Ligand-dependent serum response factor activation by the human CC chemokine receptors CCR2a and CCR2b is mediated by G proteins of the Gq family

Expression of the human CCR2 receptors, CCR2a and CCR2b, in mammalian cells results in ligand-dependent changes in the activity of multiple cellular signal transduction pathways, mediated in most cases by pertussis toxin-sensitive heterotrimeric G proteins of the G(i/o) subfamily. In addition, CCR2a and CCR2b receptors have been shown to couple to G(q) family members, triggering the canonical activation of phospholipase Cb isoenzymes. Activation of pertussis toxin-insensitive Gq proteins by cell-surface receptors is not only coupled to activation of phospholipase isoenzymes but also to Rho guanine nucleotide exchange factors, which in turn mediate activation of the Rho GTPases. Activated Rho GTPases regulate numerous cellular functions, including the organization of the actin cyto-skeleton and gene transcription, such as the transcription factor serum response factor. These findings prompted us to investigate whether CCR2a and/or CCR2b stimulate serum response factor activity. The results presented herein demonstrate that stimulation of human CCR2a- or CCR2b-expressing COS-7 cells caused a vigorous induction of serum response factor activity. This effect was specifically mediated by G(q) and/or G(14), as well as Rho A and/ or a closely related Rho GTPase. Furthermore, the stimulatory effect of CCR2a and CCR2b and G(alpha q) was sensitive to co-expression of the Gaq-interacting leukemia-associated Rho guanine nucleotide exchange factor. The findings of the work indicate a role for G alpha(q) and/or G alpha(14) and in CCR2a/CCR2b-stimulated Rho A GTPase-mediated serum response factor activation and introduce a noncanonical pathway activated by CCR2 receptors by coupling to Gq proteins.