Activation of Mitogen-activated Protein Kinase by the A2A-adenosine Receptor via a rap1-dependent and via a p21ras-dependent Pathway

Abstract

The A(2A)-adenosine receptor, a prototypical G(s)-coupled receptor, activates mitogen-activated protein (MAP) kinase in a manner independent of cAMP in primary human endothelial cells. In order to delineate signaling pathways that link the receptor to the regulation of MAP kinase, the human A(2A) receptor was heterologously expressed in Chinese hamster ovary (CHO) and HEK293 cells. In both cell lines, A(2A) agonist-mediated cAMP accumulation was accompanied by activation of the small G protein rap1. However, rap1 mediates A(2A) receptor-dependent activation of MAP kinase only in CHO cells, the signaling cascade being composed of G(s), adenylyl cyclase, rap1, and the p68 isoform of B-raf. This isoform was absent in HEK293 cells. Contrary to CHO cells, in HEK293 cells activation of MAP kinase by A(2A) agonists was not mimicked by 8-bromo-cAMP, was independent of Galpha(s), and was associated with activation of p21(ras). Accordingly, overexpression of the inactive S17N mutant of p21(ras) and of a dominant negative version of mSos (the exchange factor of p21(ras)) blocked MAP kinase stimulation by the A(2A) receptor in HEK 293 but not in CHO cells. In spite of the close homology between p21(ras) and rap1, the S17N mutant of rap1 was not dominant negative because (i) overexpression of rap1(S17N) failed to inhibit A(2A) receptor-dependent MAP kinase activation, (ii) rap1(S17N) was recovered in the active form with a GST fusion protein comprising the rap1-binding domain of ralGDS after A(2A) receptor activation, and (iii) A(2A) agonists promoted the association of rap1(S17N) with the 68-kDa isoform of B-raf in CHO cells. We conclude that the A(2A) receptor has the capacity two activate MAP kinase via at least two signaling pathways, which depend on two distinct small G proteins, namely p21(ras) and rap1. Our observations also show that the S17N version of rap1 cannot be assumed a priori to act as a dominant negative interfering mutant.