Pathways to mitogenesis in vascular smooth muscle cells stimulated with angiotensin II

This work has investigated the mitogenic signalling pathways leading from angiotensin II (AII) stimulation of the 7-transmembrane G protein - coupled, AT1 receptor in vascular smooth muscle cells (VSMCs). It has previously been shown that AII stimulates both phospholipase C (PLC) and mitogenesis to a greater extent in VSMC cultures from spontaneously hypertensive rats (SHRs) compared to normotensive (WKY) rats. Here we have investigated the role(s) of AII - stimulated mitogen activated protein kinase (MAPK) activity and phospholipase D (PLD) activity in SHR and WKY - derived VSMCs. Using a peptide kinase assay with a nonapeptide MAPK - selective substrate and a Western blot procedure using an antiserum specific for the tyrosine phosphorylated form of MAPK, combined with anion exchange chromatography, we have demonstrated that both the tyrosine phosphorylation and kinase activity of both p42mapk and p44mapk were stimulated by AII to a greater extent (approximately 2 - fold) in SHR cells than WKY cells. There was a similar enhancement in the MAPK response of SHR cells to protein kinase C (PKC) activation by phorbol myristate acetate (PMA). The enhanced MAPK response was not due to the over abundance of the MAPK protein in SHR cells, since SHR and WKY cells were discovered to possess equal MAPK immunoreactivity. These results demonstrate that the MAPK response of SHR derived cells is increased over that of WKY cells by mechanisms independent of the enhanced stimulation of PLC. The relatively specific PKC inhibitor, Ro 31 - 8220 attenuated both the early (2 min) phase of AII stimulation of MAPK and the entire stimulation caused by PMA. The PKC - sensitive components of the AII MAPK response included both p42mapk and p44mapk. At longer times of AII stimulation both isoforms of MAPK were unaffected by Ro 31 - 8220 pretreatment. Both AII and PMA stimulations of MAPK were inhibited by the tyrosine kinase inhibitor, genistein