Phorbol ester and interleukin-1 induce interleukin-6 gene expression in vascular smooth muscle cells via independent pathways

Muscle mass, muscle strength and the renin-angiotensin system

The renin-angiotensin system (RAS) is a classically known circulatory regulatory system. In addition to the previously known multi-organ circulatory form of the RAS, the existence of tissue RASs in individual organs has been well established. Skeletal muscle has also been identified as an organ with a distinct RAS. In recent years, the effects of RAS activation on skeletal muscle have been elucidated from several perspectives: differences in motor function due to genetic polymorphisms of RAS components, skeletal muscle dysfunction under conditions of excessive RAS activation such as heart failure, and the effects of the use of RAS inhibitors on muscle strength. In addition, the concept of the RAS itself has recently been expanded with the discovery of a 'protective arm' of the RAS formed by factors such as angiotensin-converting enzyme 2 and angiotensin 1-7. This has led to a new understanding of the physiological function of the RAS in skeletal muscle. This review summarizes the diverse physiological functions of the RAS in skeletal muscle and considers the potential of future therapeutic strategies targeting the RAS to overcome problems such as sarcopenia and muscle weakness associated with chronic disease.

Smooth muscle cell specific NEMO deficiency inhibits atherosclerosis in ApoE−/− mice

The development of atherosclerotic plaques is the result of a chronic inflammatory response coordinated by stromal and immune cellular components of the vascular wall. While endothelial cells and leukocytes are well-recognised mediators of inflammation in atherosclerosis, the role of smooth muscle cells (SMCs) remains incompletely understood. Here we aimed to address the role of canonical NF-κB signalling in SMCs in the development of atherosclerosis. We investigated the role of NF-κB signalling in SMCs in atherosclerosis by employing SMC-specific ablation of NEMO, an IKK complex subunit that is essential for canonical NF-κB activation, in ApoE^−/− mice. We show that SMC-specific ablation of NEMO (NEMO^SMCiKO) inhibited high fat diet induced atherosclerosis in ApoE^−/− mice. NEMO^SMCiKO/ApoE^−/− mice developed less and smaller atherosclerotic plaques, which contained fewer macrophages, decreased numbers of apoptotic cells and smaller necrotic areas and showed reduced inflammation compared to the plaques of ApoE^−/− mice. In addition, the plaques of NEMO^SMCiKO/ApoE^−/− mice showed higher expression of α-SMA and lower expression of the transcriptional factor KLF4 compared to those of ApoE^−/− mice. Consistently, in vitro, NEMO-deficient SMCs exhibited reduced proliferation and migration, as well as decreased KLF4 expression and lower production of IL-6 and MCP-1 upon inflammatory stimulus (TNF or LPS) compared to NEMO-expressing SMCs. In conclusion, NEMO-dependent activation of NF-κB signalling in SMCs critically contributes to the pathogenesis of atherosclerosis by regulating SMC proliferation, migration and phenotype switching in response to inflammatory stimuli.

Angiotensin II induces interleukin-6 expression in astrocytes: Role of reactive oxygen species and NF-κB

Previously, we showed that the bio-peptide angiotensin (Ang) II induces interleukin-6 (IL-6) in cultured astrocytes; however, the mechanism(s) involved in this effect were unknown. In the current study, we determined in brainstem and cerebellum astrocytes from the spontaneously hypertensive rat (SHR), the effect of Ang II to induce IL-6 as well as reactive oxygen species (ROS) generation. Results from this study showed that Ang II significantly induced the differential expression of IL-6 mRNA and protein levels in astrocytes from both regions of Wistar and SHRs. There were differences in the ability of Ang II to induce IL-6 mRNA and protein levels, but these differences were not apparent at all time points examined. Ang II also induced ROS generation, but there were no significant differences between ROS generation in SHR samples as compared to the Wistar samples. Ang II-induced IL-6 levels were mediated via the AT₁/Nuclear Factor Kappa beta/ROS pathway. Overall, our findings suggest that there may be dysregulation in IL-6 production from astrocytes, contributing to differences observed in SHRs versus its normotensive control. Elucidating the mechanisms involved in Ang II pro-inflammatory effects in the central nervous system may lead to the development of novel therapeutic strategies that can be harnessed not just to treat hypertension, but other Ang II-mediated diseases as well.

Angiotensin II Type-2 receptors modulate inflammation through signal transducer and activator of transcription proteins 3 phosphorylation and TNFα production

Angiotensin subtype-1 receptor (AT(1)R) influences inflammatory processes through enhancing signal transducer and activator of transcription proteins 3 (STAT3) signal transduction, resulting in increased tumor necrosis factor-α (TNF-α) production. Although angiotensin subtype-2 receptor (AT(2)R), in general, antagonizes AT(1)R-stimulated activity, it is not known if AT(2)R has any anti-inflammatory effects. In this study, we tested the hypothesis that AT(2)R activation plays an anti-inflammatory role by reducing STAT3 phosphorylation and TNF-α production. Changes in AT(2)R expression, TNF-α production, and STAT3 phosphorylation were quantified by Western blotting, Bio-Plex cytokine, and phosphoprotein cellular signaling assays in PC12W cells that express AT(2)R but not AT(1)R, in response to the AT(2)R agonist, CGP-42112 (CGP, 100 nm), or AT(2)R antagonist PD-123319 (PD, 1 μm). A 100% increase in AT(2)R expression in response to stimulation with its agonist CGP was observed. Further, AT(2)R activation reduced TNF-α production by 39% and STAT3 phosphorylation by 83%. In contrast, PD decreased AT(2)R expression by 76%, increased TNF-α production by 84%, and increased STAT3 phosphorylation by 67%. These findings suggest that increased AT(2)R expression may play a role in the observed decrease in inflammatory pathway activation through decreased TNF-α production and STAT3 signaling. Restoration of AT(2)R expression and/or its activation constitute a potentially novel therapeutic target for the management of inflammatory processes.

The frail renin-angiotensin system

Over the last few decades, the understanding of the renin-angiotensin system (RAS) has advanced dramatically. RAS is now thought to play a crucial role in physiologic and pathophysiologic mechanisms in almost every organ system and is a key regulator of hypertension, cardiovascular disease, and renal function. Angiotensin II (Ang II) promotes inflammation and the generation of reactive oxygen species and governs onset and progression of vascular senescence, which are all associated with functional and structural changes, contributing to age-related diseases. Although the vast majority of the actions of Ang II, including vascular senescence, are mediated by the Ang II type 1 receptor (AT1R), the identification, characterization, and cloning of the angiotensin type 2 receptor has focused attention on this receptor and to its antagonistic effect on the detrimental effects of AT1R. This review provides an overview of the changes in RAS with aging and age-disease interactions culminating in the development of frailty.

Resveratrol attenuates angiotensin II-induced interleukin-6 expression and perivascular fibrosis

Recent studies have shown that resveratrol (3,5,4'-trihydroxystilbene), a polyphenolic compound found in grapes and red wine, has various beneficial effects on cardiovascular diseases and prolongs the life span of mice fed a high-fat diet. We hypothesized that resveratrol may attenuate vascular inflammatory response induced by angiotensin (Ang) II. We examined the effect of resveratrol on Ang II-induced interleukin (IL)-6 expression in vascular smooth muscle cells (VSMCs). Resveratrol significantly attenuated Ang II-induced IL-6 mRNA expression and IL-6 protein in the supernatant of VSMC in a dose-dependent manner. Resveratrol suppressed the IL-6 gene promoter activity. Resveratrol inhibited the Ang II-induced cAMP-response element-binding protein and nuclear factor-kappa B activity, which are critical for Ang II-induced IL-6 gene activation. An increase in the serum concentration of IL-6 induced by Ang II infusion was attenuated by an oral administration of resveratrol. Resveratrol also inhibited Ang II-induced hypertension and perivascular fibrosis of the heart. Although hydralazine reduced blood pressure level equal to resveratrol, it did not reduce the Ang II-induced IL-6 production and perivascular fibrosis. These data suggest that the inhibition of Ang II-induced vascular inflammation and high blood pressure by resveratrol may contribute, at least in part, to the anti-atherogenic effects of resveratrol.

Antivasospastic and antiinflammatory effects of caspase inhibitor in experimental subarachnoid hemorrhage

Object

Inflammation in the subarachnoid space and apoptosis of arterial endothelial cells have been implicated in the development of delayed cerebral vasospasm after subarachnoid hemorrhage (SAH). The authors investigated mechanisms of possible antivasospastic effects of N-benzyl-oxycarbonyl-Val-Ala-Asp-fluoromethylketone (Z-VAD-FMK), a caspase inhibitor that can inhibit both inflammatory and apoptotic systems, in animal models of SAH.

Methods

Rabbits were assigned to three groups of eight animals each and were subjected to SAH by injection of blood into the cisterna magna. The experiments were performed in the following groups: SAH only, SAH + vehicle, and SAH + Z-VAD-FMK. The Z-VAD-FMK (1 mg) or vehicle (5% dimethyl sulfoxide) was intrathecally administered before SAH induction. Diameters of the basilar artery (BA) were measured on angiograms obtained before and 2 days after SAH. The BA diameter on Day 2 was expressed as a percentage of that before SAH. Interleukin (IL)–1β in the cerebrospinal fluid (CSF) was examined using Western blotting, and brains were immunohistochemically examined for caspase-1 and IL-1β. In a separate experiment, 20 rats were subjected to SAH and their brains were immunohisto-chemically assessed for caspase-1, IL-1β, and macrophages.

Results

In rabbits, Z-VAD-FMK significantly attenuated cerebral vasospasm (the BA diameter on Day 2 in SAH-only, SAH + vehicle, and SAH + Z-VAD-FMK groups was 66.6 ± 3.2%, 66.3 ± 3.7%, and 82.6 ± 4.9% of baseline, respectively), and suppressed IL-1β release into the CSF and also suppressed immunoreactivities of caspase-1 and IL-1β in macrophages infiltrating into the subarachnoid space. Immunoreactivities for caspase-1 and IL-1β were observed in immunohistochemically proven infiltrating macrophages in rats.

Conclusions

These results indicate that caspase activation may be involved in the development of SAH-induced vasospasm through inflammatory reaction.

Antihypertensive medications and differences in muscle mass in older persons: the Health, Aging and Body Composition Study

OBJECTIVES

To evaluate whether older persons using angiotensin-converting enzyme (ACE) inhibitors have a larger lower extremity muscle mass (LEMM) than users of other antihypertensive drugs.

DESIGN

Cross-sectional analysis of data from the Health, Aging and Body Composition (Health ABC) Study.

SETTING

University of Tennessee, Memphis, and University of Pittsburgh clinics.

PARTICIPANTS

A community-based sample of 2,431 well functioning participants of the Health ABC, aged 70 to 79, who were free of heart failure, were selected according to use of antihypertensive medications: ACE inhibitors (n=197), beta-blockers (n=169), thiazides (n=216), calcium-channel blockers (n=340), or none (n=1,509).

MEASUREMENTS

LEMM, assessed using dual-energy x-ray absorptiometry, compared by index drug in analysis of variance models unadjusted and adjusted for demographics, study site, height, body fat, physical activity, blood pressure, coronary artery disease, diabetes mellitus, and chronic pulmonary disease.

RESULTS

LEMM significantly differed across the study groups, being larger in users of ACE inhibitors than in users of other drugs (unadjusted and adjusted models). LEMM was comparable in users of ACE inhibitors and no drug users. A trend toward larger LEMM was also observed in sex- and ethnicity-stratified analyses and in the subgroup of noncoronary hypertensive participants.

CONCLUSION

In older persons, use of ACE inhibitors is associated cross-sectionally with larger LEMM. This finding suggests a possible explanation of the benefits of ACE inhibitors in wasting syndromes. If confirmed in longitudinal studies, this pharmacological action might have important implications for the prevention of physical disability in older patients with hypertension.

Regulation of corticotropin-releasing factor receptor type 2beta messenger ribonucleic acid by interleukin-1beta in rat vascular smooth muscle cells

OBJECTIVE

Corticotropin-releasing factor receptor type 2 (CRF R2) messenger RNA (mRNA) expression in the rodent heart or vessels is modulated by exposure to urocortin and glucocorticoids. In addition, we previously found that incubation with a variety of cytokines, such as interleukin (IL)-1 and IL-6, and tumor necrosis factor-alpha also reduced CRF R2beta mRNA expression, with IL-1beta being the most effective. In this study, we further explored the regulation of CRF R2beta mRNA levels by IL-1beta in the rat vascular smooth muscle A7r5 cells.

METHODS

A7r5 cells were incubated with IL-1beta, urocortin, or both for 6 h, after pre-incubating with or without anti-IL-1beta antibody (Ab) for 30 min, and then CRF R2beta mRNA levels were measured by RNase protection assay. Cells were incubated with lipopolysaccharides, IL-1beta, IL-6, dexamethasone, forskolin, or urocortin for 20 min, and then intracellular cAMP was measured by cAMP RIA.

RESULTS

IL-1beta produced a significant time-dependent decrease in CRF R2beta mRNA levels. Combined urocortin and IL-1beta administration did not have synergistic effects on the decrease in CRF R2beta mRNA levels. IL-1beta Ab failed to block the ability of urocortin to regulate CRF R2beta mRNA levels, suggesting that urocortin regulated CRF R2beta mRNA levels via another pathway than IL-1beta production. Urocortin induced the intracellular cAMP production in A7r5 cells, while IL-1beta failed to induce it.

CONCLUSION

The multifactorial regulation of CRF R2beta mRNA expression in the A7r5 cells serves to limit the inotropic and chronotropic effects of CRF R2 agonists such as urocortin during prolonged physical or immune challenge.

Evidence that Galpha(q)-coupled receptor-induced interleukin-6 mRNA in vascular smooth muscle cells involves the nuclear factor of activated T cells

The immunosuppressant cyclosporin A inhibits transcription mediated by the nuclear factor of activated T-cells (NFAT), a key regulator of cytokine gene expression in lymphocytes that integrates phospholipase C signaling. NFAT is also expressed in vascular smooth muscle cells, but the genes it regulates there are unknown. Here we show that Galpha(q)-coupled P2Y nucleotide receptor signaling in rat vascular smooth muscle cells increases NFAT-mediated luciferase reporter expression. It also induces interleukin (IL)-6 gene expression but not other cytokine mRNAs including IL-1, IL-2, IL-3, IL-4, IL-10, gamma-interferon, tumor necrosis factor-alpha, or tumor necrosis factor-beta. IL-6 mRNA induction by UTP is more rapid and transient then that caused by IL-1beta stimulation and is partially blocked by cyclosporin A or by expression of a trans-dominant NFAT inhibitor. Expression of recombinant NFATc1 markedly augments IL-6 mRNA induction by these and other agonists, which is partially attributable to NFAT-regulated paracrine mediators. However, trans-dominant NFkappaB inhibitors strongly interfere with IL-6 mRNA induction both by IL-1beta and by UTP, which synergistically evoke IL-6 mRNA expression. These findings suggest that NFAT is among the cofactors involved in NFkappaB-dependent IL-6 gene induction by Ca(2+)-mobilizing receptors in vascular smooth muscle cells.

Angiotensin II type 1 receptor antagonist decreases plasma levels of tumor necrosis factor alpha, interleukin-6 and soluble adhesion molecules in patients with chronic heart failure

OBJECTIVES

To evaluate the effects of an angiotensin (Ang II) type 1 receptor antagonist on immune markers in patients with congestive heart failure (CHF).

BACKGROUND

Ang II stimulates production of immune factors via the Ang II type 1 receptor in vitro, and the long-term effects of Ang II type 1 receptor antagonists on plasma markers of immune activation are unknown in patients with CHF.

METHODS

Twenty-three patients with mild to moderate CHF with left ventricular dysfunction were randomly divided into two groups: treatment with Ang II type 1 receptor (candesartan cilexetil) (n = 14) or placebo (n = 9). We measured plasma levels of immune factors such as tumor necrosis factor alpha (TNFalpha), interleukin-6 (IL-6), soluble intercellular adhesion molecule-1 (sICAM-1) and soluble vascular cell adhesion molecule-1 (sVCAM-1). We also measured plasma levels of the neurohumoral factors such as atrial natriuretic peptide (ANP) and brain natriuretic peptide (BNP) and cyclic guanosine monophosphate (cGMP), a biological marker of ANP and BNP.

RESULTS

Plasma levels of TNFalpha, IL-6, sICAM-1 and sVCAM-1 were increased in the 23 CHF patients compared with normal subjects and significantly decreased after 14 weeks of candesartan cilexetil treatment, but did not change in the placebo group. Plasma levels of BNP, which is a marker of ventricular injury, significantly decreased, and the molar ratio of plasma cGMP to cardiac natriuretic peptides (ANP + BNP) was significantly increased after candesartan cilexetil treatment, but did not change in the placebo group.

CONCLUSIONS

These findings suggest that 14 weeks of treatment with an Ang II type 1 receptor antagonist (candesartan cilexetil) decreased plasma levels of the immune markers such as TNFalpha, IL-6, sICAM-1 and sVCAM-1 and that it improved the biological compensatory action of endogenous cardiac natriuretic peptides in patients with mild to moderate CHF.

Differences in the expression and effects of interleukin-1 and -2 on androgen-sensitive and -insensitive human prostate cancer cell lines

The presence of interleukin-1 (IL-1) and IL-2 mRNA in five human prostate cancer (PCa) cell lines and their effects on cellular proliferation and prostate-specific antigen (PSA) levels were examined. IL-1 was found in androgen-unresponsive PC-3 and DU-145 but not in the androgen-responsive LNCaP, MDA-PCA-2a and MDA-PCA-2b cell lines. IL-2 message was absent while that of GAPDH (positive control) was present in all five cell lines. IL-1 decreased while IL-2 increased PSA levels of near-confluent LNCaP cells after 24 h of treatment. IL-1 inhibited whereas IL-2 stimulated the growth of sub-confluent LNCaP cells (72 h). Neither cytokine affected the proliferation of DU-145 or PC-3 cells.

Cyclosporin A protects against angiotensin II-induced end-organ damage in double transgenic rats harboring human renin and angiotensinogen genes

Leukocyte infiltration and adhesion molecule activation play a central role in the pathogenesis of angiotensin II (Ang II)-induced end-organ damage in double transgenic rats (dTGR) harboring human renin and angiotensinogen genes. We tested the hypothesis that the immunosuppressive agent cyclosporine (CsA) protects against the Ang II-induced myocardial and renal damage in dTGR. Furthermore, we investigated the influence of CsA on interleukin-6 (IL-6) and inducible nitric oxide synthase (iNOS) expression and the DNA binding activity of transcription factor necrosis factor-kappaB (NF-kappaB). The 4-week-old rats were divided into 4 groups: (1) control dTGR (n=20), (2) dTGR plus CsA (5 mg/kg SC for 3 weeks, n=15), (3) normotensive Sprague-Dawley (SD) rats (n=10), and (4) SD rats plus CsA (n=8). In dTGR, CsA completely prevented cardiovascular death (0 of 15 versus 9 of 20), decreased 24-hour albuminuria by 90% and systolic blood pressure by 35 mm Hg, and protected against the development of cardiac hypertrophy. Whole blood CsA concentrations 24 hours after the last drug treatment were 850+/-15 ng/mL. Semiquantitative ED-1 and Ki-67 (a nuclear cell proliferation-associated antigen) scoring showed that CsA prevented perivascular monocyte/macrophage infiltration and prevented cell proliferation in the kidneys and hearts of dTGR, respectively. The beneficial effects of CsA were, at least in part, mediated by the suppression of IL-6 and iNOS expression. Electrophoretic mobility shift assay revealed that CsA regulated inflammatory response in part through the NF-kappaB transcriptional pathway. In contrast to dTGR, CsA increased blood pressure in normotensive SD rats by 10 mm Hg and had no effect on cardiac mass or 24-hour urinary albumin excretion. Perivascular monocyte/macrophage infiltration, IL-6, and iNOS expression or cell proliferation were not affected by CsA in SD rats. Our findings indicate that CsA protects against Ang II-induced end-organ damage and underscore the central role of vascular inflammatory response in the pathogenesis of myocardial and renal damage in dTGR. The beneficial effects of CsA in the kidney and heart are mediated, at least in part, by suppression of IL-6 and iNOS expression via NF-kappaB transcriptional pathway.

Potentiation of lipopolysaccharide-induced IL-6 release by uridine triphosphate in macrophages: cross-interaction with cyclooxygenase-2-dependent prostaglandin E(2) production

Our previous study has demonstrated the potentiation by uridine triphosphate (UTP) of nitric oxide (NO) and prostaglandin E(2) (PGE(2)) production in lipopolysaccharide (LPS)-stimulated murine J774 macrophages. In this study, we found that the amount of interleukin-6 (IL-6) release in response to LPS stimulation was greatly enhanced in the presence of UTP. This enhancement exhibited concentration dependence and occurred after 8 h of treatment with LPS. RT-PCR analysis indicated that the steady-state level of IL-6 mRNA induced by LPS was apparently increased upon co-addition of UTP. The potentiation by UTP was inhibited by the treatment with U73122 (a phosphatidylinositol-phospholipase C inhibitor), BAPTA/AM (an intracellular Ca(2+) chelator), KN-93 (a selective inhibitor of calmodulin-dependent protein kinase) or PDTC (a nuclear factor kappaB inhibitor). To understand the cross-regulation among NO, PGE(2) and IL-6, all of which are dramatically induced after LPS stimulation, the effects of L-NAME (a nitric oxide synthase inhibitor), indomethacin (a cyclooxygenase inhibitor), NS-398 (a cycloxygenase-2 inhibitor) and IL-6 antibody were tested. The results revealed the positive regulation between PGE(2) and IL-6 synthesis because NS-398 and indomethacin inhibited LPS plus UTP-induced IL-6 release, and IL-6 antibody attenuated LPS plus UTP-induced PGE(2) release. Taken together these results reinforce the role of UTP as a regulatory element in inflamed sites by demonstrating the capacity of this nucleotide to potentiate LPS-induced release of inflammatory mediators.

Angiotensin II induces interleukin-6 transcription in vascular smooth muscle cells through pleiotropic activation of nuclear factor-kappa B transcription factors

Interleukin-6 (IL-6) is a multifunctional cytokine expressed by angiotensin II (Ang II)-stimulated vascular smooth muscle cells (VSMCs) that functions as an autocrine growth factor. In this study, we analyze the mechanism for Ang II-inducible IL-6 expression in quiescent rat VSMCs. Stimulation with the Ang II agonist Sar1 Ang II (100 nmol/L) induced transcriptional expression of IL-6 mRNA transcripts of 1.8 and 2.4 kb. In transient transfection assays of IL-6 promoter/luciferase reporter plasmids, Sar1 Ang II treatment induced IL-6 transcription in a manner completely dependent on the nuclear factor-kappaB (NF-kappaB) motif. Sar1 Ang II induced cytoplasmic-to-nuclear translocation of the NF-kappaB subunits Rel A and NF-kappaB1 with parallel changes in DNA-binding activity in a biphasic manner, which produced an early peak at 15 minutes followed by a nadir 1 to 6 hours later and a later peak at 24 hours. The early phase of NF-kappaB translocation was dependent on weak simultaneous proteolysis of the IkappaBalpha and beta inhibitors, whereas later translocation was associated with enhanced processing of the p105 precursor into the mature 50-kDa NF-kappaB1 form. Pretreatment with a potent inhibitor of IkappaBalpha proteolysis, TPCK, completely blocked Sar1 Ang IIAng II-induced NF-kappaB activation and induction of endogenous IL-6 gene expression, which indicated the essential role of NF-kappaB in mediating IL-6 expression. We conclude that Ang II is a pleiotropic regulator of the NF-kappaB transcription factor family and may be responsible for activating the expression of cytokine gene networks in VSMCs.

Protein kinase G mediates vascular endothelial growth factor-induced Raf-1 activation and proliferation in human endothelial cells

Vascular endothelial growth factor (VEGF) is an endothelium-specific, secreted protein that acts as a vasodilator, angiogenic peptide, and hyperpermeability factor. Recent reports have shown that nitric oxide synthase inhibitors block proliferation and microvascular hyperpermeability induced by VEGF. This study examined the mechanisms by which nitric oxide and its downstream signals mediate the VEGF-induced proliferative response in human umbilical vein endothelial cells (HUVECs). Nitric oxide synthase blockade by Nomega-nitro-L-arginine methyl ester prevented both the proliferative effect of VEGF and Raf-1 activation by VEGF as measured by cell counting and the capacity of immunoprecipitated Raf-1 to phosphorylate syntide 2, a Raf-1-specific synthetic substrate. VEGF-induced proliferation and Raf-1 kinase activity were also inhibited by Rp-8-pCPT-cGMPs and KT5823, inhibitors of the regulatory and catalytic subunits of cGMP-dependent protein kinase (PKG), respectively. The ability of PKG to stimulate proliferation was verified by the observation that the PKG activator, 8-pCPT-cGMPs, stimulated both Raf-1 kinase activity and endothelial proliferation in a dose-dependent manner. Furthermore, recombinant catalytically active PKG phosphorylated and activated Raf-1 in a reconstituted system. Finally, Raf-1 immunoprecipitated from VEGF-stimulated endothelial cells coprecipitated with PKG, indicating a direct protein-protein interaction in activated cells. We conclude that VEGF induces increases in both proliferation and Raf-1 kinase activity in HUVECs and these activities are dependent on NO and its downstream effector, PKG.