Received for publication May 2, 2008.
Revised August 6, 2008.
Accepted for publication August 7, 2008.

Differential Signaling Pathways in Angiotensin II- and EGF-stimulated Hepatic C9 Cells

Abstract

Caveolin1 (Cav1) is an important component of the plasma-membrane microdomains, such as caveolae/lipid rafts, that are associated with angiotensin II type1 (AT1) and epidermal growth factor (EGF) receptors in certain cell types. The interactions of Cav1 with other signaling molecules that mediate AT₁ receptor function were analyzed in angiotensin II (Ang II)- and EGF-stimulated hepatic C9 cells. This study demonstrated that cholesterol-rich domains mediate the actions of early upstream signaling molecules such as Src and intracellular Ca²⁺ in cells stimulated by Ang II, but not by EGF, and that Cav1 has a scaffolding role in the process of MAPK activation. Furthermore, Cav1 phosphorylation by Ang II and EGF was regulated by intracellular Ca²⁺ and Src, further indicating reciprocal interactions among Cav1, Src, and intracellular Ca²⁺ through the AT₁R. Phosphorylation of Cav1 and the EGFR by Ang II, but not of ERK1/2, was dependent on intracellular Ca²⁺. The PI3-kinase inhibitors, LY294002 and wortmannin, differentially modulated both Cav1 and EGF receptor activation by Ang II through intracellular Ca²⁺. These findings further demonstrate the importance of Cav1 in conjunction with the receptor-mediated signaling pathways involved in cell proliferation and survival. It is clear that differential signaling pathways are operative in Ang II- and EGF-stimulated C9 cells, and that cholesterol-enriched microdomains are essential components in cellular signaling processes that are dependent on specific agonists and/or cell types.

Key words: Growth hormone, G protein regulation, Src and other nonreceptor tyrosine kinases, MAP Kinase, Angiogenesis, Endocrine cells