Role of apelin in vascular smooth muscle cell phenotypic transition: a proatherogenic factor for atherosclerosis

Abstract Funding Acknowledgements Type of funding sources: Public grant(s) – National budget only. Main funding source(s): Swiss National Science Foundation Fondation Prévot Apelin is a small peptide identified in 1998 as the endogenous ligand for the orphan G protein-coupled receptor APJ. It is produced as a 77 amino acid preproapelin further processed into C-terminal fragments giving rise to several apelin isoforms. Apelin is widely and abundantly expressed in the cardiovascular system, in particular, in endothelial cells and smooth muscles cells (SMCs). One of the main roles of apelin is to regulate vascular tone both as an endothelium-dependent vasodilator, through nitric oxide production, and as an endothelium-independent vasoconstrictor, by acting directly on the APJ receptor of SMCs. The role of apelin in atherogenesis remains unclear. One of the hallmarks of atherosclerotic plaque formation is the transition of SMCs from contractile to synthetic phenotype. We previously isolated contractile/differentiated spindle-shaped (S) and synthetic/dedifferentiated rhomboid (R) SMCs from porcine coronary artery. R-SMCs specifically express S100A4, a small calcium binding protein, essential for their phenotypic transition. In this study, we investigated the role of apelin in this phenotypic conversion, as well as its relationship with S100A4, by inducing its expression in SMC populations isolated from porcine coronary artery. We showed that apelin was highly expressed in R-SMCs compared with S-SMCs. A nuclear expression of apelin in intimal SMCs of porcine coronary stent-induced intimal thickening was observed. To determine the effect of apelin cellular localization and its role in the phenotypic switch, two mutated preproapelin-His-tag encoding plasmids targeting apelin into the nucleus (N. Ap) and into the secretory vesicles (S. Ap) were transfected into S-SMCs (devoid of apelin). Both induced a SMC transition towards a R-phenotype, which was associated with increased proliferative activity, downregulation of α-smooth muscle actin and increased expression and release of S100A4. Unexpectedly, overexpression of N. Ap, but not S. Ap, led to nuclear localization of S100A4. Stimulation of S-SMCs with platelet-derived growth factor-BB, known to induce a transition toward the R-phenotype, yielded nuclear expression of both apelin and S100A4. After transfecting and overexpressing S100A4 in S-SMCs we did not observe any induction of apelin expression, which suggests that apelin acts upstream of S100A4. In conclusion, apelin induces a SMC phenotypic transition towards the synthetic phenotype, associated with S100A4 upregulation and release. These results suggest that apelin act as a pro-atherogenic factor. In addition, we unravel a potential new apelin pathway leading to nuclear localization of apelin and S100A4. Further investigation is needed to decipher the nature of apelin/S100A4 interaction, the mechanisms leading to apelin nuclear redirecting and whether it might constitute a potential target in toning down SMC driven atherosclerosis development.