High glucose concentrations and protein kinase C isoforms in vascular smooth muscle cells

Evolving mechanisms of vascular smooth muscle contraction highlight key targets in vascular disease

Vascular smooth muscle (VSM) plays an important role in the regulation of vascular function. Identifying the mechanisms of VSM contraction has been a major research goal in order to determine the causes of vascular dysfunction and exaggerated vasoconstriction in vascular disease. Major discoveries over several decades have helped to better understand the mechanisms of VSM contraction. Ca2+ has been established as a major regulator of VSM contraction, and its sources, cytosolic levels, homeostatic mechanisms and subcellular distribution have been defined. Biochemical studies have also suggested that stimulation of Gq protein-coupled membrane receptors activates phospholipase C and promotes the hydrolysis of membrane phospholipids into inositol 1,4,5-trisphosphate (IP3) and diacylglycerol (DAG). IP3 stimulates initial Ca2+ release from the sarcoplasmic reticulum, and is buttressed by Ca2+ influx through voltage-dependent, receptor-operated, transient receptor potential and store-operated channels. In order to prevent large increases in cytosolic Ca2+ concentration ([Ca2+]c), Ca2+ removal mechanisms promote Ca2+ extrusion via the plasmalemmal Ca2+ pump and Na+/Ca2+ exchanger, and Ca2+ uptake by the sarcoplasmic reticulum and mitochondria, and the coordinated activities of these Ca2+ handling mechanisms help to create subplasmalemmal Ca2+ domains. Threshold increases in [Ca2+]c form a Ca2+-calmodulin complex, which activates myosin light chain (MLC) kinase, and causes MLC phosphorylation, actin-myosin interaction, and VSM contraction. Dissociations in the relationships between [Ca2+]c, MLC phosphorylation, and force have suggested additional Ca2+ sensitization mechanisms. DAG activates protein kinase C (PKC) isoforms, which directly or indirectly via mitogen-activated protein kinase phosphorylate the actin-binding proteins calponin and caldesmon and thereby enhance the myofilaments force sensitivity to Ca2+. PKC-mediated phosphorylation of PKC-potentiated phosphatase inhibitor protein-17 (CPI-17), and RhoA-mediated activation of Rho-kinase (ROCK) inhibit MLC phosphatase and in turn increase MLC phosphorylation and VSM contraction. Abnormalities in the Ca2+ handling mechanisms and PKC and ROCK activity have been associated with vascular dysfunction in multiple vascular disorders. Modulators of [Ca2+]c, PKC and ROCK activity could be useful in mitigating the increased vasoconstriction associated with vascular disease.

Protein kinase Cα stimulates hypoxia‑induced pulmonary artery smooth muscle cell proliferation in rats through activating the extracellular signal‑regulated kinase 1/2 pathway

Hypoxic pulmonary hypertension (HPH) may contribute to vascular remodeling, and pulmonary artery smooth muscle cell (PASMC) proliferation has an important role in this process. However, no relevant information concerning the role and mechanism of protein kinase C (PKC)α in hypoxia-induced rat PASMC proliferation has been elucidated. The present study aimed to further investigate this by comparison of rat PASMC proliferation among normoxia for 72 h (21% O₂), hypoxia for 72 h (3% O₂), hypoxia + promoter 12-myristate 13-acetate control, hypoxia + safingol control, hypoxia + PD98059 control and hypoxia + U0126 control groups. The present study demonstrated that protein expression levels of PKCα in rat PASMCs were elevated. In conclusion, through activating the extracellular signal-regulated 1/2 signaling pathway, PKCα is involved in and initiates PASMC proliferation, thus bringing about pulmonary artery hypertension. These results add to the understanding of the mechanism PKCα in PH formation and lays a theoretical basis for prevention as well as treatment of HPH.

Protein Kinase C as Regulator of Vascular Smooth Muscle Function and Potential Target in Vascular Disorders

Vascular smooth muscle (VSM) plays an important role in maintaining vascular tone. In addition to Ca2+-dependent myosin light chain (MLC) phosphorylation, protein kinase C (PKC) is a major regulator of VSM function. PKC is a family of conventional Ca2+-dependent α, β, and γ, novel Ca2+-independent δ, ɛ, θ, and η, and atypical ξ, and ι/λ isoforms. Inactive PKC is mainly cytosolic, and upon activation it undergoes phosphorylation, maturation, and translocation to the surface membrane, the nucleus, endoplasmic reticulum, and other cell organelles; a process facilitated by scaffold proteins such as RACKs. Activated PKC phosphorylates different substrates including ion channels, pumps, and nuclear proteins. PKC also phosphorylates CPI-17 leading to inhibition of MLC phosphatase, increased MLC phosphorylation, and enhanced VSM contraction. PKC could also initiate a cascade of protein kinases leading to phosphorylation of the actin-binding proteins calponin and caldesmon, increased actin-myosin interaction, and VSM contraction. Increased PKC activity has been associated with vascular disorders including ischemia-reperfusion injury, coronary artery disease, hypertension, and diabetic vasculopathy. PKC inhibitors could test the role of PKC in different systems and could reduce PKC hyperactivity in vascular disorders. First-generation PKC inhibitors such as staurosporine and chelerythrine are not very specific. Isoform-specific PKC inhibitors such as ruboxistaurin have been tested in clinical trials. Target delivery of PKC pseudosubstrate inhibitory peptides and PKC siRNA may be useful in localized vascular disease. Further studies of PKC and its role in VSM should help design isoform-specific PKC modulators that are experimentally potent and clinically safe to target PKC in vascular disease.

ADAM-10 could mediate cleavage of N-cadherin promoting apoptosis in human atherosclerotic lesions leading to vulnerable plaque: a morphological and immunohistochemical study

Atherosclerosis remains a major cause of mortality. Whereas the histopathological progression of atherosclerotic lesions is well documented, much less is known about the development of unstable or vulnerable plaque, which can rupture leading to thrombus, luminal occlusion and infarct. Apoptosis in the fibrous cap, which is rich in vascular smooth muscle cells (VSMCs) and macrophages, and its subsequent weakening or erosion seems to be an important regulator of plaque stability. The aim of our study was to improve our knowledge on the biological mechanisms that cause plaque instability in order to develop new therapies to maintain atherosclerotic plaque stability and avoid its rupture. In our study, we collected surgical specimens from atherosclerotic plaques in the right or left internal carotid artery of 62 patients with evident clinical symptoms. Histopathology and histochemistry were performed on wax-embedded sections. Immunohistochemical localization of caspase-3, N-cadherin and ADAM-10 was undertaken in order to highlight links between apoptosis, as expressed by caspase-3 immunostaining, and possible roles of N-cadherin, a cell-cell junction protein in VSMCs and macrophages that provides a pro-survival signal reducing apoptosis, and ADAM-10, a "disintegrin and metalloproteases" that is able to cleave N-cadherin in glioblastomas. Our results showed that when apoptosis, expressed by caspase-3 immunostaining, increased in the fibrous cap, rich in VSMCs and macrophages, the expression of N-cadherin decreased. The decreased N-cadherin expression, in turn, was linked to increased ADAM-10 expression. This study shows that apoptotic events are probably involved in the vulnerability of atherosclerotic plaque.

PKCδ impaired vessel formation and angiogenic factor expression in diabetic ischemic limbs

Decreased collateral vessel formation in diabetic peripheral limbs is characterized by abnormalities of the angiogenic response to ischemia. Hyperglycemia is known to activate protein kinase C (PKC), affecting the expression and activity of growth factors such as vascular endothelial growth factor (VEGF) and platelet-derived growth factor (PDGF). The current study investigates the role of PKCδ in diabetes-induced poor collateral vessel formation and inhibition of angiogenic factors expression and actions. Ischemic adductor muscles of diabetic Prkcd(+/+) mice exhibited reduced blood reperfusion, vascular density, and number of small vessels compared with nondiabetic Prkcd(+/+) mice. By contrast, diabetic Prkcd(-/-) mice showed significant increased blood flow, capillary density, and number of capillaries. Although expression of various PKC isoforms was unchanged, activation of PKCδ was increased in diabetic Prkcd(+/+) mice. VEGF and PDGF mRNA and protein expression were decreased in the muscles of diabetic Prkcd(+/+) mice and were normalized in diabetic Prkcd(-/-) mice. Furthermore, phosphorylation of VEGF receptor 2 (VEGFR2) and PDGF receptor-β (PDGFR-β) were blunted in diabetic Prkcd(+/+) mice but elevated in diabetic Prkcd(-/-) mice. The inhibition of VEGFR2 and PDGFR-β activity was associated with increased SHP-1 expression. In conclusion, our data have uncovered the mechanisms by which PKCδ activation induced poor collateral vessel formation, offering potential novel targets to regulate angiogenesis therapeutically in diabetic patients.

Beneficial effect of genistein on lowering blood pressure and kidney toxicity in fructose-fed hypertensive rats

The study evaluates the effects of genistein on blood pressure (BP) and ultrastructural changes in kidney of fructose-fed hypertensive rats. Male Wistar rats were fed a diet containing 60 % starch or 60 % fructose as the source of carbohydrate. After 15 d, rats in each dietary group were divided into two groups and were treated with either genistein (1 mg/kg per d) in dimethylsulfoxide (DMSO) or 30 % DMSO alone. BP, pressor mechanisms, protein kinase C-βII (PKC-βII) expression, endothelial NO synthase (eNOS) expression and renal ultrastructural changes were evaluated after 60 d. Fructose-fed rats displayed significant elevation in BP and heart rate. Significant increase in plasma angiotensin-converting enzyme (ACE) activity, alterations in renal lipid profile, nitrite and kallikrein activity, enhanced expression of membrane-associated PKC-βII and decreased expression of eNOS were observed in them. Histology and electron microscopic studies showed structural changes in the kidney. Genistein administration lowered BP, restored ACE, PKC-βII and eNOS expression and preserved renal ultrastructural integrity. These findings demonstrate that genistein has effects on eNOS activity in renal cells, leading to eNOS activation and NO synthesis. These effects could have been mediated by activation of PKC-βII. The observed benefits of genistein make it a promising candidate for therapy of diabetic kidney disease.

Curcumin attenuates diabetic nephropathy by inhibiting PKC-α and PKC-β1 activity in streptozotocin-induced type I diabetic rats

SCOPE

We hypothesized that curcumin, a potent anti-oxidant, might be beneficial in ameliorating the development of diabetic nephropathy through inhibition of PKC-α and PKC-β1 activity-ERK1/2 pathway.

METHODS AND RESULTS

Diabetes was induced by a single intraperitoneal injection of streptozotocin (STZ) (55 mg/kg) in rats. Three weeks after STZ injection, rats were divided into three groups, namely, normal, diabetic and diabetic treated with curcumin at 100 mg/kg/day, p.o., for 8 wk. At 11 wk after STZ injection, diabetic rats exhibited renal dysfunction, as evidenced by reduced creatinine clearance, increased blood urea nitrogen (BUN) and proteinuria, marked increases in lipid peroxidation, NOX4 and p67phox and decrease in anti-oxidant enzyme. All of these abnormalities were significantly reversed by curcumin. Furthermore, the high-glucose-induced PKC-α and PKC-β1 activities and phosphorylated ERK1/2 was significantly diminished by curcumin. Curcumin also attenuated the expression of TGF-β1, CTGF, osteopontin, p300 and ECM proteins such as fibronectin and type IV collagen. The high-glucose-induced expression of VEGF and its receptor VEGF receptor II (flk-1) was also ameliorated by curcumin.

CONCLUSION

These results prove that curcumin produces dual blockade of both PKC-α and PKC-β1 activities, which suggests that curcumin is a potential adjuvant therapy for the prevention and treatment of diabetic nephropathy.

Renoprotective and blood pressure-lowering effect of dietary soy protein via protein kinase C beta II inhibition in a rat model of metabolic syndrome

We studied whether substitution of soy protein for casein can improve insulin sensitivity, lower blood pressure (BP), and inhibit protein kinase C betaII (PKCbetaII) activation in kidney in an acquired model of metabolic syndrome. Adult male rats were fed 4 different diets: (i) starch (60%) and casein (20%) (CCD), (ii) fructose (60%) and casein (20%) (FCD), (iii) fructose (60%) and soy protein (20%) (FSD), and (iv) starch (60%) and soy protein (20%) (CSD). Renal function parameters, BP, pressor mechanisms, PKCbetaII expression, oxidative stress, and renal histology were evaluated after 60 days. FCD rats displayed insulin resistance and significant changes in body weight, kidney weight, urine volume, plasma and urine electrolytes accompanied by significant changes in renal function parameters compared with CCD rats. Elevated BP, plasma angiotensin-converting enzyme (ACE) activity, renal oxidative stress, and reduced nitrite (NO) and kallikrein activity were observed. Western blot analysis revealed enhanced renal expression of membrane-associated PKCbetaII in the FCD group. Histology showed fatty infiltration and thickening of glomeruli while urinary protein profile revealed a 5-fold increase in albumin. Substitution of soy protein for casein improved insulin sensitivity, lowered BP and PKCbetaII activation and restored renal function. Antioxidant action, inhibitory effect on ACE and PKCbetaII activation, and increased availability of kinins and NO could be contributing mechanisms for the benefits of dietary soy protein.

Fumarate hydratase deficiency in renal cancer induces glycolytic addiction and hypoxia-inducible transcription factor 1alpha stabilization by glucose-dependent generation of reactive oxygen species

Hereditary leiomyomatosis and renal cell cancer (HLRCC) is an inherited cancer syndrome linked to biallelic inactivation of the gene encoding the tricarboxylic acid cycle enzyme fumarate hydratase (FH). Individuals with HLRCC are at risk to develop cutaneous and uterine leiomyomas and an aggressive form of kidney cancer. Pseudohypoxic drive-the aberrant activation of cellular hypoxia response pathways despite normal oxygen tension-is considered to be a likely mechanism underlying the etiology of this tumor. Pseudohypoxia requires the oxygen-independent stabilization of the alpha subunit of the hypoxia-inducible transcription factor (HIF-1alpha). Under normoxic conditions, proline hydroxylation of HIF-1alpha permits VHL recognition and subsequent targeting for proteasomal degradation. Here, we demonstrate that inactivating mutations of FH in an HLRCC-derived cell line result in glucose-mediated generation of cellular reactive oxygen species (ROS) and ROS-dependent HIF-1alpha stabilization. Additionally, we demonstrate that stable knockdown of FH in immortalized renal epithelial cells results in ROS-dependent HIF-1alpha stabilization. These data reveal that the obligate glycolytic switch present in HLRCC is critical to HIF stabilization via ROS generation.

Effects of Blocking the Renin-Angiotensin System on Expression and Translocation of Protein Kinase C Isoforms in the Kidney of Diabetic Rats

BACKGROUND

High glucose and angiotensin II (Ang II) can activate protein kinase C (PKC) in diabetes mellitus. However, it is not clear which isoform of PKC is activated by glucose or Ang II. Our study focused on the effects of angiotensin blockade, using the angiotensin-converting enzyme inhibitor fosinopril, the Ang II receptor blocker irbesartan and their combination, on the expression and translocation of PKC isoforms alpha and betaII in the renal cortex and medulla in diabetes.

METHODS

Hyperglycemia was induced with streptozotocin and diabetic rats were randomized to 4 groups: diabetic control, irbesartan group (40 mg/kg daily), fosinopril group (40 mg/kg daily) and combination group (irbesartan plus fosinopril, 20 mg/kg daily, respectively); age-matched normal rats served as normal control. After 4 weeks, expression and translocation of PKC-alpha and -betaII in the renal cortex and medulla were assessed by immunohistochemistry and Western immunoblotting.

RESULTS

The expression of PKC-alpha in the membrane and cytosol fractions from the renal cortex was significantly higher in diabetic rats (276.83 +/- 32.44% in membrane, 149.04 +/- 23.42% in cytosol) than that in normal ones. The expression of PKC-betaII in the renal cortex of diabetic rats decreased significantly in the membrane (50.00 +/- 11.68%, p < 0.05) and remained unchanged in the cytosol (94.51 +/- 11.69%, p > 0.05) compared with normal controls. Treatment with irbesartan, fosinopril and their combination partially corrected the abnormalities mentioned above. For the expression of PKC-alpha and -betaII in the medulla, no difference was detected among the 5 groups.

CONCLUSION

The renin-angiotensin system was implicated in the pathogenesis of diabetic nephropathy by regulating the activation of PKC isoforms.

Effect of chronic endothelin blockade on PKC isoform distribution in mesenteric arteries from diabetic rats

Hemodynamic changes, including increased vasoconstriction and reduced blood flow have been detected in both human diabetic patients and in animal models of diabetes. We previously demonstrated that the endothelin (ET) system was upregulated and involved in mediating the exaggerated vasoconstrictor responses in superior mesenteric artery (SMA) from diabetic rats. Chronic treatment of diabetic rats with the dual endothelin receptor antagonist, bosentan abolished the enhanced contractile responses in diabetic SMA. The biological actions of ET-1 have been shown to be coupled to the hydrolysis of phosphotidylinositol 4,5-biphosphate and phosphotidylcholine and the subsequent production of diacylglycerol (DAG). DAG is an activator of the classical and novel isoforms of PKC. Increases in PKC activity, associated with translocation of specific PKC isoforms from the cytosol to the membrane, have been implicated in the vasoconstrictor effect of ET-1. The goal of the present study was to determine whether chronic treatment of diabetic rats with bosentan influences the activation of specific PKC isoforms in SMA from diabetic rats. Elevated levels of PKCbeta2 in both the cytosol and membrane fractions and PKCepsilon in the membrane fraction were detected in SMA from diabetic rats. However, neither the levels nor the distribution between the cytosol and membrane fractions of any of these PKC isoforms were affected by the treatment of the diabetic rats with bosentan. These observations indicate that bosentan improves vascular reactivity in STZ-diabetic rats by mechanisms other than correction of increased activities of PKC isoforms.

ERK1/2 activation by angiotensin II inhibits insulin-induced glucose uptake in vascular smooth muscle cells

Clinical evidence suggests a relationship between hypertension and insulin resistance, and cross-talk between angiotensin II (Ang II) and insulin signaling pathways may take place. We now report the effect of Ang II on insulin-induced glucose uptake and its intracellular mechanisms in vascular smooth muscle cells (VSMC). We examined the translocation of glucose transporter-4 (GLUT-4) and glucose uptake in rat aortic smooth muscle cells (RASMC). Mitogen-activated protein (MAP) kinases and Akt activities, and phosphorylation of insulin receptor substrate-1 (IRS-1) at the serine and tyrosine residues were measured by immunoprecipitation and immunoblotting. As a result, Ang II inhibited insulin-induced GLUT-4 translocation from cytoplasm to the plasma membrane in RASMC. Ang II induced extracellular signal-regulated kinase (ERK) 1/2 and c-Jun N-terminal kinase (JNK) activation and IRS-1 phosphorylation at Ser307 and Ser616. Ang II-induced Ser307 and Ser616 phophorylation of IRS-1 was inhibited by a MEK inhibitor, PD98059, and a JNK inhibitor, SP600125. Ang II inhibition of insulin-stimulated IRS-1 tyrosyl phophorylation and Akt activation were reversed by PD98059 but not by SP600125. Ang II inhibited insulin-induced glucose uptake, which was also reversed by PD98059 but not by SP600125. It is shown that Ang II-induced ERK1/2 activation inhibits insulin-dependent glucose uptake through serine phophorylation of IRS-1 in RASMC.

Attenuation of extracellular matrix accumulation in diabetic nephropathy by the advanced glycation end product cross-link breaker ALT-711 via a protein kinase C-alpha-dependent pathway

This study investigated the role of advanced glycation end products (AGEs) in mediating protein kinase C (PKC) isoform expression in diabetic nephropathy. In vitro, vascular smooth muscle cells incubated in a high-glucose (25-mmol/l) medium demonstrated translocation and increased expression of PKC-alpha as compared with those from a low-glucose (5-mmol/l) environment. Coincubation with the cross-link breaker ALT-711 and, to a lesser extent, with aminoguanidine, an inhibitor of AGE formation, attenuated the increased expression and translocation of PKC-alpha. Streptozotocin-induced diabetic rats were randomized to no treatment, treatment with ALT-711, or treatment with aminoguanidine. Diabetes induced increases in PKC-alpha as well as in the -betaI, -betaII, and -epsilon isoforms. Treatment with ALT-711 and aminoguanidine, which both attenuate renal AGE accumulation, abrogated these increases in PKC expression. However, translocation of phosphorylated PKC-alpha from the cytoplasm to the membrane was reduced only by ALT-711. ALT-711 treatment attenuated expression of vascular endothelial growth factor and the extracellular matrix proteins, fibronectin and laminin, in association with reduced albuminuria. Aminoguanidine had no effect on VEGF expression, although some reduction of fibronectin and laminin was observed. These findings implicate AGEs as important stimuli for the activation of PKC, particularly PKC-alpha, in the diabetic kidney, which can be directly inhibited by ALT-711.

Selective Inhibition of Protein Kinase Cβ 2 Prevents Acute Effects of High Glucose on Vascular Cell Adhesion Molecule-1 Expression in Human Endothelial Cells

Background— Enhanced expression of adhesion molecules by the endothelium may account for vascular damage in diabetics and nondiabetic patients who develop stress hyperglycemia during acute myocardial infarction. We analyzed the phosphorylation of protein kinase Cβ 2 (PKCβ 2 ) at serine/threonine residues, which may contribute to the endothelial dysfunction during acute hyperglycemia. Furthermore, this study was designed to investigate whether selective blockade of this regulatory mechanism may prevent the development of endothelial hyperadhesiveness.

Methods and Results— Incubation of the human aortic endothelial cells with high glucose (22.2 mmol/L) resulted in significant increase of vascular cell adhesion molecule (VCAM)-1 protein expression (172±15% versus control; P <0.01). Phorbol 12-myristate 13-acetate, a potent activator of PKC, mimicked the effect of high glucose on VCAM-1 expression. High glucose led to a rapid increase (181±22% versus control; P <0.01) of membrane-bound PKCβ, reflecting activation of this enzyme. The nonselective inhibitor of PKCβ 1 and PKCβ 2 isoforms LY379196, as well as CGP53353, a highly selective inhibitor of PKCβ 2 , prevented in a dose-dependent manner upregulation of VCAM-1. Incubation with high glucose was associated with increased PKCβ 2 phosphorylation at the Ser-660 residue, and both LY379196 and CGP53353 prevented this event. Exposure of the cells to high glucose also reduced the protein level of the inhibitory subunit of nuclear factor-κB, IκBα, leading to its enhanced binding activity. Selective inhibition of PKCβ abolished IκBα degradation.

Conclusions— Our findings demonstrate for the first time that phosphorylation of Ser-660 represents a selective regulatory mechanism for glucose-induced upregulation of VCAM-1. Therefore, PKCβ 2 -selective inhibitors may be promising drugs for treatment of endothelial dysfunction during acute hyperglycemia and possibly in diabetes.

Glucose-induced TGF-beta1 and TGF-beta receptor-1 expression in vascular smooth muscle cells is mediated by protein kinase C-alpha

Sclerosis and increased matrix expression in diabetes are mediated by glucose-induced transforming growth factor (TGF)-beta1 expression. The intracellular effects of high glucose occur at least in part by way of protein kinase C (PKC). We previously described a role for PKC-alpha in glucose-induced permeability. We now investigated the hypothesis that glucose-induced expression of TGF-beta1 and its receptors (TGF-beta-R1 and -R2) are mediated by activation of this PKC isoform. TGF-beta1 and TGF-beta-R expressions were determined in vascular smooth muscle cells (VSMCs) by immunocytochemistry and Western blotting. PKC isoforms were assessed by confocal microscopy. PKC isoforms were inhibited with antisense oligodeoxynucleotides. PKC-alpha was upregulated by overexpression or microinjection. High glucose (20 mmol/L) increased VSMC TGF-beta1 and TGF-beta-R1 expression but not TGF-beta-R2 expression. PKC inhibitors and specific PKC-alpha downregulation by antisense treatment prevented this effect, whereas antisense treatment against PKC-beta, -epsilon, and -zeta had no influence. PKC-alpha overexpression increased TGF-beta1 and TGF-beta-R1 expression but not TGF-beta-R2 expression. PKC-alpha microinjection into individual VSMCs also increased TGF-beta1 and TGF-beta-R immunofluorescence. Last, VSMCs from PKC-alpha-deficient mice did not respond to high glucose compared with VSMCs from wild-type mice. We propose that high glucose-induced TGF-beta1 and TGF-beta-R1 expression is mediated by PKC-alpha. Our findings suggest an autocrine feedback mechanism and a possible role for PKC-alpha in diabetic vascular disease.

Protein kinase C changes in diabetes: is the concept relevant to neuropathy?

Protein kinase C (PKC) comprises a superfamily of isoenzymes, many of which are activated by 1,2-diacylglycerol (DAG) in the presence of phosphatidylserine. In order to be capable of DAG activation, PKC must first undergo a series of phosphorylation at three conserved sites. PKC isoforms phosphorylate a wide variety of intracellular target proteins and have multiple functions in signal transduction-mediated cellular regulation. An elevation in DAG levels and an increase in composite PKC activity and/or certain isoforms occurs in several nonneural tissues from diabetic animals, including the vasculature. The ability of isoform-specific PKC inhibitors to antagonize diabetes-induced abnormalities has implicated altered PKC beta activity in the onset of several diabetic complications, In contrast to many other tissues, DAG levels fall in diabetic nerve and a consistent pattern of change in PKC activity has not been observed. Treatments that alter PKC activity affect nerve Na+, K+-ATPase activity, but the mechanism involved is not well understood, Inhibition of PKC beta in diabetic rats appears to correct reduced nerve blood flow and decreased nerve conduction velocity. These and other findings indicate that changes in the neurovasculature exert adverse effects during the pathogenesis of diabetic neuropathy. Still unresolved is a clear-cut role for PKC in the development of abnormalities in neural cell metabolism. Further progress will depend on a more complete understanding of the functions of individual PKC isoforms in nerve. Future investigation could focus profitably on biochemical processes in nerve cells that modulate PKC activity and that are altered in diabetes, such as vascular endothelial growth factor levels and production of reactive oxygen species arising from oxidative stress.

The protein kinase C beta II exon confers mRNA instability in the presence of high glucose concentrations

Previous studies showed that short term exposure of cells to high glucose destabilized protein kinase C (PKC) betaII mRNA, whereas PKCbetaI mRNA levels remained unaltered. Because PKCbeta mRNAs share common sequences other than the PKCbetaII exon encoding a different carboxyl terminus, we examined PKCbetaII mRNA for a cis-acting region that could confer glucose-induced destabilization. A beta-globin/growth hormone reporter con struct containing the PKCbetaII exon was transfected into human aorta and rat vascular smooth muscle cells (A10) to follow glucose-induced destabilization. Glucose (25 mm) exposure destabilized PKCbetaII chimeric mRNA but not control mRNA. Deletion analysis and electrophoretic mobility shift assays followed by UV cross-linking experiments demonstrated that a region introduced by inclusion of the betaII exon was required to confer destabilization. Although a cis-acting element mapped to 38 nucleotides within the betaII exon was necessary to bestow destabilization, it was not sufficient by itself to confer complete mRNA destabilization. Yet, in intact cells antisense oligonucleotides complementary to this region blocked glucose-induced destabilization. These results suggest that this region must function in context with other sequence elements created by exon inclusion involved in affecting mRNA stability. In summary, inclusion of an exon that encodes PKCbetaII mRNA introduces a cis-acting region that confers destabilization to the mRNA in response to glucose.

Diacylglycerol production and protein kinase C activity are increased in a mouse model of diabetic embryopathy

Activation of the diacylglycerol-protein kinase C (DAG-PKC) cascade by excess glucose has been implicated in vascular complications of diabetes. Its involvement in diabetic embryopathy has not been established. We examined DAG production and PKC activities in embryos and decidua of streptozotocin (STZ)-diabetic or transiently hyperglycemic mice during neural tube formation. STZ diabetes significantly increased DAG and total PKC activity in decidua (1.5- and 1.4-fold, respectively) and embryos (1.7- and 1.3-fold, respectively) on day 9.5. Membrane-associated PKC alpha, betaII, delta, and zeta were increased in decidua by 1.25- to 2.8-fold. Maternal hyperglycemia induced by glucose injection on day 7.5, the day before the onset of neural tube formation, also increased DAG, PKC activity, and PKC isoforms (1.1-, 1.6-, and 1.5-fold, respectively) in the embryo on day 9.5. Notably, membrane-associated PKC activity was increased 24-fold in embryos of diabetic mice with structural defects. These data indicate that hyperglycemia just before organogenesis activates the DAG-PKC cascade and is correlated with congenital defects.

Phosphoinositide 3-kinase mediates protein kinase C beta II mRNA destabilization in rat A10 smooth muscle cell cultures exposed to high glucose

High-glucose exposure down-regulates protein kinaseC beta II posttranscriptionally in rat and human vascular smooth muscle cells and contributes to increased cell proliferation. High-glucose-induced mRNA destabilization is specific for PKC beta II mRNA, while PKC beta I and other PKC mRNA are not affected. This study focused on whether glucose metabolism was required. The effect was blocked by cytochalasin B, suggesting a requirement for glucose uptake. Glucosamine did not mimic the effect, indicating that metabolism via hexosamine pathway was not involved. The effect was hexokinase-independent since 3-O-methylglucose, in a dose-dependent manner, mimicked high-glucose effects. Cycloheximide did not block the effect excluding dependency on new protein synthesis. Wortmannin and LY294002, phosphoinositide 3-kinase (PI3-kinase) inhibitors, blocked glucose effects in the presence of 5,6-dichloro-1-beta-d-ribofuranosylbenzimidazole. Glucose and 3-O-methylglucose activated PI3-kinase, and LY294002 blocked glucose effects on Akt phosphorylation. In these cells, high-glucose concentrations activated a metabolically linked signaling pathway independent of glucose metabolism to regulate mRNA processing.

Mesangial cell protein kinase C isozyme activation in the diabetic milieu

High-glucose-induced activation of mesangial cell protein kinase C (PKC) contributes significantly to the pathogenesis of diabetic nephropathy. Excess glucose metabolism through the polyol pathway leads to de novo synthesis of both diacylglyerol (DAG) and phosphatidic acid, which may account for increased mesangial cell PKC-alpha, -beta, -delta, -epsilon, and -zeta activation/translocation observed within 48-h exposure to high glucose. Raised intracellular glucose causes generation of reactive oxygen species that may directly activate PKC isozymes and enhance their reactivity to vasoactive peptide signaling. In both diabetic rodent models of diabetes and cultured mesangial cells, PKC-beta appears to be the key isozyme required for the enhanced expression of transforming growth factor-beta(1), initiation of early accumulation of mesangial matrix protein, and increased microalbuminuria. Enhanced collagen IV expression by mesangial cells in response to vasoactive peptide hormone stimulation, e.g., endothelin-1, requires PKC-beta, -delta, -epsilon and -zeta. Loss of mesangial cell contractility to potent vasoactive peptides and coincident F-actin disassembly are due to high-glucose-activation of PKC-zeta. Inhibition of mesangial cell PKC isozyme activation in high glucose may prove to be the next important treatment for diabetic nephropathy.

Molecular biology of protein kinase C signaling in cardiac myocytes

The PKC family of serine/threonine kinases have been implicated in a diverse array of cellular responses. Adult cardiac myocytes express multiple PKC isozymes, which participate in the response of muscle cells to extracellular stimuli, modulate contractile properties, and promote cell growth and survival. Recently, the classification of this ubiquitous family of signaling molecules has been expanded from three to four subfamilies. This review will focus on the application of pharmacologic and molecular approaches to explore the biology of cardiac PKC isozymes. The availability of transgenic mice and peptide PKC modulators have been instrumental in identifying target substrates for activated cardiac PKC isozymes, as well as the identification of specific isozymes linked to distinct growth characteristics and cell phenotype. The rapid growth of knowledge in the area of PKC signaling and PKC substrate interactions, may result in the development of therapeutic modalities with the potential to arrest or reverse the progression of cardiovascular diseases.

Angiotensin II promotes glucose-induced activation of cardiac protein kinase C isozymes and phosphorylation of troponin I

Activation of the protein kinase C (PKC) family is a potential signaling mechanism by which high ambient glucose concentration modulates the phenotype and physiological function of cells. Recently, the cardiac renin angiotensin system (RAS) has been reported to promote PKC translocation in the diabetic heart via the angiotensin (ANG) II type 1 receptor (AT-1R). To evaluate the molecular events coupled with high glucose-induced PKC translocation and to examine the role of endogenously released ANG II in myocyte PKC signaling, primary cultures of adult rat ventricular myocytes were exposed to normal (5 mmol/l) or high (25 mmol/l) glucose for 12-24 h. Western blot analysis indicated that adult rat ventricular myocytes coexpress six PKC isozymes (alpha, beta(1,) beta(2,) delta, epsilon, and zeta). Translocation of five PKC isozymes (beta(1), beta(2), delta, epsilon, and zeta) was detected in response to 25 mmol/l glucose. Inhibition of phospholipase C with tricyclodecan-9-yl-xanthogenate blocked glucose-induced translocation of PKC-beta(2), -delta, and -zeta. Inhibition of tyrosine kinase with genistein blocked glucose-induced translocation of PKC-beta(1) and -delta, whereas chelation of intracellular Ca(2+) with 1,2-bis(2-aminophenoxy)ethane N,N,N,'N'-tetraacetic acid blocked translocation of PKC-beta(1) and -beta(2). Enzyme-linked immunosorbent assay performed on culture media from myocytes maintained in 25 mmol/l glucose detected a twofold increase in ANG II. Addition of an AT-1R antagonist (losartan; 100 nmol/l) to myocyte cultures blocked translocation of PKC-beta(1), -beta(2), -delta, and -epsilon. Phosphorylation of troponin (Tn) I was increased in myocytes exposed to 25 mmol/l glucose. Losartan selectively inhibited Tn I serine phosphorylation but did not affect phosphorylation at threonine residues. We concluded that 1) 25 mmol/l glucose triggers the release of ANG II by myocytes, resulting in activation of the ANG II autocrine pathway; 2) differential translocation of myocyte PKC isozymes occurs in response to 25 mmol/l glucose and ANG II; and 3) AT-1R-dependent PKC isozymes (beta(1), beta(2), delta, and epsilon) target Tn I serine residues.

Hyperglycemia and the O-GlcNAc transferase in rat aortic smooth muscle cells: elevated expression and altered patterns of O-GlcNAcylation

Hyperglycemia leads to vascular disease specific to diabetes mellitus. This pathology, which results from abnormal proliferation of smooth muscle cells in arterial walls, may lead to cataract, renal failure, and atherosclerosis. The hexosamine biosynthetic pathway is exquisitely responsive to glucose concentration and plays an important role in glucose-induced insulin resistance. UDP-GlcNAc: polypeptide O-N-acetylglucosaminyltransferase (O-GlcNAc transferase; OGTase) catalyzes the O-linked attachment of single GlcNAc moieties to serine and threonine residues on many cytosolic or nuclear proteins. Polyclonal antibody against OGTase was used to examine the expression of OGTase in rat aorta and aortic smooth muscle (RASM) cells. OGTase enzymatic activity and expression at the mRNA and protein levels were determined in RASM cells cultured at normal (5 mM) and at high (20 mM) glucose concentrations. OGTase mRNA and protein are expressed in both endothelial cells and smooth muscle cells in the aorta of normal rats. In both cell types, the nucleus is intensely stained, while the cytoplasm stains diffusely. Immunoelectron microscopy shows that OGTase is localized to euchromatin and around the myofilaments of smooth muscle cells. In RASM cells grown in 5 mM glucose, OGTase is also located mainly in the nucleus. Hyperglycemic RASM cells also display a relative increase in OGTase's p78 subunit and an overall increase protein and activity for OGTase. Biochemical analyses show that hyperglycemia qualitatively and quantitatively alters the glycosylation or expression of many O-GlcNAc-modified proteins in the nucleus. These results suggest that the abnormal O-GlcNAc modification of intracellular proteins may be involved in glucose toxicity to vascular tissues.

Regulation of B(2)-kinin receptors by glucose in vascular smooth muscle cells

The development of vascular disease is accelerated in hyperglycemic states. Vascular injury plays a pivotal role in the progression of atherosclerotic vascular disease in diabetes, which is characterized by increased vascular smooth muscle cell (VSMC) proliferation and extracellular matrix accumulation. We previously reported that diabetes alters the activity of the kallikrein-kinin system and results in the upregulation of kinin receptors in the vessel wall. To determine whether glucose can directly influence the regulation of kinin receptors, the independent effect of high glucose (25 mM) on B(2)-kinin receptors (B2KR) in VSMC was examined. A threefold increase in B2KR protein levels and a 40% increase in B2KR surface receptors were observed after treatment with high glucose after 24 h. The mRNA levels of B2KR were also significantly increased by high glucose as early as 4 h later. To elucidate the cellular mechanisms by which glucose regulates B2KR, we examined the role of protein kinase C (PKC). High glucose increased total PKC activity and resulted in the translocation of conventional PKC isoforms (beta(1) and beta(2)), novel (epsilon), and atypical (zeta) PKC isoforms into the membrane. Inhibition of PKC activity prevented the increase in B2KR levels induced by ambient high glucose. These findings provide the first evidence that glucose regulates the expression of B(2) receptors in VSMC and provide a rationale to further study the interaction between glucose and kinins on the pathogenesis of atherosclerotic vascular disease in diabetes.

Hexosamine regulation of glucose-mediated laminin synthesis in mesangial cells involves protein kinases A and C

Hyperglycemia leads to alterations in mesangial cell function and extracellular matrix (ECM) protein accumulation. These adverse effects of glucose may be mediated by glucose metabolism through the hexosamine biosynthesis pathway (HBP). The HBP converts fructose-6-phosphate to glucosamine-6-phosphate via the rate-limiting enzyme, glutamine:fructose-6-phosphate amidotransferase (GFA). We have investigated the effects of high glucose (HG, 25 mM) and glucosamine (GlcN, 1.5 mM) on the synthesis of the ECM protein laminin in a SV-40-transformed rat kidney mesangial (MES) cell line. The roles of protein kinases C (PKC) and A (PKA) in mediating laminin accumulation were also investigated. Treatment of MES cells with HG or GlcN for 48 h increased laminin levels in cellular extracts more than twofold compared with 5 mM glucose (low glucose; LG). The presence of the GFA inhibitor diazo-oxo-norleucine (DON, 10 microM) blocked HG but not GlcN-induced laminin synthesis. HG resulted in a time-dependent increase in total PKC and PKA activities, 57+/-11.3 (P < 0.01 vs. LG) and 85+/-17.4% (P < 0.01 vs. LG), respectively. GlcN had no effect on the total PKC activity; however, both glucose and glucosamine increased membrane-associated PKC activity by twofold compared with LG. GlcN stimulated total PKA activity by 47+/-8.4% (P < 0.01 vs. LG). Similarly, membrane- associated PKA activity was also increased by HG and GlcN approximately 1.8 and 1.5-fold, respectively. HG and GlcN increased cellular cAMP levels 2.2- and 3. 4-fold, respectively. Pharmacological downregulation of PKC by long-term incubation of MES cells with 0.5 microM phorbol 12-myristate 13-acetate (PMA) or inhibition of PKA activity by 2 microM H-8 blocked the effects of HG and GlcN on laminin synthesis. These results demonstrate that glucose-induced laminin synthesis in MES cells is mediated by flux through the HBP and that this stimulation involves PKC and PKA signaling pathways.

Hyperglycemia inhibits vascular smooth muscle cell apoptosis through a protein kinase C-dependent pathway

We hypothesized that the pathogenesis of diabetic vasculopathy involves the abnormal regulation of vascular smooth muscle cell (VSMC) apoptosis. In nondiabetic mice, a reduction in carotid artery blood flow resulted in a significant loss of medial VSMCs via apoptosis (normal flow 84+/-1 viable VSMCs, reduced flow 70+/-5 viable VSMCs; n=12, P:<0.01). In contrast, flow-induced VSMC apoptosis was markedly attenuated in streptozotocin-induced diabetic mice (normal flow 85+/-2 viable VSMC, reduced flow 82+/-4 viable VSMC; n=13, NS). In accord with our in vivo findings, the exposure of cultured rat and human VSMCs to high glucose (17.5 mmol/L) significantly attenuated the induction of apoptosis in response to serum withdrawal (rat VSMCs in normal [5.5 mmol/L] glucose 28+/-1%, high D-glucose 19+/-2%; P:<0.0001). High glucose also inhibited apoptosis induced by Fas ligand (100 ng/mL) (normal 23+/-2%, high D-glucose 13+/-2%; P:<0.006). Supplementation with the nonmetabolized enantiomer L-glucose had no effect. We confirmed reports that high glucose activates protein kinase C (PKC) and demonstrated that PKC blockade with long-term phorbol ester treatment or calphostin C prevented the antiapoptotic effect (P:<0. 001). Moreover, the upregulation of either PKCalpha or PKCbetaII expression was sufficient to inhibit serum withdrawal-induced apoptosis (control 25+/-2%, PKCalpha 11+/-2%, PKCbetaII 8+/-2%; P:<0. 0001), whereas the upregulation of PKCdelta had no significant effect. Taken together, these findings demonstrate that hyperglycemia inhibits VSMC apoptosis via a PKC-dependent pathway.

Acute glucose-induced downregulation of PKC-betaII accelerates cultured VSMC proliferation

Accelerated vascular smooth muscle cell (VSMC) proliferation contributes to the formation of atherosclerotic lesions. To investigate protein kinase C (PKC)-betaII functions with regard to glucose-induced VSMC proliferation, human VSMC from aorta (AoSMC), a clonal VSMC line of rat aorta (A10), and A10 cells overexpressing PKC-betaI (betaI-A10) and PKC-betaII (betaII-A10) were studied with the use of three techniques to evaluate glucose effects on aspects affecting proliferation. High glucose (25 mM) increased DNA synthesis and accelerated cell proliferation compared with normal glucose (5.5 mM) in AoSMC and A10 cells, but not in betaI-A10 and betaII-A10 cells. The PKC-betaII specific inhibitor CGP-53353 inhibited glucose-induced cell proliferation and DNA synthesis in AoSMC and A10 cells. In flow cytometry analysis, high glucose increased the percentage of A10 cells at 12 h after cell cycle initiation but did not increase the percentage of betaI-A10 or betaII-A10 cells entering S phase. PKC-betaII protein levels decreased before the peak of DNA synthesis, and high glucose further decreased PKC-betaII mRNA and protein levels in AoSMC and A10 cells. These results suggest that high glucose downregulates endogenous PKC-betaII, which then alters the normal inhibitory role of PKC-betaII in cell cycle progression, resulting in the stimulation of VSMC proliferation through acceleration of the cell cycle.

High glucose induces the activity and expression of Na(+)/H(+) exchange in glomerular mesangial cells

Changes in activity or expression of transporters may account for alterations in cell behavior in diabetes. We sought to ascertain if mesangial cells (MC) grown in different glucose concentrations exhibit changes in activity and expression of acid-extruding transporters, the Na(+)/H(+) and Na(+)-dependent Cl(-)/HCO(-)(3) exchanger. pH(i) was determined by the use of the fluorescent pH-sensitive dye BCECF. In MCs grown in 5 mM glucose (control), the Na(+)/H(+) exchanger was responsible for 31.8 +/- 5.1% of steady-state pH(i), whereas Na(+)-dependent Cl(-)/HCO(-)(3) contributed 62.9 +/- 4.0% (n = 11). In MCs grown in high glucose for 2 wk, Na(+)/H(+) exchange contribution to acid-extrusion increased as follows: 42.3 +/- 4.6% [n = 8, 10 mM, not significant (NS)], 51.1 +/- 5.1% (n = 8, 20 mM, P < 0.01), and 64.8 +/- 5.5% (n = 7, 30 mM, P < 0.001). The Na(+)-dependent Cl(-)/HCO(-)(3) exchanger contributed less [47.0 +/- 4.6, 38.6 +/- 5.8, and 21.1 +/- 3.8%, for 10, 20, and 30 mM glucose, respectively (n > 7)]. We sought to ascertain if the magnitude of the acute stimulated response to ANG II by the Na(+)/H(+) and Na(+)-dependent Cl(-)/HCO(-)(3) exchanger is changed. Na(+)/H(+) exchanger (1.89-fold increase in 30 vs. 5 mM, P < 0.002), but not Na(+)-dependent Cl(-)/HCO(-)(3) exchange (0. 17-fold, NS), exhibited an enhanced response to ANG II (1 microM). Na(+)/H(+) exchange (NHE1) expression was significantly different (1. 72-fold) after prolonged exposure to high glucose. These results suggest that the Na(+)/H(+) exchanger, but not Na(+)-dependent Cl(-)/HCO(-)(3) exchanger, may play an early role in the response to hyperglycemia in the diabetic state.

Urokinase-induced mitogenesis is mediated by casein kinase 2 and nucleolin

BACKGROUND

Urokinase (uPA) and the urokinase receptor (uPAR) form a multifunctional system capable of concurrently regulating pericellular proteolysis, cell-surface adhesion, and mitogenesis. The role of uPA and uPAR in directed proteolysis is well established and its function in cellular adhesiveness has recently been clarified by numerous studies. The molecular mechanisms underlying the mitogenic effects of uPA and uPAR are still unclear, however.

RESULTS

We identified mechanisms that might participate in uPA-related mitogenesis in human vascular smooth muscle cells and demonstrated that uPA induces activation of a unique signaling complex. This complex contains uPAR and two additional proteins, nucleolin and casein kinase 2, which are implicated in cell proliferation. Both proteins were isolated by affinity chromatography on uPA-conjugated cyanogen-bromide-activated Sepharose 4B and were identified using nano-electrospray mass spectrometry and immunoblotting. We used laser scanning and immunoelectron microscopy studies to further demonstrate that nucleolin and casein kinase 2 are located on the cell surface where they colocalize with the uPAR. Moreover, the proteins were co-internalized into the cell as an entire complex. Immunoprecipitation experiments in combination with an in vitro kinase assay demonstrated a specific association of uPAR with nucleolin and casein kinase 2 and revealed a uPA-induced activation of casein kinase 2, which presumably led to phosphorylation of nucleolin. Blockade of nucleolin and casein kinase 2 with specific modulators led to the inhibition of uPA-induced cell proliferation.

CONCLUSIONS

We conclude that in human vascular smooth muscle cells, uPA induces the formation and activation of a newly identified signaling complex comprising uPAR, nucleolin, and casein kinase 2, that is responsible for the uPA-related mitogenic response. The complex is not a unique feature of vascular smooth muscle cells, as it was also found in other uPAR-expressing cell types.

Differential expression of protein kinase C isoforms in streptozotocin-induced diabetic rats

BACKGROUND

The cellular effects of hyperglycemia are mediated by protein kinase C (PKC). However, PKC consists of several distinct isoforms, and their contribution to the pathogenesis of diabetic complications in different organs is not clear. We investigated the expression and translocation of PKC isoforms alpha, betaI, betaII, delta, epsilon, and zeta in kidney, heart, and aorta from diabetic rats.

METHODS

Hyperglycemia was induced with streptozotocin (70 mg/kg) in the rat. After four weeks, PKC isoform expression was assessed by Western blot after tissue fractionation and by immunohistochemistry.

RESULTS

Streptozotocin increased blood glucose from 117.0 +/- 3.6 to 510.0 +/- 19.4 mg/dl (N = 8, P < 0.01) and induced albuminuria. PKC isoforms alpha, beta, delta, epsilon, and zeta were all detected in control animals. Western blot showed increased PKC alpha expression in kidney and heart (160% and 170%, respectively). PKC betaI, betaII, and delta expression was not influenced by hyperglycemia. PKC zeta was decreased in diabetic animals in both tissues by 60%. The membrane association of PKC alpha and PKC epsilon was increased; however, the relative amount of PKC in the particulate fraction was not influenced by hyperglycemia. Immunohistochemistry revealed a marked increase in PKC alpha immunoreactivity in renal glomeruli and interstitial capillaries, cardiac capillaries, and skeletal muscle, as well as in the endothelial cells of larger arteries. PKC beta showed a small decrease in the glomeruli. PKC epsilon was increased in renal tubules in diabetic rats but was decreased in the myocardium. PKC zeta was expressed in both myocardial and glomerular cells but was decreased during hyperglycemia. Our results demonstrate that PKC isoforms are differentially regulated in kidney and heart in diabetes. High glucose increases PKC alpha expression, whereas PKC zeta is down-regulated. The finding that PKC alpha is mostly increased in endothelial cells supports a role for PKC alpha in functional endothelial disturbances observed in diabetes.

Requirement for protein kinase C in reactive oxygen species-induced apoptosis of vascular smooth muscle cells

BACKGROUND

Vascular smooth muscle cell (VSMC) apoptosis is a component of a variety of cardiovascular diseases and may be related to reactive oxygen species (ROS). This study was designed to determine the role of protein kinase C (PKC) in ROS-induced VSMC apoptosis.

METHODS AND RESULTS

Rat aortic VSMCs were exposed to H(2)O(2), and the nature of cell death was characterized in the absence or presence of different PKC inhibitors. The results demonstrate that exposure of VSMCs to H(2)O(2) led to a dose-dependent (25 to 100 micromol/L) and time-dependent (peak at 2 minutes) activation of PKC. Among the PKC isoforms alpha, beta, delta, epsilon, and zeta, only PKC-alpha and PKC-epsilon were found to change their intracellular distribution on H(2)O(2) treatment. Apoptosis was the predominant form of cell death when PKC had been activated by H(2)O(2) alone or by H(2)O(2) in the presence of 50 nmol/L phorbol 12-myristate 13-acetate. In contrast, necrosis became the predominant form of cell death when PKC had been downregulated by prolonged exposure to 200 nmol/L phorbol 12,13-dibutyrate or inhibited by 50 nmol/L staurosporine, 100 nmol/L calphostin C, or 30 micromol/L H-7. In addition, caspase-3 was activated in H(2)O(2)-induced VSMC apoptosis but not when PKC was downregulated or inhibited. Inhibition of caspase-3 by 50 micromol/L Ac-DEVD-CHO could significantly attenuate H(2)O(2)-induced apoptosis and was not associated with the induction of necrosis.

CONCLUSIONS

We conclude that in VSMCs, PKC converts the ROS-induced signals from necrotic cell death to the activation of an apoptotic cell death program. These data imply a novel and important role of PKC for the pathogenesis of such vascular diseases as atherosclerosis, restenosis, and hypertension.

Effects of advanced glycation end products on cytosolic Ca2+ signaling of cultured human mesangial cells

Advanced glycation end product (AGE) accumulation in a high glucose (HG) environment is thought to mediate some of the vascular complications of diabetes. Transmembrane signaling of contractile cells is generally inhibited by HG, with implications for systemic and target organ hemodynamics. In the kidney, glomerular mesangial cells grown in HG media are hyporesponsive to the effects of vasoconstrictor agents, possibly explaining the hyperfiltration and increased capillary pressure that eventually lead to diabetic glomerulopathy. To verify whether AGE binding to specific mesangial receptors could mediate these effects of HG, cultured human mesangial cells (HMC) were exposed to in vitro glycated bovine serum albumin (BSA) for 60 min at 37 degrees C before measurement of cytosolic Ca2+ ([Ca2+]i) by microfluorometric techniques in monolayers or single cells. AGE-BSA (2 mg/ml) reduced Ca2+ release from intracellular stores by 1 microM angiotensin II from peak [Ca2+]i levels of 843+/-117 to 390+/-50 nM in monolayers and from 689+/-68 to 291+/-36 nM in individual cells (P < 0.05). Nonglycated BSA and BSA exposed to 250 mM glucose-6-phosphate for 30 d in the presence of 250 mM aminoguanidine (AMGD), an inhibitor of nonenzymatic glycation, had no effect on the angiotensin II-induced [Ca2+]i spike (peak 766+/-104 and 647+/-87 nM, monolayers/ single cells, respectively, P = NS). AGE also inhibited store-operated Ca2+ influx through plasma membrane channels, assessed by addition of 1 to 10 mM extracellular Ca2+ to cells previously held in Ca2(+)-free media (control 339+/- 46/593 +/- 51, +AGE-BSA 236 +/- 25/390 +/- 56, +AMGD 483+/-55/ 374+/-64 nM [Ca2+]i, monolayers/single cells at 10 mM Ca2+, respectively; +AGE-BSA, P < 0.05 versus control). Contrary to HG, AGE-BSA did not translocate protein kinase C isoforms alpha, zeta, and delta to the plasma membrane. Culture of HMC in HG supplemented with 1 mM AMGD prevented downregulation of [Ca2+]i signaling. These data suggest that glycated macromolecules or matrix components may inhibit transmembrane Ca2+ signaling of glomerular cells through binding to a specific AGE receptor, thus mediating some of the known functional effects of HG on the kidney.

Effect of high glucose on mesangial cell protein kinase C-delta and -epsilon is polyol pathway-dependent

In diabetes mellitus, enhanced activity of mesangial cell protein kinase C (PKC) may contribute to nephropathy. The purpose of this study was to determine whether high glucose alters mesangial cell diacylglycerol-sensitive PKC-alpha, -beta2, -delta, and -epsilon content, cellular distribution, and activity through polyol pathway activation. Primary cultured rat mesangial cells (passage 10) were growth-arrested in 0.5% fetal bovine serum and cultured in 5.6 mM glucose (NG) or 30 mM glucose (HG) for 48 h, with or without the aldose reductase inhibitor tolrestat or ARI-509. PKC isoform content in total cell lysates, or cytosol, membrane (Triton X-soluble), and particulate (sodium dodecyl sulfate-soluble) fractions was analyzed by immunoblotting, and band density in HG was expressed as a percentage of corresponding NG values. In HG at 48 h, increased total PKC-alpha (222 +/- 17% of NG, P < 0.001), -beta2 (209 +/- 12%, P < 0.001), and -epsilon (195 +/- 19%, P < 0.001) were observed. L-Glucose had no effect on total PKC isoform content. HG caused increased membrane- and particulate-associated PKC-alpha (257 +/- 87 and 327 +/- 66%, respectively, P < 0.05), membrane-associated PKC-delta (143 +/- 10%, P < 0.05), and membrane-associated PKC-epsilon (186 +/- 11%, P < 0.001), with no change in cytosol contents. The HG effects were not mimicked by L-glucose. In NG or HG, PKC-beta2 was not detected in the cytosol fraction, and membrane and particulate association were unchanged with phorbol ester stimulation. Confocal immunofluorescence imaging revealed that in HG, PKC-alpha, -delta, and -epsilon translocate to the nucleus and plasma membrane. Total PKC activity measured by in situ 32P-phosphorylation of the epidermal growth factor receptor substrate increased from 18 +/- 1 pmol/min per mg cell protein in NG to 33 +/- 3 pmol/min per mg cell protein in HG (P < 0.002 versus NG). In NG, tolrestat and ARI-509 exposure caused increased PKC activity, enhanced accumulation of total PKC-alpha and -beta2, with no change in total or fractional recovery of PKC-delta or -epsilon. In HG, tolrestat and ARI-509 prevented the increase in total PKC-epsilon and membrane-associated PKC-delta and -epsilon. It is concluded that within 48 h of HG, enhanced mesangial cell PKC activity is associated with accumulation and cellular redistribution of diacylglycerol-sensitive PKC isoforms, and that increased PKC-epsilon content and membrane-associated PKC-delta and -epsilon are dependent on polyol pathway activation.

Calcium antagonists ameliorate ischemia-induced endothelial cell permeability by inhibiting protein kinase C

BACKGROUND

Dihydropyridines block calcium channels; however, they also influence endothelial cells, which do not express calcium channels. We tested the hypothesis that nifedipine can prevent ischemia-induced endothelial permeability increases by inhibiting protein kinase C (PKC) in cultured porcine endothelial cells.

METHODS AND RESULTS

Ischemia was induced by potassium cyanide/deoxyglucose, and permeability was measured by albumin flux. Ion channels were characterized by patch clamp. [Ca2+]i was measured by fura 2. PKC activity was measured by substrate phosphorylation after cell fractionation. PKC isoforms were assessed by Western blot and confocal microscopy. Nifedipine prevented the ischemia-induced increase in permeability in a dose-dependent manner. Ischemia increased [Ca2+]i, which was not affected by nifedipine. Instead, ischemia-induced PKC translocation was prevented by nifedipine. Phorbol ester also increased endothelial cell permeability, which was dose dependently inhibited by nifedipine. The effects of non-calcium-channel-binding dihydropyridine derivatives were similar. Analysis of the PKC isoforms showed that nifedipine prevented ischemia-induced translocation of PKC-alpha and PKC-zeta. Specific inhibition of PKC isoforms with antisense oligodeoxynucleotides demonstrated a major role for PKC-alpha.

CONCLUSIONS

Nifedipine exerts a direct effect on endothelial cell permeability that is independent of calcium channels. The inhibition of ischemia-induced permeability by nifedipine seems to be mediated primarily by PKC-alpha inhibition. Anti-ischemic effects of dihydropyridine calcium antagonists could be due in part to their effects on endothelial cell permeability.

High glucose alters the response of mesangial cell protein kinase C isoforms to endothelin-1

BACKGROUND

High glucose causes glomerular mesangial growth and increased matrix synthesis contributing to diabetic glomerulopathy. Our purpose was to determine if high glucose alters endothelin-1 (ET-1) or platelet-derived growth factor-B activation of mesangial cell diacylglycerol-sensitive protein kinase C (PKC) isoforms and subsequent stimulation of mitogen-activated protein kinase (MAPK; p42, p44).

METHODS

Rat mesangial cells in primary culture were growth arrested for 48 hours in glucose 5.6 mM (NG) or 30 mM (HG). PKC-alpha, PKC-delta, and PKC-epsilon translocation from the cytosol-to-membrane and cytosol-to-particulate (cytoskeleton, nucleus) cellular fractions were measured by immunoblot using isoform-specific monoclonal antibodies. PKC isoforms were visualized also by confocal immunofluorescence microscopy. MAPK activation was measured by immunoblot using phospho-MAPK antibody and by detection of Elk-1 fusion protein phosphorylation following phospho-MAPK immunoprecipitation.

RESULTS

In NG, ET-1 stimulated cytosol-to-membrane translocation of PKC-delta and PKC-epsilon but not PKC-alpha. In HG, the pattern of ET-1-stimulated PKC-delta and PKC-epsilon changed to a cytosol-to-particulate distribution, which was confirmed by confocal immunofluorescence imaging. Platelet-derived growth factor-B did not cause translocation of PKC-alpha, PKC-delta, or PKC-epsilon in either NG or HG. In HG, both basal and ET-1-stimulated MAPK activities were increased significantly. In HG, down-regulation of PKC isoforms with phorbol ester prevented the increased stimulation of MAPK by ET-1.

CONCLUSION

In HG, the enhanced activation of mesangial cell MAPK by ET-1 is PKC dependent and associated with altered translocation of PKC-delta and PKC-epsilon. Enhanced mesangial cell signaling responsiveness to vasoactive peptides in HG may constitute an important mechanism contributing to diabetic nephropathy.

Acute hyperglycemia regulates transcription and posttranscriptional stability of PKCbetaII mRNA in vascular smooth muscle cells

Acute hyperglycemia may contribute to the progression of atherosclerosis by regulating protein kinase C (PKC) isozymes and by accelerating vascular smooth muscle cell (VSMC) proliferation. We investigated acute glucose regulation of PKCbeta gene expression in A10 cells, a rat aortic smooth muscle cell line. Western blot analysis showed that PKCbetaII protein levels decreased with high glucose (25 mM) compared to normal glucose (5.5 mM), whereas PKCbetaI levels were unaltered. PKCbeta mRNA levels were depleted by 60-75% in hyperglycemic conditions. To elucidate whether high glucose regulated PKCbeta expression via the common promoter for PKCbetaI and PKCbetaII, deletion constructs of the PKCbeta promoter ligated to CAT as reporter gene were transfected into A10 cells. Construct D (-411 to +179CAT) showed quenching in high glucose (25 mM) and suggested the involvement of a carbohydrate response element in the 5' promoter region of the PKCbeta gene. In actinomycin D-treated A10 cells, a 60% decrease in PKCbeta mRNA with high glucose treatment indicated that posttranscriptional destabilization by glucose was also occurring. We have demonstrated that glucose-induced posttranscriptional destabilization of PKCbetaII message is mediated via a nuclease activity present in the cytosol. The specificity of glucose to posttranscriptionally destabilize PKCbetaII mRNA, but not the PKCbetaI mRNA, was confirmed in both A10 cells and primary cultures from human aorta.

Glucose or diabetes activates p38 mitogen-activated protein kinase via different pathways

Hyperglycemia can cause vascular dysfunctions by multiple factors including hyperosmolarity, oxidant formation, and protein kinase C (PKC) activation. We have characterized the effect of hyperglycemia on p38 mitogen-activated protein (p38) kinase activation, which can be induced by oxidants, hyperosmolarity, and proinflammatory cytokines, leading to apoptosis, cell growth, and gene regulation. Glucose at 16.5 mM increased p38 kinase activity in a time-dependent manner compared with 5.5 mM in rat aortic smooth muscle cells (SMC). Mannitol activated p38 kinase only at or greater than 22 mM. High glucose levels and a PKC agonist activated p38 kinase, and a PKC inhibitor, GF109203X, prevented its activation. However, p38 kinase activation by mannitol or tumor necrosis factor-alpha was not inhibited by GF109203X. Changes in PKC isoform distribution after exposure to 16.5 mM glucose in SMC suggested that both PKC-beta2 and PKC-delta isoforms were increased. Activities of p38 kinase in PKC-delta- but not PKC-beta1-overexpressed SMC were increased compared with control cells. Activation of p38 kinase was also observed and characterized in various vascular cells in culture and aorta from diabetic rats. Thus, moderate hyperglycemia can activate p38 kinase by a PKC-delta isoform-dependent pathway, but glucose at extremely elevated levels can also activate p38 kinase by hyperosmolarity via a PKC-independent pathway.

The proliferative effect of vascular endothelial growth factor requires protein kinase C-alpha and protein kinase C-zeta

The heparin-binding protein vascular endothelial growth factor (VEGF) is a highly specific growth factor for endothelial cells. VEGF binds to specific tyrosine kinase receptors, which mediate intracellular signaling. We investigated 2 hypotheses: (1) VEGF affects intracellular calcium [Ca2+]i regulation and [Ca2+]i-dependent messenger systems; and (2) these mechanisms are important for VEGF's proliferative effects. [Ca2+]i was measured in human umbilical vein endothelial cells using fura-2 and fluo-3. Protein kinase C (PKC) activity was measured by histone-like pseudosubstrate phosphorylation. PKC isoform distribution was observed with confocal microscopy and Western blot. Inhibition of PKC isoforms was assessed by specific antisense oligonucleotides (ODN) for the PKC isoforms. VEGF (10 ng/mL) induced a transient increase in [Ca2+]i followed by a sustained elevation. The sustained [Ca2+]i plateau was abolished by EGTA. Pertussis toxin also abolished the plateau phase, whereas the initial peak was not affected. The PKC isoforms alpha, delta, epsilon, and zeta were identified in endothelial cells. VEGF induced a translocation of PKC-alpha and PKC-zeta toward the nucleus and the perinuclear area, whereas cellular distribution of PKC-delta and PKC-epsilon was not influenced. Cell exposure to TPA led to a down-regulation of PKC-alpha and reduced the proliferative effect of VEGF. VEGF-induced endothelial cell proliferation also was reduced by the PKC inhibitors staurosporine and calphostin C. Specific down-regulation of PKC-alpha and PKC-zeta with antisense ODN reduced the proliferative effect of VEGF significantly. Our data show that VEGF induces initial and sustained Ca2+ influx. VEGF leads to the translocation of the [Ca2+]i-sensitive PKC isoform alpha and the atypical PKC isoform zeta. Antisense ODN for these PKC isoforms block VEGF-induced proliferation. These findings suggest that PKC isoforms alpha and zeta are important for VEGF's angiogenic effects.

PKC-dependent signalling mechanisms in differentiated smooth muscle

Protein kinase C (PKC) is now known to play an important physiological role in essentially all cell types. This review will focus on what is known about the kinase in contractile differentiated smooth muscle. Current knowledge on the molecular structure of PKC isoforms will be discussed as they relate to mechanisms of translocation and targeting of the kinase within smooth muscle cells. Studies performed on PKC-dependent signalling pathways in differentiated smooth muscle cells will be discussed with emphasis on studies form our laboratory, especially discussing thin filament linked pathways. Thick filament linked PKC-dependent pathways will be described in more detail elsewhere in this monograph.

The clinical importance of postprandial glucose

Atherosclerotic lesions develop over a long period of time and result from complex changes in the arterial wall. Although these changes are not fully understood, there is much evidence to suggest that elevated plasma glucose levels contribute to the development of atherosclerotic lesions. Many studies have shown that there is a strong correlation between elevated plasma glucose levels and the risk of developing cardiovascular disease. Effects of glucose on the arterial wall include immediate effects, which occur rapidly in response to elevated plasma glucose levels, and long-term effects, which result from non-enzymatic glycosylation of various proteins. These adverse effects of elevated plasma glucose levels suggest that tight control of blood glucose levels in patients with diabetes could possibly reduce the risk of cardiovascular complications. This is borne out by the results of clinical studies in patients with type 1 diabetes. Therapy to reduce blood glucose levels may also be appropriate in individuals with impaired glucose tolerance, as this condition is associated with postprandial hyperglycaemia and a significant risk of developing cardiovascular disease.

Protein kinase C isoforms in human aortic smooth muscle cells

PURPOSE

To identify the protein kinase C (PKC) isoforms in human arterial smooth muscle cells (SMC) and define their subcellular location in the resting state and in response to the PKC activator, 12-O-tetradecanoylphorbol 13-acetate (TPA).

METHODS

Arterial SMC cultures established from transplant donor aorta were treated with 100 nM TPA or control media, then mechanically lysed. PKC from the soluble and particulate fraction were separated by centrifugation, and protein normalized immunoblots were performed with antibodies to the PKC isoforms alpha, betaI, betaII, delta, epsilon, gamma and zeta. Bands were detected by enhanced chemiluminescence and analyzed densitometrically, with results expressed as the mean percentage of each fraction +/- SEM. Translocation was defined as a significant (p < 0.05) change in the particulate fraction for each isoform. Immunofluorescent staining of cultured SMC visualized the resting location and stimulated translocation of each isoform.

RESULTS

Isoforms alpha and betaI were detected primarily in the soluble fraction, translocating to the particulate fraction with TPA stimulation (p < 0.0001). The isoforms betaII, delta, and epsilon were found primarily in the particulate fraction and did not translocate. Immunofluorescent staining confirmed these locations. Neither gamma or zeta were detected in these SMC.

CONCLUSIONS

The PKC isoforms expressed in human arterial SMC differ from those reported in animal models. Their specific locations and response to stimulation suggest unique functions in cellular regulation and provide the groundwork for further investigation into their role in the development of vascular disease and regulation of matrix metabolism.

Leukocyte-endothelial interaction is augmented by high glucose concentrations and hyperglycemia in a NF-kB-dependent fashion

We addressed the role of hyperglycemia in leukocyte-endothelium interaction under flow conditions by exposing human umbilical vein endothelial cells for 24 h to normal (5 mM), high concentration of glucose (30 mM), advanced glycosylation end product-albumin (100 microg/ml), or hyperglycemic (174-316 mg/dl) sera from patients with diabetes and abnormal hemoglobin A1c (8.1+/-1.4%). At the end of incubation endothelial cells were perfused with total leukocyte suspension in a parallel plate flow chamber under laminar flow (1.5 dyn/cm2). Rolling and adherent cells were evaluated by digital image processing. Results showed that 30 mM glucose significantly (P < 0. 01) increased the number of adherent leukocytes to endothelial cells in respect to control (5 mM glucose; 151+/-19 versus 33+/-8 cells/mm2). A similar response was induced by endothelial stimulation with IL-1beta, here used as positive control (195+/-20 cells/mm2). The number of rolling cells on endothelial surface was not affected by high glucose level. Stable adhesion of leukocytes to glucose-treated as well as to IL-1beta-stimulated endothelial cells was preceded by short interaction of leukocytes with the endothelial surface. The distance travelled by leukocytes before arrest on 30 mM glucose, or on IL-1beta-treated endothelial cells, was significantly (P < 0.01) higher than that observed for leukocytes adhering on control endothelium (30 mM glucose: 76.7+/-3.5; IL1beta: 69.7+/-4 versus 5 mM glucose: 21.5+/-5 microm). Functional blocking of E-selectin, intercellular cell adhesion molecule-1, and vascular cell adhesion molecule-1 on endothelial cells with the corresponding mouse mAb significantly inhibited glucose-induced increase in leukocyte adhesion (67+/-16, 83+/-12, 62+/-8 versus 144+/-21 cells/ mm2). Confocal fluorescence microscopy studies showed that 30 mM glucose induced an increase in endothelial surface expression of E-selectin, intercellular cell adhesion molecule-1, and vascular cell adhesion molecule-1. Electrophoretic mobility shift assay of nuclear extracts of human umbilical vein endothelial cells (HUVEC) exposed for 1 h to 30 mM glucose revealed an intense NF-kB activation. Treatment of HUVEC exposed to high glucose with the NF-kB inhibitors pyrrolidinedithiocarbamate (100 microM) and tosyl-phe-chloromethylketone (25 microM) significantly reduced (P < 0.05) leukocyte adhesion in respect to HUVEC treated with glucose alone. A significant (P < 0.01) inhibitory effect on glucose-induced leukocyte adhesion was observed after blocking protein kinase C activity with staurosporine (5 nM). When HUVEC were treated with specific antisense oligodesoxynucleotides against PKCalpha and PKCepsilon isoforms before the addition of 30 mM glucose, a significant (P < 0.05) reduction in the adhesion was also seen. Advanced glycosylation end product-albumin significantly increased the number of adhering leukocytes in respect to native albumin used as control (110+/-16 versus 66+/-7, P < 0.01). Sera from diabetic patients significantly (P < 0.01) enhanced leukocyte adhesion as compared with controls, despite normal levels of IL-1beta and TNFalpha in these sera. These data indicate that high glucose concentration and hyperglycemia promote leukocyte adhesion to the endothelium through upregulation of cell surface expression of adhesive proteins, possibly depending on NF-kB activation.

Functional activity and transmembrane signaling mechanisms after cryopreservation of human internal mammary arteries

PURPOSE

Cryopreserved human blood vessels are important tools in bypass surgery. However, several in vitro studies have demonstrated diminished postthaw functional activity. Therefore the aim of this study was to investigate the consequences of various freezing/thawing protocols and the role of protein kinase C in the postthaw functional activity of cryopreserved human arteries.

METHODS

In vitro responses of frozen/thawed human internal mammary arteries (IMA) were used to investigate the functional activity after thawing at 15 degrees, 30 degrees, and 100 degrees C/min and after different prefreezing equilibration times (10, 60, 120, 240 minutes) with the cryomedium (Krebs-Henseleit solution containing 1.8 mol/L dimethyl sulfoxide and 0.1 mol/L sucrose) at room temperature followed by cryostorage at -196 degrees C.

RESULTS

Prefreezing equilibration for 10 to 120 minutes diminished maximal alpha-adrenoceptor-mediated responses to noradrenaline to approximately 60%, and equilibration for 240 minutes attenuated noradrenaline effects to less than 25% of that produced by unfrozen controls. Contractile responses were slightly better when thawing was performed at 15 degrees C/min compared with 100 degrees C/min. The postthaw sensitivity to direct activation of protein kinase C by phorbol 12,13-dibutyrate was enhanced. Compared with unfrozen tissues (pD2 = 7.36 +/- 0.07, n = 32) maximal sensitization to phorbol 12,13-dibutyrate was observed in IMA that had been frozen after 60 minutes of equilibration with the cryomedium (pD2 = 8.31 +/- 0.09, n = 30). Responses to phorbol 12,13-dibutyrate of cryopreserved IMA were highly susceptible to blockade of calcium influx by nifedipine, whereas those of unfrozen IMA were resistant to nifedipine. Against noradrenaline nifedipine was equipotent in cryopreserved (pD'2 = 7.75 +/- 0.15, n = 8) and unfrozen IMA (pD'2 = 7.70 +/- 0.10, n = 6). Endothelium-dependent relaxant responses to acetylcholine were significantly attenuated after cryopreservation (Emax = 26% +/- 5%, n = 4) compared with unfrozen IMA (Emax = 71% +/- 4%, n = 4, p < 0.001); endothelium-independent relaxant responses to sodium nitroprusside were unchanged.

CONCLUSIONS

Cryopreservation of human IMA under the conditions applied in this study (1) attenuated endothelial cell function and (2) induced an activation of protein kinase C, thereby increasing calcium influx through dihydropyridine-sensitive calcium channels. These experimental data suggest that postoperative administration of calcium channel blockers alone or combined with long-acting nitrates should effectively prevent the development of spasms in arterial grafts.

Integrin-induced protein kinase Calpha and Cepsilon translocation to focal adhesions mediates vascular smooth muscle cell spreading

The extracellular matrix influences the cellular spreading of vascular smooth muscle cells (VSMCs) via integrin receptors. However, the intracellular signaling mechanisms are still incompletely understood. We investigated the hypothesis that VSMCs binding to fibronectin activates the protein kinase C (PKC) pathway, causes differential intracellular PKC isoform translocation, and mediates cell spreading. VSMCs binding to poly-L-lysine or preincubated with Arg-Gly-Asp (RGD) peptides were used as controls. Diacylglycerol (DAG) and phospholipase D (PLD) activity were measured by thin-layer chromatography. Intracellular distribution of PKC isoforms was assessed by confocal microscopy. VSMCs binding to fibronectin induced focal adhesions and cell spreading within 30 minutes. Fibronectin induced a rapid increase in DAG content, peaking at 10 minutes with a sustained response for <1 hour. In contrast, PLD activity was not influenced by specific binding to fibronectin. PKC isoforms alpha, delta, epsilon, and zeta were assessed by confocal microscopy. Fibronectin induced a PKC isoform translocation to the cell nucleus and to focal adhesions within minutes. The nuclear PKCalpha immunoreactivity was transiently increased. PKC isoforms a and epsilon were both translocated to focal adhesions. The intracellular distributions of other PKC isoforms were not influenced by fibronectin. The effects of fibronectin on DAG generation, the translocation of PKCalpha and PKCepsilon, and cell spreading were all abolished by the incubation with RGD peptides. Downregulation of PKC isoforms alpha and epsilon with specific antisense oligodinucleotides resulted in a significant inhibition of cell spreading. Our results show that integrins induce intracellular signaling in VSMCs via DAG and PKC. PKC isoform a is translocated to the nucleus, whereas PKC isoforms alpha and epsilon are translocated to focal adhesions. Both isoforms seem to play a role in inside-out integrin signaling and cell spreading.

The Jak/Stat pathway and urokinase receptor signaling in human aortic vascular smooth muscle cells

The binding of urokinase plasminogen activator (uPA) to its specific receptor (uPAR) facilitates migration of vascular smooth muscle cells (VSMC). However, the signaling cascade utilized by the urokinase receptor is only incompletely understood. We investigated intracellular uPA/uPAR signaling in human aortic VSMC from the cell membrane to the nucleus. uPA binding to VSMC induced a rapid and pronounced increase in tyrosine phosphorylation of several proteins with molecular masses of 53-60, 85-90, and 130-140 kDa. By using co-immunoprecipitation techniques and in vitro kinase assays, the uPAR-associated proteins were identified as Janus (Jak) and Src non-receptor protein-tyrosine kinases (PTK) Jak1, Tyk2, and p59(fyn), p53/56(lyn), p53/59(hck), and p55(fgr). Furthermore, uPA induced a time-dependent reversible translocation of the Stat1 (signal transducer and activator of transcription) protein to the VSMC nuclei, as shown by confocal microscopy studies. Using an electrophoretic mobility shift assay, we then demonstrated that Stat1 is rapidly activated in response to stimulation with uPA and specifically binds to the DNA regulatory elements GAS (interferon-gamma activation site) and ISRE (interferon-stimulated response element). Mobility supershift experiments confirmed DNA-protein complexes containing Stat1 protein. Migration experiments with double immunofluorescence staining revealed polarization of uPAR, and colocalization with Jak1 and Tyk2 to the leading edge of the migrating cells. Under the same conditions, Jak2, Jak3, and the Src-PTKs remained randomly distributed over the entire body of the cells. Our studies therefore suggest that, in VSMC, the uPAR-signaling complex utilizes at least two different mechanisms, a direct signaling pathway utilizing the Jak/Stat cascade and a second signal transduction mechanism via Src-like protein-tyrosine kinases. uPA-induced signaling via Jak/Stat is most likely involved in the regulation of cell migration, while the functional purpose of the uPA-associated Src-PTK activation remains to be elucidated.

Calcium and protein kinase C mediate high-glucose-induced inhibition of inducible nitric oxide synthase in vascular smooth muscle cells

Abnormal vascular smooth muscle (VSMC) proliferation is a key feature in diabetes-associated atherosclerotic disease. Since nitric oxide inhibits VSMC tone, migration, adhesion, and proliferation, we examined the effects of high glucose on IL-1beta-induced NO release from VSMCs in culture. Confluent smooth muscle cells, preincubated with either 5 mmol/L (mM) or 20 mmol/L (mM) glucose for 48 hours, were stimulated with IL-1beta. Nitrite was measured in the culture medium after 24 hours. IL-1beta-induced a 15-fold increase in NO production in normal glucose medium. Glucose (10 to 30 mmol/L (mM)) significantly reduced the response to IL-1beta. High glucose (20 mmol/L (mM)) inhibited IL-1beta-evoked NO production by approximately 50%. IL-1beta-stimulated [3H] citrulline-forming activity of the nitric oxide synthase (NOS) was also significantly lower in high-glucose-exposed cells, and this was reflected in diminished cellular levels of NOS protein. To assess the role of protein kinase C (PKC), membrane PKC activity was measured, and glucose (20 mmol/L (mM)) significantly increased it. Immunoblotting of the membranes revealed a glucose-induced increase in the PKC betaII isoform. 1,2-Dioctanoyl-glycerol, a PKC activator, mimicked the high-glucose effect on IL-1beta-induced NO release, while staurosporine, a PKC inhibitor, reversed it. The role of calcium in the glucose-mediated inhibition of cytokine-induced NO release was determined by treatment with BAPTA, an intracellular chelator of calcium. BAPTA partially reversed the inhibitory effects of glucose. Increasing intracellular calcium by A23187, an ionophore or thapsigargin, an inhibitor of endoplasmic reticulum Ca2+-ATPase, significantly decreased IL-1beta-induced NO release and NOS expression. These results indicate that glucose-induced inhibition of IL-1beta-stimulated NO release and NOS expression may be mediated by PKC activation and increased intracellular calcium.

High glucose concentrations increase endothelial cell permeability via activation of protein kinase C alpha

Endothelial cell permeability is impaired in diabetes mellitus and may be increased by high extracellular glucose concentrations. High glucose activates protein kinase C (PKC), a family of kinases vital to intracellular signaling. We tested the hypothesis that high glucose concentration activates PKC in endothelial cells and leads to an increase in endothelial cell permeability via distinct PKC isoforms. Porcine aortic endothelial cells were used, and the PKC isoforms alpha, delta, epsilon, zeta, and theta were identified in these cells. Glucose caused a rapid dose-dependent increase in endothelial cell permeability, with an EC50 of 17.5 mmol/L. Phorbol 12-myristate 13-acetate (TPA) induced an increase in permeability very similar to that elicited by glucose. The effect of glucose and TPA was totally reversed by preincubating the cells with the PKC inhibitors staurosporine (10(-8) mol/L) and Goe 6976 (10(-8) mol/L). Downregulation of PKC by preincubation with TPA for 24 hours also abolished the effect of glucose and TPA on endothelial cell permeability. High glucose (20 mmol/L) caused an increase in PKC activity at 2, 10, and 30 minutes. Cell fractionation and Western blot analysis showed a glucose-induced translocation of PKC alpha and PKC epsilon. Confocal microscopy confirmed the translocation and showed an association of PKC alpha and PKC epsilon with nuclear structures and the cell membrane. Specific antisense oligodesoxynucleotides (ODNs) against PKC alpha reduced the expression of the isoform, abolished the effects of glucose on endothelial cell permeability completely, and reduced the TPA effect significantly. In contrast, specific antisense ODNs against PKC epsilon had no effect on glucose-induced permeability and only a minor effect on the TPA-induced increase in permeability. We conclude that an increase in extracellular glucose leads to a rapid dose-dependent increase in endothelial cell permeability via the activiation of PKC and that this effect is mediated by the PKC isoform alpha.