Apelin and apelin receptor at different stages of corpus luteum development and effect of apelin on progesterone secretion and 3β-hydroxysteroid dehydrogenase (3β-HSD) in pigs

Recent studies have suggested that apelin has a role in controlling female reproduction. The aims of the present study were, firstly, to investigate the gene expression (mRNA and protein) and immunolocalization of apelin and its receptor APJ in corpora lutea (CL) of pigs collected during the early (CL1), middle (CL2) and late (CL3) luteal phase. Using real time PCR and immunoblotting techniques, it was observed that apelin gene expression was similar in CL1 and CL2, and less in CL3, while relative abundance APJ mRNA and abundance of the protein were similar in CL1 and CL3 and greater in CL2. There was apelin staining in the cytoplasm of both small (SC) and large (LC) luteal cells with the greatest intensity in CL2. In the cytoplasm of CL1, only a few SC cells stained for APJ; in CL2, APJ was located in the cell membrane of LC and in the cytoplasm of SC; and in CL3 was located in the membrane with moderate cytoplasmic APJ staining. Intense APJ staining was noted in epithelium of blood vessels of CL2-3. Secondly, there was an effect of apelin on progesterone (P4) secretion in CL2 and on the molecular mechanisms of these cells. Stimulatory effects of apelin on P4 secretion, 3 beta-hydroxysteroid dehydrogenase (HSD) activity and protein abundance were observed and this was inhibited in response to APJ and adenosine 5'-monophosphate-activated protein kinase (AMPK alpha) kinase blockers. In conclusion, the presence of apelin/APJ in the CL of pigs and stimulatory effects of apelin on P4 secretion and 3 beta-HSD levels suggest potential auto/paracrine regulation by apelin in the luteal phase of the estrous cycle. Moreover, the involvement of APJ and AMPKa kinase in apelin activity in CL was confirmed.