4-Hydroxynonenal-Induced Gpr109a (Hca(2) Receptor) Activation Elicits Bipolar Responses, G(Alpha I)-Mediated Anti-Inflammatory Effects And G(Beta Gamma)-Mediated Cell Death

BACKGROUND AND PURPOSEIn this study, we examined the possibility that 4-hydroxynonenal (4-HNE) acting as a ligand for the HCA(2) receptor (GPR109A) elicits both anti-inflammatory and cell death responses.EXPERIMENTAL APPROACHAgonistic activity of 4-HNE was determined by observing the inhibition of cAMP generation in CHO-K1-GPR109A-G(i) cell line, using surface plasmon resonance (SPR) binding and competition binding assays with [H-3]-niacin. 4-HNE-mediated signalling pathways and cellular responses were investigated in cells expressing GPR109A and those not expressing these receptors.KEY RESULTSAgonistic activity of 4-HNE was stronger than that of niacin or 3-OHBA at inhibiting forskolin-induced cAMP production and SPR binding affinity. In ARPE-19 and CCD-841 cells, activation of GPR109A by high concentrations of the agonists 4-HNE (>= 10 mu M), niacin (>= 1000 mu M) and 3-OHBA (>= 1000 mu M) induced apoptosis accompanied by elevated Ca2+ and superoxide levels. This 4-HNE-induced cell death was blocked by knockdown of GPR109A or NOX4 genes, or treatment with chemical inhibitors of G(beta gamma) (gallein), intracellular Ca2+ (BAPTA-AM), NOX4 (VAS2870) and JNK (SP600125), but not by the cAMP analogue 8-CPT-cAMP. By contrast, low concentrations of 4-HNE, niacin and 3-OHBA down-regulated the expression of pro-inflammatory cytokines IL-6 and IL-8. These 4-HNE-induced inhibitory effects were blocked by a cAMP analogue but not by inhibitors of G(beta gamma)-downstream signalling molecules.CONCLUSIONS AND IMPLICATIONSThese results revealed that 4-HNE is a strong agonist for GPR109A that induces G(alpha i)-dependent anti-inflammatory and G(beta gamma)-dependent cell death responses. Moreover, the findings indicate that specific intracellular signalling molecules, but not GPR109A, can serve as therapeutic targets to block 4-HNE-induced cell death.