ABSTRACT

Apelin and its G-protein-coupled receptor APJ are potent regulators of the cardiovascular system. Recent studies have suggested that apelin-APJ reverses the function of angiotensin II (Ang II)-the Ang II type 1 receptor (AT 1 ). However, the mechanism remains unclear because of the accumulating evidences that apelin-APJ may contribute to both cardioprotection and pathological progression. In human embryonic kidney 293 cells, we found that coexpression with APJ significantly suppressed the phosphorylation of extracellular signal-regulated kinases 1/2 (ERK1/2) induced by Ang II-AT 1 , whereas apelin abolished this attenuation through activated APJ independently of its heterodimerization. Pretreatment with the Gi/o-specific inhibitor pertussis toxin (PTX) restituted the ERK1/2 phosphorylation level similar to that found with AT 1 and APJ coexpression without apelin stimulation. In contrast, coexpression of the beta-2-adrenergic receptor or the pharmacologically non-activated Ang II type 2 receptor (AT 2 ) pretreated with the AT 2 -specific antagonist, PD123319, did not affect ERK1/2 phosphorylation through AT 1 . Pretreatment with 30 nM of the AT 1 blocker (ARB) TA-606A suppressed 50% of the AT 1 -mediated ERK1/2 phosphorylation, whereas 30 nM of TA-606A achieved 75% suppression when the non-activated APJ was coexpressed without ligand or PTX. However, 120 nM of TA-606A failed to reach the target phosphorylation when it was coexpressed with activated APJ with apelin. Based on these results, we demonstrated that non-activated APJ may suppress Ang II-AT 1 signaling, whereas this ligand-independent function was diminished with apelin activation. These results may be relevant to the potential contribution of apelin-APJ to ARB treatment in the clinical realm.

Apelin and its G protein-coupled receptor (GPCR) APJ, which is most closely related to the angiotensin II (Ang II) type 1 receptor (AT 1 ) but Ang II does not bind to APJ, are potent regulators of the cardiovascular system. Although recent studies have suggested that apelin-APJ reverses the function of Ang II-AT 1 , the mechanism remains unclear because the accumulating evidences indicates that apelin-APJ may contribute to both cardioprotection and pathological progression. In APJ and AT 1 co-expression human embryonic kidney (HEK) 293 cells, we found that APJ and AT 1 created receptor heterodimers. Co-expression with APJ significantly suppressed the phosphorylated extracellular signal-regulated kinases 1/2 (pERK1/2) induced by Ang II-AT 1 , whereas apelin eliminated this ligand-independent function of APJ out of relation to its heterodimerization. The pharmacologically non-activated APJ upon apelin stimulation elicited by the Gi/o-specific inhibitor pertussis toxin (PTX) restituted the pERK1/2 level similar to that of AT 1 and APJ co-expression without apelin stimulation. However, the co-expression of the beta-2-adrenergic receptor (β 2 AR) or the pharmacologically non-activated Ang II type 2 receptor (AT 2 ) induced by the AT 2 -specific antagonist, PD123319, did not suppress pERK1/2 through Ang II-AT 1 . Pretreatment with 30 nM of the AT 1 blocker (ARB) TA-606A suppressed 50% of the AT 1 -mediated pERK1/2, whereas this suppression raised to 75% when the non-activated APJ was co-expressed. In contrast, 120 nM of TA-606A only reached 34% suppression when it was co-expressed with activated APJ with apelin. Taken together, we demonstrated that apelin may regulate the function of AT 1. Non-activated APJ may suppress Ang II-AT 1 signaling, whereas this ligand-independent function was diminished with apelin activation. These results may contribute to ARB treatment in the clinical setting.