Epidermal Growth Factor Stimulation of DNA Synthesis and its Inhibition by Tyrosine Kinase Inhibitor in Aortic Smooth Muscle Cells from SHR

Structural cardiovascular proliferative changes associated with hypertension not only may result from the effects of elevated blood pressure levels but may also take part in their development and maintenance through a positive feedback process. Genetically determined characteristics of vascular smooth muscle cells (VSMC) may be responsible for the observations showing that VSMC derived from the spontaneously hypertensive rat (SHR), grow in culture, nearly twice as fast as cells derived from the normotensive counterpart strains such as the Wistar Kyoto (WKY) or the NIH Black wistar (NBR) rat. The SHR derived VSMC in culture exhibit this characteristic of rapid growth, not only when derived from adult animals (24 weeks) with established hypertension, but also from young animals (5 weeks) before the onset of overtly elevated blood pressure levels. We examined the effects of several peptide growth factors on the [3H]-thymidine incorporation into DNA in VSMC from SHR and NBR rats. Nerve growth factor and endothelial derived growth factors were not mitogenic in either cell line. Platelet derived growth factor (PDGF) stimulated DNA synthesis to an equal degree in cells from hypertensive or normotensive models. Epidermal growth factor (EGF) on the other hand, caused in a dose response manner 3 to 7 fold increase in stimulation of thymidine incorporation into the VSMC from SHR compared to a one to two fold increase observed in NBR cells. The selective enhancement of EGF mediated DNA synthesis in VSMC from SHR was observed both in the young and adult animals. This enhanced stimulatory response to EGF in the SHR derived cells may be related to the observations of several investigators showing an increased density of EGF binding sites or EGFR receptors (EGFR) numbers on VSMC from SHR compared with WKY or NBR rats. In order to demonstrate that the increased EGFR numbers in VSMC from SHR are functionally active, we employed genistein, a specific inhibitor of EGFR mediated tyrosine kinase activity (RTK). The inhibitor concentrations required to inhibit DNA synthesis in VSMC from SHR was significantly higher (IC50, 29 ug/ml) than needed for VSMC from NBR (IC50, 23 ug/ml). These observations taken together suggest that alterations in the EGF/EGFR mediated VSMC growth stimulation through RTK activity, may contribute to the genetically determined trait responsible for early and enhanced hypertrophy and hyperplasia in the vasculature of the SHR.