Receptor-mediated mitogenic effect of thromboxane A2 in vascular smooth muscle cells

The effects of thromboxane A2 (TXA2) on the proliferation of vascular smooth muscles cells (VSMC) were examined using primary cultures of VSMC from rat aorta. U46619, a stable TXA2 mimetic, stimulated DNA synthesis of VSMC only in the presence of insulin. The effect was concentration-dependent with a half-maximal effect obtained at approximately 1 x 10(-8) M. The mitogenic effect of U46619 was larger than that of endothelin, another mitogen derived from endothelium. Among several TXA2/PGH2 analogs, the proliferative activity was detected only in the agonists, and not in the antagonists or in the metabolite of TXA2. A series of TXA2/PHG2 receptor antagonists completely suppressed the U46619-stimulated DNA synthesis as well as the [3H]SQ29,548 binding to the TXA2/PGH2 receptors in VSMC. The rank order of binding affinities to the receptors among the respective antagonists correlated well with the potencies for suppression of the proliferative effects of U46619. The mitogenic effects of U46619 were also attenuated by the presence of calcium antagonists. U46619 caused activation of phospholipase C with the production of inositol trisphosphate, leading to increases in the intracellular free Ca2+ concentration as measured with the fluorescent indicator fura-2. These results suggest that TXA2 induces mitogenic effects on VSMC through binding to its specific receptors. This effect of TXA2 on the proliferation of VSMC may be related to the development of atherosclerosis.