Tyrosine phosphorylation of specific protein kinase C isoenzymes participates in insulin stimulation of glucose transport in primary cultures of rat skeletal muscle

The phosphatidylethanolamine derivative diDCP-LA-PE mimics intracellular insulin signaling

Insulin facilitates glucose uptake into cells by translocating the glucose transporter GLUT4 towards the cell surface through a pathway along an insulin receptor (IR)/IR substrate 1 (IRS-1)/phosphatidylinositol 3 kinase (PI3K)/3-phosphoinositide-dependent protein kinase-1 (PDK1)/Akt axis. The newly synthesized phosphatidylethanolamine derivative 1,2-O-bis-[8-{2-(2-pentyl-cyclopropylmethyl)-cyclopropyl}-octanoyl]-sn-glycero-3-phosphatidylethanolamine (diDCP-LA-PE) has the potential to inhibit protein tyrosine phosphatase 1B (PTP1B) and to directly activate PKCζ, an atypical isozyme, and PKCε, a novel isozyme. PTP1B inhibition enhanced insulin signaling cascades downstream IR/IRS-1 by preventing tyrosine dephosphorylation. PKCζ and PKCε directly activated Akt2 by phosphorylating at Thr309 and Ser474, respectively. diDCP-LA-PE increased cell surface localization of GLUT4 and stimulated glucose uptake into differentiated 3T3-L1 adipocytes, still with knocking-down IR or in the absence of insulin. Moreover, diDCP-LA-PE effectively reduced serum glucose levels in type 1 diabetes (DM) model mice. diDCP-LA-PE, thus, may enable type 1 DM therapy without insulin injection.

Rutaecarpine ameliorates hyperlipidemia and hyperglycemia in fat-fed, streptozotocin-treated rats via regulating the IRS-1/PI3K/Akt and AMPK/ACC2 signaling pathways

AIM

We have shown that rutaecarpine extracted from the dried fruit of Chinese herb Evodia rutaecarpa (Juss) Benth (Wu Zhu Yu) promotes glucose consumption and anti-inflammatory cytokine expression in insulin-resistant primary skeletal muscle cells. In this study we investigated whether rutaecarpine ameliorated the obesity profiles, lipid abnormality, glucose metabolism and insulin resistance in rat model of hyperlipidemia and hyperglycemia.

METHODS

Rats fed on a high-fat diet for 8 weeks, followed by injection of streptozotocin (30 mg/kg, ip) to induce hyperlipidemia and hyperglycemia. One week after streptozotocin injection, the fat-fed, streptozotocin-treated rats were orally treated with rutaecarpine (25 mg·kg(-1)·d(-1)) or a positive control drug metformin (250 mg·kg(-1)·d(-1)) for 7 weeks. The body weight, visceral fat, blood lipid profiles and glucose levels, insulin sensitivity were measured. Serum levels of inflammatory cytokines were analyzed. IRS-1 and Akt/PKB phosphorylation, PI3K and NF-κB protein levels in liver tissues were assessed; pathological changes of livers and pancreases were examined. Glucose uptake and AMPK/ACC2 phosphorylation were studied in cultured rat skeletal muscle cells in vitro.

RESULTS

Administration of rutaecarpine or metformin significantly decreased obesity, visceral fat accumulation, water consumption, and serum TC, TG and LDL-cholesterol levels in fat-fed, streptozotocin-treated rats. The two drugs also attenuated hyperglycemia and enhanced insulin sensitivity. Moreover, the two drugs significantly decreased NF-κB protein levels in liver tissues and plasma TNF-α, IL-6, CRP and MCP-1 levels, and ameliorated the pathological changes in livers and pancreases. In addition, the two drugs increased PI3K p85 subunit levels and Akt/PKB phosphorylation, but decreased IRS-1 phosphorylation in liver tissues. Treatment of cultured skeletal muscle cells with rutaecarpine (20-180 μmol/L) or metformin (20 μmol/L) promoted the phosphorylation of AMPK and ACC2, and increased glucose uptake.

CONCLUSION

Rutaecarpine ameliorates hyperlipidemia and hyperglycemia in fat-fed, streptozotocin-treated rats via regulating IRS-1/PI3K/Akt signaling pathway in liver and AMPK/ACC2 signaling pathway in skeletal muscles.

Rac1 and ROCK are implicated in the cell surface delivery of GLUT4 under the control of the insulin signal mimetic diDCP-LA-PE

The phosphatidylethanolamine derivative 1,2-O-bis-[8-{2-(2-pentyl-cyclopropylmethyl)-cyclopropyl}-octanoyl]-sn-glycero-3-phosphatidylethanolamine (diDCP-LA-PE) promoted GLUT4 translocation to the cell surface in differentiated 3T3-L1-GLUT4myc adipocytes through a pathway along a phosphatidylinositol 3-kinase (PI3K)/3-phosphoinositide-dependent protein kinase-1 (PDK1)/Akt axis, that mimics insulin signaling. Moreover, diDCP-LA-PE-induced GLUT4 translocation was suppressed by inhibitors of the Rho GTPase Rac1 and Rho-associated coiled-coil-containing protein kinase (ROCK) or knocking-down Rac1 and ROCK1. The results of the present study show that Rac1 and ROCK are critical for regulation of GLUT4 trafficking by diDCP-LA-PE as well as insulin.

Insulin secretion and signaling in response to dietary restriction and subsequent re-alimentation in cattle

The objectives of this study were to examine systemic insulin response to a glucose tolerance test (GTT) and transcript abundance of genes of the insulin signaling pathway in skeletal muscle, during both dietary restriction and re-alimentation-induced compensatory growth. Holstein Friesian bulls were blocked to one of two groups: 1) restricted feed allowance for 125 days (period 1) (RES, n = 15) followed by ad libitum feeding for 55 days (period 2) or 2) ad libitum access to feed throughout (periods 1 and 2) (ADLIB, n = 15). On days 90 and 36 of periods 1 and 2, respectively, a GTT was performed. M. longissimus dorsi biopsies were harvested from all bulls on days 120 and 15 of periods 1 and 2, respectively, and RNA-Seq analysis was performed. RES displayed a lower growth rate during period 1 (RES: 0.6 kg/day, ADLIB: 1.9 kg/day; P < 0.001), subsequently gaining more during re-alimentation (RES: 2.5 kg/day, ADLIB: 1.4 kg/day; P < 0.001). Systemic insulin response to glucose administration was lower in RES in period 1 (P < 0.001) with no difference observed during period 2. The insulin signaling pathway in M. longissimus dorsi was enriched (P < 0.05) in response to dietary restriction but not during re-alimentation (P > 0.05). Genes differentially expressed in the insulin signaling pathway suggested a greater sensitivity to insulin in skeletal muscle, with pleiotropic effects of insulin signaling interrupted during dietary restriction. Collectively, these results indicate increased sensitivity to glucose clearance and skeletal muscle insulin signaling during dietary restriction; however, no overall role for insulin was apparent in expressing compensatory growth.

Is fat so bad? Modulation of endoplasmic reticulum stress by lipid droplet formation

LDs (lipid droplets) have long been considered as inert particles used by the cells to store fatty acids and sterols as esterified non-toxic lipid species (i.e. triacylglycerols and steryl esters). However, accumulating evidence suggests that LDs behave as a dynamic compartment, which is involved in the regulation of several aspects of the homoeostasis of their originating organelle, namely the ER (endoplasmic reticulum). The ER is particularly sensitive to physiological/pathological stimuli, which can ultimately induce ER stress. In the present review, after considering the basic mechanisms of LD formation and the signal cascades leading to ER stress, we focus on the connections between these two pathways. Taking into consideration recent data from the literature, we will try to draw possible mechanisms for the role of LDs in the regulation of ER homoeostasis and in ER-stress-related diseases.

Apurinic/apyrimidinic endonuclease 1 inhibits protein kinase C-mediated p66shc phosphorylation and vasoconstriction

AIMS

Phosphorylation of the adaptor protein p66shc is essential for p66shc-mediated oxidative stress. We investigated the role of the reducing protein/DNA repair enzyme apurinic/apyrimidinic endonuclease1 (APE1) in modulating protein kinase CβII (PKCβII)-mediated p66shc phosphorylation in cultured endothelial cells and PKC-mediated vasoconstriction of arteries.

METHODS AND RESULTS

Oxidized low-density lipoprotein (oxLDL)induced p66shc phosphorylation at serine 36 residue and PKCβII phosphorylation in mouse endothelial cells. Adenoviral overexpression of APE1 resulted in reduction of oxLDL-induced p66shc and PKCβII phosphorylation. Phorbol 12-myristate 13-acetate (PMA), which stimulates PKCs, induced p66shc phosphorylation and this was inhibited by a selective PKCβII inhibitor. Adenoviral overexpression of PKCβII also increased p66shc phosphorylation. Overexpression of APE1 suppressed PMA-induced p66shc phosphorylation. Moreover, PMA-induced p66shc phosphorylation was augmented in cells in which APE1 was knocked down. PMA increased cytoplasmic APE1 expression, compared with the basal condition, suggesting the role of cytoplasmic APE1 against p66shc phosphorylation. Finally, vasoconstriction induced by phorbol-12,13, dibutylrate, another PKC agonist, was partially inhibited by transduction of Tat-APE1 into arteries.

CONCLUSION

APE1 suppresses oxLDL-induced p66shc activation in endothelial cells by inhibiting PKCβII-mediated serine phosphorylation of p66shc, and mitigates vasoconstriction induced by activation of PKC.

Insulin stimulation of PKCδ triggers its rapid degradation via the ubiquitin-proteasome pathway

Insulin rapidly upregulates protein levels of PKCδ in classical insulin target tissues skeletal muscle and liver. Insulin induces both a rapid increase in de novo synthesis of PKCδ protein. In this study we examined the possibility that insulin may also inhibit degradation of PKCδ. Experiments were performed on L6 skeletal muscle myoblasts or myotubes in culture. Phorbol ester (PMA)- and insulin-induced degradation of PKCδ were abrogated by proteasome inhibition. Both PMA and insulin induced ubiquitination of PKCδ, but not of that PKCα or PKCε and increased proteasome activity within 5 min. We examined the role of tyrosine phosphorylation of PKCδ in targeting PKCδ for degradation by the ubiquitin-proteasome pathway. Transfection of cells with PKCδY(311)F, which is not phosphorylated, resulted in abolition of insulin-induced ubiquitination of PKCδ and increase in proteasome activity. We conclude that insulin induces degradation of PKCδ via the ubiquitin-proteasome system, and that this effect requires phosphorylation on specific tyrosine residues for targeting PKCδ for degradation by the ubiquitin-proteasome pathway. These studies provide additional evidence for unique effects of insulin on regulation of PKCδ protein levels.

Zinc and diabetes--clinical links and molecular mechanisms

Zinc is an essential trace element crucial for the function of more than 300 enzymes and it is important for cellular processes like cell division and apoptosis. Hence, the concentration of zinc in the human body is tightly regulated and disturbances of zinc homeostasis have been associated with several diseases including diabetes mellitus, a disease characterized by high blood glucose concentrations as a consequence of decreased secretion or action of insulin. Zinc supplementation of animals and humans has been shown to ameliorate glycemic control in type 1 and 2 diabetes, the two major forms of diabetes mellitus, but the underlying molecular mechanisms have only slowly been elucidated. Zinc seems to exert insulin-like effects by supporting the signal transduction of insulin and by reducing the production of cytokines, which lead to beta-cell death during the inflammatory process in the pancreas in the course of the disease. Furthermore, zinc might play a role in the development of diabetes, since genetic polymorphisms in the gene of zinc transporter 8 and in metallothionein (MT)-encoding genes could be demonstrated to be associated with type 2 diabetes mellitus. The fact that antibodies against this zinc transporter have been detected in type 1 diabetic patients offers new diagnostic possibilities. This article reviews the influence of zinc on the diabetic state including the molecular mechanisms, the role of the zinc transporter 8 and MT for diabetes development and the resulting diagnostic and therapeutic options.

Developmentally spliced PKCbetaII provides a possible link between mTORC2 and Akt kinase to regulate 3T3-L1 adipocyte insulin-stimulated glucose transport

Functional adipocyte glucose disposal is a key component of global glucose homeostasis. PKCbetaII is involved in rat skeletal muscle cell ISGT. Western blot analysis and real-time PCR revealed 3T3-L1 cells developmentally regulated PKCbeta splicing such that PKCbetaI was downregulated and PKCbetaII was upregulated during the course of differentiation. An initial glucose uptake screen using PKC inhibitor LY379196 pointed to a PKC isozyme other than PKCzeta mediating 3T3-L1 adipocyte ISGT. Subsequent use of PKCbetaII inhibitor CGP53353 pointed to a role for PKCbetaII in ISGT. Western blot analysis showed that CGP53353 specifically inhibited phosphorylation of PKCbetaII Serine 660. Subcellular fractionation and immunofluorescence demonstrated that PKCbetaII regulates GLUT4 translocation. Further Western blot, immunofluorescence and co-immunoprecipitation analysis reveal that PKCbetaII inhibition does not affect mTORC2 activity yet abrogates phosphorylation of Akt Serine 473. PKCbetaII regulates GLUT4 translocation by regulating Akt phosphorylation and thus activity.

Functional involvement of protein kinase C-betaII and its substrate, myristoylated alanine-rich C-kinase substrate (MARCKS), in insulin-stimulated glucose transport in L6 rat skeletal muscle cells

AIMS/HYPOTHESIS

Insulin stimulates phosphorylation cascades, including phosphatidylinositol-3-kinase (PI3K), phosphatidylinositol-dependent kinase (PDK1), Akt, and protein kinase C (PKC). Myristoylated alanine-rich C-kinase substrate (MARCKS), a PKCbetaII substrate, could link the effects of insulin to insulin-stimulated glucose transport (ISGT) via phosphorylation of its effector domain since MARCKS has a role in cytoskeletal rearrangements.

METHODS

We examined phosphoPKCbetaII after insulin treatment of L6 myocytes, and cytosolic and membrane phosphoMARCKS, MARCKS and phospholipase D1 in cells pretreated with LY294002 (PI3K inhibitor), CG53353 (PKCbetaII inhibitor) or W13 (calmodulin inhibitor), PI3K, PKCbetaII and calmodulin inhibitors, respectively, before insulin treatment, using western blots. ISGT was examined after cells had been treated with inhibitors, small inhibitory RNA (siRNA) for MARCKS, or transfection with MARCKS mutated at a PKC site. MARCKS, PKCbetaII, GLUT4 and insulin receptor were immunoblotted in subcellular fractions with F-actin antibody immunoprecipitates to demonstrate changes following insulin treatment. GLUT4 membrane insertion was followed after insulin with or without CG53353.

RESULTS

Insulin increased phosphoPKCbetaII(Ser660 and Thr641); LY294002 blocked this, indicating its activation by PI3K. Insulin treatment increased cytosolic phosphoMARCKS, decreased membrane MARCKS and increased membrane phospholipase D1 (PLD1), a protein regulating glucose transporter vesicle fusion resulted. PhosphoMARCKS was attenuated by CG53353 or MARCKS siRNA. MARCKS siRNA blocked ISGT. Association of PKCbetaII and GLUT4 with membrane F-actin was enhanced by insulin, as was that of cytosolic and membrane MARCKS. ISGT was attenuated in myocytes transfected with mutated MARCKS (Ser152Ala), whereas overproduction of wild-type MARCKS enhanced ISGT. CG53353 blocked insertion of GLUT4 into membranes of insulin treated cells.

CONCLUSIONS/INTERPRETATION

The results suggest that PKCbetaII is involved in mediating downstream steps of ISGT through MARCKS phosphorylation and cytoskeletal remodelling.

Mice deficient in PKCbeta and apolipoprotein E display decreased atherosclerosis

Endothelial activation is a central initiating event in atheroma formation. Evidence from our laboratory and others has demonstrated links between activation of early growth response-1 (Egr-1) and atherosclerosis and also has demonstrated that activated protein kinase C (PKC) betaII is a critical upstream regulator of Egr-1 in response to vascular stress. We tested the role of PKCbeta in regulating key events linked to atherosclerosis and show that the aortas of apoE(-/-) mice display an age-dependent increase in PKCbetaII antigen in membranous fractions vs. C57BL/6 animals with a approximately 2-fold increase at age 6 wk and a approximately 4.5-fold increase at age 24 wk. Consistent with important roles for PKCbeta in atherosclerosis, a significant decrease in atherosclerotic lesion area was evident in PKCbeta(-/-)/apoE(-/-) vs. apoE(-/-) mice by approximately 5-fold, in parallel with significantly reduced vascular transcripts for Egr-1 and matrix metalloproteinase (MMP)-2 antigen and activity vs. apoE(-/-) mice. Significant reduction in atherosclerosis of approximately 2-fold was observed in apoE(-/-) mice fed ruboxistaurin chow (PKCbeta inhibitor) vs. vehicle. In primary murine and human aortic endothelial cells, the PKCbeta-JNK mitogen-activated protein kinase pathway importantly contributes to oxLDL-mediated induction of MMP2 expression. Blockade of PKCbeta may be beneficial in mitigating endothelial perturbation and atherosclerosis.

Insulin regulates P-glycoprotein in rat brain microvessel endothelial cells via an insulin receptor-mediated PKC/NF-kappaB pathway but not a PI3K/Akt pathway

Our previous study showed that insulin restored impaired function and expression of P-glycoprotein in diabetic blood-brain barrier, and further study showed that insulin up-regulated P-glycoprotein expression and function in normal blood-brain barrier, so insulin might be one of the factors that regulated the function and expression of P-glycoprotein in blood-brain barrier of diabetes. In this study, the intracellular pathways that insulin regulated the P-glycoprotein were investigated using primarily cultured rat brain microvessel endothelial cells model. The rat brain microvessel endothelial cells were incubated in normal culture medium containing 50 mU/l insulin and different concentrations of inhibitors for 72 h. The P-glycoprotein function and expression in the rat brain microvessel endothelial cells were assessed using the uptake of P-glycoprotein substrate rhodamine 123 and western blot assay, respectively. It was found that treatment of 50 mU/l insulin significantly increased P-glycoprotein function and expression in rat brain microvessel endothelial cells. This induced effect was blocked by insulin receptor antibody, insulin receptor tyrosine kinase inhibitor I-OMe-AG538, PKC inhibitor chelerythrine and NF-kappaB inhibitor pyrrolidine dithiocarbamate ammonium (PDTC). But this induced effect was not inhibited by phosphatidylinositol 3-kinase (PI3K)/Akt inhibitor LY294002. These results indicated that insulin regulated P-glycoprotein function and expression through signal transduction pathways involving activation of PKC/NF-kappaB but not PI3K/Akt pathway.

Insulin increases nuclear protein kinase Cdelta in L6 skeletal muscle cells

Protein kinase C (PKC) isoforms are involved in the transduction of a number of signals important for the regulation of cell growth, differentiation, apoptosis, and other cellular functions. PKC proteins reside in the cytoplasm in an inactive state translocate to various membranes to become fully activated in the presence of specific cofactors. Recent evidence indicates that PKC isoforms have an important role in the nucleus. We recently showed that insulin rapidly increases PKCdelta RNA and protein. In this study we initially found that insulin induces an increase in PKCdelta protein in the nuclear fraction. We therefore attempted to elucidate the mechanism of the insulin-induced increase in nuclear PKCdelta. Studies were performed on L6 skeletal myoblasts and myotubes. The increase in nuclear PKCdelta appeared to be unique to insulin because it was not induced by other growth factors or rosiglitazone. Inhibition of transcription or translation blocked the insulin-induced increase in nuclear PKCdelta, whereas inhibition of protein import did not. Inhibition of protein export from the nucleus reduced the insulin-induced increase in PKCdelta in the cytoplasm and increased it in the nucleus. The increase in nuclear PKCdelta induced by insulin was reduced but not abrogated by treatment of isolated nuclei by trypsin digestion. Finally, we showed that insulin induced incorporation of (35)S-methionine into nuclear PKCdelta protein; this effect was not blocked by inhibition of nuclear import. Thus, these results suggest that insulin may induce nuclear-associated, or possibly nuclear, translation of PKCdelta protein.

Cytosolic protein tyrosine phosphatase-epsilon is a negative regulator of insulin signaling in skeletal muscle

Whereas positive regulatory events triggered by insulin binding to insulin receptor (IR) have been well documented, the mechanism by which the activated IR is returned to the basal status is not completely understood. Recently studies focused on the involvement of protein tyrosine phosphatases (PTPs) and how they might influence IR signaling. In this study, we examined the possibility that cytosolic PTPepsilon (cytPTPepsilon) is involved in IR signaling. Studies were performed on L6 skeletal muscle cells. cytPTPepsilon was overexpressed by using pBABE retroviral expression vectors. In addition, we inhibited cytPTPepsilon by RNA silencing. We found that insulin induced rapid association of cytPTPepsilon with IR. Interestingly, this association appeared to occur in the plasma membrane and on stimulation with insulin the two proteins internalized together. Moreover, it appeared that almost all internalized IR was associated with cytPTPepsilon. We found that knockdown of cytPTPepsilon by RNA silencing increased insulin-induced tyrosine phosphorylation of IR and IR substrate (IRS)-1 as well as phosphorylation of protein kinase B and glycogen synthase kinase-3 and insulin-induced stimulation of glucose uptake. Moreover, overexpression of wild-type cytPTPepsilon reduced insulin-induced tyrosine phosphorylation of IR, IRS-1, and phosphorylation of protein kinase B and glycogen synthase kinase-3 and insulin-induced stimulation of glucose uptake. Finally, insulin-induced tyrosine phosphorylation of IR and IRS-1 was greater in skeletal muscle from mice lacking the cytPTPepsilon gene than that from wild-type control animals. We conclude that cytPTPepsilon serves as another major candidate negative regulator of IR signaling in skeletal muscle.

Protein kinase C-dependent antilipolysis by insulin in rat adipocytes

Recently, we have shown that protein kinase C (PKC) activated by phorbol 12-myristate 13-acetate (PMA) attenuates the beta1-adrenergic receptor (beta1-AR)-mediated lipolysis in rat adipocytes. Stimulation of cells by insulin, angiotensin II, and alpha1-AR agonist is known to cause activation of PKC. In this study, we found that lipolysis induced by the beta1-AR agonist dobutamine is decreased and is no longer inhibited by PMA in adipocytes that have been treated with 20 nM insulin for 30 min followed by washing out insulin. Such effects on lipolysis were not found after pretreatment with angiotensin II and alpha1-AR agonists. The rate of lipolysis in the insulin-treated cells was normalized by the PKCalpha- and beta-specific inhibitor Gö 6976 and PKCbeta-specific inhibitor LY 333531. In the insulin-treated cells, wortmannin increased lipolysis and recovered the lipolysis-attenuating effect of PMA. Western blot analysis revealed that insulin slightly increases membrane-bound PKCalpha, betaI, and delta, and wortmannin decreases PKCbetaI, betaII, and delta in the membrane fraction. These results indicate that stimulation of insulin receptor induces a sustained activation of PKC-dependent antilipolysis in rat adipocytes.

Protein kinase C family: on the crossroads of cell signaling in skin and tumor epithelium

The protein kinase C (PKC) family represents a large group of phospholipid dependent enzymes catalyzing the covalent transfer of phosphate from ATP to serine and threonine residues of proteins. Phosphorylation of the substrate proteins induces a conformational change resulting in modification of their functional properties. The PKC family consists of at least ten members, divided into three subgroups: classical PKCs (alpha, betaI, betaII, gamma), novel PKCs (delta, epsilon, eta, theta), and atypical PKCs (zeta, iota/lambda). The specific cofactor requirements, tissue distribution, and cellular compartmentalization suggest differential functions and fine tuning of specific signaling cascades for each isoform. Thus, specific stimuli can lead to differential responses via isoform specific PKC signaling regulated by their expression, localization, and phosphorylation status in particular biological settings. PKC isoforms are activated by a variety of extracellular signals and, in turn, modify the activities of cellular proteins including receptors, enzymes, cytoskeletal proteins, and transcription factors. Accordingly, the PKC family plays a central role in cellular signal processing. Accumulating data suggest that various PKC isoforms participate in the regulation of cell proliferation, differentiation, survival and death. These findings have enabled identification of abnormalities in PKC isoform function, as they occur in several cancers. Specifically, the initiation of squamous cell carcinoma formation and progression to the malignant phenotype was found to be associated with distinct changes in PKC expression, activation, distribution, and phosphorylation. These studies were recently further extended to transgenic and knockout animals, which allowed a more direct analysis of individual PKC functions. Accordingly, this review is focused on the involvement of PKC in physiology and pathology of the skin.

Dysregulated intracellular signaling impairs CTGF-stimulated responses in human mesangial cells exposed to high extracellular glucose

High ambient glucose activates intracellular signaling pathways to induce the expression of extracellular matrix and cytokines such as connective tissue growth factor (CTGF). Cell responses to CTGF in already glucose-stressed cells may act to transform the mesangial cell phenotype leading to the development of glomerulosclerosis. We analyzed cell signaling downstream of CTGF in high glucose-stressed mesangial cells to model signaling in the diabetic milieu. The addition of CTGF to primary human mesangial cells activates cell migration which is associated with a PKC-zeta-GSK3beta signaling axis. In high ambient glucose basal PKC-zeta and GSK3beta phosphorylation levels are selectively increased and CTGF-stimulated PKC-zeta and GSK3beta phosphorylation was impaired. These effects were not induced by osmotic changes. CTGF-driven profibrotic cell signaling as determined by p42/44 MAPK and Akt phosphorylation was unaffected by high glucose. Nonresponsiveness of the PKC-zeta-GSK3beta signaling axis suppressed effective remodeling of the microtubule network necessary to support cell migration. However, interestingly the cells remain plastic: modulation of glucose-induced PKC-beta activity in human mesangial cells reversed some of the pathological effects of glucose damage in these cells. We show that inhibition of PKC-beta with LY379196 and PKC-beta siRNA reduced basal PKC-zeta and GSK3beta phosphorylation in human mesangial cells exposed to high glucose. CTGF stimulation under these conditions again resulted in PKC-zeta phosphorylation and human mesangial cell migration. Regulation of PKC-zeta by PKC-beta in this instance may establish PKC-zeta as a target for constraining the progression of mesangial cell dysfunction in the pathogenesis of diabetic nephropathy.

Glucose-transport regulation in leukemic cells: how can H2O2 mimic stem cell factor effects?

In leukemic cells, glucose transport is activated by SCF and H2O2 through a common signal cascade involving Akt, PLCgamma, Syk, and the Src family, in this order. An explanation can be provided by the phosphorylation of c-kit, the SCF receptor, elicited by either SCF or H2O2. Moreover, antioxidants prevent the SCF effect on glucose transport, confirming the involvement of H2O2 in the pathway leading to glucose-transport activation and suggesting a potential role for reactive oxygen species in leukemia proliferation.

Selective cyclooxygenase-2 inhibitors stimulate glucose transport in L6 myotubes in a protein kinase Cδ-dependent manner

Selective inhibitors of cyclooxygenase-2 (prostaglandin-endoperoxide synthase-2; COX-2) augment the rate of hexose uptake in myotubes by recruiting glucose transporter-4 (GLUT-4) to the plasma membrane in an insulin- and AMPKalpha-independent manner [Alpert E, Gruzman A, Lardi-Studler B, Cohen G, Reich R, Sasson S. Cyclooxygenase-2 (PTGS2) inhibitors augment the rate of hexose transport in L6 myotubes in an insulin- and AMPKalpha-independent manner. Diabetologia 2006;49:562-70]. We aimed at elucidating the molecular interactions that mediate this effect of COX-2 inhibitors in L6 myotubes. The effects of the inhibitors niflumic acid, nimesulide and rofecoxib on activities and phosphorylation state of key proteins in the insulin transduction pathway were determined. These inhibitors did not induce specific tyrosine phosphorylation in IRS-1, could not assemble a functional IRS-PI3K-PKB/Akt complex and did not activate GSK3alpha/beta, JNK1/2, ERK1/2, p38-MAPK or c-Cbl by site-specific phosphorylation(s). Yet, like insulin, they activated mTOR and induced downstream threonine phosphorylation in p70S6K and 4EBP1. However, rapamycin, which inhibits mTOR enzymatic activity, did not interfere with COX-2 inhibitor-induced stimulation of hexose uptake in myotube. Thus, mTOR activation was not required for COX-2 inhibitor-dependent augmentation of hexose transport in myotubes. Because PKCdelta has also been shown to activate mTOR, we asked whether COX-2 inhibitors activate mTOR by a prior activation of PKCdelta. Indeed, all three inhibitors induced tyrosine phosphorylation in PKCdelta and stimulated its kinase activity. Moreover, pharmacological inhibition of PKCdelta or the expression of a dominant-negative form of PKCdelta in myotubes completely abolished COX-2 inhibitor-dependent stimulation of hexose uptake. This study shows that selective COX-2 inhibitors activate a unique PKCdelta-dependent pathway to increase GLUT-4 abundance in the plasma membrane of myotubes and augment the rate of hexose transport.

Involvement of PKCα in insulin-induced PKCδ expression: Importance of SP-1 and NFκB transcription factors

Protein kinase C delta (PKCdelta) is a key molecule in insulin signaling essential for insulin-induced glucose transport in skeletal muscle. Recent studies in our laboratory have shown that insulin rapidly stimulates PKCdelta activity and increases PKCdelta protein and RNA levels, and that the SP-1 transcription factor is involved in insulin-induced transcription of the PKCdelta gene. Activation of SP-1 involves serine phosphorylation and translocation to the nucleus. In this study we examined the possibility that PKCalpha might be involved in serine phosphorylation and activation of SP-1. We found that insulin rapidly phosphorylates and translocates SP-1. In the cytoplasm, SP-1 was constitutively associated with PKCalpha, and insulin stimulation caused these proteins to dissociate. In contrast, in the nucleus insulin induced an increase in association between PKCalpha and SP-1. PKCalpha inhibition blocked insulin-induced serine phosphorylation of SP-1 and its association with PKCalpha in the nucleus. Inhibition of PKCalpha also reduced the insulin-induced increase in PKCdelta RNA and protein in the cytoplasmic and nuclear fractions. We also attempted to determine if another transcription factor might be involved in regulation of PKCdelta expression. We earlier showed that insulin did not affect nuclear NFkappaB levels. Inhibition of NFkappaB, however, increased insulin-induced increase in PKCdelta RNA and protein in the cytoplasmic and nuclear fractions. Surprisingly, this inhibition reduced the insulin-induced increase in cytoplasmic and nuclear PKCalpha RNA and protein. Inhibition of PKCdelta reduced IkappaBalpha phosphorylation as well as NFkappaB activation. Thus, PKCalpha regulates insulin-induced PKCdelta expression levels and this regulation involves activation of SP-1 and NFkappaB.

Angiotensin II inhibits the ROMK-like small conductance K channel in renal cortical collecting duct during dietary potassium restriction

Base-line urinary potassium secretion in the distal nephron is mediated by small conductance rat outer medullary K (ROMK)-like channels. We used the patch clamp technique applied to split-open cortical collecting ducts (CCDs) isolated from rats fed a normal potassium (NK) or low potassium (LK) diet to test the hypothesis that AngII directly inhibits ROMK channel activity. We found that AngII inhibited ROMK channel activity in LK but not NK rats in a dose-dependent manner. The AngII-induced reduction in channel activity was mediated by AT1 receptor (AT1R) binding, because pretreatment of CCDs with losartan but not PD123319 AT1 and AT2 receptor antagonists, respectively, blocked the response. Pretreatment of CCDs with U73122 and calphostin C, inhibitors of phospholipase C (PLC) and protein kinase C (PKC), respectively, abolished the AngII-induced decrease in ROMK channel activity, confirming a role of the PLC-PKC pathway in this response. Studies by others suggest that AngII stimulates an Src family protein-tyrosine kinase (PTK) via PKC-NADPH oxidase. PTK has been shown to regulate the ROMK channel. Inhibition of NADPH oxidase with diphenyliodonium abolished the inhibitory effect of AngII or the PKC activator phorbol 12-myristate 13-acetate on ROMK channels. Suppression of PTK by herbimycin A significantly attenuated the inhibitory effect of AngII on ROMK channel activity. We conclude that AngII inhibits ROMK channel activity through PKC-, NADPH oxidase-, and PTK-dependent pathways under conditions of dietary potassium restriction.

Protein kinase C beta/early growth response-1 pathway: a key player in ischemia, atherosclerosis, and restenosis

Atherosclerosis, restenosis, and the consequences of ischemia are the major causes of morbidity and mortality worldwide. Elucidation of key contributing pathways in animal models of ischemia-reperfusion injury, atherosclerosis, and restenosis consequent to vascular injury may lead to great interest in determining if blocking these pathways could prevent vascular disease in human subjects. This review details the evidence that the protein kinase C (PKC) beta/early growth response-1 axis plays a central role in the response to both acute and chronic vascular stresses in animal models and also indicates the clinical implications of a specific inhibitor of PKCbeta, ruboxistaurin (LY333531).

Protein kinase Cdelta participates in insulin-induced activation of PKB via PDK1

PKCdelta has been shown to be activated by insulin and to interact with insulin receptor and IRS. PKB(Akt) plays an important role in glucose transport and glycogen synthesis. In this study, we investigated the possibility that PKCdelta may be involved in insulin-induced activation of PKB. Studies were conducted on primary cultures of rat skeletal muscle. PKB was activated by insulin stimulation within 5min and reached a peak by 15-30min. Insulin also increased the physical association between PKCdelta with PKB and with PDK1. The insulin-induced PKCdelta-PKB association was PI3K dependent. PKB-PKCdelta association was accounted for by the involvement of PDK1. Overexpression of dominant negative PKCdelta abrogated insulin-induced association of PKCdelta with both PKB and PDK1. Blockade of PKCdelta also decreased insulin-induced Thr308 PKB phosphorylation and PKB translocation. Moreover, PKCdelta inhibition reduced insulin-induced GSK3 phosphorylation. The results indicate that insulin-activated PKCdelta interacts with PDK1 to regulate PKB.

Activation of the nuclear transcription factor SP-1 by insulin rapidly increases the expression of protein kinase C delta in skeletal muscle

SP-1, a ubiquitous transcription factor involved in regulation of target genes participating in specific signaling pathways, is utilized by insulin for induction of gene transcription. Transcriptional activation generally occurs only after several (14-24) hours. A major element rapidly activated by insulin in skeletal muscle is PKCdelta, which plays a positive regulatory role in insulin signaling. We recently reported that insulin stimulation of skeletal muscle increases PKCdelta RNA expression and PKCdelta protein levels within 5 min. These effects were blocked by inhibitors of either translation or transcription. In this study, we investigated the possibility that SP-1 may participate in this unusually rapid effect. Studies were performed on myoblasts and myotubes of the L6 skeletal muscle cell line. Insulin rapidly increased SP-1 levels and stimulated SP-1 phosphorylation in the nuclear fraction of L6 myotubes. The increase in nuclear SP-1 was blocked by inhibition of nuclear import. Inhibition of SP-1, either pharmacologically or by suppression of SP-1 by RNAi, nearly completely abrogated insulin-induced increase in PKCdelta promoter activity. Insulin induced a rapid association of SP-1 with the PKCalpha promoter. In addition, SP-1 inhibition blocked insulin-induced increases in both PKCdelta RNA expression and PKCdelta protein levels. We conclude that insulin rapidly stimulates SP-1, which mediates the ability of this hormone to induce the rapid transcription of a major target gene utilized in the insulin signaling cascade.

Modulation of mitochondrial metabolic function by phorbol 12-myristate 13-acetate through increased mitochondrial translocation of protein kinase Calpha in C2C12 myocytes

Protein kinase C (PKC) agonists including phorbol 12-myristate 13-acetate (PMA) not only induce the redistribution of cytosolic PKC to various subcellular compartments but also activate the kinase domain of the protein. In the present study we have investigated the nature of mitochondrial PKC pool and its effects on mitochondrial function in cells treated with PMA. Treatment of C2C12 myoblasts, C6 glioma and COS7 cells with PMA resulted in a dramatic redistribution of intracellular PKCalpha pool, with large fraction of the protein pool sequestered in the mitochondrial compartment. We also observed mitochondrial PKCdelta accumulation in a cell restricted manner. The intramitochondrial localization was ascertained by using a combination of protection against protease treatment of isolated mitochondria and immunofluorescence microscopy. PMA-induced mitochondrial localization of PKCalpha was accompanied by increased mitochondrial PKC activity, altered cell morphology, disruption of mitochondrial membrane potential, decreased complex I and pyruvate dehydrogenase activities, and increased mitochondrial ROS production. All of these changes could be retarded by treatment with PKC inhibitors. These results show a direct role for PMA-mediated PKCalpha translocation to mitochondria in inducing mitochondrial toxicity.

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.

Protein kinase Calpha regulates insulin receptor signaling in skeletal muscle

Certain PKC isoforms are stimulated by insulin and interact with IR as well as with IRS, but it is still not clear if specific PKC isoforms regulate IR signaling directly or through IRS-1. PKCalpha may regulate IRS activity in response to insulin. We investigated the possibility that PKCalpha may be important in insulin signaling. Studies were conducted on skeletal muscle in adult mice and on L6 skeletal cells. PKCalpha is constitutively associated with IRS-1, and insulin stimulation of PKCalpha causes disassociation of the two proteins within 5 min. Blockade of PKCalpha inhibited insulin-induced disassociation of PKCalpha from IRS1. Selective inhibition of PKCalpha increased the ability of insulin to reduce blood glucose levels. Insulin stimulation activates PKB and increases the association of PKCalpha with PKB. Blockade of PKCalpha increased threonine phosphorylation of PKB. We suggest that PKCalpha regulates insulin signaling in skeletal muscle through its disassociation from IRS-1 and association with PKB.

High glucose enhances inducible nitric oxide synthase expression. Role of protein kinase C-betaII

The aim was to determine whether high glucose levels interfere with nitric oxide (NO) production and inducible NO synthase (iNOS) protein expression in interleukin-1beta-stimulated vascular smooth muscle cells from normotensive Wistar Kyoto and spontaneously hypertensive rats. Cells were incubated with either normal (5.5 mM) or high (22 mM) d-glucose for 72 h and with interleukin-1beta (10 ng/ml) for the last 24 h. High glucose increased nitrite levels, iNOS expression and protein kinase C activity in cells from normotensive rats and had no effect in cells from hypertensive rats. High glucose effects on nitrite production and iNOS expression was abolished by the selective inhibitor for the protein kinase C-betaII, 5,21:12,17-dimetheno-18H-dibenzo[i,o]pyrrolo[3,4-1] [1,8]diacyclohexadecine-18,20 (19H)-dione, 8-[(dimethylamino) methyl]-6,7,8,9,10,11-hexahydro-monomethanesulfonate (LY379196, 30 nM). Calphostin C (1 microM) and LY379196 (10 microM) reduced nitrite levels and iNOS expression only in cells from normotensive rats treated with both media. These results suggest that high glucose increases inducible nitric oxide synthase induction and subsequent NO production by activating the protein kinase C-betaII; this mechanism seems to be altered in hypertension.

Functional and pharmacological mechanisms of nucleoside transport across the basolateral membrane of rabbit tracheal epithelial cells

The role of basolateral membrane nucleoside transport in primary cultured rabbit tracheal epithelial cells (RTEC) was studied. Primary cultured RTEC were grown on permeable support at an air-interface. Transport studies were conducted in the uptake, efflux, and transepithelial transport configurations using (3)H-uridine as a model substrate. Time, temperature and concentration dependency of (3)H-uridine transport were evaluated in parallel to the metabolism of this substrate using scintillation counting and thin layer chromatography. Inhibition of (3)H-uridine uptake from basolateral fluid was estimated in presence of all unlabeled natural nucleosides as well as analogs and nucleobases. Functional modulation pathways of (3)H-uridine uptake were studied after treatment of RTEC with pharmacological levels of A23187, forskolin, tamoxifen, H89 and colchicine. The basolateral aspect has a low-affinity and high-capacity transport system that exhibits characteristics of bi-directionality, temperature/concentration dependency, and broad specificity towards purines and pyrimidines without requiring Na(+). Basolateral equilibrative-sensitive/insensitive (es/ei) type transport machinery manifested as a biphasic dose response to nitro-benzyl-mercapto-purine-ribose (NBMPR) inhibition. In addition, a number of therapeutically relevant nucleoside analogs appeared to compete with the uptake of uridine from basolateral fluid. Short-term pre-incubation of primary cultured RTEC with the calcium ionophore A23187 inhibited basolateral uridine uptake without affecting the J(max) and K(m). The inhibitory effect was not reversible with a protein kinase C (PKC) antagonist, tamoxifen. In contrast, basolateral uridine uptake was increased by adenylyl cyclase activator forskolin (reversible with protein kinase A (PKA) inhibitor H89), resulting in a decreased K(m), but a lower J(max). Uridine exit across the basolateral membrane of primary cultured RTEC occurs via a facilitative diffusion carrier, which can be modulated by intracellular Ca(2+) levels and PKA. Information about these carriers will help improve the transportability of antitumor and antiviral nucleoside analogs in the pulmonary setting.

Protein kinase C-alpha regulates insulin action and degradation by interacting with insulin receptor substrate-1 and 14-3-3 epsilon

Protein kinase C (PKC)-alpha exerts a regulatory function on insulin action. We showed by overlay blot that PKCalpha directly binds a 180-kDa protein, corresponding to IRS-1, and a 30-kDa molecular species, identified as 14-3-3epsilon. In intact NIH-3T3 cells overexpressing insulin receptors (3T3-hIR), insulin selectively increased PKCalpha co-precipitation with IRS-1, but not with IRS-2, and with 14-3-3epsilon, but not with other 14-3-3 isoforms. Overexpression of 14-3-3epsilon in 3T3-hIR cells significantly reduced IRS-1-bound PKCalpha activity, without altering IRS-1/PKCalpha co-precipitation. 14-3-3epsilon overexpression also increased insulin-stimulated insulin receptor and IRS-1 tyrosine phosphorylation, followed by increased activation of Raf1, ERK1/2, and Akt/protein kinase B. Insulin-induced glycogen synthase activity and thymidine incorporation were also augmented. Consistently, selective depletion of 14-3-3epsilon by antisense oligonucleotides caused a 3-fold increase of IRS-1-bound PKCalpha activity and a similarly sized reduction of insulin receptor and IRS-1 tyrosine phosphorylation and signaling. In turn, selective inhibition of PKCalpha expression by antisense oligonucleotides reverted the negative effect of 14-3-3epsilon depletion on insulin signaling. Moreover, PKCalpha inhibition was accompanied by a >2-fold decrease of insulin degradation. Similar results were also obtained by overexpressing 14-3-3epsilon. Thus, in NIH-3T3 cells, insulin induces the formation of multimolecular complexes, including IRS-1, PKCalpha, and 14-3-3epsilon. The presence of 14-3-3epsilon in the complex is not necessary for IRS-1/PKCalpha interaction but modulates PKCalpha activity, thereby regulating insulin signaling and degradation.

Insulin rapidly upregulates protein kinase Cdelta gene expression in skeletal muscle

Recent studies in our laboratories have shown that Protein Kinase C delta (PKCdelta) is essential for insulin-induced glucose transport in skeletal muscle, and that insulin rapidly stimulates PKCdelta activity skeletal muscle. The purpose of this study was to examine mechanisms of regulation of PKCdelta protein availability. Studies were done on several models of mammalian skeletal muscle and utilized whole cell lysates of differentiated myotubes. PKCdelta protein levels were determined by Western blotting techniques, and PKCdelta RNA levels were determined by Northern blotting, RT-PCR and Real-Time RT-PCR. Insulin stimulation increased PKCdelta protein levels in whole cell lysates. This effect was not due to an inhibition by insulin of the rate of PKCdelta protein degradation. Insulin also increased 35S-methionine incorporation into PKCdelta within 5-15 min. Pretreatment of cells with transcription or translation inhibitors abrogated the insulin-induced increase in PKCdelta protein levels. We also found that insulin rapidly increased the level of PKCdelta RNA, an effect abolished by inhibitors of transcription. The insulin-induced increase in PKCdelta expression was not reduced by inhibition of either PI3 Kinase or MAP kinase, indicating that these signaling mechanisms are not involved, consistent with insulin activation of PKCdelta. Studies on cells transfected with the PKCdelta promoter demonstrate that insulin activated the promoter within 5 min. This study indicates that the expression of PKCdelta may be regulated in a rapid manner during the course of insulin action in skeletal muscle and raise the possibility that PKCdelta may be an immediate early response gene activated by insulin.

Two different pathways link G-protein-coupled receptors with tyrosine kinases for the modulation of growth and survival in human hematopoietic progenitor cells

The G-protein-coupled receptor agonists CXCL12 (SDF-1, a chemokine) and thrombin showed opposite effects on growth and survival of multipotent and erythroid human hematopoietic progenitor cells. CXCL12 promoted growth in multipotent cells by activating the RhoA-Rho kinase pathway. Its effect was largely blocked by Y-27632, a specific inhibitor of Rho kinase, and by clostridial toxin B, a specific inhibitor of Rho family proteins. Rho activation required a G(i)-mediated stimulation of tyrosine kinases, which was blocked by PP2 and tyrphostin AG 490, inhibitors of Src and Jak type kinases, respectively. By contrast, in erythroid cells, inhibitors of Src family and c-Abl tyrosine kinases (tyrphostin AG 82, PP2, imatinib) enhanced protein kinase C (PKC)-dependent cell growth and antagonized thrombin-promoted apoptosis by specifically stimulating PKCbeta activity. The PKC activating phorbol ester PMA (a growth factor in erythroid cells) induced the activation of Lyn and c-Abl tyrosine kinases, thus establishing a feedback inhibition of PKCbeta. Hence, developmental stage-specific crosstalk between PKC subtypes and tyrosine kinases appear to determine whether growth and survival of hematopoietic cells are promoted or inhibited by G-protein-coupled receptor agonists.

Src tyrosine kinase regulates insulin-induced activation of protein kinase C (PKC) delta in skeletal muscle

Insulin stimulation of skeletal muscle results in rapid activation of protein kinase Cdelta (PKCdelta), which is associated with its tyrosine phosphorylation and physical association with insulin receptor (IR). The mechanisms underlying tyrosine phosphorylation of PKCdelta have not been determined. In this study, we investigated the possibility that the Src family of nonreceptor tyrosine kinases may be involved upstream insulin signaling. Studies were done on differentiated rat skeletal myotubes in primary culture. Insulin caused an immediate stimulation of Src and induced its physical association with both IR and PKCdelta. Inhibition of Src by treatment with the Src family inhibitor PP2 reduced insulin-stimulated Src-PKCdelta association, PKCdelta tyrosine phosphorylation and PKCdelta activation. PP2 inhibition of Src also decreased insulin-induced IR tyrosine phosphorylation, IR-PKCdelta association and association of Src with both PKCdelta and IR. Finally, inhibition of Src decreased insulin-induced glucose uptake. We conclude that insulin activates Src tyrosine kinase, which regulates PKCdelta activity. Thus, Src tyrosine kinase may play an important role in insulin-induced tyrosine phosphorylation of both IR and PKCdelta. Moreover, both Src and PKCdelta appear to be involved in IR activation and subsequent downstream signaling.

Circadian rhythm of glucose uptake in cultures of skeletal muscle cells and adipocytes in Wistar-Kyoto, Wistar, Goto-Kakizaki, and spontaneously hypertensive rats

Hypertension and noninsulin-dependent diabetes mellitus are usually associated with marked glucose intolerance. Hypertensive and even nonhypertensive diabetic individuals display disturbances of the normal circadian blood pressure rhythm. However, little is known about circadian changes of the glucose uptake in muscle and fat cells, the major glucose utilizing tissues. Therefore, we investigated circadian rhythms of glucose uptake in primary muscle and fat cell cultures of hypertensive and type II diabetic rats and their respective control strains. 2-Deoxy-D-(1-3H)glucose uptake was measured over 48 h after synchronization of cells by means of medium change with and without addition of insulin, phloretine, and/or staurosporine. The circadian changes of glucose uptake were assessed by fitting cosine curves to the uptake values. Insulin stimulation of deoxyglucose uptake was only present in control animals, not in hypertensive and diabetic rats. Deoxyglucose uptake displayed a circadian rhythm in control animals, and was markedly disturbed in hypertensive and diabetic animals. Blocking of glucose transporters by phloretine abolished the circadian pattern of deoxyglucose uptake indicating a role of glucose transporters in its generation. Inhibition of kinases by staurosporine inhibited the insulin-stimulated deoxyglucose uptake, but did not dampen the circadian rhythmicity of basal deoxyglucose uptake. The generation of the circadian rhythm of glucose uptake in muscle and fat cell cultures is therefore probably insulin independent and independent of protein kinases. In summary, our results show for the first time: (a) a circadian rhythm of deoxyglucose uptake in glucose utilizing muscle and fat cells in vitro, (b) a disruption of this rhythm in cells of hypertensive and diabetic rats.

Effect of exercise on protein kinase C activity and localization in human skeletal muscle

To investigate the effect of exercise on protein kinase C (PKC) activity and localization in human skeletal muscle, eight healthy men performed cycle ergometer exercise for 40 min at 76 +/- 1% the peak pulmonary O(2) uptake , with muscle samples obtained at rest and after 5 and 40 min of exercise. PKC expression, phosphorylation and activities were examined by immunoblotting and in vitro kinase assays of fractionated and whole tissue preparations. In response to exercise, total PKC activity was slightly higher at 40 min in an enriched membrane fraction, and using a pSer-PKC-substrate motif antibody it was revealed that exercise increased the serine phosphorylation of an approximately 50 kDa protein. There were no changes in conventional PKC (cPKC) or PKC activities; however, atypical PKC (aPKC) activity was approximately 70% higher at 5 and 40 min, and aPKC expression and Thr(410/403) phosphorylation were unaltered by exercise. There were no effects of exercise on the abundance of PKCalpha, PKCdelta, PKC and aPKC within cytosolic or enriched membrane fractions of skeletal muscle. These data indicate that aPKC, but not cPKC or PKC, are activated by exercise in contracting muscle suggesting a potential role for aPKC in the regulation of skeletal muscle function and metabolism during exercise in humans.

Depolarization of skeletal muscle cells induces phosphorylation of cAMP response element binding protein via calcium and protein kinase Calpha

Membrane depolarization of skeletal muscle cells induces slow inositol trisphosphate-mediated calcium signals that regulate the activity of transcription factors such as the cAMP-response element-binding protein (CREB), jun, and fos. Here we investigated whether such signals regulate CREB phosphorylation via protein kinase C (PKC)-dependent pathways. Western blot analysis revealed the presence of seven isoforms (PKCalpha, -betaI, -betaII, -delta, -epsilon, -, and -zeta) in rat primary myotubes. The PKC inhibitors bisindolymaleimide I and Gö6976, blocked CREB phosphorylation. Chronic exposure to phorbol ester triggered complete down-regulation of several isoforms, but reduced PKCalpha levels to only 40%, and did not prevent CREB phosphorylation upon myotube depolarization. Immunocytochemical analysis revealed selective and rapid PKCalpha translocation to the nucleus following depolarization, which was blocked by 2-amino-ethoxydiphenyl borate, an inositol trisphosphate receptor inhibitor, and by the phospholipase C inhibitor U73122. In C2C12 cells, which expressed PKCalpha,-epsilon, and -zeta, CREB phosphorylation also depended on PKCalpha. These results strongly implicate nuclear PKCalpha translocation in CREB phosphorylation induced by skeletal muscle membrane depolarization.

PKCbeta regulates ischemia/reperfusion injury in the lung

Activation of PKCbetaII is associated with the response to ischemia/reperfusion (I/R), though its role, either pathogenic or protective, has not been determined. In a murine model of single-lung I/R, evidence linking PKCbeta to maladaptive responses is shown in the following studies. Homozygous PKCbeta-null mice and WT mice fed the PKCbeta inhibitor ruboxistaurin subjected to I/R displayed increased survival compared with controls. In PKCbeta-null mice, phosphorylation of extracellular signal-regulated protein kinase-1 and -2 (ERK1/2), JNK, and p38 MAPK was suppressed in I/R. Expression of the immediate early gene, early growth response-1 (Egr-1), and its downstream target genes was significantly increased in WT mice in I/R, particularly in mononuclear phagocytes (MPs), whereas this expression was attenuated in PKCbeta-null mice or WT mice fed ruboxistaurin. In vitro, hypoxia/reoxygenation-mediated induction of Egr-1 in MPs was suppressed by inhibition of PKCbeta, ERK1/2, and JNK, but not by inhibition of p38 MAPK. These findings elucidate key roles for PKCbetaII activation in I/R by coordinated activation of MAPKs (ERK1/2, JNK) and Egr-1.

Effects of insulin, contraction, and phorbol esters on mitogen-activated protein kinase signaling in skeletal muscle from lean and ob/ob mice

Effects of diverse stimuli, including insulin, muscle contraction, and phorbol 12-myristate-13-acetate (PMA), were determined on phosphorylation of mitogen-activated protein kinase (MAPK) signaling modules (c-Jun NH(2)-terminal kinase [JNK], p38 MAPK, and extracellular signal-related kinase [ERK1/2]) in skeletal muscle from lean and ob/ob mice. Insulin increased phosphorylation of JNK, p38 MAPK, and ERK1/2 in isolated extensor digitorum longus (EDL) and soleus muscle from lean mice in a time- and dose-dependent manner. Muscle contraction and PMA also elicited robust effects on these parallel MAPK modules. Insulin action on JNK, p38 MAPK, and ERK1/2 phosphorylation was significantly impaired in EDL and soleus muscle from ob/ob mice. In contrast, muscle contraction-mediated JNK, p38 MAPK, and ERK1/2 phosphorylation was preserved. PMA effects on phosphorylation of JNK and ERK1/2 were normal in ob/ob mice, whereas effects on p38 MAPK were abolished. In conclusion, insulin, contraction, and PMA activate MAPK signaling in skeletal muscle. Insulin-mediated responses on MAPK signaling are impaired in skeletal muscle from ob/ob mice, whereas the effect of contraction is generally well preserved. In addition, PMA-induced phosphorylation of JNK and ERK1/2 are preserved, whereas p38 MAPK pathways are impaired in skeletal muscle from ob/ob mice. Thus, appropriate MAPK responses can be elicited in insulin-resistant skeletal muscle via an insulin-independent mechanism.

Pervanadate-induced reverse translocation and tyrosine phosphorylation of phorbol ester-stimulated protein kinase C betaII are mediated by Src-family tyrosine kinases in porcine neutrophils

Protein kinase C (PKC), upon activation, translocates from the cytosol to the plasma membrane. Phorbol 12-myristate 13-acetate (PMA), a potent PKC activator, is known to induce irreversible translocation of PKC to the plasma membrane, in contrast to the reversible translocation resulting from physiological stimuli and subsequent rapid return to the cytosol (reverse translocation). However, we have previously shown that tyrosine phosphatase (PTPase) inhibitors induce reverse translocation of PMA-stimulated PKCbetaII in porcine polymorphonuclear leukocytes (PMNs). In the present study, we showed that pervanadate, a potent PTPase inhibitor, also induces tyrosine phosphorylation of PMA-stimulated PKCbetaII in porcine PMNs. Furthermore, PP2, a specific inhibitor of Src-family tyrosine kinases (PTKs), was found to inhibit both pervanadate-induced reverse translocation and tyrosine phosphorylation of PMA-stimulated PKCbetaII, suggesting that these two pervanadate-induced responses are mediated by Src-family PTKs. Our findings provide novel insight into the relationship between the subcellular localization and tyrosine phosphorylation of PKC.

Tumor necrosis factor alpha produces insulin resistance in skeletal muscle by activation of inhibitor kappaB kinase in a p38 MAPK-dependent manner

Insulin stimulation produced a reliable 3-fold increase in glucose uptake in primary neonatal rat myotubes, which was accompanied by a similar effect on GLUT4 translocation to plasma membrane. Tumor necrosis factor (TNF)-alpha caused insulin resistance on glucose uptake and GLUT4 translocation by impairing insulin stimulation of insulin receptor (IR) and IR substrate (IRS)-1 and IRS-2 tyrosine phosphorylation, IRS-associated phosphatidylinositol 3-kinase activation, and Akt phosphorylation. Because this cytokine produced sustained activation of stress and proinflammatory kinases, we have explored the hypothesis that insulin resistance by TNF-alpha could be mediated by these pathways. In this study we demonstrate that pretreatment with PD169316 or SB203580, inhibitors of p38 MAPK, restored insulin signaling and normalized insulin-induced glucose uptake in the presence of TNF-alpha. However, in the presence of PD98059 or SP600125, inhibitors of p42/p44 MAPK or JNK, respectively, insulin resistance by TNF-alpha was still produced. Moreover, TNF-alpha produced inhibitor kappaB kinase (IKK)-beta activation and inhibitor kappaB-beta and -alpha degradation in a p38 MAPK-dependent manner, and treatment with salicylate (an inhibitor of IKK) completely restored insulin signaling. Furthermore, TNF-alpha produced serine phosphorylation of IR and IRS-1 (total and on Ser(307) residue), and these effects were completely precluded by pretreatment with either PD169316 or salicylate. Consequently, TNF-alpha, through activation of p38 MAPK and IKK, produces serine phosphorylation of IR and IRS-1, impairing its tyrosine phosphorylation by insulin and the corresponding activation of phosphatidylinositol 3-kinase and Akt, leading to insulin resistance on glucose uptake and GLUT4 translocation.

Insulin regulates protein kinase CbetaII alternative splicing in multiple target tissues: development of a hormonally responsive heterologous minigene

Cells respond to external signals like insulin to alter metabolic pathways in response to varying physiological environments. Insulin stimulates the protein kinase C beta (PKCbeta) isozymes and preferentially switches the expression to PKCbetaII isozyme, which is shown to have a crucial role in glucose uptake, cellular proliferation, and differentiation. We have developed an insulin-responsive PKCbetaII heterologous minigene to identify cis-elements in vivo in eukaryotes by cloning the PKCbetaII exon and its flanking intronic sequences into the splicing vector pSPL3. The transfected minigene mimicked the endogenous insulin response of PKCbetaII alternative splicing in five distinct cell types, i.e. L6 skeletal muscle, 3T3-L1 pre-adipocytes, HepG2 human hepatoma cells, A10 vascular smooth muscle cells, and murine embryonic fibroblasts within 30 min of insulin stimulation. Sequential deletions of the flanking introns in the minigene demonstrated that insulin regulated elements within the 5'-intron flanking the PKCbetaII exon. Mutational studies indicated the SRp40 binding site promotes splice site selection. In these cases, splicing appears to be regulated by a phosphatidylinositol 3-kinase signaling pathway because LY294002 and wortmannin, its specific inhibitors, blocked exon inclusion. Cotransfection with constitutively active Akt2 kinase mimicked insulin action. Signal-dependent regulation of splicing by insulin is unique from tissue-specific and developmentally regulated mechanisms previously reported and serves as a prototype for studies of alternative splicing involving protein phosphorylation.

The diacylglycerol and protein kinase C pathways are not involved in insulin signalling in primary rat hepatocytes

Diacylglycerol (DAG) and protein kinase C (PKC) isoforms have been implicated in insulin signalling in muscle and fat cells. We evaluated the involvement of DAG and PKC in the action of insulin in adult rat hepatocytes cultured with dexamethasone, but in the absence of serum, for 48 h. Our results show that although insulin stimulated glycolysis and glycogen synthesis, it had no effect on DAG mass or molecular species composition. Epidermal growth factor showed the expected insulin-mimetic effect on glycolysis, whereas ATP and exogenous phospholipase C acted as antagonists and abolished the insulin signal. Similarly to insulin, epidermal growth factor had no effect on DAG mass or molecular species composition. In contrast, both ATP and phospholipase C induced a prominent increase in several DAG molecular species, including 18:0/20:4, 18:0/20:5, 18:0/22:5 and a decrease in 18:1/18:1. These changes were paralleled by an increase in phospholipase D activity, which was absent in insulin-treated cells. By immunoblotting or by measuring PKC activity, we found that neither insulin nor ATP translocated the PKCalpha, -delta, -epsilon or -zeta isoforms from the cytosol to the membrane in cells cultured for six or 48 h. Similarly, insulin had no effect on immunoprecipitable PKCzeta. Suppression of the glycogenic insulin signal by phorbol 12-myristate 13-acetate, but not by ATP, could be completely alleviated by bisindolylmaleimide. Finally, insulin showed no effect on DAG mass or translocation of PKC isoforms in the perfused liver, although it reduced the glucagon-stimulated glucose output by 75%. Together these results indicate that phospholipases C and D or multiple PKC isoforms are not involved in the hepatic insulin signal chain.

Dehydroepiandrosterone stimulates glucose uptake in human and murine adipocytes by inducing GLUT1 and GLUT4 translocation to the plasma membrane

Dehydroepiandrosterone (DHEA) has been shown to modulate glucose utilization in humans and animals, but the mechanisms of DHEA action have not been clarified. We show that DHEA induces a dose- and time-dependent increase in glucose transport rates in both 3T3-L1 and human adipocytes with maximal effects at 2 h. Exposure of adipocytes to DHEA does not result in changes of total GLUT4 and GLUT1 protein levels. However, it does result in significant increases of these glucose transporters in the plasma membrane. In 3T3-L1 adipocytes, DHEA increases tyrosine phosphorylation of insulin receptor substrate (IRS)-1 and IRS-2 and stimulates IRS-1- and IRS-2-associated phosphatidylinositol (PI) 3-kinase activity with no effects on either insulin receptor or Akt phosphorylation. In addition, DHEA causes significant increases of cytosolic Ca(2+) concentrations and a parallel activation of protein kinase C (PKC)-beta(2). The effects of DHEA are abrogated by pretreatment of adipocytes with PI 3-kinase and phospholipase C gamma inhibitors, as well as by inhibitors of Ca(2+)-dependent PKC isoforms, including a specific PKC-beta inhibitor. Thus, DHEA increases glucose uptake in both human and 3T3-L1 adipocytes by stimulating GLUT4 and GLUT1 translocation to the plasma membrane. PI 3-kinase, phospholipase C gamma, and the conventional PKC-beta(2) seem to be involved in DHEA effects.

Immunohistochemical identification and fiber type specific localization of protein kinase C isoforms in equine skeletal muscle

OBJECTIVE

To investigate whether protein kinase C (PKC) isoforms are expressed in equine skeletal muscle and determine their distribution in various types of fibers by use of immunofluorescence microscopy.

ANIMALS

5 healthy adult Dutch Warmblood horses.

PROCEDURE

In each horse, 2 biopsy specimens were obtained from the vastus lateralis muscle. Cryosections of equine muscle were stained with PKC isoform (alpha, beta1, beta2, delta, epsilon, or zeta)-specific polyclonal antibodies and examined by use of a fluorescence microscope. Homogenized muscle samples were evaluated via western blot analysis.

RESULTS

The PKC alpha, beta1, beta2, delta, epsilon, and zeta isoforms were localized within the fibers of equine skeletal muscle. In addition, PKC alpha and beta2 were detected near or in the plasma membrane of muscle cells. For some PKC isoforms, distribution was specific for fiber type. Staining of cell membranes for PKC alpha was observed predominantly in fibers that reacted positively with myosin heavy chain (MHC)-IIa; PKC delta and epsilon staining were more pronounced in MHC-I-positive fibers. In contrast, MHC-I negative fibers contained more PKC zeta than MHC-I-positive fibers. Distribution of PKC beta1 was equal among the different fiber types.

CONCLUSIONS AND CLINICAL RELEVANCE

Results indicated that PKC isoforms are expressed in equine skeletal muscle in a fiber type-specific manner. Therefore, the involvement of PKC isoforms in signal transduction in equine skeletal muscle might be dependent on fiber type.

Physical and functional interaction between protein kinase C delta and Fyn tyrosine kinase in human platelets

An increasing number of tyrosine kinases have been shown to associate with isoforms of the protein kinase C (PKC) family. Here, we show evidence for physical and functional interaction between PKCdelta and the Src family kinase Fyn in human platelets activated by alboaggregin-A, a snake venom capable of activating both GPIb-V-IX and GPVI adhesion receptors. This interaction involves phosphorylation of PKCdelta on tyrosine and is specific in that other isoforms of PKC, PKCepsilon and lambda, which also become tyrosine-phosphorylated, do not interact with Fyn. In addition, PKCdelta does not interact with other platelet-expressed tyrosine kinases Syk, Src, or Btk. Stimulation also leads to activation of both Fyn and PKCdelta and to serine phosphorylation of Fyn within a PKC consensus sequence. Alboaggregin-A-dependent activation of Fyn is blocked by bisindolylmaleimide I, suggesting a role for PKC isoforms in regulating Fyn activity. Platelet activation with alboaggregin-A induces translocation of the two kinases from cytoplasm to the plasma membrane of platelets, as observed by confocal immunofluorescence microscopy. Translocation of Fyn and PKCdelta are blocked by PP1 and bisindolylmaleimide I, showing a dependence upon Src and PKC kinase activities. Although PKC activity is required for translocation, it is not required for association between the two kinases, because this was not blocked by bisindolylmaleimide I. Rottlerin, which inhibited PKCdelta activity, did not block translocation of either PKCdelta or Fyn but potentiated platelet aggregation, 5-hydroxytryptamine secretion, and the calcium response induced by alboaggregin-A, indicating that this kinase plays a negative role in the control of these processes.