Double blockade of cell cycle progression by coptisine in vascular smooth muscle cells

Coptisine, an isoquinoline alkaloid isolated from rhizome of Coptis japonica, inhibits proliferation of vascular smooth muscle cells (VSMCs). The aim of this study was to evaluate the action of coptisine, along with berberine (a structurally similar isoquinoline alkaloid), on progression of the cell cycle in VSMCs. Coptisine displayed antiproliferative action against VSMCs by blocking the cell cycle at G1 and G2/M phases. The G1 block was shown by inhibition of [3H]thymidine incorporation into VSMCs at coptisine concentrations higher than 15 μM. The mechanism underlying the G1 arrest involved a decrease in cyclin D1 protein, although cyclin E, A, and B were not affected by coptisine treatment. The selective reduction in cyclin D1 protein was mainly attributable to accelerated proteolysis via proteasome-dependent pathway, since it was inhibited by a proteasome inhibitor, N-carbobenzoxy-l-leucinyl-l-leucinyl-l-norleucinal (MG132) and further the mRNA level of cyclin D1, protein synthesis, and mitogen-activated protein kinase (MAPK) activity remained unaltered. The mechanism underlying the G2/M arrest involved partial inhibition of tubulin polymerization, which was apparent at coptisine concentration of 3 μM. Berberine arrested the cell cycle at G1 phase via a mechanism identical with coptisine, but did not cause block at G2/M phase. The results demonstrate that a small difference in the structure between isoquinoline alkaloids produces a big difference in activity, and that coptisine has a unique double action in arresting the cell cycle of VSMCs.