Cell cycle related inhibition of mouse vascular smooth muscle cell proliferation by prostaglandin E₁: Relationship between prostaglandin E₁ and intracellular cAMP levels

Elucidation of the cellular and molecular mechanisms which regulate vascular smooth muscle cell proliferation is critical to the understanding of atherogenesis. The present studies were conducted to evaluate the relationship between prostaglandin E₁ (PGE₁) and cAMP in the regulation of DNA synthesis in mouse vascular smooth muscle cells (SMCs). Quiescent cultures of SMCs were challenged with 10% fetal bovine serum to initiate cell cycle transit and PGE₁ (10 μM) or dibutyryl cAMP (1, 10, 100 μM) added at 0, 8, 16, 24, and 32 h. DNA synthesis as measured by [³H] thymidine incorporation and intracellular cAMP levels were measured 24 h following individual treatments. PGE₁ modulated DNA synthesis in a cell cycle related fashion, with inhibition only observed in cells challenged 16 h or longer following initiation of cell cycle transit. The decrease in DNA synthesis induced by PGE₁ was associated with increased intracellular cAMP levels at 16 and 24 h, but not 32 h. Exposure of SMCs to dibutyryl-cAMP also inhibited DNA synthesis in a cell cycle related fashion, with the most pronounced effect seen at 16 h. These results demonstrate that the effects of PGE₁ are restricted to a defined period within the cell cycle following S phase entry and implicate modulation of intracellular cAMP levels in the inhibitory response.