Activation of protein kinase A by atrial natriuretic peptide in neonatal rat cardiac fibroblasts: role in regulation of the local renin-angiotensin system

Myocardial toxicity induced by silica nanoparticles in a transcriptome profile

The deleterious effects of silica nanoparticles (SiNPs) on human health and the ecological system have gradually gained attention owing to their heavy annual output and extensive global flux. The updated epidemiological or experimental investigations have demonstrated the potential myocardial toxicity triggered by SiNPs, but the underlying mechanisms and long-lasting cardiac effects are still poorly understood. Here, a rat model of sub-chronic respiratory exposure to SiNPs was conducted, and the histopathological analysis and ultrastructural investigation of heart tissues were carried out. More importantly, a comprehensive analysis of whole-genome transcription was utilized in rat heart to uncover key biological and cellular mechanisms triggered by SiNPs. The widening of myocardial space and partial fiber rupture were clearly manifested in rat heart after prolonged SiNPs exposure, particularly accompanied by mitochondrial swelling and cristae rupture. With the aid of Affymetrix GeneChips, 3153 differentially expressed genes (DEGs) were identified after SiNPs exposure, including 1916 down- and 1237 up-regulated genes. GO and KEGG analysis illustrated many important biological processes and pathways perturbed by SiNPs, mainly specializing in cellular stress, energy metabolism, actin filament dynamics and immune response. Signal-net analysis revealed that Prkaca (PKA) plays a core role in the cardiac toxification process of prolonged exposure of SiNPs to rats. Furthermore, qRT-PCR verified that PKA-mediated calcium signaling is probably responsible for SiNPs-induced cardiac injury. Conclusively, our study revealed that SiNPs caused myocardial injury, and particularly, provided transcriptomic insight into the role of PKA-calcium signaling triggered by SiNPs, which would facilitate SiNPs-based nanosafety assessment and biomedicine development.

Diabetes, heart damage, and angiotensin II. What is the relationship link between them? A minireview

Cardiovascular complications are the main cause of mortality and morbidity in the diabetic patients, in whom changes in myocardial structure and function have been described. Numerous molecular mechanisms have been proposed that could contribute to the development of a cardiac damage. In this regard, angiotensin II (Ang II), a proinflammatory peptide that constitutes the main effector of the renin-angiotensin system (RAS) has taken a relevant role. The aim of this review was to analyze the role of Ang II in the different biochemical pathways that could be involved in the development of cardiovascular damage during diabetes. We performed an exhaustive review in the main databases, using the following terms: angiotensin II, cardiovascular damage, renin angiotensin system, inflammation, and diabetes mellitus. Classically, the RAS has been defined as a complex system of enzymes, receptors, and peptides that help control the blood pressure and the fluid homeostasis. However, in recent years, this concept has undergone substantial changes. Although this system has been known for decades, recent discoveries in cellular and molecular biology, as well as cardiovascular physiology, have introduced a better understanding of its function and relationship to the development of the diabetic cardiomyopathy.

Cardiac fibrosis: Myofibroblast-mediated pathological regulation and drug delivery strategies

Cardiac fibrosis remains an unresolved problem in heart diseases. After initial injury, cardiac fibroblasts (CFs) are activated and subsequently differentiate into myofibroblasts (myoFbs) that are major mediator cells in the pathological remodeling. MyoFbs exhibit proliferative and secretive characteristics, and contribute to extracellular matrix (ECM) turnover, collagen deposition. The persistent functions of myoFbs lead to fibrotic scars and cardiac dysfunction. The anti-fibrotic treatment is hindered by the elusive mechanism of fibrosis and lack of specific targets on myoFbs. In this review, we will outline the progress of cardiac fibrosis and its contributions to the heart failure. We will also shed light on the role of myoFbs in the regulation of adverse remodeling. The communication between myoFbs and other cells that are involved in the heart injury and repair respectively will be reviewed in detail. Then, recently developed therapeutic strategies to treat fibrosis will be summarized such as i) chimeric antigen receptor T cell (CAR-T) therapy with an optimal target on myoFbs, ii) direct reprogramming from stem cells to quiescent CFs, iii) "off-target" small molecular drugs. The application of nano/micro technology will be discussed as well, which is involved in the construction of cell-based biomimic platforms and "pleiotropic" drug delivery systems.

Effect of phenylacetamide isolated from lepidium apetalum on myocardial injury in spontaneously hypertensive rats and its possible mechanism

Context: In the antihypertensive study of phenylacetamide (PA) on spontaneously hypertensive rats (SHR), it was occasionally found that PA prevents myocardial injury.Objective: Clarify the protective mechanism of PA on myocardial injury in SHR rats.Materials and methods: In vivo, SHR rats were treated with or without PA (15, 30, 45 mg/kg) for 3 weeks (12 per group). In vitro, H9c2 cells were treated with PA (1, 5, 10 μM) for 24 h, and then stimulated with H₂O₂ (300 μM) for 4 h. Molecular mechanisms were explored through cardiac pathology, cardiac function and biochemical markers.Results: In vivo, PA (15, 30, 45 mg/kg) reduced CVF from 14.8 ± 1.62 to 9.94 ± 1.56, 8.6 ± 1.33, 8.14 ± 1.45%; increased the LVEF relative level from 0.8 ± 0.06 to 0.83 ± 0.04, 0.86 ± 0.05, 0.9 ± 0.04. All three doses can improve the cardiac pathological structure and function (LVEDD, LVESD, LVFS, heart index, NT-proBNP, CKMB, SBP); however, 45 mg/kg works best. But different doses show different molecular mechanisms. PA (15 mg/kg) improves RAAS system (REN, ACE), inflammation (ET-1, IL-1β) and MAPK pathway (p-ERK/ERK, p-JNK/JNK) better. PA (45 mg/kg) improves oxidative stress (SOD, NOX1) and TGF-β pathway (Smad3) better. In vitro, PA improved cell viability, oxidative stress (SOD, NOX1) and Smad3 protein expression.Discussion and conclusions: PA regulates different mechanisms at different concentrations to improve myocardial injury, and high dose is the best. This experiment provides a theoretical basis for the development of new clinical drugs for cardiovascular disease.

FasL expression in cardiomyocytes activates the ERK1/2 pathway, leading to dilated cardiomyopathy and advanced heart failure

Increase in the apoptotic molecule Fas ligand (FasL) in serum and cardiomyocytes has been shown to be associated with progressive dilated cardiomyopathy (DCM) and congestive heart failure (CHF) in humans. However, the underlying mechanism(s) of FasL-related deterioration of heart function remain obscure. The aim of the present study is to determine roles of myocardial FasL in the activation of alternative pathways such as extracellular-signal-regulated kinase 1/2 (ERK1/2), inflammation or fibrosis and to identify effective treatments of progressive DCM and advanced CHF. Transgenic mice with cardiomyocyte-specific overexpression of FasL were investigated and treated with an ERK1/2 inhibitor (U-0126), losartan (los), prednisolone (pred) or placebo. Morpho-histological and molecular studies were subsequently performed. FasL mice showed significantly higher mortality compared with wild-type (WT) littermates due to DCM and advanced CHF. Prominent perivascular and interstitial fibrosis, increased interleukin secretion and diffuse CD3-positive cell infiltration were evident in FasL hearts. Up-regulation of the short form of Fas-associated death domain (FADD)-like interleukin 1β-converting enzyme (FLICE) inhibitory protein (s-FLIP), RIP (receptor-interacting protein) and ERK1/2 and down-regulation of transforming growth factor beta 1 (TGFβ1) and nuclear factor-κB (NF-κB) was determined in the myocardium, whereas expression of ERK1/2, periostin (Postn) and osteopontin increased in cardiac fibroblasts. U-0126 and los increased CHF survival by 75% compared with pred and placebo groups. U-0126 had both anti-fibrotic and anti-apoptotic effects, whereas los reduced fibrosis only. Myocardial FasL expression in mice activates differential robust fibrotic, apoptotic and inflammatory responses via ERK1/2 in cardiomyocytes and cardiac fibroblasts inducing DCM and CHF. Blocking the ERK1/2 pathway prevented progression of FasL-induced DCM and CHF by reducing fibrosis, inflammation and apoptosis in the myocardium.

Role of microtubules in attenuation of PepG-induced vascular endothelial dysfunction by atrial natriuretic peptide

Apart from control of circulating fluid, atrial natriuretic peptide (ANP) exhibits anti-inflammatory effects in the lung. However, molecular mechanisms of ANP anti-inflammatory effects are not well-understood. Peripheral microtubule (MT) dynamics is essential for agonist-induced regulation of vascular endothelial permeability. Here we studied the role of MT-dependent signaling in ANP protective effects against endothelial cell (EC) barrier dysfunction and acute lung injury induced by Staphylococcus aureus-derived peptidoglican-G (PepG). PepG-induced vascular endothelial dysfunction was accompanied by MT destabilization and disruption of MT network. ANP attenuated PepG-induced MT disassembly, NFκB signaling and activity of MT-associated Rho activator GEF-H1 leading to attenuation of EC inflammatory activation reflected by expression of adhesion molecules ICAM1 and VCAM1. ANP-induced EC barrier preservation and MT stabilization were linked to phosphorylation and inactivation of MT-depolymerizing protein stathmin. Expression of stathmin phosphorylation-deficient mutant abolished ANP protective effects against PepG-induced inflammation and EC permeability. In contrast, siRNA-mediated stathmin knockdown prevented PepG-induced peripheral MT disassembly and endothelial barrier dysfunction. ANP protective effects in a murine model of PepG-induced lung injury were associated with increased phosphorylation of stathmin, while exacerbated lung injury in the ANP knockout mice was accompanied by decreased pool of stable MT. Stathmin knockdown in vivo reversed exacerbation of lung injury in the ANP knockout mice. These results show a novel MT-mediated mechanism of endothelial barrier protection by ANP in pulmonary EC and animal model of PepG-induced lung injury via stathmin-dependent control of MT assembly.

Hyperglycemia and angiotensin II cooperate to enhance collagen I deposition by cardiac fibroblasts through a ROS-STAT3-dependent mechanism

Cardiac fibroblasts significantly contribute to diabetes-induced structural and functional changes in the myocardium. The objective of the present study was to determine the effects of high glucose (alone or supplemented with angiotensin II) in the activation of the JAK2/STAT3 pathway and its involvement in collagen I production by cardiac fibroblasts. We observed that the diabetic environment 1) enhanced tyrosine phosphorylation of JAK2 and STAT3; 2) induced nuclear localization of tyrosine phosphorylated STAT3 through a reactive oxygen species-mediated mechanism, with angiotensin II stimulation further enhancing STAT3 nuclear accumulation; and 3) stimulated collagen I production. The effects were inhibited by depletion of reactive oxygen species or silencing of STAT3 in high glucose alone or supplemented with exogenous angiotensin II. Combined, our data demonstrate that increased collagen I deposition in the setting of high glucose occurred through a reactive oxygen species- and STAT3-dependent mechanism. Our results reveal a novel role for STAT3 as a key signaling molecule of collagen I production in cardiac fibroblasts exposed to a diabetic environment.

Intracardiac intracellular angiotensin system in diabetes

The renin-angiotensin system (RAS) has mainly been categorized as a circulating and a local tissue RAS. A new component of the local system, known as the intracellular RAS, has recently been described. The intracellular RAS is defined as synthesis and action of ANG II intracellularly. This RAS appears to differ from the circulating and the local RAS, in terms of components and the mechanism of action. These differences may alter treatment strategies that target the RAS in several pathological conditions. Recent work from our laboratory has demonstrated significant upregulation of the cardiac, intracellular RAS in diabetes, which is associated with cardiac dysfunction. Here, we have reviewed evidence supporting an intracellular RAS in different cell types, ANG II's actions in cardiac cells, and its mechanism of action, focusing on the intracellular cardiac RAS in diabetes. We have discussed the significance of an intracellular RAS in cardiac pathophysiology and implications for potential therapies.

Cardiac fibroblast: the renaissance cell

The permanent cellular constituents of the heart include cardiac fibroblasts, myocytes, endothelial cells, and vascular smooth muscle cells. Previous studies have demonstrated that there are undulating changes in cardiac cell populations during embryonic development, through neonatal development and into the adult. Transient cell populations include lymphocytes, mast cells, and macrophages, which can interact with these permanent cell types to affect cardiac function. It has also been observed that there are marked differences in the makeup of the cardiac cell populations depending on the species, which may be important when examining myocardial remodeling. Current dogma states that the fibroblast makes up the largest cell population of the heart; however, this appears to vary for different species, especially mice. Cardiac fibroblasts play a critical role in maintaining normal cardiac function, as well as in cardiac remodeling during pathological conditions such as myocardial infarct and hypertension. These cells have numerous functions, including synthesis and deposition of extracellular matrix, cell-cell communication with myocytes, cell-cell signaling with other fibroblasts, as well as with endothelial cells. These contacts affect the electrophysiological properties, secretion of growth factors and cytokines, as well as potentiating blood vessel formation. Although a plethora of information is known about several of these processes, relatively little is understood about fibroblasts and their role in angiogenesis during development or cardiac remodeling. In this review, we provide insight into the various properties of cardiac fibroblasts that helps illustrate their importance in maintaining proper cardiac function, as well as their critical role in the remodeling heart.

Cardiac fibroblasts: at the heart of myocardial remodeling

Cardiac fibroblasts are the most prevalent cell type in the heart and play a key role in regulating normal myocardial function and in the adverse myocardial remodeling that occurs with hypertension, myocardial infarction and heart failure. Many of the functional effects of cardiac fibroblasts are mediated through differentiation to a myofibroblast phenotype that expresses contractile proteins and exhibits increased migratory, proliferative and secretory properties. Cardiac myofibroblasts respond to proinflammatory cytokines (e.g. TNFalpha, IL-1, IL-6, TGF-beta), vasoactive peptides (e.g. angiotensin II, endothelin-1, natriuretic peptides) and hormones (e.g. noradrenaline), the levels of which are increased in the remodeling heart. Their function is also modulated by mechanical stretch and changes in oxygen availability (e.g. ischaemia-reperfusion). Myofibroblast responses to such stimuli include changes in cell proliferation, cell migration, extracellular matrix metabolism and secretion of various bioactive molecules including cytokines, vasoactive peptides and growth factors. Several classes of commonly prescribed therapeutic agents for cardiovascular disease also exert pleiotropic effects on cardiac fibroblasts that may explain some of their beneficial outcomes on the remodeling heart. These include drugs for reducing hypertension (ACE inhibitors, angiotensin receptor blockers, beta-blockers), cholesterol levels (statins, fibrates) and insulin resistance (thiazolidinediones). In this review, we provide insight into the properties of cardiac fibroblasts that underscores their importance in the remodeling heart, including their origin, electrophysiological properties, role in matrix metabolism, functional responses to environmental stimuli and ability to secrete bioactive molecules. We also review the evidence suggesting that certain cardiovascular drugs can reduce myocardial remodeling specifically via modulatory effects on cardiac fibroblasts.

Diversity of pathways for intracellular angiotensin II synthesis

PURPOSE OF REVIEW

The renin-angiotensin system (RAS) has undergone continuous advancement since the initial identification of renin as a pressor agent. Traditionally considered a circulatory system, the RAS is now known to exist as a tissue system as well. Recently, the tissue RAS has been further categorized as intracellular and extracellular. Owing to the unique location, the intracellular RAS encompasses new components, such as cathepsin D and chymase, which participate in intracellular angiotensin (Ang) II synthesis. In this review, evidence of the intracellular RAS and the mechanism of Ang II synthesis in various cell types will be discussed.

RECENT FINDINGS

A physiological role for intracellular Ang II in vascular and cardiac cells has recently been demonstrated. Evidence of intracellular Ang II generation has been shown in several cell types, particularly cardiac, renal, and vascular. Importantly, intracellular synthesis of Ang II is more prominent in hyperglycemic conditions and generally involves angiotensin-converting enzyme-dependent and angiotensin-converting enzyme-independent mechanisms.

SUMMARY

There is significant diversity in the mechanism of intracellular synthesis of Ang II in various cell types and pathological conditions. These observations suggest that a therapeutic intervention to block the RAS should take into consideration the nature of the disorder and the cell type involved.

Intracellular angiotensin II production in diabetic rats is correlated with cardiomyocyte apoptosis, oxidative stress, and cardiac fibrosis

OBJECTIVE

Many of the effects of angiotensin (Ang) II are mediated through specific plasma membrane receptors. However, Ang II also elicits biological effects from the interior of the cell (intracrine), some of which are not inhibited by Ang receptor blockers (ARBs). Recent in vitro studies have identified high glucose as a potent stimulus for the intracellular synthesis of Ang II, the production of which is mainly chymase dependent. In the present study, we determined whether hyperglycemia activates the cardiac intracellular renin-Ang system (RAS) in vivo and whether ARBs, ACE, or renin inhibitors block synthesis and effects of intracellular Ang II (iAng II).

RESEARCH DESIGN AND METHODS

Diabetes was induced in adult male rats by streptozotocin. Diabetic rats were treated with insulin, candesartan (ARB), benazepril (ACE inhibitor), or aliskiren (renin inhibitor).

RESULTS

One week of diabetes significantly increased iAng II levels in cardiac myocytes, which were not normalized by candesartan, suggesting that Ang II was synthesized intracellularly, not internalized through AT(1) receptor. Increased intracellular levels of Ang II, angiotensinogen, and renin were observed by confocal microscopy. iAng II synthesis was blocked by aliskiren but not by benazepril. Diabetes-induced superoxide production and cardiac fibrosis were partially inhibited by candesartan and benazepril, whereas aliskiren produced complete inhibition. Myocyte apoptosis was partially inhibited by all three agents.

CONCLUSIONS

Diabetes activates the cardiac intracellular RAS, which increases oxidative stress and cardiac fibrosis. Renin inhibition has a more pronounced effect than ARBs and ACE inhibitors on these diabetes complications and may be clinically more efficacious.

Stretch-induced regulation of angiotensinogen gene expression in cardiac myocytes and fibroblasts: opposing roles of JNK1/2 and p38alpha MAP kinases

The cardiac renin-angiotensin system (RAS) has been implicated in mediating myocyte hypertrophy, remodeling, and fibroblast proliferation in the hemodynamically overloaded heart. However, the intracellular signaling mechanisms responsible for regulation of angiotensinogen (Ao), a substrate of the RAS system, are largely unknown. Here we report the identification of JNK1/2 as a negative, and p38alpha as a major positive regulator of Ao gene expression. Isolated neonatal rat ventricular myocytes (NRVM) and fibroblasts (NRFB) plated on deformable membranes coated with collagen IV, were exposed to 20% equiaxial static-stretch (0-24 h). Mechanical stretch initially depressed Ao gene expression (4 h), whereas after 8 h, Ao gene expression increased in a time-dependent manner. Blockade of JNK1/2 with SP600125 increased basal Ao gene expression in NRVM (10.52+/-1.98 fold, P<0.001) and NRFB (13.32+/-2.07 fold, P<0.001). Adenovirus-mediated expression of wild-type JNK1 significantly inhibited, whereas expression of dominant-negative JNK1 and JNK2 increased basal and stretch-mediated (24 h) Ao gene expression, showing both JNK1 and JNK2 to be negative regulators of Ao gene expression in NRVM and NRFB. Blockade of p38alpha/beta by SB202190 or p38alpha by SB203580 significantly inhibited stretch-induced (24 h) Ao gene expression, whereas expression of wild-type p38alpha increased stretch-induced Ao gene expression in both NRVM (8.41+/-1.50 fold, P<0.001) and NRFB (3.39+/-0.74 fold, P<0.001). Conversely, expression of dominant-negative p38alpha significantly inhibited stretch response. Moreover, expression of constitutively active MKK6b (E) significantly stimulated Ao gene expression in the absence of stretch, indicating that p38 activation alone is sufficient to induce Ao gene expression. Taken together p38alpha was demonstrated to be a positive regulator, whereas JNK1/2 was found to be a negative regulator of Ao gene expression. Prolonged stretch diminished JNK1/2 activation, which was accompanied by a reciprocal increase in p38 activation and Ao gene expression. This suggests that a balance in JNK1/2 and p38alpha activation determines the level of Ao gene expression in myocardial cells.

Epac/Rap and PKA are novel mechanisms of ANP-induced Rac-mediated pulmonary endothelial barrier protection

Acute lung injury, sepsis, lung inflammation, and ventilator-induced lung injury are life-threatening conditions associated with lung vascular barrier dysfunction, which may lead to pulmonary edema. Increased levels of atrial natriuretic peptide (ANP) in lung circulation reported in these pathologies suggest its potential role in the modulation of lung injury. Besides well recognized physiological effects on vascular tone, plasma volume, and renal function, ANP may exhibit protective effects in models of lung vascular endothelial cell (EC) barrier dysfunction. However, the molecular mechanisms of ANP protective effects are not well understood. The recently described cAMP-dependent guanine nucleotide exchange factor (GEF) Epac activates small GTPase Rap1, which results in activation of small GTPase Rac-specific GEFs Tiam1 and Vav2 and Rac-mediated EC barrier protective responses. Our results show that ANP stimulated protein kinase A and the Epac/Rap1/Tiam/Vav/Rac cascade dramatically attenuated thrombin-induced pulmonary EC permeability and the disruption of EC monolayer integrity. Using pharmacological and molecular activation and inhibition of cAMP-and cGMP-dependent protein kinases (PKA and PKG), Epac, Rap1, Tiam1, Vav2, and Rac we linked ANP-mediated protective effects to the activation of Epac/Rap and PKA signaling cascades, which dramatically inhibited the Rho pathway of thrombin-induced EC hyper-permeability. These results suggest a novel mechanism of ANP protective effects against agonist-induced pulmonary EC barrier dysfunction via inhibition of Rho signaling by Epac/Rap1-Rac and PKA signaling cascades.

Activation of the intracellular renin-angiotensin system in cardiac fibroblasts by high glucose: role in extracellular matrix production

The occurrence of a functional intracellular renin-angiotensin system (RAS) has emerged as a new paradigm. Recently, we and others demonstrated intracellular synthesis of ANG II in cardiac myocytes and vascular smooth muscle cells that was dramatically stimulated in high glucose conditions. Cardiac fibroblasts significantly contribute to diabetes-induced diastolic dysfunction. The objective of the present study was to determine the existence of the intracellular RAS in cardiac fibroblasts and its role in extracellular matrix deposition. Neonatal rat ventricular fibroblasts were serum starved and exposed to isoproterenol or high glucose in the absence or presence of candesartan, which was used to prevent receptor-mediated uptake of ANG II. Under these conditions, an increase in ANG II levels in the cell lysate represented intracellular synthesis. Both isoproterenol and high glucose significantly increased intracellular ANG II levels. Confocal microscopy revealed perinuclear and nuclear distribution of intracellular ANG II. Consistent with intracellular synthesis, Western analysis showed increased intracellular levels of renin following stimulation with isoproterenol and high glucose. ANG II synthesis was catalyzed by renin and angiotensin-converting enzyme (ACE), but not chymase, as determined using specific inhibitors. High glucose resulted in increased transforming growth factor-beta and collagen-1 synthesis by cardiac fibroblasts that was partially inhibited by candesartan but completely prevented by renin and ACE inhibitors. In conclusion, cardiac fibroblasts contain a functional intracellular RAS that participates in extracellular matrix formation in high glucose conditions, an observation that may be helpful in developing an appropriate therapeutic strategy in diabetic conditions.

High-glucose-induced regulation of intracellular ANG II synthesis and nuclear redistribution in cardiac myocytes

The prevailing paradigm is that cardiac ANG II is synthesized in the extracellular space from components of the circulating and/or local renin-angiotensin system. The recent discovery of intracrine effects of ANG II led us to determine whether ANG II is synthesized intracellularly in neonatal rat ventricular myocytes (NRVM). NRVM, incubated in serum-free medium, were exposed to isoproterenol or high glucose in the absence or presence of candesartan, which was used to prevent angiotensin type 1 (AT1) receptor-mediated internalization of ANG II. ANG II was measured in cell lysates and the culture medium, which represented intra- and extracellularly synthesized ANG II, respectively. Isoproterenol increased ANG II concentration in cell lysates and medium of NRVM in the absence or presence of candesartan. High glucose markedly increased ANG II synthesis only in cell lysates in the absence and presence of candesartan. Western analysis showed increased intracellular levels of angiotensinogen, renin, and chymase in high-glucose-exposed cells. Confocal immunofluorocytometry confirmed the presence of ANG II in the cytoplasm and nucleus of high-glucose-exposed NRVM and along the actin filaments in isoproterenol-exposed cells. ANG II synthesis was dependent on renin and chymase in high-glucose-exposed cells and on renin and angiotensin-converting enzyme in isoproterenol-exposed cells. In summary, the site of ANG II synthesis, intracellular localization, and the synthetic pathway in NRVM are stimulus dependent. Significantly, NRVM synthesized and retained ANG II intracellularly, which redistributed to the nucleus under high-glucose conditions, suggesting a role for an intracrine mechanism in diabetic conditions.

Evidence for amylase release by cGMP via cAMP-dependent protein kinase in rat parotid acinar cells

Amylase release from the rat parotid gland is primarily mediated by a cAMP-dependent protein kinase (PKA). We previously reported that cGMP/cGMP-dependent protein kinase (PKG) signaling evokes amylase release. In the present study, we investigated whether cGMP-mediated amylase release might be due to cGMP/PKA signaling, as well as cGMP/PKG pathway. Activation of PKA by cGMP was required 100-1000-fold greater concentration than activation by cAMP in a parotid cytosol fraction. Synergistic activation of PKA by the combination of physiological cAMP and low concentration of cGMP was observed. Amylase release from intact acinar cells was synergistically stimulated by the combination of diBu-cAMP and 8-pCPT-cGMP. cGMP dose-dependently stimulated amylase release from saponin-permeabilized parotid acinar cells. Phosphorylation by cGMP produced phosphorylated proteins of the same size as those produced by cAMP. Phosphorylation by cGMP was inhibited by the addition of PKA inhibitor, H-89. These results suggest that cGMP activates both PKG and PKA. Thus, it appears that both cGMP/PKG and cGMP/PKA pathways mediate amylase release from rat parotid acinar cells.

Intracellular angiotensin II induces cell proliferation independent of AT1 receptor

We recently reported intracrine effects of angiotensin II (ANG II) on cardiac myocyte growth and hypertrophy that were not inhibited by the ANG II type 1 receptor (AT1) antagonist, losartan. To further determine the role of AT1 in intracrine effects, we studied the effect of intracellular ANG II (iANG II) on cell proliferation in native Chinese hamster ovary (CHO) cells and those stably transfected with AT1 receptor (CHO-AT1). CHO-AT1, but not CHO cells, showed enhanced proliferation following exposure to extracellular ANG II (eANG II). However, when transiently transfected with an iANG II expression vector, both cell types showed significantly enhanced proliferation, compared with those transfected with a scrambled peptide. Losartan blocked eANG II-induced cell proliferation, but not that induced by iANG II. To further confirm these findings, CHO and CHO-AT1 cells were stably transfected for iANG II expression (CHO-iA and CHO-AT1-iA, respectively). Cells grown in serum-free medium were counted every 24 h, up to 72 h. CHO-iA and CHO-AT1-iA cells showed a steeper growth curve compared with CHO and CHO-AT1, respectively. These observations were confirmed by Wst-1 assay. The AT1 receptor antagonists losartan, valsartan, telmisartan, and candesartan did not attenuate the faster growth rate of CHO-iA and CHO-AT1-iA cells. eANG II showed an additional growth effect in CHO-AT1-iA cells, which could be selectively blocked by losartan. These data demonstrate that intracrine ANG II can act independent of AT1 receptors and suggest novel intracellular mechanisms of action for ANG II.