A adenosine receptor-triggered intracellular calcium mobilization: Cell type-dependent involvement of G, G, G proteins and protein kinase C.

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