Angiotensin II-induced transactivation of epidermal growth factor receptorregulates fibronectin and transforming growth factor-beta synthesis via transcriptional and posttranscriptional mechanisms

The signaling cascade elicited by angiotensin II (Ang II) resembles that ch aracteristic of a growth factor, and recent evidence indicates transactivat ion of epidermal growth factor receptor (EGF-R) by G protein-coupled recept ors. Here, we report the involvement of EGF-R in Ang II-induced synthesis o f fibronectin and transforming growth factor-beta (TGF-beta) in cardiac fib roblasts, Ang II stimulated fibronectin mRNA levels dose dependently, with a maximal increase (approximate to 5-fold) observed after 12 hours of incub ation. Fibronectin synthesis induced by Ang II or calcium ionophore was com pletely abolished by tyrosine kinase inhibitors and intracellular Ca2+ chel ating agents. Ang II-induced fibronectin mRNA was not affected by protein k inase C inhibitors or protein kinase C depletion, whereas specific inhibiti on of EGF-R function by a dominant negative EGF-R mutant and tyrphostin AG1 478 abolished induction of fibronectin mRNA. We isolated the rat fibronecti n gene, including the 5'-flanking region, and found that the activator prot ein-1 (AP-1) binding site present in the promoter region was responsible fo r the Ang II responsiveness of this gene. A gel retardation assay revealed the binding of nuclear protein to the AP-1 site, which was supershifted wit h anti-c-fos and anti-c-jun but not anti-activating transcription factor (A TF)-2 antibodies. Conditioned medium from Ang II-treated cells contained TG F-beta bioactivity, and addition of neutralizing TGF-beta antibody modestly (46%) inhibited induction of fibronectin. Ang II-induced synthesis of TGF- beta was also abolished by inhibition of EGF-R function. The effect of TGF- P was exerted by stabilizing fibronectin mRNA without affecting the promote r activity and required de novo protein synthesis. We concluded that Ang II -induced expression of fibronectin and TGF-beta is mediated by downstream s ignaling of EGF-R transactivated by Ca2+-dependent tyrosine kinase and that Ang II-induced fibronectin mRNA expression is regulated by 2 different mec hanisms, which are transcriptional control by binding of the c-fos/c-jun co mplex to the AP-1 site and posttranscriptional control by mRNA stabilizatio n due to autocrine or paracrine effects of TGF-beta. Thus, this study sugge sts that the action of Ang II on extracellular matrix formation should be i nterpreted in association with the EGF-R signaling cascade.