Intracellular targeting and protein kinase C in vascular smooth muscle cells: specific effects of different membrane-bound receptors

Intratubular, Intracellular, and Mitochondrial Angiotensin II/AT1 (AT1a) Receptor/NHE3 Signaling Plays a Critical Role in Angiotensin II-Induced Hypertension and Kidney Injury

Hypertension is well recognized to be the most important risk factor for cardiovascular diseases, stroke, and end-stage kidney failure. A quarter of the world’s adult populations and 46% of the US adults develop hypertension and currently require antihypertensive treatments. Only 50% of hypertensive patients are responsive to current antihypertensive drugs, whereas remaining patients may continue to develop cardiovascular, stroke, and kidney diseases. The mechanisms underlying the poorly controlled hypertension remain incompletely understood. Recently, we have focused our efforts to uncover additional renal mechanisms, pathways, and therapeutic targets of poorly controlled hypertension and target organ injury using novel animal models or innovative experimental approaches. Specifically, we studied and elucidated the important roles of intratubular, intracellular, and mitochondrial angiotensin II (Ang II) system in the development of Ang II-dependent hypertension. The objectives of this invited article are to review and discuss our recent findings that (a) circulating and intratubular Ang II is taken up by the proximal tubules via the (AT₁) AT_1a receptor-dependent mechanism, (b) intracellular administration of Ang II in proximal tubule cells or adenovirus-mediated overexpression of an intracellular Ang II fusion protein selectively in the mitochonria of the proximal tubules induces blood pressure responses, and (c) genetic deletion of AT₁ (AT_1a) receptors or the Na⁺/H⁺ exchanger 3 selectively in the proximal tubules decreases basal blood pressure and attenuates Ang II-induced hypertension. These studies provide a new perspective into the important roles of the intratubular, intracellular, and mitochondrial angiotensin II/AT₁ (AT_1a) receptor signaling in Ang II-dependent hypertensive kidney diseases.

Tonic and phasic smooth muscle contraction is not regulated by the PKCα - CPI-17 pathway in swine stomach antrum and fundus

Regulation of myosin light chain phosphatase (MLCP) via protein kinase C (PKC) and the 17 kDa PKC-potentiated inhibitor of myosin light chain phosphatase (CPI-17) has been reported as a Ca²⁺ sensitization signaling pathway in smooth muscle (SM), and thus may be involved in tonic vs. phasic contractions. This study examined the protein expression and spatial-temporal distribution of PKCα and CPI-17 in intact SM tissues. KCl or carbachol (CCh) stimulation of tonic stomach fundus SM generates a sustained contraction while the phasic stomach antrum generates a transient contraction. In addition, the tonic fundus generates greater relative force than phasic antrum with 1 µM phorbol 12, 13-dibutyrate (PDBu) stimulation which is reported to activate the PKCα – CPI-17 pathway. Western blot analyses demonstrated that this contractile difference was not caused by a difference in the protein expression of PKCα or CPI-17 between these two tissues. Immunohistochemical results show that the distribution of PKCα in the longitudinal and circular layers of the fundus and antrum do not differ, being predominantly localized near the SM cell plasma membrane. Stimulation of either tissue with 1 µM PDBu or 1 µM CCh does not alter this peripheral PKCα distribution. There are no differences between these two tissues for the CPI-17 distribution, but unlike the PKCα distribution, CPI-17 appears to be diffusely distributed throughout the cytoplasm under relaxed tissue conditions but shifts to a primarily peripheral distribution at the plasma membrane with stimulation of the tissues with 1 µM PDBu or 1 µM CCh. Results from double labeling show that neither PKCα nor CPI-17 co-localize at the adherens junction (vinculin/talin) at the membrane but they do co-localize with each other and with caveoli (caveolin) at the membrane. This lack of difference suggests that the PKCα - CPI-17 pathway is not responsible for the tonic vs. phasic contractions observed in stomach fundus and antrum.

Differential expression of protein kinase C isoforms in coronary arteries of diabetic mice lacking the G-protein Gα11

Background

Diabetes mellitus counts as a major risk factor for developing atherosclerosis. The activation of protein kinase C (PKC) is commonly known to take a pivotal part in the pathogenesis of atherosclerosis, though the influence of specific PKC isozymes remains unclear. There is evidence from large clinical trials suggesting excessive neurohumoral stimulation, amongst other pathways leading to PKC activation, as a central mechanism in the pathogenesis of diabetic heart disease. The present study was therefore designed to determine the role of G_q-protein signalling via Gα₁₁ in diabetes for the expression of PKC isozymes in the coronary vessels.

Methods

The role of Gα₁₁ in diabetes was examined in knockout mice with global deletion of Gα₁₁ compared to wildtype controls. An experimental type 1-diabetes was induced in both groups by injection of streptozotocin. Expression and localization of the PKC isozymes α, βII, δ, ε, and ζ was examined by quantitative immunohistochemistry.

Results

8 weeks after induction of diabetes a diminished expression of PKC ε was observed in wildtype animals. This alteration was not seen in Gα₁₁ knockout animals, however, these mice showed a diminished expression of PKCζ. Direct comparison of wildtype and knockout control animals revealed a diminished expression of PKC δ and ε in Gα₁₁ knockout animals.

Conclusion

The present study shows that expression of the nPKCs δ and ε in coronary vessels is under control of the g-protein Gα₁₁. The reduced expression of PKC ζ that we observed in coronary arteries from Gα₁₁-knockout mice compared to wildtype controls upon induction of diabetes could reduce apoptosis and promote plaque stability. These findings suggest a mechanism that may in part underlie the therapeutic benefit of RAS inhibition on cardiovascular endpoints in diabetic patients.

PKC isozymes in chronic cardiac disease: possible therapeutic targets?

Cardiovascular disease is the leading cause of death in the United States. Therefore, identifying therapeutic targets is a major focus of current research. Protein kinase C (PKC), a family of serine/threonine kinases, has been identified as playing a role in many of the pathologies of heart disease. However, the lack of specific PKC regulators and the ubiquitous expression and normal physiological functions of the 11 PKC isozymes has made drug development a challenge. Here we discuss the validity of therapeutically targeting PKC, an intracellular signaling enzyme. We describe PKC structure, function, and distribution in the healthy and diseased heart, as well as the development of rationally designed isozyme-selective regulators of PKC functions. The review focuses on the roles of specific PKC isozymes in atherosclerosis, fibrosis, and cardiac hypertrophy, and examines principles of pharmacology as they pertain to regulators of signaling cascades associated with these diseases.

The detrusor muscle: an innocent victim of bladder outlet obstruction

OBJECTIVES

Benign prostatic hyperplasia (BPH) is considered a frequent cause of bladder outlet obstruction (BOO) and lower urinary tract symptoms (LUTS), although the physiopathologic mechanism through which BPH causes LUTS is not clear. Several morphologic and functional modifications of the bladder detrusor have been described in patients with BPH and could play a direct role in determining symptoms. The opinion is spreading that the enlarged prostates in patients with LUTS is nothing more than a mere bystander. Evidence has accumulated, however, supporting the role of BPH-related BOO as the direct cause determining bladder dysfunction and indirectly causing urinary symptoms. The present review addresses the bladder response to BOO, particularly focusing on the physiopathologic cascade that links obstructive BPH to bladder dysfunction.

METHODS

A literature review of peer-reviewed articles has been performed, including both in vivo and in vitro studies on human tissue and animal model experiments.

RESULTS

Epithelial and smooth muscle cells in the bladder wall are mechanosensitive, and in response to mechanical stretch stress caused by BOO, undergo modifications of gene expression and protein synthesis. This process involves several transduction mechanisms and finally alter the ultrastructure and physiology of cell membranes, cytoskeleton, contractile proteins, mitochondria, extracellular matrix, and neuronal networks.

CONCLUSIONS

BOO is the initiator of a physiopathologic cascade leading to deep changing of bladder structure and function. Before being a direct cause of storing-phase urinary symptoms, the bladder is the first innocent victim of prostatic obstruction.

Effect of protein kinase C activation on intracellular Ca2+ signaling and integrity of intestinal epithelial cells

Protein kinase C (PKC) activation and increases in cytosolic Ca(2+) cause intestinal injury. Since PKC activation can alter Ca(2+) homeostasis and increase Ca(2+) levels, we examined the effects of PKC activation on intestinal cellular integrity and the role of Ca(2+) signaling in this response. The epithelial cell line, IEC-18 was incubated with the PKC activator phorbol myristate acetate (PMA; 0.1-1.0 microM). In some experiments, cells were incubated in Ca(2+)-free medium. PMA treatment produced a concentration-dependent increase in cell injury and PKC activity. This response was attenuated by addition of the pan-specific PKC inhibitor, GF 109203X. Furthermore, cell viability was maintained in cells preincubated with PKC isoform-specific inhibitors to PKCalpha, PKCdelta and PKCepsilon. Cell injury was also reduced if cells were incubated in Ca(2+)-free medium or in the presence of the Ca(2+) channel antagonist, verapamil or the intracellular chelator BAPTA-AM. PMA, but not the inactive phorbol ester, 4alphaPMA, induced a dose-dependent increase in cellular Ca(2+) that was characterized by a rapid, transient spike followed by a tonic plateau phase which approximated control levels. These responses were eliminated by the addition of BAPTA-AM. Furthermore the increase in the Ca(2+) spike was reduced or eliminated by co-incubation with the PKCdelta antagonist, rottlerin. Inhibition of PKCalpha or PKCepsilon was less effective or ineffective in this regard. These data suggest that PKC activation via PMA challenge affects the integrity of rat intestinal epithelial cells. PKCdelta, but not PKCepsilon or PKCalpha activation appears to mediate this effect via an increase in cellular Ca(2+).

PKC-epsilon translocation in enteric neurons and interstitial cells of Cajal in response to muscarinic stimulation

Interstitial cells of Cajal in the deep muscular plexus (ICC-DMP) of the small intestine express excitatory neurotransmitter receptors. We tested whether ICC-DMP are functionally innervated by cholinergic neurons in the murine intestine. Muscles were stimulated by intrinsic nerves and ACh and processed for immunohistochemistry to determine these effects on PKC-epsilon activation. Under control conditions, PKC-epsilon-like immunoreactivy (PKC-epsilon-LI) was only observed in myenteric neurons within the tunica muscularis. Electrical field stimulation or ACh caused translocation of neural PKC-epsilon-LI from the cytosol to a peripheral compartment. After stimulation, PKC-epsilon-LI was found in spindle-shaped cells in the DMP. These cells were identified as ICC-DMP by Kit-LI and vimentin-LI. PKC-epsilon-LI in ICC-DMP and translocation of PKC epsilon-LI in neurons were blocked by tetrodotoxin or atropine, suggesting that these responses were due to activation of muscarinic receptors. Western blots also confirmed translocation of PKC-epsilon-LI. In conclusion, PKC-epsilon translocation is linked to muscarinic receptor activation in ICC-DMP and a subpopulation of myenteric neurons. These studies demonstrate that ICC-DMP are functionally innervated by excitatory motoneurons.

Fibroblast growth factor mediates hypoxia-induced endothelin-- a receptor expression in lung artery smooth muscle cells

We have previously demonstrated that endothelin (ET)-1 and its subtype A receptor (ET-AR) expression are increased in lung under hypoxic conditions and that activation of ET-AR by ET-1 is a major mediator of hypoxia-induced pulmonary hypertension in the rat. The present study tested the hypothesis that the hypoxia-responsive tyrosine kinase receptor-activating growth factors fibroblast growth factor (FGF)-1, FGF-2, and platelet-derived growth factor (PDGF)-BB stimulate expression of the ET-AR in pulmonary arterial smooth muscle cells (PASMCs). Quiescent rat PASMCs were incubated under hypoxia (1% O2), or with FGF-1, FGF-2, PDGF-BB, vascular endothelial growth factor, ET-1, angiotensin II, or atrial natriuretic peptide under normoxic conditions for 24 h. FGF-1 and -2 and PDGF-BB, but not hypoxia, vascular endothelial growth factor, ET-1, angiotensin II, or atrial natriuretic peptide, significantly increased ET-AR mRNA levels. FGF-1-induced ET-AR expression was inhibited by FGF-receptor inhibitor PD-166866, MEK inhibitor U-0126, transcription inhibitor actinomycin D, and translation inhibitor cycloheximide. In contrast, the stimulatory effect of FGF-1 on ET-AR mRNA expression was not altered by PI3 kinase, PKA, PKC, or adenylate cyclase inhibitors. PASMC ET-AR gene transcription, assessed by nuclear-runoff analysis, was increased by FGF-1. These results provide novel finding that ET-AR in PASMCs in vitro is unresponsive to hypoxia per se but is robustly simulated by tyrosine kinase receptor-associated growth factors (FGF-1, FGF-2, PDGF-BB) that themselves are stimulated by hypoxia in lung. This observation suggests a novel signaling mechanism that may be responsible for overexpression of ET-AR in lung, and may contribute to the hypoxia-induced pulmonary vasoconstriction, hypertension, and vascular remodeling in hypoxia-adapted animal.

Regulation of arterial tone by smooth muscle myosin type II

The initiation of contractile force in arterial smooth muscle (SM) is believed to be regulated by the intracellular Ca2+ concentration and SM myosin type II phosphorylation. We tested the hypothesis that SM myosin type II operates as a molecular motor protein in electromechanical, but not in protein kinase C (PKC)-induced, contraction of small resistance-sized cerebral arteries. We utilized a SM type II myosin heavy chain (MHC) knockout mouse model and measured arterial wall Ca2+ concentration ([Ca2+](i)) and the diameter of pressurized cerebral arteries (30-100 microm) by means of digital fluorescence video imaging. Intravasal pressure elevation caused a graded [Ca2+](i) increase and constricted cerebral arteries of neonatal wild-type mice by 20-30%. In contrast, intravasal pressure elevation caused a graded increase of [Ca2+](i) without constriction in (-/-) MHC-deficient arteries. KCl (60 mM) induced a further [Ca2+](i) increase but failed to induce vasoconstriction of (-/-) MHC-deficient cerebral arteries. Activation of PKC by phorbol ester (phorbol 12-myristate 13-acetate, 100 nM) induced a strong, sustained constriction of (-/-) MHC-deficient cerebral arteries without changing [Ca2+](i). These results demonstrate a major role for SM type II myosin in the development of myogenic tone and Ca2+ -dependent constriction of resistance-sized cerebral arteries. In contrast, the sustained contractile response did not depend on myosin and intracellular Ca2+ but instead depended on PKC. We suggest that SM myosin type II operates as a molecular motor protein in the development of myogenic tone but not in pharmacomechanical coupling by PKC in cerebral arteries. Thus PKC-dependent phosphorylation of cytoskeletal proteins may be responsible for sustained contraction in vascular SM.

PKC-alpha shows variable patterns of translocation in response to different stimulatory agents

Reports from numerous laboratories suggest that protein kinase C (PKC) translocation to substrate target sites may vary depending on cell type and experimental conditions. We have proposed that acutely variable targeting of PKC to different substrate sites could greatly expand the functional properties of individual isoforms in individual cell types (Li et al., 2001). Confocal microscopy and PKC alpha-enhanced green fluorescent protein (PKC alpha-EGFP) fusion protein expression were utilized to investigate the spatial and temporal pattern of PKC alpha translocation to different stimulating agents in A7r5 smooth muscle cells. Phorbol 12, 13 dibutyrate (PDBu 10(-8) M) caused a slow but irreversible relocation of the fusion protein from the cytosol to the plasmalemma. By comparison, thapsigargin (10(-5) M) and A23 187 (2 x 10(-5) M) induced a rapidly transient translocation to the cell membrane which was completed within 4 min. In contrast to these agents, angiotensin II (Ang II, 10(-6) M) caused only partial relocalization of cytosolic PKC alpha-EGFP to brightly fluorescing patches at the cell periphery. Localization at peripheral patches was completed within seconds and the fusion protein returned to the cytosol within 2 min. The PKC inhibitor staurosporine blocked cellular contraction to PDBu but not A(23 187) and had no effect on PKC alpha-EGFP translocation. By comparison, the calcium chelators EDTA and BAPTA-AM blocked the contraction to A(23 187), attenuated the contraction to PDBu, and abolished the translocation of PKC alpha-EGFP by both agents. The results show that in a single cell type the spatial and temporal characteristics of individual PKC isoform translocation may differ markedly. This further suggests the existence of potentially complex mechanisms which regulate the rate and location of target site availability.

Protein kinase C and cerebral vasospasm

Twenty-five years after the discovery of protein kinase C (PKC), the physiologic function of PKC, and especially its role in pathologic conditions, remains a subject of great interest with 30,000 studies published on these aspects. In the cerebral circulation, PKC plays a role in the regulation of myogenic tone by sensitization of myofilaments to calcium. Protein kinase C phosphorylates various ion channels including augmenting voltage-dependent Ca2+ channels and inhibiting K+ channels, which both lead to vessel contraction. These actions of PKC amplify vascular reactivity to different agonists and may be critical in the regulation of cerebral artery tone during vasospasm. Evidence accumulated during at least the last decade suggest that activation of PKC in cerebral vasospasm results in a delayed but prolonged contraction of major arteries after subarachnoid hemorrhage. Most of the experimental results in vitro or in animal models support the view that PKC is involved in cerebral vasospasm. Implication of PKC in cerebral vasospasm helps explain increased arterial narrowing at the signal transduction level and alters current perceptions that the pathophysiology is caused by a combination of multiple receptor activation, hemoglobin toxicity, and damaged neurogenic control. Activation of protein kinase C also interacts with other signaling pathways such as myosin light chain kinase, nitric oxide, intracellular Ca2+, protein tyrosine kinase, and its substrates such as mitogen-activated protein kinase. Even though identifying PKC revolutionized the understanding of cerebral vasospasm, clinical advances are hampered by the lack of clinical trials using selective PKC inhibitors.

Expression of protein kinase C isozymes in nonpregnant and pregnant human myometrium

OBJECTIVE

The aim of this study was to compare the distributions of protein kinase C isozymes in human nonpregnant and pregnant myometrial tissues and primary cell cultures.

STUDY DESIGN

Myometrial tissues were obtained at hysterectomy from nonpregnant women and at cesarean delivery from women both before and during early labor at term. Western immunoblot analysis was performed on homogenates of myometrial tissues and primary cell cultures with monoclonal antibodies specific for protein kinase C isozymes. Redistribution and translocation of protein kinase C were examined by means of immunocytochemical methods.

RESULTS

Nonpregnant myometrial tissues contained protein kinase C isozymes alpha, gamma, delta, mu, iota, and zeta but not beta(1), beta(2), theta, or epsilon. Pregnant myometrial tissues both before and during early labor contained the same protein kinase C isozymes and also beta(1) and beta(2). The protein kinase C isozyme distribution in primary myometrial cell cultures was identical to that in the myometrial tissues. Protein kinase C redistribution and translocation were demonstrated in these cultured myometrial cells.

CONCLUSION

Both human myometrial tissues and primary cell cultures expressed a broad range of protein kinase C isozymes. Protein kinase C isozymes beta(1) and beta(2) were absent in nonpregnant myometrium but were induced during pregnancy. Labor at term did not alter protein kinase C isozyme expression.

Mechanical stress-initiated signal transductions in vascular smooth muscle cells

Mechanical force is an important modulator of cellular morphology and function in a variety of tissues, and is particularly important in cardiovascular systems. Vascular smooth muscle cell (VSMC) hypertrophy and proliferation contribute to the development of atherosclerosis, hypertension, and restenosis, where mechanical forces are largely disturbed. How VSMCs sense and transduce the extracellular mechanical signals into the cell nucleus resulting in quantitative and qualitative changes in gene expression is an interesting and important research field. Recently, it has been demonstrated that mechanical stress rapidly induced phosphorylation of platelet-derived growth factor (PDGF) receptor, activation of integrin receptor, stretch-activated cation channels, and G proteins, which might serve as mechanosensors. Once mechanical force is sensed, protein kinase C and mitogen-activated protein kinases (MAPKs) were activated, leading to increased c-fos and c-jun gene expression and enhanced transcription factor AP-1 DNA-binding activity. Interestingly, physical forces also rapidly resulted in expression of MAPK phosphatase-1 (MKP-1), which inactivates MAPKs. Thus, mechanical stresses can directly stretch the cell membrane and alter receptor or G protein conformation, thereby initiating signalling pathways, usually used by growth factors. These findings have significantly enhanced our knowledge of the pathogenesis of arteriosclerosis and provided promising information for therapeutic interventions for vascular diseases.

Regulation of vascular smooth muscle cell proliferation by nuclear factor-kappaB and its inhibitor, I-kappaB

Proliferation of vascular smooth muscle cells (SMC) is a crucial event in the formation of atherosclerotic tissues and is regulated by nuclear transcriptional factors including nuclear factor-kappaB (NF-kappaB). We constructed a reporter gene assay to measure NF-kappaB-dependent transcriptional activity in SMC. Thrombin receptor-activating peptide (TRAP) and basic fibroblast growth factor (bFGF) stimulated SMC proliferation and rapidly enhanced the NF-kappaB transcriptional activity in a dose-dependent manner. 4-Cyano-5,5-bis-(methoxyphenyl)4-pentenoic acid (E5510) significantly inhibited SMC proliferation and also suppressed NF-kappaB transcription stimulated by TRAP and bFGF. In contrast, although tumor necrosis factor (TNF)-alpha activated NF-kappaB transcription, E5510 had no effect on TNF-alpha-induced activation. NF-kappaB was activated after the stimulation of TRAP, bFGF, and TNF-alpha in electrophoretic mobility shift assay, and E5510 suppressed the NF-kappaB activation induced by TRAP and bFGF but not the activation by TNF-alpha. Western blot analysis of I-kappaBalpha and I-kappaBbeta, inhibitors of NF-kappaB, indicated that I-kappaBalpha degradation, rather than I-kappaBbeta degradation, was important in NF-kappaB activation after the stimulation of TRAP and bFGF. PD98059, an inhibitor of extracellular signal-regulated kinase (ERK) kinase, suppressed NF-kappaB transcriptional activity and SMC proliferation. The phosphorylation of ERK1/2 was rapidly induced by TRAP and bFGF but not by TNF-alpha. These results indicate that TRAP and bFGF induced I-kappaB degradation and NF-kappaB activation through a distinct pathway from TNF-alpha and that ERK1/2 may play an important role in NF-kappaB activation induced by TRAP and bFGF.