Background: Protease-activated receptor 4 (PAR4) is a seven transmembrane G-protein coupled receptor (GPCR) activated by endogenous proteases, such as thrombin. PAR4 is involved in various pathophysiologies including cancer, inflammation, pain, and thrombosis. Although regulators of G-protein signaling (RGS) are known to modulate GPCR/Gα-mediated pathways, their specific effects on PAR4 are not fully understood at present. We previously reported that RGS proteins attenuate PAR1-and PAR2-mediated signaling through interactions with these receptors in conjunction with distinct Gα subunits. Methods: We employed a bioluminescence resonance energy transfer technique and confocal microscopy to examine potential interactions among PAR4, RGS, and Gα subunits. The inhibitory effects of RGS proteins on PAR4mediated downstream signaling and cancer progression were additionally investigated by using several assays including ERK phosphorylation, calcium mobilization, RhoA activity, cancer cell proliferation, and related gene expression. Results: In live cells, RGS2 interacts with PAR4 in the presence of Gα q while RGS4 binding to PAR4 occurs in the presence of Gα q and Gα 12/13. Co-expression of PAR4 and Gα q induced a shift in the subcellular localization of RGS2 and RGS4 from the cytoplasm to plasma membrane. Combined PAR4 and Gα 12/13 expression additionally promoted translocation of RGS4 from the cytoplasm to the membrane. Both RGS2 and RGS4 abolished PAR4-activated ERK phosphorylation, calcium mobilization and RhoA activity, as well as PAR4-mediated colon cancer cell proliferation and related gene expression. Conclusions: RGS2 and RGS4 forms ternary complex with PAR4 in Gα-dependent manner and inhibits its downstream signaling. Our findings support a novel physiological function of RGS2 and RGS4 as inhibitors of PAR4-mediated signaling through selective PAR4/RGS/Gα coupling.

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