I. Leduc et al., INVOLVEMENT OF A TYROSINE KINASE PATHWAY IN THE GROWTH-PROMOTING EFFECTS OF ANGIOTENSIN-II ON AORTIC SMOOTH-MUSCLE CELLS, Molecular pharmacology, 48(4), 1995, pp. 582-592
Angiotensin II (All) is a growth factor that stimulates protein synthe
sis and induces cellular hypertrophy in aortic smooth muscle cells (SM
C). This trophic effect is mediated by the AT(1) subtype of All recept
ors. However, very little is known about the cellular signaling pathwa
ys involved in this response. In the present study, we examined the ro
le of protein tyrosine phosphorylation in the growth-promoting effects
of All on rat aortic SMC. The addition of All to quiescent aortic SMC
induced tyrosine phosphorylation of multiple substrates, as revealed
by antiphosphotyrosine immunoblotting. This response was blocked by pr
eincubation with the AT(1)-selective antagonist losartan. To explore t
he functional role of this signaling pathway, we performed experiments
with two mechanistically distinct tyrosine kinase inhibitors. Treatme
nt of quiescent aortic SMC with genistein and herbimycin A abolished t
he stimulatory effect of All on overall protein tyrosine phosphorylati
on. Similarly, the two inhibitors prevented All-induced tyrosine phosp
horylation of the cytoskeletal protein paxillin. Under the same condit
ions, incubation with genistein or herbimycin A did not interfere with
All binding to the AT(1) receptor and did not significantly affect Al
l-stimulated inositol-l,4,5-trisphosphate production and Ca2+ mobiliza
tion. In parallel to their selective action on tyrosine phosphorylatio
n, both genistein and herbimycin A completely inhibited All-stimulated
protein synthesis in a dose-dependent manner. In contrast, the two in
hibitors were much less potent in preventing the trophic effect of pho
rbol-12-myristate 13-acetate in these cells. We further demonstrate th
at genistein and herbimycin A did not prevent mitogen-activated protei
n kinase activation and c-fos gene induction, which is consistent with
the notion that these downstream effecters do not link All-induced ty
rosine phosphorylation to protein synthesis. These results provide evi
dence that tyrosine phosphorylation has a critical role in cellular hy
pertrophy and is involved in All action in vascular SMC.