Thromboxane/Prostaglandin Endoperoxide–Induced Hypertrophy of Rat Vascular Smooth Muscle Cells Is Signaled by Protein Kinase C–Dependent Increases in Transforming Growth Factor-β

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Abstract
In the present study, we examined the effect of the thromboxane/prostaglandin endoperoxide analogue U46619 on proliferation and hypertrophy in cultured rat vascular smooth muscle cells and the roles of protein kinase C and transforming growth factor-β (TGF-β) in the mediation of the hypertrophic response to U46619. Since an increase in basic fibroblast growth factor (bFGF) was previously shown to mediate the hypertrophic response to U46619, we also assessed the relationship between bFGF and TGF-β in the expression of U46619 actions. U46619 increased [ 35 S]methionine incorporation into protein and protein content of vascular smooth muscle cells but had no effect on cell number. A role for TGF-β was supported by the following observations: (1) exogenous human TGF-β1 increased protein synthesis; (2) antibody to TGF-β blocked both TGF-β– and U46619-induced increases in protein content; (3) U46619 increased active and total TGF-β bioactivities; and (4) the actions of U46619 on protein content and TGF-β bioactivity were blocked by the thromboxane/prostaglandin endoperoxide receptor antagonist SQ 29,548. Previous observations had demonstrated a role for bFGF in the expression of U46619 actions on protein synthesis. Results of the present study suggest that TGF-β and bFGF interact in mediating the protein synthetic response to U46619. First, the concentration of exogenous TGF-β (10 pmol/L) alone required to produce a protein synthetic response equivalent to that induced by U46619 was much higher than the concentration of endogenous active TGF-β that accumulated in the media in response to U46619 (0.7 pmol/L). Second, bFGF (20 ng/mL) increased total TGF-β bioactivity and stimulated protein synthesis. The hypertrophic response to bFGF was blocked by anti–TGF-β. The ability of U46619 and bFGF to increase protein synthesis and protein content in vascular smooth muscle cells was associated with TGF-β–induced suppression of proliferation, as evidenced by the ability of antibody to TGF-β to enhance U46619- and bFGF-induced increases in [ 3 H]thymidine incorporation into DNA. Results of the present study also supported a role for protein kinase C in the expression of U46619 and bFGF actions. U46619 increased protein kinase C activity in the particulate fraction of vascular smooth muscle cells. Moreover, the protein kinase C inhibitors GF109203X and staurosporine blocked U46619- and bFGF-induced increases in protein synthesis as well as active and total TGF-β bioactivities. By contrast, the protein kinase C inhibitors did not prevent the increases in protein synthesis induced by exogenous TGF-β. The results demonstrate that thromboxane/prostaglandin endoperoxide signals increased TGF-β bioactivity via protein kinase C. Increases in both bFGF and TGF-β are required for an optimal hypertrophic response to U46619. The hypertrophic response to TGF-β occurs through a protein kinase C–independent pathway.