Angiotensin-II-induced enhanced expression of Gi proteins is attenuated by losartan in A10 vascular smooth muscle cells: role of AT1 receptors

Implication of multiple signaling pathways in the regulation of angiotensin II induced enhanced expression of Giα proteins in vascular smooth muscle cells

We have previously shown that A10 vascular smooth muscle cells (VSMC) exposed to angiotensin II (Ang II) exhibited overexpression of Giα proteins. In the present study, we examined the involvement of different signaling pathways in regulating Ang II induced enhanced expression of Giα proteins in VSMC by using pharmacological inhibitors. Ang II induced increased expression of Giα proteins in A10 VSMC was markedly attenuated by actinomycin D, losartan (an AT(1) receptor antagonist), dibutyryl cAMP, phospholipase C (PLC) inhibitor U73122, protein kinase C (PKC) inhibitors staurosporine and GP109203X, but not by PD123319 (an AT(2) receptor antagonist). In addition, BAPTA-AM and TMB-8 (chelators of intracellular Ca(2+)); and nifedipine (a blocker of L-type Ca(2+) channels) significantly inhibited Ang II induced enhanced expression of Giα proteins. On the other hand, extracellular Ca(2+) chelation using EGTA did not affect the Ang II evoked enhanced levels of Giα proteins. Furthermore, pretreatment of A10 VSMC with calmidazolium (an inhibitor of calmodulin), or KN93 (an inhibitor of CaM kinase), or genistein (an inhibitor of protein tyrosine kinase, PTK), also attenuated the increased levels of Giα proteins induced by Ang II. These results suggest that Ang II induced enhanced expression of Giα proteins may be regulated by different signaling pathways through AT(1) receptors in A10 VSMC.

Contribution of epidermal growth factor receptor transactivation in angiotensin II-induced enhanced expression of Gi protein and proliferation in A10 vascular smooth muscle cellsThis article is one of a selection of papers published in a special issue on Advances in Cardiovascular Research

We have recently shown that vasoactive peptides such as angiotensin II (Ang II) and endothelin-1 (ET-1) increased the expression of Gi proteins and proliferation of A10 vascular smooth muscle cells (VSMC) through MAP kinase / PI3 kinase pathways. The present study was undertaken to examine the implication of growth factor receptor activation in Ang II-induced enhanced expression of Gi proteins and proliferation of A10 VSMC and to further investigate the underlying mechanisms responsible for these increases. Cell proliferation was determined by [3H]thymidine incorporation, and the expression of Gi proteins and the phosphorylation of ERK1/2 and epidermal growth factor receptor (EGFR) was determined by Western blotting. Treatment of A10 VSMC with Ang II enhanced the expression of Gi proteins, which was attenuated by Ang II AT1 receptor antagonist but not by AT2 receptor antagonist. The inhibitor of EGFR also attenuated the enhanced expression of Gi proteins induced by Ang II to control levels. In addition, Ang II enhanced the phosphorylation of EGFR in A10 VSMC, and this was restored to control levels by the EGFR inhibitor and antioxidants. Furthermore, Ang II also augmented the proliferation and ERK1/2 phosphorylation of A10 VSMC, which were restored to control levels by the EGFR inhibitor. These data suggest that the Ang II-induced increase in oxidative stress transactivates EGFR, which through MAP kinase signaling may contribute to the enhanced expression of Gi proteins and thereby to the increased proliferation of A10 VSMC.

Role of oxidative stress in angiotensin II-induced enhanced expression of Gi(alpha) proteins and adenylyl cyclase signaling in A10 vascular smooth muscle cells

We have previously reported that angiotensin II (ANG II) treatment of A10 vascular smooth muscle cells (VSMCs) increased inhibitory G proteins (G(i) protein) expression and associated adenylyl cyclase signaling which was attributed to the enhanced MAP kinase activity. Since ANG II has been shown to increase oxidative stress, we investigated the role of oxidative stress in ANG II-induced enhanced expression of G(i)alpha proteins and examined the effects of antioxidants on ANG II-induced enhanced expression of G(i)alpha proteins and associated adenylyl cyclase signaling in A10 VSMCs. ANG II treatment of A10 VSMCs enhanced the production of O(2)(-) and the expression of Nox4 and P47(phox), different subunits of NADPH oxidase, which were attenuated toward control levels by diphenyleneiodonium (DPI). In addition, ANG II augmented the expression of G(i)alpha-2 and G(i)alpha-3 proteins in a concentration- and time-dependent manner; the maximal increase in the expression of G(i)alpha was observed at 1 to 2 h and at 0.1-1.0 microM. The enhanced expression of G(i)alpha-2 and G(i)alpha-3 proteins was restored to control levels by antioxidants such as N-acetyl-L-cysteine, alpha-tocopherol, DPI, and apocynin. In addition, ANG II also enhanced the ERK1/2 phosphorylation that was restored to control levels by DPI. Furthermore, the inhibition of forskolin-stimulated adenylyl cyclase activity by low concentrations of 5'-O-(3-triotriphosphate) (receptor-independent G(i) functions) and ANG II-, des(Glu(18),Ser(19),Glu(20),Leu(21),Gly(22))atrial natriuretic peptide(4-23)-NH(2) (natriuretic peptide receptor-C agonist), and oxotremorine-mediated inhibitions of adenylyl cyclase (receptor-dependent functions) that were augmented in ANG II-treated VSMCs was also restored to control levels by antioxidant treatments. In addition, G(s)alpha-mediated diminished stimulation of adenylyl cyclase by stimulatory hormones in ANG II-treated cells was also restored to control levels by DPI. These results suggest that ANG II-induced enhanced levels of G(i)alpha proteins and associated functions in VSMCs may be attributed to the ANG II-induced enhanced oxidative stress, which exerts its effects through mitogen-activated protein kinase signaling pathway.

Cleavage of the angiotensin II type 1 receptor and nuclear accumulation of the cytoplasmic carboxy-terminal fragment

Our published studies show that the distribution of the ANG II type 1 (AT(1)) receptor (AT(1)R), expressed as a enhanced yellow fluorescent fusion (YFP) protein (AT(1)R/EYFP), is altered upon cellular treatment with ANG II or coexpression with intracellular ANG II. AT(1)R accumulates in nuclei of cells only in the presence of ANG II. Several transmembrane receptors are known to accumulate in nuclei, some as holoreceptors and others as cleaved receptor products. The present study was designed to determine whether the AT(1)R is cleaved before nuclear transport. A plasmid encoding a rat AT(1)R labeled at the amino terminus with enhanced cyan fluorescent protein (CFP) and at the carboxy terminus with EYFP was employed. Image analyses of this protein in COS-7 cells, CCF-STTG1 glial cells, and A10 vascular smooth muscle cells show the two fluorescent moieties to be largely spatially colocalized in untreated cells. ANG II treatment, however, leads to a separation of the fluorescent moieties with yellow fluorescence accumulating in more than 30% of cellular nuclei. Immunoblot analyses of extracts and conditioned media from transfected cells indicate that the CFP domain fused to the extracellular amino-terminal AT(1)R domain is cleaved from the membrane and that the YFP domain, together with the intracellular cytoplasmic carboxy terminus of the AT(1)R, is also cleaved from the membrane-bound receptor. The carboxy terminus of the AT(1)R is essential for cleavage; cleavage does not occur in protein deleted with respect to this region. Overexpressed native AT(1)R (nonfusion) is also cleaved; the intracellular 6-kDa cytoplasmic domain product accumulates to a significantly higher level with ANG II treatment.

Prevention of adipose tissue depletion during food deprivation in angiotensin type 2 receptor-deficient mice

Angiotensin (Ang) II is produced locally in various tissues, but its role in the regulation of tissue metabolism is still unclear. Recent studies have revealed the role of type 2 Ang II receptor (AT2R) in the control of energy homeostasis and lipid metabolism. The contribution of the AT2R to adaptation to starvation was tested using AT2R-deficient (AT2R (y)(/-)) mice. Fasted AT2R (y)(/-) mice exhibited a lower loss of adipose tissue weight associated to a decreased free fatty acid (FFA) release from stored lipids than the controls. In vitro studies show that Ang II causes an AT1R-mediated antilipolytic effect in isolated adipocytes. AT1R expression is up-regulated by fasting in both genotypes, but the increase is more pronounced in AT2R (y/-) mice. In addition, the increased muscle beta-oxidation displayed in AT2R (y/-) mice on a fed state, persists after fasting compared with wild-type mice. In liver from fed mice, AT2R deficiency did not modify the expression of genes involved in fatty acid oxidation. However, in response to fasting, the large increase of the expression of this subset of genes exhibited by wild-type mice, was impaired in AT2R (y/-) mice. Taken together, decreased lipolytic capacity and increased muscle fatty acid oxidation participate in the decreased plasma FFA observed in fasted AT2R (y/-) mice and could account for the lower FFA metabolism in the liver. These data reveal an important physiological role of AT2R in metabolic adaptations to fasting.

Oxidative stress contributes to the enhanced expression of Gialpha proteins and adenylyl cyclase signaling in vascular smooth muscle cells from spontaneously hypertensive rats

OBJECTIVE

We have previously shown an enhanced expression of Gialpha proteins in spontaneously hypertensive rats (SHR) that precedes the development of hypertension. Since oxidative stress has been shown to be increased in SHR, the present studies were undertaken to examine the role of oxidative stress in enhanced expression of Gialpha proteins in SHR.

METHODS

Aortic vascular smooth muscle cells (VSMC) from 12-week-old SHR and Wistar-Kyoto (WKY) rats were used for the present studies. The levels of inhibitory guanine nucleotide regulatory proteins (Gialpha-2 and Gialpha-3) and stimulatory proteins (Gsalpha) were determined by western blotting techniques. Adenylyl cyclase activity was determined by measuring [32P]cAMP formation from [alpha-32P]ATP.

RESULTS

VSMC from SHR exhibited enhanced expression of Gialpha-2 and Gialpha-3 proteins as compared with age-matched WKY rats; however, the levels of Gsalpha proteins were not different between the two groups. The levels of superoxide anion (O2-) were also increased in SHR as compared with WKY rats, and losartan, an AT1 receptor antagonist, restored the enhanced levels to control WKY rat levels. Treatment of VSMC with antioxidants such as N-acetyl-L-cysteine (NAC) or diphenyleneiodonium (DPI) for 24 h decreased the enhanced expression of Gialpha-2 and Gialpha-3 proteins in a concentration-dependent manner in VSMC from SHR. In addition, the inhibition of forskolin-stimulated enzyme activity by low concentrations of GTPgammaS (receptor-independent Gi functions) and C-ANP4-23-mediated inhibition of adenylyl cyclase (receptor-dependent Gi functions) that were significantly enhanced in SHR were restored to WKY rat levels by NAC and DPI treatments. Similarly, diminished stimulation of adenylyl cyclase by GTPgammaS, isoproterenol and sodium fluoride in SHR was also restored towards control WKY rat levels by NAC and DPI treatments. Furthermore, PD98059, a selective inhibitor of mitogen-activated protein kinase, was able to restore the enhanced expression of Gialpha proteins in VSMC from SHR towards WKY rat levels. In addition, the enhanced activity of extracellular signal-regulated kinase 1/2 in SHR as compared with WKY rats, as demonstrated by enhanced phosphorylation of extracellular signal-regulated kinase 1/2, was also restored to WKY rat levels by NAC or DPI.

CONCLUSIONS

These results suggest that enhanced levels of Gialpha proteins and associated functions in SHR may be attributed to the enhanced oxidative stress present in SHR, which exerts its effects through the mitogen-activated protein kinase signaling pathway.

Angiotensin II-induced increase of T-type Ca2+ current and decrease of L-type Ca2+ current in heart cells

The effect of angiotensin II (Ang II) on the T- and L-type calcium currents (I(Ca)) in single ventricular heart cells of 18-week-old fetal human and 10-day-old chick embryos was studied using the whole-cell voltage clamp technique. Our results showed that in both, human and chick cardiomyocytes, Ang II (10(-7)M) increased the T-type calcium current and decreased the L-type I(Ca). The effect of Ang II on both types of currents was blocked by the AT1 peptidic antagonist, [Sar1, Ala8] Ang II (2 x 10(-7)M). Protein kinase C activator, phorbol 12,13-dibutyrate, mimicked the effect of Ang II on the T- and L-type calcium currents. These results demonstrate that in fetal human and chick embryo cardiomyocytes Ang II affects the T- and L-type Ca2+ currents differently, and this effect seems to be mediated by the PKC pathway.

Attenuation of the serotonin-induced increase in intracellular calcium in rat aortic smooth muscle cells by sarpogrelate

Although serotonin (5-HT) induced proliferation of vascular smooth muscle cells is considered to involve changes in intracellular Ca2+([Ca2+]i), the mechanism of Ca2+mobilization by 5-HT is not well defined. In this study, we examined the effect of 5-HT on rat aortic smooth muscle cells (RASMCs) by Fura-2 microfluorometry for [Ca2+]imeasurements. 5-HT was observed to increase the [Ca2+]iin a concentration- and time-dependent manner. This action of 5-HT was dependent upon the extracellular concentration of Ca2+([Ca2+]e) and was inhibited by both Ca2+channel antagonists (verapamil and diltiazem) and inhibitors of sarcoplasmic reticular Ca2+pumps (thapsigargin and cyclopia zonic acid). The 5-HT-induced increase in [Ca2+]iwas blocked by sarpogrelate, a 5-HT2A-receptor antagonist, but not by different agents known to block other receptor sites. 5-HT-receptor antagonists such as ketanserin, cinanserin, and mianserin, unlike methysergide, were also found to inhibit the 5-HT-induced Ca2+mobilization, but these agents were less effective in comparison to sarpogrelate. On the other hand, the increase in [Ca2+]iin RASMCs by ATP, angiotensin II, endothelin-1, or phorbol ester was not affected by sarpogrelate. These results indicate that Ca2+mobilization in RASMCs by 5-HT is mediated through the activation of 5-HT2Areceptors and support the view that the 5-HT-induced increase in [Ca2+]iinvolves both the extracellular and intracellular sources of Ca2+.Key words: sarpogrelate, serotonin, vascular smooth muscle cells, intracellular Ca2+.