HEME OXYGENASE-1 IS REGULATED BY ANGIOTENSIN-II IN RAT VASCULAR SMOOTH-MUSCLE CELLS

Recently, heme oxygenase-l (HO-I) has been shown to be present in vasc ular smooth muscle cells. In the present study, we examined the effect of angiotensin Il (Ang II) on HO-1 in rat vascular smooth muscle cell s. After treatment with 100 nmol/L Ang II, HO-1 mRNA levels were decre ased. with a nadir at 2 hours (39+/-9% of the control level, P<.01). T his downregulation was completely blocked by the Ang II type 1 recepto r antagonist losartan. Western blot analysis showed that HO-I protein is also significantly downregulated, with a nadir at 4 hours (52+/-6% of the control level, P<.01). Heme oxygenase activity was also signifi cantly decreased at 4 hours (control, 0.35+/-0.86 nmol bilirubin/mg pe r hour: Ang II, 0.10+/-0.06). This downregulation was observed in seru m-starved cells to a similar extent as in serum-supplemented cells. In hibitors of protein kinase C, lipoxygenase, cyclooxygenase, cytochrome P450 monooxygenase, and phospholipase A(2) did not block this downreg ulation. However, this effect was not observed in the absence of calci um and presence of EGTA (2 mmol/L). Furthermore, a 2-hour incubation w ith calcium ionophore or arginine vasopressin decreased HO-1 mRNA leve ls, suggesting that an increase of intracellular calcium mediates the downregulation. In conclusion, Ang II decreases HO-1 mRNA in a calcium -dependent manner in vascular smooth muscle cells, which may provide a novel mechanism for the modulation of vascular tone and oxidative str ess.