Submitted on June 2, 2008
Revised on February 23, 2009
Accepted on March 3, 2009
Regulation of cardiac fibroblast collagen synthesis by adenosine: Roles for Epac and PI3K
Francisco J. Villarreal1*, Sara Ann Epperson1, Israel Ramirez-Sanchez1, Katrina G. Yamazaki1, and Laurence L Brunton1
1 University of California, San Diego
* To whom correspondence should be addressed. E-mail: fvillarr{at}ucsd.edu.
Rat cardiac fibroblasts (CF) express multiple adenosine (ADO) receptors. Pharmacological evidence suggests that activation of A2 receptors may inhibit collagen synthesis via adenylyl cyclase-induced elevation of cellular cAMP. We have characterized the signaling pathways involved in ADO-mediated inhibition of collagen synthesis in primary cultures of adult rat CF. Angiotensin II (Ang II) stimulates collagen production in these cells. Co-incubation with agents that elevate cellular cAMP (the ADO agonist, NECA, and forskolin) inhibited the stimulatory effects of Ang II. However, direct stimulators and inhibitors of PKA do not alter Ang II-induced collagen synthesis indicating that PKA does not mediate NECA's inhibitory effects. Inhibitors of AMP-kinase (AMPK) and ERK1/2 blockers do not alter NECA-inhibited collagen synthesis. However, activation of Epac (exchange protein activated by cAMP) mimicked the effects of NECA on Ang II-stimulated collagen synthesis. Inhibition of phosphoinositol-3 kinase (PI3K) reduced the inhibitory effects of NECA on Ang II-induced collagen synthesis, suggesting that NECA acts via PI3K. Furthermore, inhibition of PI3K also relieves the inhibitory effect of Epac activation on Ang II-stimulated collagen synthesis. Thus, it appears that ADO activates the A2R-Gs-adenylyl cyclase pathway and that the resultant cAMP reduces collagen synthesis via a PKA-independent, Epac-dependent pathway that feeds through PI3K.
