Midori Okumura

Angiotensin II Type-1 Receptor Blocker Valsartan Enhances Insulin Sensitivity in Skeletal Muscles of Diabetic Mice

Abstract—Angiotensin II has been shown to contribute to the pathogenesis of insulin resistance; however, the mechanism is not well understood. The present study was undertaken to investigate the potential effect of an angiotensin II type-1 (AT1) receptor blocker, valsartan, to improve insulin resistance and to explore the signaling basis of cross-talk of the AT1 receptor- and insulin-mediated signaling in type 2 diabetic KK-Ay mice. Treatment of KK-Ay mice with valsartan at a dose of 1 mg/kg per day, which did not influence systolic blood pressure, significantly increased insulin-mediated 2-[3H]deoxy-d-glucose (2-[3H]DG) uptake into skeletal muscle and attenuated the increase in plasma glucose concentration after a glucose load and plasma concentrations of glucose and insulin. In contrast, insulin-mediated 2-[3H]DG uptake into skeletal muscle was not influenced in AT2 receptor null mice, and an AT2 receptor blocker, PD123319, did not affect 2-[3H]DG uptake and superoxide production in skeletal muscle of KK-Ay mice. Moreover, we observed that valsartan treatment exaggerated the insulin-induced phosphorylation of IRS-1, the association of IRS-1 with the p85 regulatory subunit of phosphoinositide 3 kinase (PI 3-K), PI 3-K activity, and translocation of GLUT4 to the plasma membrane. It also reduced tumor necrosis factor-&agr; (TNF-&agr;) expression and superoxide production in skeletal muscle of KK-Ay mice. Specific AT1 receptor blockade increases insulin sensitivity and glucose uptake in skeletal muscle of KK-Ay mice via stimulating the insulin signaling cascade and consequent enhancement of GLUT4 translocation to the plasma membrane.

Deletion of Angiotensin II Type 2 Receptor Exaggerated Atherosclerosis in Apolipoprotein E–Null Mice

Background—The role of angiotensin II (Ang II) type 2 (AT2) receptor in atherosclerosis was explored with the use of AT2 receptor/apolipoprotein E (ApoE)–double-knockout (AT2/ApoE-DKO) mice, with a focus on oxidative stress. Methods and Results—After treatment with a high-cholesterol diet (1.25% cholesterol) for 10 weeks, ApoE-knockout (KO) mice developed atherosclerotic lesions in the aorta. In AT2/ApoE-DKO mice receiving a high-cholesterol diet, the atherosclerotic changes were further exaggerated, without significant changes in plasma cholesterol level and blood pressure. In the atherosclerotic lesion, an increase in superoxide production, NADPH oxidase activity, and expression of p47phox was observed. These changes were also greater in AT2/ApoE-DKO mice. An Ang II type 1 (AT1) receptor blocker, valsartan, inhibited atherosclerotic lesion formation, superoxide production, NADPH oxidase activity, and p47phox expression; these inhibitory effects were significantly weaker in AT2/ApoE-KO mice. We further examined the signaling mechanism of the AT2 receptor–mediated antioxidative effect in cultured fetal vascular smooth muscle cells. NADPH oxidase activity and phosphorylation and translocation of p47phox induced by Ang II were inhibited by valsartan but enhanced by an AT2 receptor blocker, PD123319. Conclusions—These results suggest that AT2 receptor stimulation attenuates atherosclerosis through inhibition of oxidative stress and that the antiatherosclerotic effect of valsartan could be at least partly due to AT2 receptor stimulation by unbound Ang II.

Inhibitory Effects of AT1 Receptor Blocker, Olmesartan, and Estrogen on Atherosclerosis Via Anti-Oxidative Stress

The present study explored the possibility that estrogen enhances the inhibitory effect of an angiotensin II type-1 (AT1) receptor blocker (ARB), olmesartan, on atherosclerosis, focusing on oxidative stress using apolipoprotein E knockout mice (ApoEKO). After 6 weeks on a high-cholesterol diet, marked atherosclerotic lesion formation with an increase in oxidative stress, such as superoxide production, NAD(P)H oxidase activity and expression of p47phox mRNA and rac-1 mRNA, were observed in the proximal aorta in both male and female ApoEKO mice, whereas these changes were less marked in female mice. Ovariectomy enhanced these parameters, the changes of which were reversed by 17&bgr;-estradiol (80 &mgr;g/kg per day) replacement. Treatment with olmesartan (3 mg/kg per day) significantly inhibited oxidative stress and atherosclerosis, whereas its inhibitory effects were more marked in female than in male or ovariectomized mice. Smaller doses of olmesartan (0.5 mg/kg per day) or 17&bgr;-estradiol (20 &mgr;g/kg per day) did not influence atherosclerosis and oxidative stress in ovariectomized mice, whereas co-administration of olmesartan and 17&bgr;-estradiol at these doses attenuated these parameters. An angiotensin-converting enzyme (ACE) inhibitor, temocapril, also inhibited atherosclerotic changes similarly to olmesartan. Moreover, angiotensin II–mediated activation of NAD(P)H oxidase in cultured vascular smooth muscle cells was attenuated by 17&bgr;-estradiol. These results indicate that estrogen and an ARB synergistically attenuate atherosclerosis at least partly via inhibition of oxidative stress.

Fluvastatin Enhances the Inhibitory Effects of a Selective AT1 Receptor Blocker, Valsartan, on Atherosclerosis

We investigated the effects of a 3-hydroxy-3-methylglutaryl–coenzyme A reductase inhibitor (statin) on the inhibitory effects of an angiotensin II type-1 receptor (AT1) blocker on atherosclerosis and explored cellular mechanisms. We gave apolipoprotein E null mice a high-cholesterol diet for 10 weeks and measured atherosclerotic plaque area and lipid deposition. Neither 1 mg/kg per day of valsartan nor 3 mg/kg per day of fluvastatin had any effect on blood pressure or cholesterol concentration; however, both drugs decreased plaque area and lipid deposition after 10 weeks. We then reduced the doses of both drugs to 0.1 mg/kg per day and 1 mg/kg per day, respectively. At these doses, neither drug had an effect on atherosclerotic lesions. When both drugs were combined at these doses, a significant reduction in atherosclerotic lesions was observed. Similar inhibitory effects of valsartan or fluvastatin on the expressions of nicotinamide-adenine dinucleotide/nicotinamide-adenine dinucleotide phosphate oxidase subunits p22phox and p47phox, production of superoxide anion, the expression of monocyte chemoattractant protein-1, and intercellular adhesion molecule-1 expression were observed. These results suggest that concomitant AT1 receptor and cholesterol biosynthesis blockade, particularly when given concomitantly, blunts oxidative stress and inflammation independent of blood pressure or cholesterol-related effects.

Effect of Estrogen and AT1 Receptor Blocker on Neointima Formation

Abstract—The present study explored the possibility that estrogen may enhance the inhibitory effect of an angiotensin (Ang) II type 1 (AT1) receptor blocker on neointima formation in vascular injury, and investigated the signaling mechanism involved in their actions. Polyethylene cuff placement around the femoral artery of mice induced neointima formation and increased bromodeoxyuridine (BrdU) incorporation into vascular smooth muscle cells. These changes were significantly smaller in female mice than in male mice. Ovariectomy enhanced neointima formation and BrdU incorporation in the injured artery, which were reversed by 17&bgr;-estradiol (80 &mgr;g/kg per day) replacement. Treatment with a selective AT1 receptor blocker, olmesartan (3 mg/kg per day), significantly inhibited neointima formation and BrdU incorporation, whereas the inhibitory effects of olmesartan were more marked in intact female mice than in male or ovariectomized mice. Phosphorylation of extracellular signal–regulated kinase (ERK), signal transducer and activator of transcription (STAT) 1, and STAT3 was increased in the injured artery. These increases were significantly smaller in intact female mice than in male or ovariectomized mice. Olmesartan or estrogen attenuated the phosphorylation of ERK and STAT in the injured artery, whereas these inhibitory effects were greater in intact female mice. Lower doses of olmesartan (0.5 mg/kg per day) or 17&bgr;-estradiol (20 &mgr;g/kg per day) did not influence neointima formation, BrdU incorporation, and ERK and STAT phosphorylation in ovariectomized mice, whereas coadministration of olmesartan and 17&bgr;-estradiol at these doses attenuated these parameters. These results indicate that estrogen and an AT1 receptor blocker synergistically attenuate vascular remodeling, which is at least partly via inhibition of ERK and STAT activity.

Sex Difference in Vascular Injury and the Vasoprotective Effect of Valsartan Are Related to Differential AT2 Receptor Expression

The angiotensin II type 2 (AT2) receptor is upregulated in pathological conditions such as vascular injury and exerts antagonistic effects against AT1 receptor–mediated actions. We examined the possibility that the sex difference in vascular remodeling is associated with altered AT2 receptor expression, which is located on the X chromosome. In this study, we examined this possibility by using AT2 receptor–null (Agtr2−) mice. Vascular injury was induced by polyethylene cuff placement around the femoral artery of wild-type (Agtr2+) and Agtr2− mice. In Agtr2+ mice, AT2 receptor expression in the injured artery was enhanced, and this increase was greater in female than in male mice, with no significant difference in AT1 receptor expression between male and female mice. Increases in neointimal formation, DNA synthesis, expression of monocyte chemoattractant protein-1, production of superoxide anion, and NADPH oxidase activity in the injured artery were attenuated in female compared with male mice. These parameters were augmented in Agtr2− mice, whereas the sex differences in these parameters were smaller in Agtr2− than in Agtr2+ mice. Treatment with a nonhypotensive dose of the AT1 receptor blocker valsartan decreased these parameters significantly in Agtr2+ mice, and these inhibitory effects of valsartan were greater in female mice. This sex difference in valsartan’s inhibitory effect was less marked in Agtr2− mice. Our results suggest that the sex difference in response to vascular injury could be at least partially attributed to the exaggerated AT2 receptor expression in the injured vessel in female mice.

Nicotine Enhances Angiotensin II-Induced Mitogenic Response in Vascular Smooth Muscle Cells and Fibroblasts

Objective—The pathogenetic mechanism of tobacco-related cardiovascular diseases is still not well defined. We examined the potential possibility of an interaction between nicotine, a major component of cigarette smoke, and angiotensin II (Ang II), which plays an important role in the pathogenesis of cardiovascular diseases characterized by Ang II type 1 (AT1) receptor-mediated abnormal growth of vascular smooth muscle cells (VSMC) and fibroblasts. Methods and Results—Nicotine or Ang II-stimulated [3H]thymidine incorporation and c-fos expression in adult rat aortic VSMC and adventitial fibroblast. The nicotine-induced DNA synthesis was not affected by valsartan, an AT1 receptor-specific blocker, or PD123319, an Ang II type 2 (AT2) receptor-specific antagonist. Nicotine or Ang II stimulation rapidly increased extracellular signal-regulated kinase (ERK) activation, tyrosine- and serine-phosphorylation of signal transducer and activator of transcription (STAT)1 and STAT3, and p38 mitogen-activated protein kinase (p38 MAPK), in both cell types. Interestingly, co-administration of nicotine and Ang II at lower doses, which did not affect cell growth, induced DNA synthesis and c-fos expression accompanied by enhancement of ERK, STAT, and p38MAPK activity. PD98059, a mitogen-activated protein kinase/ERK kinase inhibitor, or SB23058, a p38MAPK inhibitor, significantly attenuated the vasotrophic effect of nicotine and Ang II. Conclusions—These results suggest that nicotine exerts a growth-promoting effect on vascular cells and enhances the Ang II-induced vasotrophic effect, which is at least partly mediated by the activation of ERK, STAT, and p38MAPK.

Attenuation of inflammatory vascular remodeling by angiotensin II type 1 receptor-associated protein.

To explore the role of angiotensin II Type 1 receptor–associated protein (ATRAP) in vascular remodeling, we developed transgenic mice for mouse ATRAP cDNA and examined remodeling after inflammatory vascular injury induced by polyethylene cuff placement. In ATRAP transgenic (ATRAP-Tg) mice, ATRAP mRNA was increased 3- to 4-fold in the heart, aorta, and femoral artery. ATRAP-Tg mice showed no significant change in body weight, systolic blood pressure, heart rate, and heart/body weight ratio. However, cell proliferation and neointimal formation in the injured artery were attenuated in ATRAP-Tg mice. The increase in NADPH oxidase activity and the expression of p22phox, a reduced nicotinamide-adenine dinucleotide/reduced nicotinamide-adenine dinucleotide phosphate oxidase subunit, after cuff placement was also attenuated in ATRAP-Tg mice. Moreover, activation of extracellular signal–regulated kinase, signal transducer and activator of transcription 1, and signal transducer and activator of transcription 3 after cuff placement was significantly reduced in ATRAP-Tg mice. Pressor response and cardiac hypertrophy induced by angiotensin II infusion and pressure overload were also attenuated in ATRAP-Tg mice. These results suggest that ATRAP plays an important role in vascular remodeling as a negative regulator.

Possible involvement of AT2 receptor dysfunction in age-related gender difference in vascular remodeling.

This study explored the possible involvement of AT(2) receptor stimulation in the age-related gender difference in vascular remodeling of mouse femoral artery induced by cuff placement. In the young adult group of wild-type mice (10 weeks of age), the increase in DNA synthesis, neointimal formation, expression of chemokines, and superoxide anion production in the injured femoral artery were smaller in female than in male mice. These gender differences were smaller in the aged group (50-55 weeks of age) of wild-type mice, because vascular responses of female mice in the aged group were stronger than those in the young group. Treatment with 17β-estradiol attenuated vascular remodeling in aged female mice. AT(2) receptor expression in the injured artery was higher in female than in male in the young group. AT(2) receptor expression in the injured artery of female mice was lower in the aged group than in the young group. Lack of AT(2) receptor increased neointimal formation in the aged group and reduced the inhibitory action of 17β-estradiol in aged female mice. Our findings suggest a possibility that the change in AT(2) receptor stimulation by aging might be involved in the response to estrogen and improvement of vascular remodeling in the aged female group.