Regulation of cardiac fibroblast collagen synthesis by adenosine: roles for Epac and PI3K

Rat cardiac fibroblasts (CF) express multiple adenosine (ADO) receptors. Pharmacological evidence suggests that activation of A(2) 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. ANG II stimulates collagen production in these cells. Coincubation with agents that elevate cellular cAMP the ADO agonist, 5'-N-ethylcarboxamidoadensoine (NECA), and forskolin inhibited the stimulatory effects of ANG II. However, direct stimulators and inhibitors of protein kinase A (PKA) did not alter ANG II-induced collagen synthesis, indicating that PKA does not mediate the inhibitory effects of NECA. Inhibitors of AMP-kinase (AMPK) and extracellular signal-regulated kinase 1/2 (ERK1/2) do not alter NECA-inhibited collagen synthesis. However, activation of exchange factor directly activated by cAMP (Epac) 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 relieved the inhibitory effect of Epac activation on ANG II-stimulated collagen synthesis. Thus it appears that ADO activates the A(2)R-G(s)-adenylyl cyclase pathway and that the resultant cAMP reduces collagen synthesis via a PKA-independent, Epac-dependent pathway that feeds through PI3K.