cGMP antagonizes angiotensin-mediated phosphatidylcholine hydrolysis and C kinase activation in mesangial cells

Natriuretic peptides suppress protein kinase C activity to reduce evoked dopamine efflux from pheochromocytoma (PC12) cells

The observation that natriuretic peptides and protein kinase C activators influence evoked neurotransmitter efflux by diametrically opposed mechanisms prompted an investigation of the influence of natriuretic peptides on protein kinase C activity and the potential involvement of this pathway in neuromodulatory responses to natriuretic peptides. C-Type natriuretic peptide attenuated both evoked dopamine efflux and protein kinase C activity in a concentration-dependent manner consistent with a 10% diminution in protein kinase C activity producing a 4.6-6.2% reduction in evoked dopamine efflux. The ability of C-type natriuretic peptide to suppress evoked dopamine efflux was abolished by treatment with the protein kinase C inhibitors chelerythrine (10 micro M) and staurosporine (10 nM). Both chelerythrine and staurosporine attenuated protein kinase C activity at the concentrations used. The natriuretic peptide C receptor (NPR-C) appeared to mediate the attenuation of protein kinase C activity, because the effect was mimicked by a pentadecapeptide fragment of the NPR-C, and the effect of C-type natriuretic peptide was attenuated by an antibody generated against the same region of the receptor. These data suggest that C-type natriuretic peptide attenuates neurotransmitter efflux by a mechanism involving suppression of neuronal protein kinase C activity via an interaction with the NPR-C.

High glucose stimulates synthesis of fibronectin via a novel protein kinase C, Rap1b, and B-Raf signaling pathway

The molecular mechanism(s) by which high glucose induces fibronectin expression via G-protein activation in the kidney are largely unknown. This investigation describes the effect of high glucose (HG) on a small GTP-binding protein, Rap1b, expression and activation, and the relevance of protein kinase C (PKC) and Raf pathways in fibronectin synthesis in cultured renal glomerular mesangial cells (MCs). In vivo experiments revealed a dose-dependent increase in Rap1b expression in glomeruli of diabetic rat kidneys. Similarly, in vitro exposure of MCs to HG led to an up-regulation of Rap1b with concomitant increase in fibronectin (FN) mRNA and protein expression. The up-regulation of Rap1b mRNA was mitigated by the PKC inhibitors, calphostin C, and bisindolymaleimide, while also reducing HG- induced FN expression in non-transfected MCs. Overexpression of Rap1b by transfection with pcDNA 3.1/Rap1b in MCs resulted in the stimulation of FN synthesis; however, the PKC inhibitors had no significant effect in reducing FN expression in Rap1b-transfected MCs. Transfection of Rap1b mutants S17N (Ser --> Asn) or T61R (Thr --> Arg) in MCs inhibited the HG-induced increased FN synthesis. B-Raf and Raf-1 expression was investigated to assess whether Rap1b effects are mediated via the Raf pathway. B-Raf, and not Raf-1, expression was increased in MCs transfected with Rap1b. HG also caused activation of Rap1b, which was largely unaffected by anti-platelet-derived growth factor (PDGF) antibodies. HG-induced activation of Rap1b was specific, since Rap2b activation and expression of Rap2a and Rap2b were unaffected by HG. These findings indicate that hyperglycemia and HG cause an activation and up-regulation of Rap1b in renal glomeruli and in cultured MCs, which then stimulates FN synthesis. This effect appears to be PKC-dependent and PDGF-independent, but involves B-Raf, suggesting a novel PKC-Rap1b-B-Raf pathway responsible for HG-induced increased mesangial matrix synthesis, a hallmark of diabetic nephropathy.

Vascular mechanisms of increased arterial pressure in preeclampsia: lessons from animal models

Normal pregnancy is associated with reductions in total vascular resistance and arterial pressure possibly due to enhanced endothelium-dependent vascular relaxation and decreased vascular reactivity to vasoconstrictor agonists. These beneficial hemodynamic and vascular changes do not occur in women who develop preeclampsia; instead, severe increases in vascular resistance and arterial pressure are observed. Although preeclampsia represents a major cause of maternal and fetal morbidity and mortality, the vascular and cellular mechanisms underlying this disorder have not been clearly identified. Studies in hypertensive pregnant women and experimental animal models suggested that reduction in uteroplacental perfusion pressure and the ensuing placental ischemia/hypoxia during late pregnancy may trigger the release of placental factors that initiate a cascade of cellular and molecular events leading to endothelial and vascular smooth muscle cell dysfunction and thereby increased vascular resistance and arterial pressure. The reduction in uterine perfusion pressure and the ensuing placental ischemia are possibly caused by inadequate cytotrophoblast invasion of the uterine spiral arteries. Placental ischemia may promote the release of a variety of biologically active factors, including cytokines such as tumor necrosis factor-alpha and reactive oxygen species. Threshold increases in the plasma levels of placental factors may lead to endothelial cell dysfunction, alterations in the release of vasodilator substances such as nitric oxide (NO), prostacyclin (PGI(2)), and endothelium-derived hyperpolarizing factor, and thereby reductions of the NO-cGMP, PGI(2)-cAMP, and hyperpolarizing factor vascular relaxation pathways. The placental factors may also increase the release of or the vascular reactivity to endothelium-derived contracting factors such as endothelin, thromboxane, and ANG II. These contracting factors could increase intracellular Ca(2+) concentrations ([Ca(2+)](i)) and stimulate Ca(2+)-dependent contraction pathways in vascular smooth muscle. The contracting factors could also increase the activity of vascular protein kinases such as protein kinase C, leading to increased myofilament force sensitivity to [Ca(2+)](i) and enhancement of smooth muscle contraction. The decreased endothelium-dependent mechanisms of vascular relaxation and the enhanced mechanisms of vascular smooth muscle contraction represent plausible causes of the increased vascular resistance and arterial pressure associated with preeclampsia.

Relationship between protein kinase C alterations and nitric oxide overproduction in cirrhotic rat aortas

BACKGROUND

Although nitric oxide (NO) overproduction and protein kinase C (PKC) alterations may play a role in systemic haemodynamic changes in cirrhotic rat aortas, the relationship between NO synthase (NOS) hyperactivation and PKC hypoactivation is unknown. Therefore, the relationships between NOS and PKC activities were studied in cirrhotic rat aortas.

METHODS

The effects of NOS inhibition by Nw-nitro-L-arginine (LNNA) on the contractile response to phorbol myristate acetate (PMA), a PKC activator, were studied. The effects of NOS inhibition and those of S-nitroso-N acetyl-DL-penicillamine (SNAP), an NO donor, on PKC activity were also evaluated. The effects of PKC activation and inhibition on total NOS and inducible NOS (iNOS) activities were measured. Nitric oxide synthase inhibition caused an increase in PMA-induced contraction and an increase in PKC activity in cirrhotic rat aortas. S-nitroso-N acetyl-DL-penicillamine induced downregulation of PKC activity. Total basal aortic NOS activity was significantly higher in cirrhotic rats than in control rats and activation of PKC by PMA induced a decrease in total aortic NOS activity. Protein kinase C downregulation caused an increase in both total aortic NOS and iNOS activities only in control rats, whereas only iNOS activity increased in cirrhotic rats.

CONCLUSION

In cirrhotic rat aortas, NO overproduction plays a role in the decreased PKC activation that leads to reduced aortic contraction. Overactivation of aortic NOS in cirrhotic rats may be because of, in part, the reduced PKC activity.

A cGMP-dependent protein kinase is implicated in wild-type motility in C. elegans

In mammals, cyclic GMP and cGMP-dependent protein kinases (cGKs) have been implicated in the regulation of many neuronal functions including long-term potentiation and long-term depression of synaptic efficacy. To develop Caenorhabditis elegans as a model system for studying the neuronal function of the cGKs, we cloned and characterized the cgk-1 gene. A combination of approaches showed that cgk-1 produces three transcripts, which differ in their first exon but are similar in length. Northern analysis of C. elegans RNA, performed with a probe designed to hybridize to all three transcripts, confirmed that a major 3.0 kb cgk-1 transcript is present at all stages of development. To determine if the CGK-1C protein was a cGMP-dependent protein kinase, CGK-1C was expressed in SF:9 cells and purified. CGK-1C shows a K(a) of 190 +/- 14 nM for cGMP and 18.4 +/- 2 microM for cAMP. Furthermore, CGK-1C undergoes autophosphorylation in a cGMP-dependent manner and is inhibited by the commonly used cGK inhibitor, KT5823. To determine which cells expressed CGK-1C, a 2.4-kb DNA fragment from the promoter of CGK-1C was used to drive GFP expression. The CGK-1C reporter construct is strongly expressed in the ventral nerve cord and in several other neurons as well as the marginal cells of the pharynx and intestine. Finally, RNA-mediated interference of CGK-1 resulted in movement defects in nematode larvae. These results provide the first demonstration that cGMP-dependent protein kinase is present in neurons of C. elegans and show that this kinase is required for normal motility.

Pregnancy-associated reduction in vascular protein kinase C activity rebounds during inhibition of NO synthesis

Vascular reactivity has been shown to be reduced during pregnancy and to be enhanced during chronic inhibition of nitric oxide (NO) synthesis in pregnant rats; however, the cellular mechanisms involved are unclear. The purpose of this study was to investigate whether the pregnancy-induced changes in vascular reactivity are associated with changes in the amount and/or activity of vascular protein kinase C (PKC). Active stress as well as the amount and activity of PKC was measured in deendothelialized thoracic aortic strips from virgin and pregnant rats untreated or treated with the NO synthase inhibitor N(G)-nitro-L-arginine methyl ester (L-NAME). In virgin rats, the PKC activator phorbol 12,13-dibutyrate (PDBu, 10(-6) M) and the alpha-adrenergic agonist phenylephrine (Phe, 10(-5) M) caused significant increases in active stress and PKC activity that were inhibited by the PKC inhibitors staurosporine and calphostin C. Western blot analysis in aortic strips of virgin rats showed significant amount of the alpha-PKC isoform. Both PDBu and Phe caused significant translocation of alpha-PKC from the cytosolic to the particulate fraction. Compared with virgin rats, the PDBu- and Phe-stimulated active stress and PKC activity as well as the amount and the PDBu- and Phe-induced translocation of alpha-PKC were significantly reduced in late pregnant rats but significantly enhanced in pregnant rats treated with L-NAME. The PDBu- and Phe-induced changes in active stress and the amount, distribution, and activity of alpha-PKC in virgin rats treated with L-NAME were not significantly different from that in virgin rats, whereas the changes in pregnant rats treated with L-NAME + the NO synthase substrate L-arginine were not significantly different from that in pregnant rats. These results provide evidence that a PKC-mediated contractile pathway in vascular smooth muscle is reduced during pregnancy and significantly enhanced during chronic inhibition of NO synthesis. The results suggest that one possible mechanism of the pregnancy-associated changes in vascular reactivity may involve changes in the amount and activity of the alpha-PKC isoform.

Ca(2+)-insensitive vascular protein kinase C during pregnancy and NOS inhibition

Pregnancy-induced hypertension is associated with increased vascular resistance; however, the cellular mechanisms involved are unclear. We have previously found that the relation between Ca(2+) entry and the developed force in vascular smooth muscle is altered during normal pregnancy and in a rat model of pregnancy-induced hypertension produced by long-term treatment with the nitric oxide synthase inhibitor N(G)-nitro-L-arginine methyl ester (L-NAME). The purpose of this study was to investigate whether the pregnancy-associated changes in the vascular Ca(2+) entry-force relation reflect changes in the amount and/or activity of Ca(2+)-insensitive protein kinase C (PKC) isoforms. Active stress and the amount and activity of PKC were measured in deendothelialized aortic strips from nonpregnant and pregnant rats untreated or treated with L-NAME and incubated in Ca(2+)-free (2 mmol/L EGTA) Krebs solution. In nonpregnant rats, the PKC activator phorbol 12,13-dibutyrate (PDBu, 10(-6) mol/L) and the alpha-adrenergic agonist phenylephrine (Phe, 10(-5) mol/L) caused significant, maintained increases in active stress and PKC activity that were inhibited by the PKC inhibitors staurosporine and calphostin C. Western blots in aortic strips of nonpregnant rats revealed the Ca(2+)-insensitive delta-PKC and zeta-PKC isoforms. Both PDBu and Phe caused translocation of delta-PKC from the cytosolic to the particulate fraction. Compared with nonpregnant rats, the amount of delta-PKC and zeta-PKC and the PDBu-stimulated and Phe-stimulated stress, PKC activity and translocation of delta-PKC were significantly reduced in late pregnant rats but significantly enhanced in pregnant rats treated with L-NAME. The PDBu-induced and Phe-induced responses in nonpregnant rats treated with L-NAME were not significantly different from nonpregnant rats, whereas the responses in pregnant rats treated with L-NAME+L-arginine were not significantly different from pregnant rats. These results provide evidence that a signaling pathway in vascular smooth muscle possibly involving the Ca(2+)-insensitive delta-PKC and zeta-PKC isoforms is reduced in late pregnancy and enhanced during long-term inhibition of nitric oxide synthesis. The changes in the amount and activity of vascular PKC isoforms may, in part, explain the changes in vascular resistance during normal pregnancy and pregnancy-induced hypertension.

Human mesangial cells express inducible macrophage scavenger receptor

BACKGROUND

Type A scavenger receptors (Scr) mediate the uptake of modified low-density lipoproteins by macrophages. The accumulation of lipids via this process is thought to lead to foam cell formation in atherosclerotic plaques. Human mesangial cells (HMCs) have not been previously shown to express Scr in normal culture. We therefore investigated whether there is an inducible form of Scr in a human mesangial cell line (HMCL).

METHODS

Scr activity was analyzed by cellular uptake of fluorescently labeled acetylated low-density lipoprotein using a flow cytometer. Scr mRNA expression was examined using reverse transcription-polymerase chain reaction, followed by Southern blotting. To investigate the molecular mechanism of Scr expression, several reporter gene constructs were designed. The first contained a full Scr promoter, the second a part of the Scr promoter that has both AP-1 and ets transcription factor binding sites. Other constructs were identical to the second, except that they contained either AP-1 or ets motif mutations.

RESULTS

Phorbol 12-Myristate 13-acetate (PMA) and angiotensin II (Ang II) increased both the percentage of Scr-positive cells and the Scr mean fluorescence intensity. PMA and Ang II also increased Scr mRNA and promoter activity in a time- and dose-responsive manner. Protein kinase C and calmodulin transduction pathways were involved in Scr up-regulation induced by PMA and Ang II. Additionally, a serine/threonine kinase was involved in PMA stimulation. Functional analysis showed that both AP-1 and ets motifs were specific response elements to PMA stimulation in HMCLs.

CONCLUSIONS

This study suggests that HMCs may express an inducible Scr, by which cells can acquire lipids and convert to foam cells in developing glomerulosclerosis.

Angiotensin II-induced hypertrophy of adult rat cardiomyocytes is blocked by nitric oxide

The aim of the present study was to test the hypothesis that bradykinin-stimulated release of nitric oxide (NO) and/or prostacyclin from endothelium blocks myocyte hypertrophy in vitro. Angiotensin II increased [3H]phenylalanine incorporation by 21 +/- 2% in myocytes cocultured with endothelial cells; this was abolished by bradykinin in the presence of endothelial cells. Bradykinin increased cytosolic concentrations of cGMP by 29 +/- 4% in myocytes cocultured with endothelial cells. This was abolished by inhibition of NO synthase and by a cyclooxygenase inhibitor. Angiotensin II also increased [3H]phenylalanine incorporation by 28 +/- 3% in myocytes cultured in the absence of endothelial cells. This effect of angiotensin II in monoculture was abolished by donors of NO but not by bradykinin. Neither the stable analog of prostacyclin (iloprost) nor the prostacyclin second messanger analog 8-bromo-cAMP (8-BrcAMP) blocked the effect of angiotensin II. Furthermore, 8-BrcAMP and iloprost individually increased [3H]phenylalanine incorporation. The antihypertrophic effects of bradykinin are critically dependent on endothelium-derived NO.

Glomerular mesangial cells: electrophysiology and regulation of contraction

Mesangial cells are smooth muscle-like pericytes that abut and surround the filtration capillaries within the glomerulus. Studies of the fine ultrastructure of the glomerulus show that the mesangial cell and the capillary basement membrane form a biomechanical unit capable of regulating filtration surface area as well as intraglomerular blood volume. Structural and functional studies suggest that mesangial cells regulate filtration rate in both a static and dynamic fashion. Mesangial excitability enables a homeostatic intraglomerular stretch reflex that integrates an increase in filtration pressure with a reduction in capillary surface area. In addition, mesangial tone is regulated by diverse vasoactive hormones. Agonists, such as angiotensin II, contract mesangial cells through a signal transduction pathway that releases intracellular stores of Ca2+, which subsequently activate nonselective cation channels and Cl- channels to depolarize the plasma membrane. The change in membrane potential activates voltage-gated Ca2+ channels, allowing Ca2+ cell entry and further activation of depolarizing conductances. Contraction and entry of cell Ca2+ are inhibited only when Ca2+-activated K+ channels (BK(Ca)) are activated and the membrane is hyperpolarized toward the K+ equilibrium potential. The mesangial BK(Ca) is a weak regulator of contraction in unstimulated cells; however, the gain of the feedback is increased by atrial natriuretic peptide, nitric oxide, and the second messenger cGMP, which activates protein kinase G and decreases both the voltage and Ca2+ activation thresholds of BK(Ca) independent of sensitivity. This enables BK(Ca) to more effectively counter membrane depolarization and voltage-gated Ca2+ influx. After hyperpolarizing the membrane, BK(Ca) rapidly inactivates because of dephosphorylation by protein phosphatase 2A. Regulation of ion channels has been linked casually to hyperfiltration during early stages of diabetes mellitus. Determining the signaling pathways controlling the electrophysiology of glomerular mesangial cells is important for understanding how glomerular filtration rate is regulated in health and disease.

Tonic protein kinase C-mediated vasoconstriction is unmasked when nitric oxide synthase is inhibited in cerebral microvessels

Recent evidence indicates that nitric oxide participates in the modulation of vascular tone in a variety of vascular beds, including the parenchymal microvasculature of the brain. The present study examined the role of protein kinase activity in the induction and maintenance of the contractile response when endogenous nitric oxide production is inhibited in parenchymal microvessels of the rat hippocampus. Microvessels in in vitro slices of the hippocampus were monitored using computer-assisted video microscopy. The effects of inhibitors of two kinases, protein kinase C and calcium/calmodulin-dependent protein kinase, on the vasoconstrictor response to NG-nitro-L-arginine (L-NNA) were investigated. The resting luminal diameter of the microvessels examined in this study ranged from 9 to 29 microns. Addition of 100 microM L-NNA to the medium superfusing the slice constricted microvessels by 38.8 +/- 0.6%. The addition of protein kinase inhibitors reversed this constriction in a dose-dependent manner. H-7 (50 microM), a relatively non-selective protein kinase C inhibitor, elicited an 81.4 +/- 10.0% reversal of the L-NNA-induced constriction. Bisindolylmaleimide (5 microM), a selective protein kinase C inhibitor, reversed the constriction by 69.1 +/- 13.7%. KN-62, an inhibitor of calcium/calmodulin-dependent protein kinase II, elicited a smaller yet statistically significant reversal of 17.1 +/- 5.1%. Pretreatment with H-7 or bisindolyl-maleimide blocked the LNNA-induced constriction entirely, while KN-62 did not significantly inhibit the response. These findings indicate that the contractile response observed upon removal of endogenous nitric oxidergic vasodilation is mediated by protein kinase activity, and the contribution of protein kinase C to this effect is greater than that of calcium/calmodulin-dependent protein kinase II. The results suggest that a tonic nitric oxidergic influence serves to mask the potential for protein kinase C-mediated vasoconstriction in cerebral microvessels.