conditions important for effective insulin stimulation. Studies attempting to demonstrate a definitive role for any of the isoforms have been performed on different cells, ranging from appropriate model systems for skeletal muscle, liver and fat, such as primary cultures, and cell lines and even in vivo studies, including transgenic mice with selective deletion of specific PKC isoforms. In addition, studies have been done on certain expression systems such as CHO or HEK293 cells, which are far removed from the tissues themselves and serve mainly as vessels for potential protein-protein interactions. Thus, a clear picture for many of the isoforms remains elusive in spite of over two decades of intensive research. The recent intrusion of transgenic and precise molecular biology technologies into the research armamentarium has opened a wide range of additional possibilities for direct involvement of individual isoforms in the insulin signaling cascade. As we hope to discuss within the context of this review, whereas many of the long sought-after answers to specific questions are not yet clear, major advances have been made in our understanding of precise roles for individual PKC isoforms in mediation of insulin effects. In this review, in which we shall focus our attention on isoforms in the conventional and novel categories, a clear case will be made to show that these isoforms are not only expressed but are importantly involved in regulation of insulin metabolic effects.

Effect of tumor necrosis factor-alpha on insulin signal transduction in rat adipocytes: relation to PKCbeta and zeta translocation

Although much evidence has been accumulated suggesting that tumor necrosis factor-alpha (TNF-alpha) is an important mediator of insulin resistance, the precise mechanism involved is still unclear. Recently, it has been reported that insulin-induced glucose uptake is mediated by activation of second messengers such as insulin receptor substrate 1 (IRS-1), phosphatidylinositol 3-kinase (PI3K), and diacylglycerol (DG)-protein kinase C (PKC). We have examined the effect of TNF-alpha on insulin-induced glucose uptake and activations of tyrosine kinase, IRS-1, PI3K and PKC in rat adipocytes. Pretreatment with 0.1-100 nM TNF-alpha for 60 min resulted in a significant decrease in 10 nM insulin- or 1 microM 12-O-tetradecanoyl phorbol-13-acetate (TPA)-induced [3H]2-deoxyglucose uptake without affecting basal glucose uptake. 10 nM insulin-stimulated activation of tyrosine kinase, IRS-1 and PI3K was suppressed by preincubation with 0.1-10 nM TNF-alpha for 60 min. 10 nM TNF-alpha pretreatment also suppressed 10 nM insulin- and 1 microM TPA-induced increases in membrane-associated PKCbeta and PKCzeta. Furthermore, 10 nM TNF-alpha, by itself, altered PKCbeta translocation from the membrane to cytosol. These results suggest that TNF-alpha inhibits insulin-stimulated activation of both the tyrosine kinase-IRS-1-PI3K-PKCzeta pathway and DG-PKC pathway. Finally, TNF-alpha contributes to insulin resistance in rat adipocytes.