Effect of hypertension on fibronectin expression in the rat aorta

Selective induction of an embryonic fibronectin isoform in the rat aorta in vitro

The temporal changes in the expression of fibronectin and other extracellular matrix genes were studied in rat aortic rings incubated in vitro in a serum-free medium. Changes in all forms of fibronectin mRNA increased progressively during the 24-hour incubation period, although an increase in the alternatively spliced form of fibronectin designated EIIIA was most pronounced. Both collagen and elastin mRNA levels decreased markedly during the 24-hour interval, as did alpha-actin mRNA. The increase in the relative amount of the EIIIA isoform after a 24-hour incubation was also shown using ribonuclease protection assays. In situ hybridization showed the distribution of the induced fibronectin mRNA to be within all cell types, including endothelial cells, medial smooth muscle cells, and adventitial fibroblasts. Localization in the media was not uniform and was clearly identified mainly in clusters of cells distributed throughout the media. The early induction of fibronectin mRNA was inhibited by genistein, implicating tyrosine kinase activation as a causative factor in fibronectin expression. The in vitro changes reported may reflect a phenotypic change in vascular cell types that is both similar to and different from the changes reported in vivo under conditions in which vascular injury and repair occur.

Long-term effects of angiotensin-converting enzyme inhibition on the arterial wall of adult spontaneously hypertensive rats

The effects of long-term angiotensin-converting enzyme (ACE) inhibitor treatment with perindopril 2 mg/kg/day, by gavage, for 3 months on the mechanical function and structure of large arteries were studied in adult spontaneously hypertensive rats with established hypertension. Hemodynamic parameters, including instantaneous aortic blood flow and pressure, were recorded under anesthesia at the end of the treatment period. Systemic arterial compliance was calculated from aortic pressure and flow recordings; passive mechanical properties of the in situ localized carotid artery were measured. Histologic and morphologic parameters of the aortic media, including cross-sectional area and thickness, size, and density of smooth muscle nuclei and of elastin and collagen fibers, were measured using an automated image analysis system. ACE inhibitor treatment significantly decreased mean arterial pressure (-27%, p < 0.001) and total peripheral resistance (-30%, p < 0.05) while cardiac output was increased (29%, p < 0.05). Systemic arterial compliance and carotid compliance were both increased by treatment (63%, p < 0.05, and 83%, p < 0.05, respectively). Morphometric assessment of vascular structure showed that ACE inhibitor treatment significantly decreased medial cross-sectional area (-36%, p < 0.001) and thickness (-16%, p < 0.001) by affecting smooth muscle cell hypertrophy (nucleus size decreased by 26%, p < 0.05) without changes in smooth cell number. Collagen density was decreased by treatment (-42%, p < 0.05), whereas elastin density was not affected by treatment.(ABSTRACT TRUNCATED AT 250 WORDS)

Expression and alternative splicing of fibronectin mRNA in human diploid endothelial cells during aging in vitro

Different mRNAs for fibronectin arise from the variable processing of a single primary transcript. We used ribonuclease protection assay to investigate the changes occurring in fibronectin expression and the alternative splicing of mRNA precursor during aging in vitro of human diploid endothelial cells. Senescent endothelial cells release more protein and contain 4-5-fold more fibronectin mRNA than young cells. The pattern of alternative splicing of fibronectin mRNA, with the EDA and the CS1 segments largely included (35% and 77%, respectively) and the EDB segment undetectable, correlates well with previous studies at the protein level both in vitro and in vivo. No changes in the splicing pattern of fibronectin mRNA precursor were detected during endothelial cellular senescence. The increased expression of fibronectin in senescent cells may be a result of the activity of interleukin-1 alpha, which is overexpressed in senescent endothelial cells. It could be also important in vivo during aging and in atherosclerotic lesions.

Age-related changes in fibronectin expression in spontaneously hypertensive, Wistar-Kyoto, and Wistar rat hearts

The effects of age and blood pressure on fibronectin expression in the rat heart were studied in the normotensive Wistar and Wistar-Kyoto (WKY) strains and in the spontaneously hypertensive rat (SHR). Fibronectin mRNA expression decreased threefold between 10 and 40 weeks of age in Wistar hearts, with changes of similar magnitude occurring between 6 and 24 weeks in WKY rats. In contrast, no decrease in fibronectin mRNA was observed in SHR hearts during this time span. These results are in contrast to changes observed previously in the aorta, where an increase in fibronectin mRNA occurred with age in all three rat strains. Ribonuclease protection analysis showed a small age-specific increase in the relative content of EIIIA+ fibronectin mRNA isoforms in hearts from Wistar rats, whereas no change was found in the relative amount of either EIIIA or EIIIB isoforms in SHR hearts. Changes similar to those observed for fibronectin mRNA, although of different magnitudes, were observed in mRNA levels for collagen alpha 1(III) and beta 1 integrin. In Wistar hearts, collagen alpha 1(III) mRNA levels decreased fivefold to sixfold between 10 and 40 weeks of age, whereas a twofold to threefold decrease in beta 1 integrin was observed in WKY hearts between 6 and 24 weeks of age. Western blot analysis revealed a positive correlation between fibronectin mRNA and protein for age-dependent changes in ventricular tissue but not in the atria, suggesting that the regulation of fibronectin expression during the changes common to both aging and hypertrophy could involve both transcriptional and posttranscriptional mechanisms.

Effect of aging on macrophage adherence to extracellular matrix proteins

Fibronectin, type I, type IV and type V collagens were compared for their abilities to promote mouse peritoneal resident macrophage adherence and the effect of aging on macrophage adhesion to these matrix proteins was examined. Adherence of macrophages to fibronectin was remarkable and more than 5-fold compared to type I, type IV or type V collagens. Adherence of macrophages to fibronectin and type I collagen increased during aging. However, no age-related changes were observed in macrophage adherence to type IV and type V collagens. The percent of inhibition of macrophage adherence to fibronectin by arginine-glycine-aspartic acid-serine (RGDS) peptide (58.1 +/- 2.7%) was significantly (P < 0.01) higher than that in cells from young mice (23.1 +/- 5.7%). These data suggest that macrophage attach preferentially to fibronectin and that the ability of macrophage to attach to fibronectin increases during aging. The age-related increase in macrophage attachment to fibronectin is related to the concentration of cell surface receptors of macrophage which recognize RGDS sequence within fibronectin during aging. Adherence of macrophage to fibronectin implies retention of macrophages in subendothelial space. Age-related increase in macrophage ability to attach to fibronectin may be related to atherogenesis during aging.

Trandolapril inhibits atherosclerosis in the Watanabe heritable hyperlipidemic rabbit

The effects of trandolapril (0.25 mg/kg body wt per 48 hours) on aortic atherosclerosis were examined in the Watanabe heritable hyperlipidemic rabbit treated from 3 to 12 months of age. Trandolapril caused a significant decrease in atherosclerotic involvement of the intimal surface of total aorta from 56.3 +/- 5.0% in control Watanabe rabbits to 35.0 +/- 4.1% with treatment (p less than 0.01). The largest reductions were observed in descending thoracic aorta where 21.8 +/- 5.7% of intimal surface was involved in the trandolapril-treated animals versus 54.4 +/- 7.7% in the control group (p less than 0.01). Significant decreases also occurred in ascending aorta/arch and abdominal aortic segments. Cholesterol content of descending thoracic aorta was also significantly reduced in the trandolapril-treated rabbits. The atherosclerotic plaques in aorta from trandolapril-treated rabbits appeared to contain less foam cells and relatively greater amounts of connective tissue than those from control animals. These studies indicate that trandolapril inhibits aortic atherosclerosis in the Watanabe heritable hyperlipidemic rabbit. The similarity in results between the current study and that using captopril suggests that the antiatherosclerotic action of trandolapril and captopril represents a class effect related to angiotensin converting enzyme inhibition.

Hypertension induces alternatively spliced forms of fibronectin in rat aorta

Fibronectin expression was shown recently to increase in the rat aorta in response to experimental hypertension. Fibronectin is known to alter the phenotype of vascular smooth muscle and endothelial cells, and relative changes in the expression of different isoforms of fibronectin, generated by alternative splicing and distinguished by the absence or presence of inserts designated as EIIIA, EIIIB, and V, may reflect a change in cell phenotype. In the present study we examined the expression of alternatively spliced forms of aortic fibronectin during deoxycorticosterone-salt hypertension. Aortic RNA was analyzed quantitatively using Northern blot analysis and ribonuclease protection assays. Using Northern blot analysis, deoxycorticosterone-salt treatment for 21 days led to a 4.9-fold increase in EIIIA fibronectin messenger RNA, while EIIIB and V forms increased by 2.6- and 2.5-fold, respectively. As determined by ribonuclease protection assays, the percentage of fibronectin transcripts containing either EIIIA, EIIIB, or V in control aorta was 7.3%, 19%, and 40%, respectively. The percentage of EIIIA transcripts increased 42% over control levels after 21 days of deoxycorticosterone-salt treatment, whereas no proportionate change in the other alternatively spliced forms was found. Thus, all forms increased, but a selective increase in the EIIIA form was induced. Analogous increases in each of the fibronectin isoforms were found in the spontaneously hypertensive rats when compared with age-matched Wistar-Kyoto or Wistar rats, and 40-week-old animals showed increases over 10-week-old animals in all strains, consistent with an age-dependent increase in aortic fibronectin expression.

Vascular effects of systemic hypertension

Hypertension is a major risk factor for cardiovascular diseases, including coronary artery disease (CAD), stroke, left ventricular hypertrophy (LVH), congestive heart failure, peripheral vascular disease, renal failure, and aortic aneurysms. It is also a potent promoter of atherosclerosis. Observational studies have shown a linear relationship between a wide range of blood pressures and the risk for CAD and stroke. Clinical trials have indicated that hypertension reduction leads to the predicted reduction in stroke incidence, but that CAD incidence is affected to a lesser extent than predicted. The modest effect of traditional antihypertensive drugs on CAD may be due to several factors, including failure to reverse well-established coronary atherosclerosis, particularly if multiple risk factors are not reduced as well. Metabolic side effects of antihypertensive drugs or excessive lowering of blood pressure leading to inadequate myocardial perfusion, especially in patients with increased left ventricular (LV) mass, also may play important roles. Hypertension is a major cause of renal failure, particularly in black males, but control of the hypertension does not necessarily prevent deterioration of renal function. Increased glomerular pressure is thought to play a causative role in the development of renal failure in hypertensive and diabetic patients. Antihypertensive drugs may have a direct effect on the arterial wall, which may be independent of their antihypertensive action. Beta-adrenergic blockers, calcium antagonists, and angiotensin-converting enzyme (ACE) inhibitors inhibit the development of vascular lesions in response to hypercholesterolemia or to iatrogenic balloon injury, but the clinical importance of these observations remains to be determined.

Smooth muscle cell growth factors

While the roles of the platelet-derived growth factors (PDGFs) in vascular smooth muscle cells (SMCs) continue to be elucidated, these cells, especially in their activated 'synthetic' state, have also been found to express, and proliferate in response to, many of the other families of polypeptide growth factors, such as the fibroblast growth factors. Other stimulators of DNA synthesis, and particularly of SMC hypertrophy, include the vasoconstrictor hormones such as angiotensin II, as well as physical forces, especially stretch or tension. For many of these ligands, multiple receptors have been identified and their means of signal transduction are being characterized rapidly. Regulatory regions of these genes are being identified as are transcription factors. Complex post-transcriptional regulation has also been shown by the findings that some growth factors are phosphorylated, or translocated to the nucleus or the extracellular matrix. Inhibitors have also been identified. These include some prostaglandins, calcium antagonists, agonists that activate guanylate and adenylate cyclases, inhibitors of angiotensin-converting enzyme, interferon gamma, and heparin. Future studies are likely to show that tyrosine phosphatases and recessive oncogenes also regulate growth. The existence of so many autocrine/paracrine mitogens--together with some experimental data--suggests some redundancy in the system as well as some additive effects. Redundancy may limit the efficacy of antibodies to a single growth factor to block cell proliferation. Their evolutionary conservation implies some unique roles for each growth factor but these have not been apparent from in vitro studies to date. Further insights are apt to come from the increasing recognition that growth factors have other effects--on cell attachment, migration, survival, production of extracellular matrix, thrombosis, vaso-constriction, regulation of cytokine synthesis, and inhibition of growth. Many of these effects may prove to be context-dependent, as with the case of growth inhibition by transforming growth factor-beta. Studies in monolayer cultures may not obtain the same results as studies using cocultures of endothelial and smooth muscle cells, or 3-dimensional matrix cultures, organ cultures, or in the intact animal. In vivo descriptive studies of growth factors expressed in vascular embryogenesis, hypertension, atherosclerosis, acute balloon injury and thrombosis are being supplemented by interventions such as infusions with growth factors, antibodies, and toxin conjugates. These studies, and studies using transgenic mice and homologous recombination, should yield information as to mechanisms and may also suggest new therapies.