Inhibition of plasminogen activator inhibitor-1 expression in vascular smooth muscle cells by protoporphyrins through a heme oxygenase-independent mechanism.

Heme oxygenase-1 (HO-1), a rate-limiting enzyme in heme catabolism, has been shown to play a regulatory role in the expression of plasminogen activator inhibitor-1 (PAI-1), a risk factor for vascular disease. Accordingly, we examined the effect of protoporphyrins, both HO inhibitors and activators, on PAI-1 expression in human vascular smooth muscle cells (VSMCs). Tin-protoporphyrin (SnPP) markedly inhibited the transforming growth factor beta1 (TGFbeta1)-induced expression of PAI-1 protein. Protoporphyrins, whether they are inhibitors or activators of HO, produced a similar inhibitory effect. However, SnPP had no effect on the level of PAI-1 mRNA transcripts. Knockdown of human HO-1 with a specific siRNA did not reduce the PAI-1 protein level in TGFbeta1-treated cells. In addition, the proteasome inhibitor lactacystin reversed the inhibitory effect of SnPP on PAI-1 protein expression. Both cobalt-protoporphyrin (CoPP) and CoCl2 markedly induced HO-1 expression. However, CoPP did not affect PAI-1 gene expression, whereas CoCl2 upregulated PAI-1 mRNA in a dose-dependent manner. Our results demonstrate that protoporphyrins can block the TGFbeta1-mediated induction of PAI-1 protein in VSMCs and that this inhibitory effect is independent of HO activity.