Effects of thiol protease inhibitors on cell cycle and proliferation of vascular smooth muscle cells in culture.

Smooth muscle proliferation is a prominent feature of the vascular response to mechanical injury. Accordingly, modulation of proliferation has important therapeutic implications for angioplasty restenosis. We have identified a subclass of thiol protease inhibitors (TPIs) that reversibly inhibit bovine aortic smooth muscle cell (BASMC) proliferation in vitro. To define the nature of this inhibition, an evaluation of selected steps in the cell cycle was undertaken. Treatment of BASMCs with benzyloxycarbonyl-Leu-norleucinal (calpeptin) at 100 microM and acetyl-Leu-Leu-norleucinal (TPI-1) at 50 microM was shown to cause a block of platelet-derived growth factor-BB as well as serum-inducible cell cycle progression at a point before the G1-S boundary, reducing the percentage of bromodeoxyuridine-positive cells from 87% to 5% over a 24-hour labeling period. Addition of TPI-1 at various times after serum addition to serum-deprived BASMCs showed 80% of the maximal block of DNA synthesis even when added 6 hours after serum. The cell cycle progression block was gradually lost as the delay from serum to TPI-1 application was increased from 6 to 12 hours. By Northern analysis of mRNA after serum addition, TPI-1 caused a fourfold decrease in the transient elevation of fos and myc proto-oncogene as well as a decrease in the levels of both muscle and nonmuscle actin mRNA induced early after serum addition. Flow cytometric analysis of DNA content and synthesis in BASMCs treated with TPI-1 or calpeptin additionally revealed the presence of a distinct cell cycle block in the G2-M compartment. In the aggregate, these results suggest the existence of more than one molecular site potentially involved in inhibition by TPI of cell cycling in BASMCs.