A2B adenosine receptor activation and modulation by protein kinase C

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 Giβγ and Gαq/11 proteins is a common mechanism to trigger cytosolic Ca2+ increase. We and others reported that Gαq/11 inhibitor FR900358 (FR) can inhibit both and Gαq- and, surprisingly, Giβγ-mediated intracellular Ca2+ mobilization. Thus, the Gαi-Gβγ-PLCβ-Ca2+ signaling axis depends entirely on the presence of active Gαq, which reasonably explained FR-inhibited Giβγ-induced Ca2+ release. However, the conclusion that Giβγ signaling is controlled by Gαq derives mostly from HEK293 cells. Here we show that indeed in HEK293 cells both Gαq/11 siRNA and Gαq/11 inhibitors diminished Ca2+ increase triggered by native Gq-coupled P2Y1 receptors, or by transfected Gi-coupled A1-or Gs-coupled A2B adenosine receptors (ARs). However, in T24 bladder cancer cells, Gi inhibitor PTX, but not Gαq/11 inhibitors, FR, YM254890 (YM) or Gq/11 siRNA, inhibited Ca2+ increase triggered by native A2BAR activation. Simultaneous inactivation of Gi and Gs further suppressed A2BAR-triggered Ca2+ increase in T24 cells. The Gαq/11 inhibitor YM fully and partially inhibited endogenous P2Y1- and β2-adrenergic receptor-induced Ca2+ increase in T24 cells, respectively. PKC activator PMA partially diminished A2BAR-triggered but completely diminished β2-adrenergic receptor-triggered Ca2+ increase in T24 cells. Neither β-arrestin1 nor β-arrestin2 siRNA affected A2BAR-mediated Ca2+ increase. Unlike in T24 cells, YM inhibited native A2BAR-triggered calcium mobilization in MDA-MB-231 breast cancer cells. Thus, Gαq/11 is vital for Ca2+ increase in some cell types, but Giβγ-mediated Ca2+ signaling can be Gαq/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.

Genetic and functional modulation by agonist MRS5698 and allosteric enhancer LUF6000 at the native A3 adenosine receptor in HL-60 cells

The A3 adenosine receptor (AR) is an important inflammatory and immunological target. However, the underlying mechanisms are not fully understood. Here, we report the gene regulation in HL-60 cells treated acutely with highly selective A3AR agonist MRS5698, positive allosteric modulator (PAM) LUF6000, or both. Both pro- and anti-inflammatory genes, such as IL-1a, IL-1β, and NFκBIZ, are significantly upregulated. During our observations, LUF6000 alone produced a lesser effect, while the MRS5698 + LUF6000 group demonstrated generally greater effects than MRS5698 alone, consistent with allosteric enhancement. The number of genes up- and down-regulated are similar. Pathway analysis highlighted the critical involvement of signaling molecules, including IL-6 and IL-17. Important upstream regulators include IL-1a, IL-1β, TNF-α, NF-κB, etc. PPAR, which modulates eicosanoid metabolism, was highly downregulated by the A3AR agonist. Considering previous pharmacological results and mathematical modeling, LUF6000's small enhancement of genetic upregulation suggested that MRS5698 is a nearly full agonist, which we demonstrated in both cAMP and calcium assays. The smaller effect of LUF6000 on MRS5698 in comparison to its effect on Cl-IB-MECA was shown in both HL-60 cells endogenously expressing the human (h) A3AR and in recombinant hA3AR-expressing CHO cells, consistent with its HL-60 cell genetic regulation patterns. In summary, by using both selective agonists and PAM, we identified genes that are closely relevant to immunity and inflammation to be regulated by A3AR in differentiated HL-60 cells, a cell model of neutrophil function. In addition, we demonstrated the previously uncharacterized allosteric signaling-enhancing effect of LUF6000 in cells endogenously expressing the hA3AR.

A2B adenosine receptor signaling and regulation

The A2B adenosine receptor (A2BR) is one of the four adenosine-activated G protein-coupled receptors. In addition to adenosine, protein kinase C (PKC) was recently found to activate the A2BR. The A2BR is coupled to both Gs and Gi, as well as Gq proteins in some cell types. Many primary cells and cell lines, such as bladder and breast cancer, bronchial smooth muscle, skeletal muscle, and fat cells, express the A2BR endogenously at high levels, suggesting its potentially important role in asthma, cancer, diabetes, and other conditions. The A2BR has been characterized as both pro- and anti-inflammatory, inducing cell type-dependent secretion of IL-6, IL-8, and IL-10. Theophylline and enprofylline have long been used for asthma treatment, although it is still not entirely clear if their A2BR antagonism contributes to their therapeutic effects or side effects. The A2BR is required in ischemic cardiac preconditioning by adenosine. Both A2BR and protein kinase C (PKC) contribute to cardioprotection, and both modes of A2BR signaling can be blocked by A2BR antagonists. Inhibitors of PKC and A2BR are in clinical cancer trials. Sulforaphane and other isothiocyanates from cruciferous vegetables such as broccoli and cauliflower have been reported to inhibit A2BR signaling via reaction with an intracellular A2BR cysteine residue (C210). A full, A2BR-selective agonist, critical to elucidate many controversial roles of the A2BR, is still not available, although agonist-bound A2BR structures have recently been reported.