Gβγ subunits inhibit Epac-induced melanoma cell migration

Recently we reported that activation of Epac1, an exchange protein activated by cAMP, increases melanoma cell migration via Ca ²⁺ release from the endoplasmic reticulum (ER). G-protein βγ subunits (Gβγ) are known to act as an independent signaling molecule upon activation of G-protein coupled receptor. However, the role of Gβγ in cell migration and Ca ²⁺ signaling in melanoma has not been well studied. Here we report that there is crosstalk of Ca ²⁺ signaling between Gβγ and Epac in melanoma, which plays a role in regulation of cell migration.

SK-Mel-2 cells, a human metastatic melanoma cell line, were mainly used in this study. Intracellular Ca ²⁺ was measured with Fluo-4AM fluorescent dyes. Cell migration was examined using the Boyden chambers.

The effect of Gβγ on Epac-induced cell migration was first examined. Epac-induced cell migration was inhibited by mSIRK, a Gβγ -activating peptide, but not its inactive analog, L9A, in SK-Mel-2 cells. Guanosine 5', α-β-methylene triphosphate (Gp(CH2)pp), a constitutively active GTP analogue that activates Gβγ, also inhibited Epac-induced cell migration. In addition, co-overexpression of β1 and γ2, which is the major combination of Gβγ, inhibited Epac1-induced cell migration. By contrast, when the C-terminus of β adrenergic receptor kinase (βARK-CT), an endogenous inhibitor for Gβγ, was overexpressed, mSIRK's inhibitory effect on Epac-induced cell migration was negated, suggesting the specificity of mSIRK for Gβγ. We next examined the effect of mSIRK on Epac-induced Ca ²⁺ response. When cells were pretreated with mSIRK, but not with L9A, 8-(4-Methoxyphenylthio)-2'-O-methyladenosine-3',5'-cyclic monophosphate (8-pMeOPT), an Epac-specific agonist, failed to increase Ca ²⁺ signal. Co-overexpression of β1 and γ2 subunits inhibited 8-pMeOPT-induced Ca ²⁺ elevation. Inhibition of Gβγ with βARK-CT or guanosine 5'-O-(2-thiodiphosphate) (GDPβS), a GDP analogue that inactivates Gβγ, restored 8-pMeOPT-induced Ca ²⁺ elevation even in the presence of mSIRK. These data suggested that Gβγ inhibits Epac-induced Ca ²⁺ elevation. Subsequently, the mechanism by which Gβγ inhibits Epac-induced Ca ²⁺ elevation was explored. mSIRK activates Ca ²⁺ influx from the extracellular space. In addition, W-5, an inhibitor of calmodulin, abolished mSIRK's inhibitory effects on Epac-induced Ca ²⁺ elevation, and cell migration. These data suggest that, the mSIRK-induced Ca ²⁺ from the extracellular space inhibits the Epac-induced Ca ²⁺ release from the ER, resulting suppression of cell migration.

We found the cross talk of Ca ²⁺ signaling between Gβγ and Epac, which plays a major role in melanoma cell migration.