Protein phosphatase 2a is involved in regulation of the secondary sustained phase of map kinase activity in response to vascular smooth muscle hypertrophic and mitogenic stimuli
Angiotensin II (A-II) and cc-thrombin are growth factors that have been previously shown in this laboratory to induce hypertrophy and mitogenesis of cultured vascular smooth muscle cells (VSMC), respectively. The intracellular signaling pathways through which these factors regulate growth and proliferation of VSMC are complex, but known to involve mitogen-activated protein kinase (MAPK). Furthermore, it has been shown that a sustained activation of MAPK is required for mitogenic growth in fibroblasts (Kahan et al. JBC 267:13369-13375, 1992). In this study, we tested the hypothesis that protein phosphatase 2A (PP2A), a known MAPK phosphatase, regulates MAPK activity differently in response to VSMC hypertrophie and mitogenic stimuli. MAPK and MAPK kinase (MEK) activities in growth arrested VSMC were markedly increased within 5 min. of stimulation with either A-II or a-thrombin. However, only athrombin stimulated a secondary sustained phase of activity. In vitro phosphatase assays with cytosolic extracts and 32P-MAPK showed that the A-II induced dephosphorylation of MAPK was inhibited by okadaic acid (OA), a potent PP2A inhibitor. It was also demonstrated that after 3 H of a-thrombin treatment, there was a significant decrease in PP2A activity in vivo. The a-thrombin-induced sustained increase in MAPK activity was rapidly abolished by treatment with the thrombm antagonists hirudin, and this effect could be reversed with the addition of OA. These data provide evidence that growth factors can differentially affect the level of PP2A activity and thus influence MAPK activity. This in turn may explain the different growth responses (i.e. hypertrophie or mitogenic) of the VSMC.
Hershev, JC; Madsen, CS; Partridge, JM; Owens, GK; Haystead, TAJ
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