A2B adenosine receptor-triggered intracellular calcium mobilization: Cell type-dependent involvement of Gi, Gq, Gs proteins and protein kinase C

Activation of PLC beta enzymes by G(i beta gamma) and G(alpha q/11) proteins is a common mechanism to trigger cytosolic Ca2+ increase. We and others reported that G(alpha q/11) inhibitor FR900359 (FR) can inhibit both G(alpha q)- and, surprisingly, G(i beta gamma)-mediated intracellular Ca2+ mobilization. Thus, the G(alpha i)-G(beta gamma)-PLC beta-Ca2+ signaling axis depends entirely on the presence of active G(alpha q), which reasonably explained FR-inhibited G(i beta gamma)-induced Ca2+ release. However, the conclusion that G(i beta gamma) signaling is controlled by G(alpha q) derives mostly from HEK293 cells. Here we show that indeed in HEK293 cells both G(alpha q/11) siRNA and G(alpha q/11) inhibitors diminished Ca2+ increase triggered by native G(q)-coupled P2Y(1) receptors, or by transfected G(i)-coupled A(1)- or G(s)-coupled A(2B) adenosine receptors (ARs). However, in T24 bladder cancer cells, G(i) inhibitor PTX, but not G(alpha q/11) inhibitors, FR, YM254890 (YM) or G(q/11) siRNA, inhibited Ca2+ increase triggered by native A(2B)AR activation. Simultaneous inactivation of G(i) and G(s) further suppressed A(2B)AR-triggered Ca2+ increase in T24 cells. The G(alpha q/11) inhibitor YM fully and partially inhibited endogenous P2Y(1)- and beta(2)-adrenergic receptor-induced Ca2+ increase in T24 cells, respectively. PKC activator PMA partially diminished A(2B)AR-triggered but completely diminished beta(2)-adrenergic receptor-triggered Ca2+ increase in T24 cells. Neither beta-arrestin1 nor beta-arrestin2 siRNA affected A(2B)AR-mediated Ca2+ increase. Unlike in T24 cells, YM inhibited native A(2B)AR-triggered calcium mobilization in MDA-MB-231 breast cancer cells. Thus, G(alpha q/11) is vital for Ca2+ increase in some cell types, but G(i beta gamma)-mediated Ca2+ signaling can be G alpha(q/11)-dependent or independent based on cell type and receptor activated. Besides G proteins, PKC also modulates cytosolic Ca2+ increase depending on cell type and receptor.