SYMPOSIUM: Experimental Biology 1995 Role of Mesangial Cell Ion Transport in Glomerular Physiology and Disease: ANGIOTENSIN II-INDUCED TYROSINE PHOSPHORYLATION IN MESANGIAL AND VASCULAR SMOOTH MUSCLE CELLS

Vascular mechanotransduction

This review aims to survey the current state of mechanotransduction in vascular smooth muscle cells (VSMCs) and endothelial cells (ECs), including their sensing of mechanical stimuli and transduction of mechanical signals that result in the acute functional modulation and longer-term transcriptomic and epigenetic regulation of blood vessels. The mechanosensors discussed include ion channels, plasma membrane-associated structures and receptors, and junction proteins. The mechanosignaling pathways presented include the cytoskeleton, integrins, extracellular matrix, and intracellular signaling molecules. These are followed by discussions on mechanical regulation of transcriptome and epigenetics, relevance of mechanotransduction to health and disease, and interactions between VSMCs and ECs. Throughout this review, we offer suggestions for specific topics that require further understanding. In the closing section on conclusions and perspectives, we summarize what is known and point out the need to treat the vasculature as a system, including not only VSMCs and ECs but also the extracellular matrix and other types of cells such as resident macrophages and pericytes, so that we can fully understand the physiology and pathophysiology of the blood vessel as a whole, thus enhancing the comprehension, diagnosis, treatment, and prevention of vascular diseases.

Angiotensin II, a unique vasoactive agent dissociates myosin light chain phosphorylation from contraction

Angiotensin II (100 nM) induced bi-phasic increases in cytosolic Ca²⁺ level ([Ca²⁺]_i) through the activation of angiotensin II type 1 receptor. Pharmacological examinations using 10 µM verapamil, 30 µM La³⁺, and 1 µM thapsigargin indicated that the first phase of the [Ca²⁺]_i-increase was mediated by Ca²⁺ release from sarcoplasmic reticulum (SR) and Ca²⁺ influx independently of voltage dependent Ca²⁺ channel (VDC). In contrast, the second phase of [Ca²⁺]_i-increase was mediated by Ca²⁺ influx through VDC. Although both [Ca²⁺]_i and myosin light chain (MLC)-phosphorylation at the first phase was apparently exceeded the threshold for contraction as estimated by high K⁺-induced responses, there was no appreciable contraction, indicating the dissociation between MLC phosphorylation and force during this phase. In contrast, the second phase of [Ca²⁺]_i was associated with the increases in both MLC phosphorylation and force. These results suggest that angiotensin II is a unique agonist which dissociates MLC-phosphorylation from muscle force during the Ca²⁺ releases from SR.

Steroid-sparing effects of angiotensin-II inhibitors in glioblastoma patients

BACKGROUND

The standard of care in patients with glioblastoma (GBM) relies on surgical resection, radiation therapy (RT), and temozolomide. Steroids are required in almost all patients to reduce peritumoral edema, but are associated with numerous side effects. Vascular endothelial growth factor (VEGF) is a key driver of peritumoral edema and angiogenesis in human GBM. Recently, angiotensin-II inhibitors were reported to reduce VEGF secretion and tumor growth in some animal models.

METHODS

To investigate whether angiotensin-II inhibitors might have a similar effect in humans and before undertaking a prospective study, we retrospectively investigated a series of 87 consecutive, newly diagnosed GBM patients, treated in a single center. Amongst these patients, 29 (33%) were already treated before RT for high blood pressure (HBP), 18 of them (21%) with an angiotensin-II inhibitor. In all patients, performance status, surgical procedures, and steroid dosages were documented.

RESULTS

Patients treated with angiotensin-II inhibitors, but not other antihypertensive drugs, required half of the steroids of the other patients during radiotherapy (P = 0.005 in multivariate analysis, considering other antihypertensive treatments, surgical resection, and performance status). This effect of angiotensin-II inhibitors was also significant at the beginning of radiotherapy (P = 0.03 in multivariate analysis). Treatment with angiotensin-II inhibitors had no effect on survival (16.2 vs. 17.9 months for the treated and the non-treated group, respectively, P = 0.77).

CONCLUSION

Angiotensin-II inhibitors might display significant steroid-sparing effects in brain tumor patients. Given the morbidity associated with steroids, this finding might have important practical consequences in these patients and warrants a randomized study.

Src and Pyk2 mediate angiotensin II effects in cultured rat astrocytes

Angiotensin II (Ang II)-induced proliferation of rat astrocytes is mediated by multiple signaling pathways. In the present study, we investigated the role of non-receptor tyrosine kinases on Ang II-signaling and proliferation of astrocytes cultured from neonatal rat pups. Ang II stimulated astrocyte growth, ERK1/2 phosphorylation and the phosphorylation of Src and proline-rich tyrosine kinase-2 (Pyk2), in astrocytes obtained from brainstem and cerebellum. Pretreatment with 10 microM PP2, a selective Src inhibitor, inhibited Ang II stimulated ERK1/2 phosphorylation by 59% to 91% both in brainstem and cerebellum astrocytes. PP2 also inhibited Ang II induction of brainstem (76% inhibition) and cerebellar (64% inhibition) astrocyte growth. Similarly, pretreatment with 25 microM dantrolene, the Pyk2 inhibitor, attenuated ERK1/2 activity in brainstem (62% inhibition) and in cerebellum astrocytes (44% inhibition). Interestingly, inhibition of Pyk2 inhibited Ang II-induced Src activation suggesting that these two non-receptor tyrosine kinases may be acting in concert to mediate Ang II effects in astrocytes. In summary, we found that Ang II stimulates the non-receptor tyrosine kinases Src and Pyk2 which mediate Ang II-induced ERK1/2 activation leading to stimulation of astrocyte growth. In addition, these two tyrosine kinases may be interacting to regulate effects of the peptide in these cells.

Role of the JAK/STAT signaling pathway in diabetic nephropathy

Excessive cellular growth is a major contributor to pathological changes associated with diabetic nephropathy. In particular, high glucose-induced growth of glomerular mesangial cells is a characteristic feature of diabetes-induced renal complications. Glomerular mesangial cells respond to traditional growth factors, although in diabetes this occurs in the context of an environment enriched in both circulating vasoactive mediators and high glucose. For example, the vasoactive peptide ANG II has been implicated in the pathogenesis of diabetic renal disease, and recent findings suggest that high glucose and ANG II activate intracellular signaling processes, including the polyol pathway and generation of reactive oxygen species. These pathways activate the Janus kinase (JAK)/signal transducers and activators of transcription (STAT) signaling cascades in glomerular mesangial cells. Activation of the JAK/STAT signaling cascade can stimulate excessive proliferation and growth of glomerular mesangial cells, contributing to diabetic nephropathy. This review focuses on some of the key elements in the diabetic microenvironment, especially high glucose and the accumulation of advanced glycoxidation end products and considers their impact on ANG II and other vasoactive peptide-mediated signaling events in vitro and in vivo.

Protein kinases in vascular smooth muscle tone--role in the pulmonary vasculature and hypoxic pulmonary vasoconstriction

Hypoxic pulmonary vasoconstriction (HPV) is an adaptive mechanism that in the normal animal diverts blood away from poorly ventilated areas of the lung, thereby maintaining optimal ventilation-perfusion matching. In global hypoxia however, such as in respiratory disease or at altitude, it causes detrimental increases in pulmonary vascular resistance and pulmonary artery (PA) pressure. The precise intracellular pathways and mechanisms underlying HPV remain unclear, although it is now recognised that both an elevation in smooth muscle intracellular [Ca2+] and a concomitant increase in Ca2+ sensitivity are involved. Several key intracellular protein kinases have been proposed as components of the signal transduction pathways leading to development of HPV, specifically Rho kinase, non-receptor tyrosine kinases (NRTK), p38 mitogen activated protein (MAP) kinase, and protein kinase C (PKC). All of these have been implicated to a greater or lesser extent in pathways leading to Ca2+ sensitisation, and in some cases regulation of intracellular [Ca2+] as well. In this article, we review the role of these key protein kinases in the regulation of vascular smooth muscle (VSM) constriction, applying what is known in the systemic circulation to the pulmonary circulation and HPV. We conclude that the strongest evidence for direct involvement of protein kinases in the mechanisms of HPV concerns a central role for Rho kinase in Ca2+ sensitisation, and a potential role for Src-family kinases in both modulation of Ca2+ entry via capacitative Ca2+ entry (CCE) and activation of Rho kinase, though others are likely to have indirect or modulatory influences. In addition, we speculate that Src family kinases may provide a central interface between the proposed hypoxia-induced generation of reactive oxygen species by mitochondria and both the elevation in intracellular [Ca2+] and Rho kinase mediated Ca2+ sensitisation.

Regulation of the cystic fibrosis transmembrane conductance regulator channel by beta-adrenergic agonists and vasoactive intestinal peptide in rat smooth muscle cells and its role in vasorelaxation

The signaling events that regulate vascular tone include voltage-dependent Ca(2+) influx and the activities of various ionic channels; which molecular entities are involved and their role are still a matter of debate. Here we show expression of the cystic fibrosis transmembrane conductance regulator (CFTR) Cl(-) channel in rat aortic smooth muscle cells. Immunoprecipitation and in vitro protein kinase A phosphorylation show the appearance of mature band C of CFTR. An immunohistochemistry study shows CFTR proteins in smooth muscles of aortic rings but not in skeletal muscles. Using the iodide efflux method, a combination of agonists and pharmacological agents was used to dissect the function of CFTR. Agonists of the cAMP pathway, the beta-adrenergic agonist isoproterenol, and the neuropeptide vasoactive intestinal peptide activate CFTR-dependent transport from cells maintained in a high but not low extracellular potassium-rich saline, suggesting that depolarization of smooth muscle is critical to CFTR activation. Smooth muscle CFTR possesses all of the pharmacological attributes of its epithelial homologues: stimulation by the CFTR pharmacological activators MPB-07 (EC(50) = 158 microm) and MPB-91 (EC(50) = 20 microm) and inhibition by glibenclamide and diphenylamine-2-carboxylic acid but not by 5,11,17,23-tetrasulfonato-25,26,27,28-tetramethoxy-calix[4]arene. Contraction measurements on isolated aortic rings were performed to study the contribution of CFTR to vascular tone. With aortic rings (without endothelium) preconstricted by high K(+) saline or by the alpha-adrenergic agonist norepinephrine, CFTR activators produced a concentration-dependent relaxation. These results identify for the first time the expression and function of CFTR in smooth muscle where it plays an unexpected but fundamental role in the autonomic and hormonal regulation of the vascular tone.

Angiotensin II evokes calcium-mediated signaling events in isolated dog pancreatic epithelial cells

INTRODUCTION

Calcium-activated chloride conductance has been identified in normal pancreatic duct cells. Recent in vitro evidence suggests that angiotensin II (AngII) stimulates pancreatic secretion in both cystic fibrosis (CFPAC) and transformed pancreatic cells.

AIMS

To investigate calcium-mediated stimulatory effects of AngII in both nontransformed dog pancreatic duct epithelial (DPDE) and CFPAC cells.

METHODS

Western blots were performed in both cells seeking AngII receptors. In additional studies, DPDE and CFPAC cells were grown on vitrogen-coated glass cover slips and loaded with Indo-1-AM dye. Cells were placed in a confocal microscope's perfusion chamber and perfused with 100 microM AngII or ATP (control). Cells were excited with UV light, and intracellular calcium ([Ca+2]i) was read using fluorescence emission at 405 and 530 nm. Finally, single channels in the DPDE cells were examined using cell-attached patch clamps. Current amplitude histograms provided estimates of the conductance and open probability of channels.

RESULTS

Western blots demonstrated presence of both AT and AT AngII receptors in DPDE and CFPAC cells; the density of AT receptors appeared lower than that of AT receptors. Basal intracellular calcium concentrations did not differ between DPDE (109 +/- 11 nM) and CFPAC (103 +/- 8 nM) cells. AngII significantly increased measured intracellular calcium concentrations in both DPDE (909 +/- 98 nM) and CFPAC (879 +/- 207 nM) cells, as did ATP (DPDE = 1722 +/- 228 nM; CFPAC = 1522 +/- 245 nM). In the patch clamp studies, a variety of different channels were observed; they appeared to be an 11pS nonselective cation (NSC) channel, a 4.6pS Na+ channel, a 3pS anion channel, and an 8pS chloride channel. The latter channel had characteristics similar to cystic fibrosis transmembrane conductance regulator (CFTR). Apical or basolateral application of AngII activated both the 11pS NSC and the 3pS channels.

CONCLUSION

In nontransformed DPDE and CFPAC cells, specific AngII receptors mediate increases in [Ca ]. The latter effect of AngII may elicit activation of calcium-mediated chloride channels, suggesting a role for AngII as an alternative mediator of pancreatic ductal secretion.

Regulation of ion channels by protein tyrosine phosphorylation

Ion channels are regulated by protein phosphorylation and dephosphorylation of serine, threonine, and tyrosine residues. Evidence for the latter process, tyrosine phosphorylation, has increased substantially since this topic was last reviewed. In this review, we present a comprehensive summary and synthesis of the literature regarding the mechanism and function of ion channel regulation by protein tyrosine kinases and phosphatases. Coverage includes the majority of voltage-gated, ligand-gated, and second messenger-gated channels as well as several types of channels that have not yet been cloned, including store-operated Ca2+ channels, nonselective cation channels, and epithelial Na+ and Cl- channels. Additionally, we discuss the critical roles that channel-associated scaffolding proteins may play in localizing protein tyrosine kinases and phosphatases to the vicinity of ion channels.

Contractile effects of angiotensin peptides in rat aorta are differentially dependent on tyrosine kinase activity

It has been suggested that tyrosine kinase activity participates in the regulation of signal transduction associated with angiotensin II (Ang II)-induced pharmaco-mechanical coupling in rat aortic smooth muscle. We further tested the effects of genistein, a tyrosine-kinase inhibitor, and its inactive analogue, daidzein, on angiotensin I (Ang I), angiotensin III (Ang III) and angiotensin IV (Ang IV) contractions, as compared with those on Ang II. Genistein partially inhibited Ang II- and Ang I-induced contractions. The genistein-induced inhibition was more evident on Ang III and especially important on Ang IV contractile effects. Thus, Ang IV- and Ang III-induced contractions seem to be more dependent on tyrosine kinase activity than those evoked by Ang II or Ang I. Daidzein did not significantly affect the contractile effects of any of angiotensin peptides tested. These results clearly suggest that the inhibition of the action of angiotensin peptides actions by genistein is mediated by inhibition of endogenous tyrosine kinase activity. Furthermore, our data show that the type and/or intensity of tyrosine kinase activity is differentially associated with the contractile effects of different angiotensin peptides in rat aorta. Nifedipine, a blocker of membrane L-type Ca2+ channels, strongly inhibited Ang IV-induced contractions. At the same time, it significantly inhibited Ang III contractile effects as compared with Ang II and Ang I contractions. Meanwhile, we observed a close relationship between calcium influx and tyrosine kinase phosphorylation activity under the stimulatory effects of angiotensin peptides. Furthermore, genistein did not significantly influence the phasic contractions induced by angiotensin peptides in Ca2+-free Krebs-Henseleit solution. Thus, it appears that Ca2+ influx, rather than the release of Ca2+ from IP3-sensitive stores, may play a major role in the contractile effects of angiotensin peptides in rat aorta via tyrosine kinase activation. One argument against a direct action of genistein on the Ca2+ channel itself is that it did not markedly affect the K+-induced contraction (depolarisation) in rat aorta. At the same time, a potential role for tyrosine kinase activity in the process of calcium entry is suggested. An elevation of intracellular calcium via tyrosine kinase-mediated processes may mediate the actions of G-protein coupled receptor agonists in smooth muscle, including angiotensin peptides.

Invited review: focal adhesion and small heat shock proteins in the regulation of actin remodeling and contractility in smooth muscle

Smooth muscle cells are able to adapt rapidly to chemical and mechanical signals impinging on the cell surface. It has been suggested that dynamic changes in the actin cytoskeleton contribute to the processes of contractile activation and mechanical adaptation in smooth muscle. In this review, evidence for functionally important changes in actin polymerization during smooth muscle contraction is summarized. The functions and regulation of proteins associated with "focal adhesion complexes" (membrane-associated dense plaques) in differentiated smooth muscle, including integrins, focal adhesion kinase (FAK), c-Src, paxillin, and the 27-kDa small heat shock protein (HSP27) are described. Integrins in smooth muscles are key elements of mechanotransduction pathways that communicate with and are regulated by focal adhesion proteins that include FAK, c-Src, and paxillin as well as proteins known to mediate cytoskeletal remodeling. Evidence that functions of FAK and c-Src protein kinases are closely intertwined is discussed as well as evidence that focal adhesion proteins mediate key signal transduction events that regulate actin remodeling and contraction. HSP27 is reviewed as a potentially significant effector protein that may regulate actin dynamics and cross-bridge function in response to activation of p21-activated kinase and the p38 mitogen-activated protein kinase signaling pathway by signaling pathways linked to integrin proteins. These signaling pathways are only part of a large number of yet to be defined pathways that mediate acute adaptive responses of the cytoskeleton in smooth muscle to environmental stimuli.

Role of chloride in constriction of descending vasa recta by angiotensin II

We investigated the dependence of ANG II (10−8 M)-induced constriction of outer medullary descending vasa recta (OMDVR) on membrane potential (Ψm) and chloride ion. ANG II depolarized OMDVR, as measured by fully loading them with the voltage-sensitive dye bis[1,3-dibutylbarbituric acid-(5)] trimethineoxonol [DiBAC4(3)] or selectively loading their pericytes. ANG II was also observed to depolarize pericytes from a resting value of −55.6 ± 2.6 to −26.2 ± 5.4 mV when measured with gramicidin D-perforated patches. When measured with DiBAC4(3) in unstimulated vessels, neither changing extracellular Cl− concentration ([Cl−]) nor exposure to the chloride channel blocker indanyloxyacetic acid 94 (IAA-94; 30 μM) affected Ψm. In contrast, IAA-94 repolarized OMDVR pretreated with ANG II. Neither IAA-94 (30 μM) nor niflumic acid (30 μM, 1 mM) affected the vasoactivity of unstimulated OMDVR, whereas both dilated ANG II-preconstricted vessels. Reduction of extracellular [Cl−] from 150 to 30 meq/l enhanced ANG II-induced constriction. Finally, we identified a Cl−channel in OMDVR pericytes that is activated by ANG II or by excision into extracellular buffer. We conclude that constriction of OMDVR by ANG II involves pericyte depolarization due, in part, to increased activity of chloride channels.

Angiotensin II inhibits HCO-3 absorption via a cytochrome P-450-dependent pathway in MTAL

The role of ANG II in the regulation of ion reabsorption by the renal thick ascending limb is poorly understood. Here, we demonstrate that ANG II (10(-8) M in the bath) inhibits HCO-3 absorption by 40% in the isolated, perfused medullary thick ascending limb (MTAL) of the rat. The inhibition by ANG II was abolished by pretreatment with eicosatetraynoic acid (10 microM), a general inhibitor of arachidonic acid metabolism, or 17-octadecynoic acid (10 microM), a highly selective inhibitor of cytochrome P-450 pathways. Bath addition of 20-hydroxyeicosatetraenoic acid (20-HETE; 10(-8) M), the major P-450 metabolite in the MTAL, inhibited HCO-3 absorption, whereas pretreatment with 20-HETE prevented the inhibition by ANG II. The addition of 15-HETE (10(-8) M) to the bath had no effect on HCO-3 absorption. The inhibition of HCO-3 absorption by ANG II was reduced by >50% in the presence of the tyrosine kinase inhibitors genistein (7 microM) or herbimycin A (1 microM). We found no role for cAMP, protein kinase C, or NO in the inhibition by ANG II. However, addition of the exogenous NO donor S-nitroso-N-acetylpenicillamine (SNAP; 10 microM) or the NO synthase (NOS) substrate L-arginine (1 mM) to the bath stimulated HCO-3 absorption by 35%, suggesting that NO directly regulates MTAL HCO-3 absorption. Addition of 10(-11) to 10(-10) M ANG II to the bath did not affect HCO-3 absorption. We conclude that ANG II inhibits HCO-3 absorption in the MTAL via a cytochrome P-450-dependent signaling pathway, most likely involving the production of 20-HETE. Tyrosine kinase pathways also appear to play a role in the ANG II-induced transport inhibition. The inhibition of HCO-3 absorption by ANG II in the MTAL may play a key role in the ability of the kidney to regulate sodium balance and extracellular fluid volume independently of acid-base balance.

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.