Inhibition of PDGF-mediated proliferation of vascular smooth muscle cells by calcium antagonists.

BACKGROUND AND PURPOSE

The mechanism by which calcium antagonists (CAs) inhibit proliferation in vascular smooth muscle cells (VSMCs) is not yet fully understood. We investigated the effects of four CAs (clentiazem, verapamil, diltiazem, and nifedipine) on signal transduction pathways activated by platelet-derived growth factor (PDGF). To determine these effects, the levels of inositol phosphates (IPs), protein kinase C (PKC), and the induction of the transcription factor activator protein-1 (AP-1) were measured.

METHODS

The mitogenic effect of PDGF on VSMCs was measured by [3H]thymidine incorporated into DNA. IP production was monitored by [3H]myo-inositol incorporation. PKC activation was determined by measurement of myristoylated, alanine-rich C kinase substrate (MARCKS) phosphorylation in digitonin-permeabilized VSMCs. The induction of AP-1 complex was detected by electrophoretic mobility shift assays.

RESULTS

Each CA significantly inhibited the [3H]thymidine incorporation into DNA in unstimulated cells. Similar significant decreases in [3H]thymidine incorporation by CAs were observed when cells were stimulated by rPDGF-BB. The phosphorylation of MARCKS mediated by rPDGF-BB was significantly reduced by each CA. Clentiazem and verapamil significantly reduced the expression of AP-I induced by rPDGF-BB (P < .01, P < .05). Clentiazem also significantly reduced the expression of AP-1 induced by rPDGF-AB (P < .05).

CONCLUSIONS

PDGF-mediated proliferation of VSMCs correlates with activation of PKC but not with induction of the AP-1 complexes. In addition, our results suggest that CAs block proliferation of VSMCs by inhibiting DNA synthesis, possibly via PKC.