MECHANISMS OF ANG II-INDUCED MITOGENIC RESPONSES - ROLE OF 12-LIPOXYGENASE AND BIPHASIC MAP KINASE

The potential mechanisms of angiotensin II (ANG II)-induced mitogenesi s were studied in a Chinese hamster ovary fibroblast cell line overexp ressing the rat vascular type la ANG II receptor (CHO-AT(1a)). ANG II had potent mitogenic effects in these CHO-AT(1a) cells, leading to a s ustained increase in cell number as well as a dose-dependent increase in DNA synthesis. ANG II treatment also induced a biphasic elevation o f mitogen-activated protein (MAP) kinase activity of both p42(MAPK) an d p44(MAPK) With a rapid early peak at 5 min (2- to B-fold) followed b y a second sustained increase that reached a peak at 3 h (1.5- to S-fo ld). We have previously shown that the 12-lipoxygenase (12-LO) pathway of arachidonate metabolism plays a key role in ANG II-induced growth of vascular smooth muscle and adrenal cells. In the present study, ANG II (10(-7) M) increased the formation of the 12-LO product, 12-hydrox yeicosatetraenoic acid (12-HETE). ANG II-induced DNA synthesis was inh ibited by a specific LO inhibitor, cinnamyl-3,4-dihydroxy-alpha-cyanoc innamate (CDC, 10 mu M). In contrast, a cyclooxygenase blocker of arac hidonate metabolism such as ibuprofen had no effect on ANG II-induced DNA synthesis. ANG II-induced DNA synthesis was also partially (32%) b locked by pertussis toxin (PTX). CDC and PTX also selectively blocked only the late (3 h) peak of ANG II-induced MAP kinase activity, sugges ting that the late sustained peak of MAP kinase activity may be linked to the mitogenic effect of ANG II. Direct addition of 12-HETE (10(-7) M) led to a sustained increase in cell number similar to the effect o f ANG II. 12-HETE also caused an increase in MAP kinase activity, and 12-HETE effects were blocked by PTX. These results suggest that ANG II -induced mitogenic response is associated with sustained MAP kinase ac tivation and that LO activation may play a key role in this process.