Angiotensin II-induced changes in guanine nucleotide binding and regulatory proteins

Modulation of Gi Proteins in Hypertension: Role of Angiotensin II and Oxidative Stress

Guanine nucleotide regulatory proteins (G-proteins) play a key role in the regulation of various signal transduction systems including adenylyl cyclase/cAMP and phospholipase C (PLC)/phosphatidyl inositol turnover (PI). These are implicated in the modulation of a variety of physiological functions such as platelet functions, cardiovascular functions, including arterial tone and reactivity. Several abnormalities in adenylyl cyclase activity, cAMP levels and G proteins have shown to be responsible for the altered cardiac performance and vascular functions observed in cardiovascular disease states. The enhanced or unaltered levels of inhibitory G-proteins (Giα-2 and Giα-3) and mRNA have been reported in different models of hypertension, whereas Gsα levels were shown to be unaltered. These changes in G-protein expression were associated with Gi functions. The enhanced levels of Giα proteins precede the development of blood pressure and suggest that overexpression of Gi proteins may be one of the contributing factors for the pathogenesis of hypertension. The augmented levels of vasoactive peptides, including angiotensin II (AngII), were shown to contribute to enhanced expression of Giα proteins and associated adenylyl cyclase signaling and thereby increased blood pressure. In addition, enhanced oxidative stress in hypertension due to Ang II may also be responsible for the enhanced expression of Giα proteins observed in hypertension. The mechanism by which oxidative stress enhances the expression of Gi proteins appears to be through the activation of mitogen activated protein (MAP) kinase activity.

Abnormal regulation of G protein alpha(i2) subunit in skin fibroblasts from insulin-resistant hypertensive individuals

BACKGROUND

Studies in experimental animals and human cells have demonstrated increased intracellular calcium (Ca(i2) signalling and Galphai signal transduction associated with hypertension. We have recently shown that angiotensin II-induced mobilization of Ca(i2) is enhanced in fibroblasts from hypertensive individuals in comparison with that in normotensive individuals and that it is blunted by insulin and pertussis toxin in insulin-sensitive, but not in insulin-resistant, patients. This suggests that G(i)-mediated signal transduction is reduced in insulin-resistant hypertension.

OBJECTIVE

To investigate the expression and regulation of Galpha(i2) subunit in insulin-sensitive and insulin-resistant hypertensive individuals.

METHODS

G protein alpha(i2) subunit mRNA was measured in cultured skin fibroblasts from patients with insulin-sensitive and insulin-resistant hypertension, by real-time reverse transcriptase polymerase chain reaction. We also investigated the effects of short-term exposure to fetal calf serum, angiotensin II and insulin, alone and in combination, on the expression of Galpha(i2) in vitro. Spectrofluorophotometric measurement of free Cai was performed in monolayers of 24 h serum-deprived cells in basal conditions and after exposure to angiotensin II, with and without pre-incubation with insulin.

RESULTS

Expression of Galpha(i2) was significantly greater in fibroblasts from hypertensive individuals than in normotensive individuals and the increase was unrelated to age and body mass. The difference was largely accounted for by greater values in insulin-sensitive than in insulin-resistant hypertensive individuals. In fibroblasts from those with insulin-sensitive hypertension, angiotensin II and insulin were additive to fetal calf serum in increasing the expression of Galpha(i2). In these patients, insulin blunted the angiotensin-II induced Cai transient. In contrast, in those with insulin-resistant hypertension, Galpha(i2) was lower and unresponsive to angiotensin II and insulin. Finally, in fibroblasts from insulin-resistant patients, insulin was unable to reduce the angiotensin II-induced Cai peak.

CONCLUSIONS

A subnormal Galpha(i2)-mediated signal transduction may be involved in the pathogenesis of cellular insulin resistance in hypertension. This novel Galpha(i2)-mediated signal transduction associated with insulin sensitivity in fibroblasts may help to control excessive angiotensin II signalling.

Angiotensin II enhances the expression of Gialpha in A10 cells (smooth muscle): relationship with adenylyl cyclase activity

In the present studies, we have investigated the effect of angiotensin II (AII) on guanine nucleotide regulatory protein (G protein) expression and functions in A10 smooth muscle cells. AII treatment of A10 cells enhanced the levels of inhibitory guanine nucleotide regulatory protein (Gi) as well as Gi mRNA and not of stimulatory guanine nucleotide regulatory protein (Gs) in a concentration-dependent manner as determined by immunoblot and Northern blot analysis, respectively. AII-evoked increased expression of Gialpha-2 and Gialpha-3 was inhibited by actinomycin D treatment (RNA synthesis inhibitor). The increased expression of Gialpha-2 and Gialpha-3 by AII was not reflected in functions, because the GTPgammaS-mediated inhibition of forskolin-stimulated adenylyl cyclase and the receptor-mediated inhibition of adenylyl cyclase by AII and C-ANP4-23 [des(Gln18, Ser19, Gln20, Leu21, Gly22) ANP4-23-NH2] were not augmented but attenuated in AII-treated A10 cells. The attenuation was prevented by staurosporine (a protein kinase C inhibitor) treatment. On the other hand, AII treatment did not affect the expression and functions of stimulatory guanine nucleotide regulatory protein (Gs), however, the stimulatory effects of 5'-O-(3-thiotriphosphate), isoproterenol, and N-ethylcarboxamide adenosine (NECA) on adenylyl cyclase activity were inhibited to various degrees by AII treatment. Staurosporine reversed the AII-evoked attenuation of isoproterenol- and NECA-stimulated enzyme activity. From these results, it can be suggested that AII, whose levels are increased in hypertension, may be one of the possible contributing factors responsible for exhibiting an enhanced expression of Gi protein in hypertension.

G-proteins in rat blood vessels--I. Identification

1. The purpose of this investigation was to identify various types of conventional, low and high molecular weight G-proteins in purified membranes of rat aorta and mesenteric artery. 2. In both blood vessels, immunoblotting of G-proteins using AS/7 antibody specific for Gi-1/2, EC/2 antibody specific for Gi-3 and RM/1 antibody specific for Gs-type conventional G-proteins demonstrated the presence of M(r) 28-43 kDa, M(r) 41 and 48 kDa, and M(r) 36-46 kDa polypeptides, respectively. Immunoblotting using a common antibody (GA/1) for the Gs and Gi alpha-subunits also revealed the existence of multiple polypeptides (M(r) 24-52 kDa) in both aorta and mesenteric artery, some of which were identified by the specific antibodies. The intensity and number of bands detected by most of the antibodies were greater in mesenteric artery than in aorta. 3. Cholera toxin (CT) catalyzed ADP-ribosylation of two Gs alpha (M(r) 43, 46 kDa) in both types of blood vessels with similar intensities of bands. Pertussis toxin (PT), on the other hand, catalyzed ADP-ribosylation of one Gi alpha (M(r) 41 kDa) polypeptide, and the intensity of this band was greater in aorta than in mesenteric artery. The polypeptides ADP-ribosylated by the toxins corresponded with their identification by antibodies. 4. Nitrocellulose blot overlay with [35S]GTP gamma S identified at least seven low molecular weight G-proteins (M(r) 21-30 kDa) in both aorta and mesenteric artery, with greater intensity of bands in mesenteric artery.(ABSTRACT TRUNCATED AT 250 WORDS)

G-proteins in rat blood vessels--II. Assessment of functional involvement

1. In this study, we compared G-protein-mediated contractile responses of rat aorta and mesenteric artery rings induced by the alpha-adrenoceptor agonist, norepinephrine (NE) and by the direct G-protein activator, sodium fluoride, using various probes. 2. Activator of the stimulatory G-protein (Gs), cholera toxin (CT), attenuated the sensitivity and maximum contractile response of both aorta and mesenteric artery to NE and sodium fluoride. The effect of the toxin on the NE-sensitivity was greater in mesenteric artery. 3. Pretreatment of tissues with the inhibitor of Gi-protein, pertussis toxin (PT), reduced the sensitivity as well as maximum contraction of both the aorta and mesenteric artery to sodium fluoride, and of the mesenteric artery to NE. PT attenuated only the sensitivity but not the maximum contraction of the aorta to NE. The inhibitory effect of PT on sensitivity to NE or sodium fluoride was greater in the aorta. 4. NE and sodium fluoride-induced contractions were reduced by the sulfhydryl G-protein inhibitor, N-ethylmaleimide (NEM) in both blood vessels. NEM produced greater inhibitory effect on the sensitivity of the aorta to both contractile agents. 5. These data demonstrate that CT, PT and NEM-sensitive G-proteins are involved in NE- and sodium fluoride-induced contractile responses of the rat aorta and mesenteric artery. The differential effects of the G-protein probes indicate that certain variations in G-protein-mediated contractile responses exist among the two blood vessels, suggesting that G-protein involvement in functional responses may vary with the type of blood vessel investigated.

Ontogenesis of sympathetic responsiveness in spontaneously hypertensive rats. II. Renal G proteins in male and female rats

Previously we have reported an increased renal alpha 1- and beta-adrenergic receptor expression in male spontaneously hypertensive rats that occurred ontogenetically in parallel with blood pressure elevation. However, increased receptor numbers were not accompanied by enhanced stimulation of inositol phosphate and cyclic AMP formation, respectively, indicating relative desensitization. We have now quantified alpha-subunits of the G proteins Gs (Gs short and Gs long), G(i), and Gq by immunoblotting and pertussis toxin-catalyzed ADP-ribosylation in renal membranes from 3-, 6-, 8-, and 28-week-old normotensive and spontaneously hypertensive male Wistar-Kyoto rats; additionally, 28-week-old female normotensive and spontaneously hypertensive rats were studied. During ontogenesis of male normotensive rats, Gs short increased, Gs long remained unchanged, and G(i) alpha and Gq alpha decreased. In adult normotensive rats no sex differences were detected for Gs short, Gs long, and G(i) alpha. When male rats from the normotensive and spontaneously hypertensive strains were compared, all G protein alpha-subunits were similar in the prehypertensive phase (3 weeks). In established hypertension (28 weeks), Gs long and Gq alpha were reduced, whereas Gs short and G(i) alpha remained unchanged. Gs long was also reduced during the development of hypertension (6 and 8 weeks), whereas Gs short and G(i) alpha were not consistently altered in this phase. The reduction in Gs long seen in male adult hypertensive rats was not detectable in female hypertensive rats.(ABSTRACT TRUNCATED AT 250 WORDS)

G protein coupling of human platelet V1 vascular vasopressin receptors

We used several approaches to identify the G protein coupled to V1 vascular arginine vasopressin (AVP) receptors of human platelets. In purified platelet membranes, high-affinity specific binding of [3H]AVP but not that of the V1 vascular antagonist [3H]d(CH2)5Tyr(Me)AVP was modulated by guanosine 5'-O-(3-thiotriphosphate) or sodium fluoride both in the presence and absence of MgCl2. AVP failed to modify the [alpha-32P]GTP labeling pattern or the cytosolic translocation of the 24- to 27-kDa GTP-binding proteins. AVP-stimulated GTPase activity of platelet membranes was blocked by antibodies specific for the COOH-terminal of the Gq alpha protein. AVP increased labeling of a 42-kDa platelet membrane protein by the photoreactive GTP analogue [alpha-32P]azidoanilido GTP. Immunoblotting of platelet proteins with various G protein-specific antibodies revealed that the 42-kDa protein labeled with [alpha-32P]azidoanilido GTP was immunoblotted only by antibodies specific for the alpha-subunit of GQ-11. Thus V1 vascular AVP receptors of human platelets are coupled in a divalent cation-dependent manner to a G protein belonging to the Gq-11 family.

Altered expression of G-protein mRNA in spontaneously hypertensive rats

We have recently demonstrated that the decreased ability of hormones, forskolin and GTP to stimulate adenylate cyclase in heart and aorta from spontaneously hypertensive rats (SHR), as compared to their age-matched Wistar-Kyoto control rats (WKY), was associated with enhanced levels of Gi- and not with Gs-regulatory proteins. In the present studies we have investigated the expression of Gi-regulatory proteins at the mRNA level by Northern blotting. Total RNA of heart ventricle and aorta from WKY and SHR was probed with radiolabeled cDNA inserts encoding Gi alpha-2 and Gi alpha-3. The Gi alpha-2 and Gi alpha-3 probes detected a message of 2-3 and 3-5 kb, respectively, in both WKY and SHR, however, the message was significantly enhanced in SHR, as compared by WKY. On the other hand the cDNA probe encoding Gs alpha detected a message of 1.8 kb in heart and aorta from both WKY and SHR, however, no difference in the levels of Gs alpha mRNA was detected in SHR and WKY tissues. These results indicate that the mRNA levels of Gi alpha-2 and Gi alpha-3 and not of Gs are overexpressed in heart and aorta from SHR, which may be responsible for the increased levels of Gi as shown earlier by immunoblotting techniques. It may be suggested that the enhanced vascular tone and impaired cardiac contractility in hypertension may partly be the consequences of increased levels of Gi in heart and aorta.