Effect of dehydroepiandrosterone on insulin sensitivity in Otsuka Long-Evans Tokushima-fatty rats.

In order to clarify the effect of dehydroepiandrosterone (DHEA) on improvement of insulin resistance, we examined the effects of overexpression of wild-type protein kinase C-zeta (wt-PKCzeta)/3-phosphoinositide-dependent protein kinase-1 (wt-PDK1) and kinase-inactive PKCzeta/PDK1 (DeltaPKCzeta/DeltaPDK1) on DHEA-induced [(3)H]2-deoxyglucose (DOG) uptake using the electroporation method in rat adipocytes. Overexpression of wt-PKCzeta and wt-PDK1 significantly increased in DHEA-induced [(3)H]2-DOG uptake. Wortmannin completely suppressed DHEA-induced [(3)H]2-DOG uptake in wt-PKCzeta- and wt-PDK1-transfected adipocytes. Overexpression of neither DeltaPKCzeta nor DeltaPDK1 increased DHEA-induced [(3)H]2-DOG uptake. Otsuka Long-Evans fatty rats (OLETF), animal models of type 2 diabetes, and Long-Evans Tokushima rats (LETO) as control, were treated with 0.4% DHEA for 2 weeks. Insulin-induced [(3)H]2-DOG uptakes, activations of PI 3-kinase and PKCzeta of adipocytes were significantly increased in DHEA-treated OLETF rats. Moreover, plasma glucose levels in OLETF rats after treatment with DHEA for 2 weeks were significantly lower than treatment without DHEA, but not in LETO rats. These results indicate that DHEA treatment may improve glucose tolerance through a PI 3-kinase-PKCzeta pathway and downregulates adiposity in OLETF rats.

TNFalpha reduces the expression of peroxisome proliferator-activated receptor gamma (PPARgamma) via the production of ceramide and activation of atypical PKC.

Although tumor necrosis factor alpha (TNFalpha) decreases the expression of peroxisome proliferator-activated receptor gamma (PPARgamma), its mechanism is not understood. We evaluated the effect of ceramide, the second messenger of TNFalpha, on the expression of PPARgamma in primary cultured adipocytes. PPARgamma mRNA and aP2 mRNA levels were measured with real-time PCR. The PPARgamma protein level was measured with immunoblot. C6- and C2-ceramide, but not dihydroC6-ceramide, reduced the expression of PPARgamma in a time and concentration dependent manner. The application of 1 microM C6-ceramide for 36 h reduced PPARgamma mRNA level, aP2 mRNA level, and PPARgamma protein level to 56.3%, 80.4% and 62.1%, respectively. Since ceramide is known to activate atypical PKC, we also studied the role of atypical PKC on the PPARgamma reducing effect. Overexpression of wild type PKCzeta magnified and accelerated the effect of TNFalpha and C6-ceramide on PPARgamma mRNA levels, whereas overexpression of dominant negative PKCzeta abolished the effect. We also found that the overexpression of constitutive active PKCzeta reduced PPARgamma mRNA level, aP2 mRNA level, and PPARgamma protein level to 61.4%, 70.3% and 81.6%, respectively. Furthermore, TNFalpha activated nuclear factor-kappaB (NF-kappaB), known as a downstream effector of PKCzeta to 256.6%, which was enhanced with overexpression of wild-type PKCzeta. On the other hand, treatment with phorbol 12-myristate 13-acetate, another activator of NF-kappaB, also reduced the expression of PPARgamma to 57.8%. These results indicate that the reducing effect of TNFalpha is mediated through ceramide, atypical PKC and NF-kappaB pathway.

Protein kinase C (PKC) beta modulates serine phosphorylation of insulin receptor substrate-1 (IRS-1)--effect of overexpression of PKCbeta on insulin signal transduction.

In vitro phosphorylation of 180-kDa protein, obtained by immunoprecipitation of adipocyte homogenate with anti-IRS-1 antibody was increased with the addition of conventional PKC in the presence of Ca2+, phosphatidylserine (PS) and diolein (DL). Human purified IRS-1 was phosphorylated by purified conventional PKC (cPKC) in the presence of Ca2+/PS/DL. These results suggest that PKC may have a role in the serine phosphorylation of IRS-1. In order to clarify the inhibitory effect of cPKC on glucose transport mechanism, we examined the overexpression of PKCbeta in cultured adipocytes. Overexpression of PKCbeta in adipocytes markedly induced mobility shift and serine phosphorylation of IRS-1, whereas overexpression of dominant negative PKCbeta (DNPKCbeta) blocked this mobility shift and serine phosphorylation of IRS-1. Insulin (10 nM) increased [3H]2-deoxyglucose (2-DOG) uptake to 200% from basal level (100%) in cultured adipocytes transfected with a vector alone. Overexpression of PKCbeta in adipocytes decreased insulin-induced 2-DOG uptake to 110%, whereas overexpression of DNPKCbeta increased it to 230%. These results suggest that PKCbeta negatively regulates glucose uptake via serine phosphorylation of IRS-1 in rat adipocytes.

Depressive state and paresthesia dramatically improved by intravenous MgSO4 in Gitelman's syndrome.

A 69-year-old woman was referred to our department for evaluation of hypokalemia, which had been treated by oral potassium for more than ten years. She complained of headache, knee joint pain, sleeplessness and paresthesia in extremities and, most prominently, depression. Laboratory data suggested Gitelman's syndrome, which is caused by mutations in the gene encoding the thiazide-sensitive Na-Cl cotransporter. Direct sequencing of the gene in this patient revealed homozygous mutation R964Q in exon 25. Intravenous supplement of MgSO4 dramatically improved both the depression and the paresthesia, suggesting that hypomagnesemia played a role in the clinical manifestations.

Role of adipocyte-derived factors in enhancing insulin signaling in skeletal muscle and white adipose tissue of mice lacking Acyl CoA:diacylglycerol acyltransferase 1.

Mice that lack acyl CoA:diacylglycerol acyltransferase 1 (DGAT1), a key enzyme in mammalian triglyceride synthesis, have decreased adiposity and increased insulin sensitivity. Here we show that insulin-stimulated glucose transport is increased in the skeletal muscle and white adipose tissue (WAT) of chow-fed DGAT1-deficient mice. This increase in glucose transport correlated with enhanced insulin-stimulated activities of phosphatidylinositol 3-kinase, protein kinase B (or Akt), and protein kinase Clambda (PKC-lambda), three key molecules in the insulin-signaling pathway, and was associated with decreased levels of serine-phosphorylated insulin receptor substrate 1 (IRS-1), a molecule implicated in insulin resistance. Similar findings in insulin signaling were also observed in DGAT1-deficient mice fed a high-fat diet. Interestingly, the increased PKC-lambda activity and decreased serine phosphorylation of IRS-1 were observed in chow-fed wild-type mice transplanted with DGAT1-deficient WAT, consistent with our previous finding that transplantation of DGAT1-deficient WAT enhances glucose disposal in wild-type recipient mice. Our findings demonstrate that DGAT1 deficiency enhances insulin signaling in the skeletal muscle and WAT, in part through altered expression of adipocyte-derived factors that modulate insulin signaling in peripheral tissues.

Insulin-induced activation of atypical protein kinase C, but not protein kinase B, is maintained in diabetic (ob/ob and Goto-Kakazaki) liver. Contrasting insulin signaling patterns in liver versus muscle define phenotypes of type 2 diabetic and high fat-induced insulin-resistant states.

Insulin resistance in type 2 diabetes is characterized by defects in muscle glucose uptake and hepatic overproduction of both glucose and lipids. These hepatic defects are perplexing because insulin normally suppresses glucose production and increases lipid synthesis in the liver. To understand the mechanisms for these seemingly paradoxical defects, we examined the activation of atypical protein kinase C (aPKC) and protein kinase B (PKB), two key signaling factors that operate downstream of phosphatidylinositol 3-kinase and regulate various insulin-sensitive metabolic processes. Livers and muscles of three insulin-resistant rodent models were studied. In livers of type 2 diabetic non-obese Goto-Kakazaki rats and ob/ob-diabetic mice, the activation of PKB was impaired, whereas activation of aPKC was surprisingly maintained. In livers of non-diabetic high fatfed mice, the activation of both aPKC and PKB was maintained. In contrast to the maintenance of aPKC activation in the liver, insulin activation of aPKC was impaired in muscles of Goto-Kakazaki-diabetic rats and ob/ob-diabetic and non-diabetic high fat-fed mice. These findings suggest that, at least in these rodent models, (a) defects in aPKC activation contribute importantly to skeletal muscle insulin resistance observed in both high fat feeding and type 2 diabetes; (b) insulin signaling defects in muscle are not necessarily accompanied by similar defects in liver; (c) defects in hepatic PKB activation occur in association with, and probably contribute importantly to, the development of overt diabetes; and (d) maintenance of hepatic aPKC activation may explain the continued effectiveness of insulin for stimulating certain metabolic actions in the liver.

Protein kinase C-lambda knockout in embryonic stem cells and adipocytes impairs insulin-stimulated glucose transport.

Atypical protein kinase C (aPKC) isoforms have been suggested to mediate insulin effects on glucose transport in adipocytes and other cells. To more rigorously test this hypothesis, we generated mouse embryonic stem (ES) cells and ES-derived adipocytes in which both aPKC-lambda alleles were knocked out by recombinant methods. Insulin activated PKC-lambda and stimulated glucose transport in wild-type (WT) PKC-lambda(+/+), but not in knockout PKC-lambda(-/-), ES cells. However, insulin-stimulated glucose transport was rescued by expression of WT PKC-lambda in PKC-lambda(-/-) ES cells. Surprisingly, insulin-induced increases in both PKC-lambda activity and glucose transport were dependent on activation of proline-rich tyrosine protein kinase 2, the ERK pathway, and phospholipase D (PLD) but were independent of phosphatidylinositol 3-kinase (PI3K) in PKC-lambda(+/+) ES cells. Interestingly, this dependency was completely reversed after differentiation of ES cells to adipocytes, i.e. insulin effects on PKC-lambda and glucose transport were dependent on PI3K, rather than proline-rich tyrosine protein kinase 2/ERK/PLD. As in ES cells, insulin effects on glucose transport were absent in PKC-lambda(-/-) adipocytes but were rescued by expression of WT PKC-lambda in these adipocytes. Our findings suggest that insulin activates aPKCs and glucose transport in ES cells by a newly recognized PI3K-independent ERK/PLD-dependent pathway and provide a compelling line of evidence suggesting that aPKCs are required for insulin-stimulated glucose transport, regardless of whether aPKCs are activated by PI3K-dependent or PI3K-independent mechanisms.

Defective Activation of Protein Kinase C-z in Muscle by Insulin and Phosphatidylinositol-3,4,5,-(PO(4))(3) in Obesity and Polycystic Ovary Syndrome.

Insulin resistance occurs frequently in metabolic syndrome components, obesity, and the polycystic ovary syndrome, and is partly due to impaired glucose transport into skeletal muscle, but underlying mechanisms are uncertain. Atypical protein kinase C and protein kinase B, operating downstream of phosphatidylinositol 3-kinase, mediate insulin effects on glucose transport, but their importance in these syndromes is poorly understood. Presently, we examined these signaling factors in muscle biopsies obtained during euglycemic/hyperinsulinemic clamp studies. In lean subjects, insulin provoked approximately twofold increases in muscle atypical protein kinase C activity. In obese subjects and obese subjects who had evidence of the polycystic ovary syndrome, insulin-stimulated glucose disposal and atypical protein kinase C activation were diminished, whereas activation of insulin receptor substrate-1-dependent phosphatidylinositol 3-kinase and protein kinase B trended lower, but not significantly. Interestingly, direct activation of atypical protein kinase C by phosphatidylinositol-3,4,5-(PO(4))(3), the lipid product of phosphatidylinositol 3-kinase, was readily apparent in immunoprecipitates prepared from muscles of lean subjects, but to a lesser degree or poorly if at all in subjects who were obese or had the obesity/polycystic ovary syndrome. Our findings suggest that activation of muscle atypical protein kinase C by insulin and phosphatidylinositol-3,4,5-(PO(4))(3) is defective and may contribute to skeletal muscle insulin resistance in women who are obese, or have obesity associated with the polycystic ovary syndrome.

Activation of protein kinase C-zeta by insulin and phosphatidylinositol-3,4,5-(PO4)3 is defective in muscle in type 2 diabetes and impaired glucose tolerance: amelioration by rosiglitazone and exercise.

Insulin resistance in type 2 diabetes is partly due to impaired glucose transport in skeletal muscle. Atypical protein kinase C (aPKC) and protein kinase B (PKB), operating downstream of phosphatidylinositol (PI) 3-kinase and its lipid product, PI-3,4,5-(PO(4))(3) (PIP(3)), apparently mediate insulin effects on glucose transport. We examined these signaling factors during hyperinsulinemic-euglycemic clamp studies in nondiabetic subjects, subjects with impaired glucose tolerance (IGT), and type 2 diabetic subjects. In nondiabetic control subjects, insulin provoked twofold increases in muscle aPKC activity. In both IGT and diabetes, aPKC activation was markedly (70-80%) diminished, most likely reflecting impaired activation of insulin receptor substrate (IRS)-1-dependent PI 3-kinase and decreased ability of PIP(3) to directly activate aPKCs; additionally, muscle PKC-zeta levels were diminished by 40%. PKB activation was diminished in patients with IGT but not significantly in diabetic patients. The insulin sensitizer rosiglitazone improved insulin-stimulated IRS-1-dependent PI 3-kinase and aPKC activation, as well as glucose disposal rates. Bicycle exercise, which activates aPKCs and stimulates glucose transport independently of PI 3-kinase, activated aPKCs comparably to insulin in nondiabetic subjects and better than insulin in diabetic patients. Defective aPKC activation contributes to skeletal muscle insulin resistance in IGT and type 2 diabetes, rosiglitazone improves insulin-stimulated aPKC activation, and exercise directly activates aPKCs in diabetic muscle.

Defective activation of atypical protein kinase C zeta and lambda by insulin and phosphatidylinositol-3,4,5-(PO4)(3) in skeletal muscle of rats following high-fat feeding and streptozotocin-induced diabetes.

UNLABELLED: Insulin-stimulated glucose transport in skeletal muscle is thought to be effected at least partly through atypical protein kinase C isoforms (aPKCs) operating downstream of phosphatidylinositol (PI) 3-kinase and 3-phosphoinositide-dependent protein kinase-1 (PDK-1). However, relatively little is known about the activation of aPKCs in physiological conditions or insulin-resistant states. Presently, we studied aPKC activation in vastus lateralis muscles of normal chow-fed and high-fat-fed rats and after streptozotocin (STZ)-induced diabetes. In normal chow-fed rats, dose-dependent increases in aPKC activity approached maximal levels after 15-30 min of stimulation by relatively high and lower, presumably more physiological, insulin concentrations, achieved by im insulin or ip glucose administration. Insulin-induced activation of aPKCs was impaired in both high-fat-fed and STZ-diabetic rats, but, surprisingly, IRS-1-dependent and IRS-2-dependent PI 3-kinase activation was not appreciably compromised. Most interestingly, direct in vitro activation of aPKCs by PI-3,4,5-(PO(4))(3), the lipid product of PI 3-kinase, was impaired in both high-fat-fed and STZ-diabetic rats. Defects in activation of aPKCs by insulin and PI-3,4,5-(PO(4))(3) could not be explained by diminished PDK-1-dependent phosphorylation of threonine-410 in the PKC-zeta activation loop, as this phosphorylation was increased even in the absence of insulin treatment in high-fat-fed rats. CONCLUSIONS: 1) muscle aPKCs are activated at relatively low, presumably physiological, as well as higher supraphysiological, insulin concentrations; 2) aPKC activation is defective in muscles of high-fat-fed and STZ-diabetic rats; and 3) defective aPKC activation in these states is at least partly due to impaired responsiveness to PI-3,4,5-(PO(4))(3), apparently at activation steps distal to PDK-1-dependent loop phosphorylation.

Inhibitory effect of ceramide on insulin-induced protein kinase Czeta translocation in rat adipocytes.

Ceramide has been confirmed to be a signal mediator of apoptosis that is induced by tumor necrosis factor-alpha (TNF-alpha). It has also been reported that ceramide may induce insulin resistance as well as TNF-alpha. We investigated the effect of ceramide on insulin signaling pathways, such as insulin receptor (IR) beta-subunit, insulin receptor substrate 1 (IRS-1), phosphatidylinositol 3-kinase (PI3K), and protein kinase Czeta (PKCzeta) in rat adipocytes. We examined insulin-stimulated [(3)H]2-deoxyglucose (2-DOG) uptake in rat adipocytes pretreated with N-hexanoylsphingosine (C(6)-ceramide, 10 to 30 micromol/L). Insulin-induced 2-DOG uptake was significantly reduced by C(6)-ceramide pretreatment. We also examined the effect of various concentrations of C(6)-ceramide pretreatment on insulin-induced autophosphorylation of the IR beta-subunit, tyrosine phosphorylation of IRS-1, enzyme activity of PI3K, and membrane-associated PKCzeta immunoreactivity. Pretreatment with C(6)-ceramide significantly reduced autophosphorylation of the IR beta-subunit, tyrosine phosphorylation of IRS-1, and enzyme activity of PI3K. Moreover, membrane-associated PKCzeta immunoreactivity and immunoprecipitable PKCzeta enzyme activity, downstream of PI3K, were significantly suppressed by C(6)-ceramide pretreatment. These results suggest that ceramide may induce insulin resistance via the suppression of IRS-1-PI3K signaling, and subsequent activation of PKCzeta.

Dehydroepiandrosterone down-regulates the expression of peroxisome proliferator-activated receptor gamma in adipocytes.

Dehydroepiandrosterone (DHEA) is expected to have a weight-reducing effect. In this study, we evaluated the effect of DHEA on genetically obese Otsuka Long Evans Fatty rats (OLETF) compared with Long-Evans Tokushima rats (LETO) as control. Feeding with 0.4% DHEA-containing food for 2 wk reduced the weight of sc, epididymal, and perirenal adipose tissue in association with decreased plasma leptin levels in OLETF. Adipose tissue from OLETF showed increased expression of peroxisome proliferator-activated receptor gamma (PPARgamma) protein, which was prevented by DHEA treatment. Further, we examined the effect of DHEA on PPARgamma in primary cultured adipocytes and monolayer adipocytes differentiated from rat preadipocytes. PPARgamma protein level was decreased in a time- and concentration-dependent manner, and DHEA significantly reduced mRNA levels of PPARgamma, adipocyte lipid-binding protein, and sterol regulatory element-binding protein, but not CCAAT/enhancer binding protein alpha. DHEA-sulfate also reduced the PPARgamma protein, but dexamethasone, testosterone, or androstenedione did not alter its expression. In addition, treatment with DHEA for 5 d reduced the triglyceride content in monolayer adipocytes. These results suggest that DHEA down-regulates adiposity through the reduction of PPARgamma in adipocytes.

Skeletal muscle insulin resistance in obesity-associated type 2 diabetes in monkeys is linked to a defect in insulin activation of protein kinase C-zeta/lambda/iota.

Rhesus monkeys frequently develop obesity and insulin resistance followed by type 2 diabetes when allowed free access to chow. This insulin resistance is partly due to defective glucose transport into skeletal muscle. In this study, we examined signaling factors required for insulin-stimulated glucose transport in muscle biopsies taken during euglycemic-hyperinsulinemic clamps in nondiabetic, obese prediabetic, and diabetic monkeys. Insulin increased activities of insulin receptor substrate (IRS)-1-dependent phosphatidylinositol (PI) 3-kinase and its downstream effectors, atypical protein kinase Cs (aPKCs) (zeta/lambda/iota) and protein kinase B (PKB) in muscles of nondiabetic monkeys. Insulin-induced increases in glucose disposal and aPKC activity diminished progressively in prediabetic and diabetic monkeys. Decreases in aPKC activation appeared to be at least partly due to diminished activation of IRS-1-dependent PI 3-kinase, but direct activation of aPKCs by the PI 3-kinase lipid product PI-3,4,5-(PO(4))(3) was also diminished. In conjunction with aPKCs, PKB activation was diminished in prediabetic muscle but, differently from aPKCs, seemed to partially improve in diabetic muscle. Interestingly, calorie restriction and avoidance of obesity largely prevented development of defects in glucose disposal and aPKC activation. Our findings suggest that defective activation of aPKCs contributes importantly to obesity-dependent development of skeletal muscle insulin resistance in prediabetic and type 2 diabetic monkeys.

Cbl, IRS-1, and IRS-2 mediate effects of rosiglitazone on PI3K, PKC-lambda, and glucose transport in 3T3/L1 adipocytes.

The thiazolidenedione, rosiglitazone, increases basal and/or insulin-stimulated glucose transport in various cell types by diverse but uncertain mechanisms that may involve insulin receptor substrate (IRS)-1-dependent PI3K. Presently, in 3T3/L1 adipocytes, rosiglitazone induced sizable increases in basal glucose transport that were: dependent on PI3K, 3-phosphoinositide-dependent protein kinase-1 (PDK-1), and PKC-lambda; accompanied by increases in tyrosine phosphorylation of Cbl and Cbl-dependent increases in PI3K and PKC-lambda activity; but not accompanied by increases in IRS-1/2-dependent PI3K or protein kinase B activity. Additionally, rosiglitazone increased IRS-1 and IRS-2 levels, thereby enhancing insulin effects on IRS-1- and IRS-2-dependent PI3K and downstream signaling factors PKC-lambda and protein kinase B. Our findings suggest that Cbl participates in mediating effects of rosiglitazone on PI3K, PDK-1, and PKC-lambda and the glucose transport system and that this Cbl-dependent pathway complements the IRS-1 and IRS-2 pathways for activating PI3K, PDK-1, and PKC-lambda during combined actions of rosiglitazone and insulin in 3T3/L1 cells.

Activation of the ERK pathway and atypical protein kinase C isoforms in exercise- and aminoimidazole-4-carboxamide-1-beta-D-riboside (AICAR)-stimulated glucose transport.

Exercise increases glucose transport in muscle by activating 5'-AMP-activated protein kinase (AMPK), but subsequent events are unclear. Presently, we examined the possibility that AMPK increases glucose transport through atypical protein kinase Cs (aPKCs) by activating proline-rich tyrosine kinase-2 (PYK2), ERK pathway components, and phospholipase D (PLD). In mice, treadmill exercise rapidly activated ERK and aPKCs in mouse vastus lateralis muscles. In rat extensor digitorum longus (EDL) muscles, (a) AMPK activator, 5-aminoimidazole-4-carboxamide-1-beta-d-riboside (AICAR), activated PYK2, ERK and aPKCs; (b) effects of AICAR on ERK and aPKCs were blocked by tyrosine kinase inhibitor, genistein, and MEK1 inhibitor, PD98059; and (c) effects of AICAR on aPKCs and 2-deoxyglucose (2-DOG) uptake were inhibited by genistein, PD98059, and PLD-inhibitor, 1-butanol. Similarly, in L6 myotubes, (a) AICAR activated PYK2, ERK, PLD, and aPKCs; (b) effects of AICAR on ERK were inhibited by genistein, PD98059, and expression of dominant-negative PYK2; (c) effects of AICAR on PLD were inhibited by MEK1 inhibitor UO126; (d) effects of AICAR on aPKCs were inhibited by genistein, PD98059, 1-butanol, and expression of dominant-negative forms of PYK2, GRB2, SOS, RAS, RAF, and ERK; and (e) effects of AICAR on 2DOG uptake/GLUT4 translocation were inhibited by genistein, PD98059, UO126, 1-butanol, cell-permeable myristoylated PKC-zeta pseudosubstrate, and expression of kinase-inactive RAF, ERK, and PKC-zeta. AMPK activator dinitrophenol had effects on ERK, aPKCs, and 2-DOG uptake similar to those of AICAR. Our findings suggest that effects of exercise on glucose transport that are dependent on AMPK are mediated via PYK2, the ERK pathway, PLD, and aPKCs.

Sorbitol activates atypical protein kinase C and GLUT4 glucose transporter translocation/glucose transport through proline-rich tyrosine kinase-2, the extracellular signal-regulated kinase pathway and phospholipase D.

Sorbitol, "osmotic stress", stimulates GLUT4 glucose transporter translocation to the plasma membrane and glucose transport by a phosphatidylinositol (PI) 3-kinase-independent mechanism that reportedly involves non-receptor proline-rich tyrosine kinase-2 (PYK2) but subsequent events are obscure. In the present study, we found that extracellular signal-regulated kinase (ERK) pathway components, growth-factor-receptor-bound-2 protein, son of sevenless (SOS), RAS, RAF and mitogen-activated protein (MAP) kinase/ERK kinase, MEK(-1), operating downstream of PYK2, were required for sorbitol-stimulated GLUT4 translocation/glucose transport in rat adipocytes, L6 myotubes and 3T3/L1 adipocytes. Furthermore, sorbitol activated atypical protein kinase C (aPKC) through a similar mechanism depending on the PYK2/ERK pathway, independent of PI 3-kinase and its downstream effector, 3-phosphoinositide-dependent protein kinase-1 (PDK-1). Like PYK2/ERK pathway components, aPKCs were required for sorbitol-stimulated GLUT4 translocation/glucose transport. Interestingly, sorbitol stimulated increases in phospholipase D (PLD) activity and generation of phosphatidic acid (PA), which directly activated aPKCs. As with aPKCs and glucose transport, sorbitol-stimulated PLD activity was dependent on the ERK pathway. Moreover, PLD-generated PA was required for sorbitol-induced activation of aPKCs and GLUT4 translocation/glucose transport. Our findings suggest that sorbitol sequentially activates PYK2, the ERK pathway and PLD, thereby increasing PA, which activates aPKCs and GLUT4 translocation. This mechanism contrasts with that of insulin, which primarily uses PI 3-kinase, D3-PO(4) polyphosphoinositides and PDK-1 to activate aPKCs.

Knockout of PKC alpha enhances insulin signaling through PI3K.

Insulin stimulates glucose transport and certain other metabolic processes by activating atypical PKC isoforms (lambda, zeta, iota) and protein kinase B (PKB) through increases in D3-polyphosphoinositides derived from the action of PI3K. The role of diacylglycerol-sensitive PKC isoforms is less clear as they have been suggested to be both activated by insulin and yet inhibit insulin signaling to PI3K. Presently, we found that insulin signaling to insulin receptor substrate 1-dependent PI3K, PKB, and PKC lambda, and downstream processes, glucose transport and activation of ERK, were enhanced in skeletal muscles and adipocytes of mice in which the ubiquitous conventional diacylglycerol-sensitive PKC isoform, PKC alpha, was knocked out by homologous recombination. On the other hand, insulin provoked wortmannin-insensitive increases in immunoprecipitable PKC alpha activity in adipocytes and skeletal muscles of wild-type mice and rats. We conclude that 1) PKC alpha is not required for insulin-stimulated glucose transport, and 2) PKC alpha is activated by insulin at least partly independently of PI3K, and largely serves as a physiological feedback inhibitor of insulin signaling to the insulin receptor substrate 1/PI3K/PKB/PKC lambda/zeta/iota complex and dependent metabolic processes.

PKC-zeta mediates insulin effects on glucose transport in cultured preadipocyte-derived human adipocytes.

Insulin-stimulated glucose transport is impaired in the early phases of type 2 diabetes mellitus. Studies in rodent cells suggest that atypical PKC (aPKC) isoforms (zeta, lamda, and iota) and PKB, and their upstream activators, PI3K and 3-phosphoinositide-dependent protein kinase-1 (PDK-1), play important roles in insulin-stimulated glucose transport. However, there is no information on requirements for aPKCs, PKB, or PDK-1 during insulin action in human cell types. Presently, by using preadipocyte-derived adipocytes, we were able to employ adenoviral gene transfer methods to critically examine these requirements in a human cell type. These adipocytes were found to contain PKC-zeta, rather than PKC-lamda/iota, as their major aPKC. Expression of kinase-inactive forms of PDK-1, PKC-zeta, and PKC-lamda (which functions interchangeably with PKC-zeta) as well as chemical inhibitors of PI 3-kinase and PKC-zeta/lamda, wortmannin and the cell-permeable myristoylated PKC-zeta pseudosubstrate, respectively, effectively inhibited insulin-stimulated glucose transport. In contrast, expression of a kinase-inactive, activation-resistant, triple alanine mutant form of PKB-alpha had little or no effect, and expression of wild-type and constitutively active PKC-zeta or PKC-lamda increased glucose transport. Our findings provide convincing evidence that aPKCs and upstream activators, PI 3-kinase and PDK-1, play important roles in insulin-stimulated glucose transport in preadipocyte-derived human adipocytes.