P314Osteopontin stimulates cell cycle in neonatal cardiomyocytes

Abstract

Abstract Introduction Macrophages control myocardial regeneration in the neonatal heart and repair in the adult heart. Osteopontin (OPN) is a macrophage-derived protein which is upregulated following tissue injury. While OPN regulates cell adhesion, spreading and migration, its role in myocardial regeneration is unknown. Purpose We aimed to test the hypothesis that OPN stimulates cardiomyocyte proliferation. Methods and results Mouse neonatal cardiomyocytes were treated with a serum-free medium for 24 hours. Immunofluorescent staining showed that these cells expressed the OPN receptor CD44 (Fig. 1A). To assess cell expansion, we used MTT, 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide, colorimetric assay. OPN preserved cardiomyocyte count in a low glucose medium for 24 hours, compared with no OPN treatment (Fig. 1B). Staining for phosphohistone (pH)3, a marker of nucleus division, showed a higher expression of pH3 in cardiomyocytes after OPN treatment (Fig. 1C). Immunoblotting revealed that OPN induced the phosphorylation of cell-cycle activity proteins, known as yes-associated protein (YAP)1, and extracellular signal-regulated kinase (ERK)2, while it decreased the phosphorylation of the YAP1 inhibitor, the large tumor-suppressor kinase (LATS)1/2 (Figs. 1D and 1E). Gene expression analysis revealed that OPN upregulated the transcriptional enhancer factor TEF-1 (TEAD1), which interacts with YAP1 (Fig. 1F), connective tissue growth factor (CTGF) and cyclin-dependent kinase 1 (CDK1), which are YAP downstream targets (Figs. 1G and 1H). Furthermore, OPN significantly upregulated cyclin B1, a regulator of the mitotic (M) phase (Fig. 1I). Moreover, OPN may promote positive feedback by upregulating the expression of CD44, SPP-1 (Figs. 1J and 1K). Finally, administration of OPN to adult mouse improved cardiac remodeling and function after myocardial infarction. Conclusions OPN stimulates cell-cycle activity in neonatal cardiomyocytes through YAP1 activity, independently from the HIPPO-YAP signaling pathway. Because macrophages are a major source of OPN after injury, our findings could explain the essential role of macrophages in neonatal heart regeneration and adult heart repair. Acknowledgement/Funding Israel Science Foundation