Regulation of RhoA Signaling by the cAMP-dependent Phosphorylation of RhoGDIα

RhoA plays a pivotal role in regulating cell shape and movement. Protein kinase A (PKA) inhibits RhoA signaling and thereby induces a characteristic morphological change, cell rounding. This has been considered to result from cAMP-induced phosphorylation of RhoA at Ser-188, which induces a stable RhoA-GTP-RhoGDI alpha complex and sequesters RhoA to the cytosol. However, few groups have shown RhoA phosphorylation in intact cells. Here we show that phosphorylation of RhoGDI alpha but not RhoA plays an essential role in the PKA-induced inhibition of RhoA signaling and in the morphological changes using cardiac fibroblasts. The knockdown of RhoGDI alpha by siRNA blocks cAMP-induced cell rounding, which is recovered by RhoGDI alpha-WT expression but not when a RhoGDI alpha-S174A mutant is expressed. PKA phosphorylates RhoGDI alpha at Ser-174 and the phosphorylation of RhoGDI alpha is likely to induce the formation of a active RhoA-RhoGDI alpha complex. Our present results thus reveal a principal molecular mechanism underlying G(g)/cAMP-induced cross-talk with G(q)/G(13)/RhoA signaling.