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

Activation of PLCβ enzymes by G _iβγ and G _αq/11 proteins is a common mechanism to trigger cytosolic Ca ²⁺ increase. We and others reported that G _αq/11 inhibitor FR900358 (FR) can inhibit both and G _αq - and, surprisingly, G _iβγ -mediated intracellular Ca ²⁺ mobilization. Thus, the G _αi -G _βγ -PLCβ-Ca ²⁺ signaling axis depends entirely on the presence of active G _αq , which reasonably explained FR-inhibited G _iβγ -induced Ca ²⁺ release. However, the conclusion that G _iβγ signaling is controlled by G _αq derives mostly from HEK293 cells. Here we show that indeed in HEK293 cells both G _αq/11 siRNA and G _αq/11 inhibitors diminished Ca ²⁺ 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 _αq/11 inhibitors, FR, YM254890 (YM) or G _q/11 siRNA, inhibited Ca ²⁺ increase triggered by native A _2B AR activation. Simultaneous inactivation of G _i and G _s further suppressed A _2B AR-triggered Ca ²⁺ increase in T24 cells. The G _αq/11 inhibitor YM fully and partially inhibited endogenous P2Y ₁ - and β ₂ -adrenergic receptor-induced Ca ²⁺ increase in T24 cells, respectively. PKC activator PMA partially diminished A _2B AR-triggered but completely diminished β ₂ -adrenergic receptor-triggered Ca ²⁺ increase in T24 cells. Neither β-arrestin1 nor β-arrestin2 siRNA affected A _2B AR-mediated Ca ²⁺ increase. Unlike in T24 cells, YM inhibited native A _2B AR-triggered calcium mobilization in MDA-MB-231 breast cancer cells. Thus, G _αq/11 is vital for Ca ²⁺ increase in some cell types, but G _iβγ -mediated Ca ²⁺ signaling can be Gα _q/11 -dependent or independent based on cell type and receptor activated. Besides G proteins, PKC also modulates cytosolic Ca ²⁺ increase depending on cell type and receptor.