Growth factor expression in aorta of normotensive and hypertensive rats

Stimulation of endothelin mRNA and secretion in rat vascular smooth muscle cells: a novel autocrine function

Endothelin (ET), a peptide originally isolated from the supernatants of cultured endothelial cells, exerts a wide variety of biological effects in different tissues. Endothelial-cell-synthesized ET-1 has been proposed to act in a paracrine manner on adjacent smooth muscle cells (SMC) in vivo, with effects that include both vascular reactivity (vasodilation/vasoconstriction) and mitogenesis. This study, by the use of immunocytochemically characterized SMC (rVSMC) isolated from the aortas of spontaneously hypertensive rats, has investigated a possible autocrine role for ET in regulation of the vasculature. Although quiescent cultures of rVSMC apparently did not constitutively express prepro ET-1mRNA, ET-specific transcripts could be induced by a variety of growth factors (transforming growth factor beta [TGF-beta]; platelet-derived growth factor-AA homodimer [PDGF-A chain]) and vasoactive hormones (angiotensin II [Ang II], arginine-vasopressin, and ET-1 itself). The kinetics for prepro ET-1mRNA induction in rVSMC were characteristically rapid in onset and transient. Down-regulation of protein kinase C by 48 h pretreatment of rVSMC with phorbol ester markedly reduced the subsequent ability of rVSMC to express ET-1 transcripts and secrete ET-1 peptide in response to Ang II. Inducible prepro ET-1mRNA expression was accompanied by a cycloheximide-inhibitable release of ET-1 peptide into the medium of rVSMC. ET-1 peptide was determined by both radioreceptor- and radioimmunoassay. Stimulated rVSMC accumulated ET-1 (approximately 200 pg.10(6) cells-1 x 4 h-1) at levels that attained biological relevance (approximately 10(-10) M). Sep-pak C18 extracts of medium from stimulated rVSMC elicited contraction of isolated endothelium-denuded rat mesenteric resistance vessels, and this response was characteristically protracted and difficult to "wash out." Synthetic (porcine) ET-1 promoted the expression of transcripts for PDGF-A chain, TGF-beta, and thrombospondin in quiescent rVSMC. Such effects of ET-1 on gene expression may be relevant to the mitogenic potential of ET-1 on VSMC. Our findings imply a role for ET-1 in the control of vascular function via both paracrine and autocrine regulatory mechanisms. The expression of prepro ET-1mRNA and peptide biosynthesis by rVSMC may have both short-term (e.g., vasoconstriction) and long-term (e.g., structural remodeling) consequences. A sustained loop of autocrine stimulation by ET-1 in SMC could contribute toward the pathogenesis of vasospasm and/or atherosclerosis.

Role of PDGF-A expression in the control of vascular smooth muscle cell growth by transforming growth factor-beta

Transforming growth factor-beta (TGF-beta) is a multifunctional regulatory peptide that can inhibit or promote the proliferation of cultured vascular smooth muscle cells (SMCs), depending on cell density (Majack, R. A. 1987. J. Cell Biol. 105:465-471). In this study, we have examined the mechanisms underlying the growth-promoting effects of TGF-beta in confluent SMC cultures. In mitogenesis assays using confluent cells, TGF-beta was found to potentiate the stimulatory effects of serum, PDGF, and basic fibroblast growth factor (bFGF), and was shown to act individually as a mitogen for SMC. In gene and protein expression experiments, TGF-beta was found to regulate the expression of PDGF-A and thrombospondin, two potential mediators of SMC proliferative events. The induction of thrombospondin protein and mRNA was density-dependent, delayed relative to its induction by PDGF and, based on cycloheximide experiments, appeared to depend on the de novo synthesis of an intermediary protein (probably PDGF-A). The relationship between PDGF-A expression and TGF-beta-mediated mitogenesis was investigated, and it was determined that a PDGF-like activity (probably PDGF-A) was the biological mediator of the growth-stimulatory effects of TGF-beta on confluent SMC. The effects of purified homodimers of PDGF-A on SMC replication were investigated, and it was determined that PDGF-AA was mitogenic for cultured SMC, particularly when used in combination with other growth factors such as bFGF and PDGF-BB. The data suggest several molecular mechanisms that may account for the ability of TGF-beta to promote the growth of confluent SMC in culture.

1989 Corcoran lecture: adaptive and maladaptive responses of the arterial wall to hypertension

This study reviews recent experimental data from our own and other laboratories on the effects of hypertension on the arterial wall and the potential mechanisms by which hypertension can induce vascular injury and accelerate atherosclerosis. The findings suggest that the responses of the arterial media to hypertension reflect appropriate adaptations to increased intramural tension with resultant medial thickening secondary to an increase in both cellular mass and extracellular matrix. The role of growth factors in this process and their effects on arterial contractility are discussed as well as the potential importance of the changes in extracellular matrix constituents. The intimal changes induced by hypertension have many similarities to those caused by aging or hypercholesterolemia and can in part reflect general arterial responses to injury. They make the arterial wall more vulnerable to the effects of hypercholesterolemia, however, and as noted in our studies with the Watanabe heritable hyperlipidemic rabbit, pronounced acceleration of atherosclerosis is induced when hypertension is combined with hypercholesterolemia. Antihypertensive drugs can affect the arterial response to hypercholesterolemia. In the present study, new data are provided indicating that captopril inhibits aortic atherosclerosis in the Watanabe heritable hyperlipidemic rabbit in association with a pronounced reduction in cellularity of lesions.

The effects of ACE inhibitors and other antihypertensive drugs on cardiovascular risk factors and atherogenesis

The effects of antihypertensive therapy on the atherosclerotic process and differences among various treatment approaches have been studied in several animal models. Studies in cholesterol-fed rabbits have indicated that beta blockers and calcium antagonists can inhibit the development of aortic atherosclerosis induced by marked hypercholesterolemia. However, no such antiatherosclerotic action has been apparent with these agents in the Watanabe heritable hyperlipidemic (WHHL) rabbit. A study was conducted to determine the effects of the angiotensin-converting enzyme (ACE) inhibitor captopril administered orally for 9 months on the development of atherosclerosis in normotensive WHHL. Compared with controls, the treated animals had a significant reduction in atherosclerosis of the descending thoracic aorta. These effects were associated with significantly lower blood pressure, though within the normal range, in captopril-treated versus control animals. In the captopril group, the atherosclerotic plaques were thinner and less cellular than plaques of similar gross severity in control WHHL. The findings indicate that the ACE inhibitor captopril can inhibit atherogenesis in the WHHL, which has been resistant to such therapy with other antihypertensive drugs.

Induction of insulin-like growth factor I messenger RNA in rat aorta after balloon denudation

Insulin-like growth factor I (IGF-I) is a widely distributed mitogen that mediates the growth-promoting effects of platelet-derived growth factor in mesenchymal cells. We show that rat aortic IGF-I messenger RNA (mRNA) is induced 24 hours after deendothelialization, at a time when smooth muscle cell proliferation within the intima is still not apparent. After 7 days, IGF-I mRNA induction peaks at about ninefold control levels and then falls to about threefold 14 days after denudation when smooth muscle cell proliferation is at its peak. We also show that, of the 5' untranslated IGF-I mRNA transcripts, only the class C transcript is expressed and regulated in aortic tissue. In contrast, treatment of rats with supraphysiological doses of growth hormone, the major endocrine regulator of IGF-I gene expression, elicited only twofold induction of aortic IGF-I mRNA. Our findings suggest that IGF-I may be an important autocrine or paracrine regulator of smooth muscle cell proliferation and that it may be significant in determining the cellular response to arterial wall injury.

Antiatherogenic effect of captopril in the Watanabe heritable hyperlipidemic rabbit

The effects of 9 months of orally administered captopril (25-50 mg/kg body wt/day) on aortic atherosclerosis was examined in normotensive Watanabe heritable hyperlipidemic rabbits. Captopril caused a significant decrease in aortic atherosclerosis. Total aortic surface involvement by lesions was reduced from 48 +/- 3.6% in control Watanabe rabbits to 30 +/- 3.9% with captopril treatment (p less than 0.01). Most of the decrease could be accounted for by a marked reduction in atherosclerosis of descending thoracic aortas from 49 +/- 5.2% to 15 +/- 3.9% in control and captopril-related groups, respectively (p less than 0.001). Significant decrease in cholesterol content of descending thoracic aorta was also observed in captopril-treated rabbits. Microscopic examination of the arterial lesions in captopril-treated animals suggested a relative decrease in cellularity and increase in extracellular matrix as compared with untreated animals. These studies indicate that captopril has a potent antiatherosclerotic action in the Watanabe heritable hyperlipidemic rabbit.