Sex steroid regulation of thromboxane A2 receptors in cultured rat aortic smooth muscle cells

The Inflammatory Cytokine Imbalance for Miscarriage, Pregnancy Loss and COVID-19 Pneumonia

Pregnancy can be defined a vascular event upon endocrine control. In the human hemo-chorial placentation the chorionic villi penetrate the wall of the uterine spiral arteries, to provide increasing amounts of nutrients and oxygen for optimal fetal growth. In any physiological pregnancy the natural maternal response is of a Th1 inflammatory type, aimed at avoiding blood loss through the arteriolar wall openings. The control of the vascular function, during gestation as in any other condition, is achieved through the action of two main types of prostanoids: prostaglandin E2 and thromboxane on the one hand (for vasoconstriction and coagulation), prostacyclin on the other (for vasodilation and blood fluidification). The control of the maternal immune response is upon the responsibility of the fetus itself. Indeed, the chorionic villi are able to counteract the natural maternal response, thus changing the inflammatory Th1 type into the anti-inflammatory Th2. Clinical and experimental research in the past half century address to inflammation as the leading cause of abortion, pregnancy loss, premature delivery and related pulmonary, cerebral, intestinal fetal syndromes. Increased level of Interleukin 6, Interleukin 1-beta, Tumor Necrosis Factor-alfa, Interferon-gamma, are some among the well-known markers of gestational inflammation. On the other side, COVID-19 pneumonia is a result of extensive inflammation induced by viral replication within the cells of the respiratory tract. As it may happen in the uterine arteries in the absence of an effective fetal control, viral pneumonia triggers pulmonary vascular coagulation. The cytokines involved in the process are the same as those in gestational inflammation. As the fetus breathes throughout the placenta, fetal death from placental thrombosis is similar to adult death from pulmonary thrombosis. Preventing and counteracting inflammation is mandatory in both conditions. The most relevant literature dealing with the above-mentioned concepts is reviewed in the present article.

Effects of insulin resistance and testosterone on the participation of cyclooxygenase isoforms in vascular reactivity

Testosterone plays an important role in mediating hypertension and altered vascular reactivity associated with insulin resistance. In addition to other pathways, testosterone-dependent changes in aortic cyclooxygenase (COX-2) mRNA levels affect blood pressure following insulin resistance. However their effects on vascular tone are unclear. We studied the changes in contraction response to phenylephrine (PE) in the aorta and superior mesenteric artery (SMA) from intact and gonadectomized fructose-fed rats. Constriction response to PE was studied in tissues incubated with the COX-1 and COX-2-selective antagonists, SC-560 and NS-398, respectively, and indomethacin, in addition to assessing its role in endothelium-dependent relaxation. Finally changes in COX-2 protein expression and plasma thromboxane A2 (TXA2), a downstream vasoconstrictor metabolite of COX-2, were measured. In fructose-fed rats, castration prevented the increase in blood pressure but not insulin resistance. The involvement of COX-2 in mediating the alpha-adrenergic vasoconstriction was higher in intact rat aorta compared to COX-1, which was prevented by castration. However, in the SMA, COX-2 participation was dependent on testosterone alone. Fructose-induced attenuation of endothelial relaxation was restored by indomethacin, which suggests a pro-vasoconstrictor role for COX. Both diet and testosterone did not alter vascular COX-2 expression thus suggesting the involvement of downstream testosterone-dependent pathways. This is supported by increased plasma TXA2 in the castrated rats compared to intact rats. Isoform-specific actions of COX are tissue-selective in states of insulin resistance and involve potential testosterone-dependent downstream targets. Further studies are needed to investigate the role of androgens and insulin resistance in vascular arachidonic acid metabolism.

Testosterone treatment increases thromboxane function in rat cerebral arteries

We previously showed that testosterone, administered in vivo, increases the tone of cerebral arteries. A possible underlying mechanism is increased vasoconstriction through the thromboxane A2 (TxA2) pathway. Therefore, we investigated the effect of chronic testosterone treatment (4 wk) on TxA2 synthase levels and the contribution of TxA2 to vascular tone in rat middle cerebral arteries (MCAs). Using immunofluorescence and confocal microscopy, we demonstrated that TxA2 synthase is present in MCA segments in both smooth muscle and endothelial layers. Using Western blot analysis, we found that TxA2 synthase protein levels are higher in cerebral vessel homogenates from testosterone-treated orchiectomized (ORX+T) rats compared with orchiectomized (ORX) control animals. Functional consequences of changes in cerebrovascular TxA2 synthase were determined using cannulated, pressurized MCA segments in vitro. Constrictor responses to the TxA2 mimetic U-46619 were not different between the ORX+T and ORX groups. However, dilator responses to either the selective TxA2 synthase inhibitor furegrelate or the TxA2-endoperoxide receptor (TP) antagonist SQ-29548 were greater in the ORX+T compared with ORX group. In endothelium-denuded arteries, the dilation to furegrelate was attenuated in both the ORX and ORX+T groups, and the difference between the groups was abolished. These data suggest that chronic testosterone treatment enhances TxA2-mediated tone in rat cerebral arteries by increasing endothelial TxA2 synthesis without altering the TP receptors mediating constriction. The effect of in vivo testosterone on cerebrovascular TxA2 synthase, observed here after chronic hormone administration, may contribute to the risk of vasospasm and thrombosis related to cerebrovascular disease.

Estrogen potentiates vasopressin-induced contraction of female rat aorta by enhancing cyclooxygenase-2 and thromboxane function

To determine the roles of estrogen and constrictor prostanoids in vasopressin (VP)-induced contraction of female rat aorta, vascular reactivity to VP was determined in thoracic aortas of intact, ovariectomized, and ovariectomized + estrogen-replaced female rats in the presence of indomethacin (Indo), NS-398, SQ-29,548, or vehicle control. The effects of estrogen on vascular reactivity to the thromboxane A(2) analog U-46619 were also examined. Maximal contractile response to VP in intact female rats (5,567 +/- 276 mg/mg of aortic ring wt) was markedly attenuated by ovariectomy (2,485 +/- 394 mg; P < 0.001) and restored by estrogen replacement with 17beta-estradiol (5,059 +/- 194 mg; P > 0.1). Indo and NS-398 significantly attenuated maximal responses to VP in intact female rats to a similar extent [3,176 +/- 179 (P < 0.0001) and 3,258 +/- 152 mg (P < 0.0001), respectively]. Ovariectomy abolished and estrogen replacement restored the inhibitory effects of Indo, NS-398, and SQ-29,548. Contractile responses of rat aorta to U-46619 were significantly greater (P < 0.0001) in females (5,040 +/- 238 mg) than in males (3,679 +/- 96 mg). Ovariectomy markedly attenuated (3,923 +/- 84 mg; P < 0.01) and estrogen replacement restored (5,024 +/- 155 mg; P > 0.1) responses to U-46619 in female aortas. These data reveal that estrogen is an important regulator of the contractile responses of female rat aorta to VP, which appears to potentiate both cyclooxygenase-2 and constrictor prostanoid function in the vascular wall.

Androgens and cardiovascular disease

Globally, cardiovascular disease will continue causing most human deaths for the foreseeable future. The consistent gender gap in life span of approximately 5.6 yr in all advanced economies must derive from gender differences in age-specific cardiovascular death rates, which rise steeply in parallel for both genders but 5-10 yr earlier in men. The lack of inflection point at modal age of menopause, contrasting with unequivocally estrogen-dependent biological markers like breast cancer or bone density, makes estrogen protection of premenopausal women an unlikely explanation. Limited human data suggest that testosterone exposure does not shorten life span in either gender, and oral estrogen treatment increases risk of cardiovascular death in men as it does in women. Alternatively, androgen exposure in early life (perinatal androgen imprinting) may predispose males to earlier onset of atherosclerosis. Following the recent reevaluation of the estrogen-protection orthodoxy, empirical research has flourished into the role of androgens in the progression of cardiovascular disease, highlighting the need to better understand androgen receptor (AR) coregulators, nongenomic androgen effects, tissue-specific metabolic activation of androgens, and androgen sensitivity. Novel therapeutic targets may arise from understanding how androgens enhance early plaque formation and cause vasodilatation via nongenomic androgen effects on vascular smooth muscle, and how tissue-specific variations in androgen effects are modulated by AR coregulators as well as metabolic activation of testosterone to amplify (via 5alpha-reductase to form dihydrotestosterone acting on AR) or diversify (via aromatization to estradiol acting upon estrogen receptor alpha/beta) the biological effects of testosterone on the vasculature. Observational studies show that blood testosterone concentrations are consistently lower among men with cardiovascular disease, suggesting a possible preventive role for testosterone therapy, which requires critical evaluation by further prospective studies. Short-term interventional studies show that testosterone produces a modest but consistent improvement in cardiac ischemia over placebo, comparable to the effects of existing antianginal drugs. By contrast, testosterone therapy has no beneficial effects in peripheral arterial disease but has not been evaluated in cerebrovascular disease. Erectile dysfunction is most frequently caused by pelvic arterial insufficiency due to atherosclerosis, and its sentinel relationship to generalized atherosclerosis is insufficiently appreciated. The commonality of risk factor patterns and mechanisms (including endothelial dysfunction) suggests that the efficacy of antiatherogenic therapy is an important challenge with the potential to enhance men's motivation for prevention and treatment of cardiovascular diseases.

Cardiovascular effects of androgens

In the process of atherosclerosis sex steroids play a complex role in the vascular vessel wall system. Although a number of experimental studies have clearly documented an atheroprotective effect of estrogens, in recent clinical studies, estrogen replacement therapy has failed to reduce cardiovascular mortality. The effects of androgens on the cardiovascular system and cardiovascular diseases are even more controversial. Whereas in the past, androgens were mainly believed to exert adverse effects on the cardiovascular system, recent studies in men have documented a number of beneficial actions of testosterone in the arterial vascular system. Androgens affect lipid metabolism (e.g., LDL and HDL cholesterol, Lp(a)) and hemostasis (e.g., platelet aggregation and fibrinolytic activity). In addition, several other physiological and pathophysiological processes in the arterial vessel wall are influenced by androgens. Acute hemodynamic effects of testosterone on coronary vasomotion and stress-test-induced ischemia were reported. Additionally, recent animal and in vitro studies have further documented an inhibitory effect of androgens on neointimal plaque formation. This review discusses different and, in part, contradictory effects of androgens on the cardiovascular system including potential signal transduction pathways in androgen target cells.

Sex differences and the effects of sex hormones on hemostasis and vascular reactivity

Thrombus formation and vasospasm are involved in the initiation of acute ischemic events in the heart. Gender differences in persons with coronary artery disease and the incidence of myocardial ischemia have been clearly documented. In addition, it is well established that sex hormones influence the risk of developing coronary artery disease. Epidemiologic studies suggest that estrogen may exert a protective effect, yet the results of recently completed and ongoing prospective trials of estrogen and hormone (estrogen + progesterone) replacement suggest that these hormones can increase thrombotic events in postmenopausal women. This review focuses on sex (gender) differences in hemostasis and vascular reactivity and on the influence that sex hormones have on these physiologic systems. This review takes the novel approach of focusing on sex differences in hemostasis and vascular reactivity in healthy premenopausal women and men of a similar age. By comparing men and women in this age group, the confounding issues of age, pathology, or decline in sex hormone levels are avoided. Animal and in vitro investigations pertinent to examining potential cellular mechanism(s) of sex hormones in mediating these sex differences are discussed. We assume there is a relationship between the normal physiologic and pathologic effects of sex hormones; elucidating sex differences in normal cardiovascular function will help clarify the basis for sex differences in the incidence and manifestations of coronary heart disease and will aid in the future development of gender-specific therapies for cardiovascular disease.

Influence of gender on control of arterial tone in experimental hypertension

Endothelial dysfunction has been found to be less severe in female than in male spontaneously hypertensive rats (SHR), which could contribute to the gender differences observed in the extent and rate of progression of hypertension in SHR. However, the influence of gender on the roles of different endothelium-derived mediators in the arterial responses in hypertension have not been evaluated in detail. Therefore, contractile and relaxation responses of mesenteric arterial rings in vitro were studied in female and male SHR, with normotensive female and male Wistar-Kyoto rats (WKY) serving as controls. In norepinephrine (NE)-precontracted arterial rings, endothelium-dependent relaxations to ACh as well as endothelium-independent dilations to sodium nitroprusside were more pronounced in female than in male SHR, whereas relaxations to the beta-adrenoceptor agonist isoproterenol remained equally impaired in female and male SHR. The cyclooxygenase inhibitor diclofenac, which reduces the synthesis of dilating and constricting prostanoids, markedly enhanced the relaxations to ACh in male SHR but not in the other groups. The nitric oxide (NO) synthase inhibitor NG-nitro-L-arginine methyl ester attenuated the relaxations to ACh more effectively in female SHR and WKY than in the male groups. However, when endothelium-dependent hyperpolarization was prevented by precontracting the preparations with KCl, no significant differences were found in relaxations to ACh among the study groups. In conclusion, release of cyclooxygenase-derived constricting factors appeared to be more pronounced in male than in female SHR. In addition, the relative role of NO in endothelium-dependent arterial relaxation seemed to be higher in female than in male SHR, and relaxation induced by an NO donor also was more pronounced in female than in male SHR.

Cloning and characterization of an endogenous COS-7 cell thromboxane A2 receptor

A cDNA for a thromboxane A2 (TXA2) receptor was cloned from an SV40 transformed African Green Monkey kidney cell line (COS-7). The sequence is 98% homologous with the isoform of the human TXA2 receptor and has agonist and antagonist ligand binding characteristics that are not significantly different from the human receptor. Stimulation of the COS-7 cells with the TXA2 receptor agonist, ONO 11113 resulted in a significant increase in cAMP formation that was blocked by a receptor antagonist. The results raise the question of the utility of the COS-7 cell line for studies of cloned and expressed TXA2 receptor signalling mechanisms.

Gender-related differences in androgen regulation of thromboxane A2 receptors in rat aortic smooth-muscle cells

Thromboxane A2 (TXA2) has been implicated as an important mediator of cardiovascular diseases. Aortas obtained from male rats are more sensitive to TXA2 mimetics compared with those obtained from females. A similar phenomenon has been reported in canine coronary arteries. To determine whether there is a gender-related difference in the regulation of TXA2 receptors by androgenic steroids, we determined the effect of testosterone and dihydrotestosterone (DHT) on TXA2 receptor density in cultured rat aortic smooth-muscle (RASM) cells and guinea pig coronary artery smooth-muscle (CASM) cells. TXA2 receptor density (B(max)) and dissociation constant (Kd) were determined by radioligand binding studies with (125)I-BOP, a TXA2 receptor agonist. Testosterone significantly (p < 0.05) increased TXA2 receptor density in cultured RASM cells and guinea pig CASM cells. DHT significantly (p < 0.005) increased the B(max) in male RASM cells (62 +/- 2 vs. 40 +/- 3 fmol/mg protein; n = 7; p < 0.005). DHT increased the B(max) values in both male and female RASM cells, but the increase was significantly (p < 0.05) less in female than in male RASM cells (57 +/- 10% increase for male and 31 +/- 5% for female). Androgen-receptor protein was detected in RASM cells by Western blot and was less in the female RASM cells than in the male. The results indicate that RASM cells possess an androgen receptor and that gender-related differences exist in the regulation of expression of TXA2 receptors by androgens.

Long-term estradiol replacement decreases contractility of guinea pig coronary arteries to the thromboxane mimetic U46619

BACKGROUND

Estradiol replacement therapy reduces the incidence of coronary artery disease. Current evidence suggests that estradiol may stimulate the production of endothelium-derived NO and thereby reduce the contractile response of vascular smooth muscle. We investigated the effect of long-term replacement of estradiol on NO release and its effect on coronary artery contractility.

METHODS AND RESULTS

Female guinea pigs were ovariectomized and allowed to recover for 100 days. Pellets containing 17 beta-estradiol (0.25, 0.5, 1.5, and 7.5 mg released over 21 days) were placed subcutaneously for 19 to 20 days. Animals were then anesthetized, and the coronary arteries were excised and cut into ring segments. Rings were placed in small-vessel myographs for measurement of isometric force. Contractile responses of coronary arteries to cumulative addition of U46619 (10(-10) to 10(-5) mol/L), a thromboxane mimetic, were measured in the presence and absence of nitro-L-arginine (LNA), a selective NO synthase inhibitor, and methylene blue, a guanylate cyclase inhibitor. Low (0.25-mg) but not high (0.5-, 1.5-, or 7.5-mg) doses of estradiol inhibited the maximal contractile responses to U46619 compared with arteries from untreated castrated animals. In addition, both LNA and methylene blue potentiated contractile responses to U46619 of arteries from animals receiving 0.25 and 0.5 mg but not 1.5 and 7.5 mg estradiol. Negative log EC50 values were significantly inhibited at 0.25 and 7.5 mg but unaffected at 0.5 and 1.5 mg estradiol compared with castrated animals.

CONCLUSIONS

Estradiol at low doses may protect against vasospasm by stimulating endothelium-derived NO release and inhibiting coronary artery contractility.

Estrogen increases male rat aortic endothelial cell (RAEC) PGI2 release

Estrogen has been proposed as a negative risk factor for development of peripheral vascular disease yet mechanisms of this protection are not known. This study examines the hypothesis that estrogen stimulates rat aortic endothelial cell (RAEC) release of PGI2. Male Sprague-Dawley rat abdominal aortic 1-mm rings were placed on 35 mm matrigel plates, and incubated for 1 week. The cells were transferred to a Primaria 60-mm dish and maintained from passage 3 in RAEC complete media and experiments performed between passages 4-10. Cells were incubated with Krebs-Henseleit buffer (pH 7.4) containing carrier or increasing concentrations of beta-estradiol or testosterone for 60 min. The effluent was analyzed for eicosanoid release of 6-keto-PGF1 alpha (6-keto, PGI2 metabolite), PGE2 and thromboxane B2 (TXB2) by EIA (hormone stimulated-basal). Cells were analyzed for total protein by the Bradford method and for cyclooxygenase-1 (COX-1) and prostacyclin synthase (PS) content by Western blot analysis and densitometry. Testosterone did not alter RAEC 6-keto-PGF1 alpha release, whereas estrogen increased RAEC 6-keto-PGF1 alpha release in a dose-related manner. Estrogen preincubation (10 ng/ml) decreased COX-1 and PS content by 40% suggesting that the estrogen-induced increase in male RAEC PGI2 release was not due to increased synthesis of COX-1 or PS. These data support the hypothesis that estrogen stimulation can increase endogenous male RAEC release of PGI2.

Androstenedione increases thromboxane A2 receptors in human erythroleukemia cells

Previous studies have demonstrated an increased thromboxane A2 (TXA2) receptor expression in human erythroleukemia (HEL) cells and rat aortic smooth muscle (RASM) cells in response to testosterone treatment. HEL cells have served as a model for megakaryocytes, the progenitor cell for platelets. Platelets have previously been shown to convert androstenedione to testosterone. This study investigated the effects of androstenedione on the TXA2 receptor density in HEL and cultured RASM cells. Both cell lines were incubated with vehicle, 150 nM testosterone or 250, 500 or 750nM androstenedione for 48 hours. Co-incubation with testosterone or androstenedione significantly (p<0.05) increased the maximum number of TXA2 binding sites (Bmax) in HEL cells compared to controls. There was no significant change in Kd values. In a separate series of experiments, HEL cells were incubated with the androgen receptor antagonist hydroxyflutamide (2.5mM). Treatment with androstenedione (500nM) significantly (p<0.05) increased the Bmax value by 35% compared to control and hydroxyflutamide completely antagonized this effect of androstenedione. Incubation with hydroxyflutamide alone had no effect on the Bmax values compared to control. RASM cells also showed an increase in Bmax values by 25% and 23% over control (95+/-6.6, 118+/-7.2 and 117+/-5.1 fmoles/mg protein, control, testosterone and androstenedione, n=3). Both cell lines converted androstenedione to testosterone. The results raise the possibility that the adrenal androgen, androstenedione can regulate the expression of TXA2 receptors either on its own or via conversion to testosterone and through an androgen receptor.