Differential effect of simvastatin on various signal transduction intermediates in cultured human smooth muscle cells

The underlying mechanism of the antiproliferative effect of S (simvastatin) , a HMG-CoA reductase inhibitor, in vascular smooth muscle cells (SMC) is s till poorly understood. In the present study, we used synchronized human SM C, isolated from left interior mammary artery, as an in vitro model to test the effects of S on platelet-derived growth factor (PDGF)-induced DNA synt hesis, extracellular-regulated kinase 1/2 (ERK1/2), p38/stress-activated pr otein kinase 2 (SAPK2), RhoA and Rac1 activation. ERK1/2 phosphorylation wa s triggered within 2 min of PDGF stimulation (early G1 phase) and was block ed by PD98059, a specific inhibitor of the ERK1/2 pathway, which also stron gly inhibited PDGF-induced DNA synthesis (IC50 = 10 mu mol/L). PDGF quickly induced p38 phosphorylation (early G1 phase) and SB203580, a specific inhi bitor of the p38/SAPK2 pathway, also blocked PDGF-induced DNA synthesis (IC 50 = 0.3 mu mol/L). Translocation to the plasma membrane of small GTPases, such as RhoA and Rac1, could not be detected within 15 min of stimulation w ith PDGF or lysophosphatidic acid (LPA) (early G1 phase), but occurred afte r 24 hr of PDGF stimulation (late G1/S phase). S inhibited PDGF-induced DNA synthesis (IC50 = 3.5 mu mol/L). and this effect was dependent on intracel lular mevalonate, farnesyl pyrophosphate, and geranylgeranyl pyrophosphate availability. The critical time period for the reversal of the S effect by mevalonate comprised both the early and late G1 phase of the SMC cycle. PDG F-induced ERK1/2 phosphorylation and PDGF-induced p38 phosphorylation were not markedly affected by S during the whole G1 phase. However, S treatment blocked the PDGF- and LPA-induced membrane translocation of RhoA that occur red during the late G1/S phase, In the case of Rac1, the same process was a lso inhibited by S treatment. We concluded from these results that, in SMC, the early events associated with ERK1/2 and p38 signal transduction pathwa ys, recruited for PDGF-mediated DNA synthesis, were insensitive to S action , whereas the mevalonate-dependent. posttranslational modification of RhoA and Rac1 molecules, required for PDGF-induced membrane translocation, was b locked by this drug. These results suggest that the antiproliferative effec t of S can be explained not only by the blockage of RhoA-mediated signaling events but also by Rac1-mediated signaling events. (C) 2001 Elsevier Scien ce Inc. All rights reserved.