Runx1 is upregulated by STAT3 and promotes proliferation of neonatal rat cardiomyocytes

Though it is well known that mammalian cardiomyocytes exit cell cycle soon after birth, the mechanisms that regulate proliferation remain to be fully elucidated. Recent studies reported that cardiomyocytes undergo dedifferentiation before proliferation, indicating the importance of dedifferentiation in cardiomyocyte proliferation. Since Runx1 is expressed in dedifferentiated cardiomyocytes, Runx1 is widely used as a dedifferentiation marker of cardiomyocytes; however, little is known about the role of Runx1 in the proliferation of cardiomyocytes. The purpose of this study was to clarify the functional significance of Runx1 in cardiomyocyte proliferation. qRT-PCR analysis and immunoblot analysis demonstrated that Runx1 expression was upregulated in neonatal rat cardiomyocytes when cultured in the presence of FBS. Similarly, STAT3 was activated in the presence of FBS. Interestingly, knockdown of STAT3 significantly decreased Runx1 expression, indicating Runx1 is regulated by STAT3. We next investigated the effect of Runx1 on proliferation. Immunofluorescence microscopic analysis using an anti-Ki-67 antibody revealed that knockdown of Runx1 decreased the ratio of proliferating cardiomyocytes. Conversely, Runx1 overexpression using adenovirus vector induced cardiomyocyte proliferation in the absence of FBS. Finally, RNA-sequencing analysis revealed that Runx1 overexpression induced upregulation of cardiac fetal genes and downregulation of genes associated with fatty acid oxidation. Collectively, Runx1 is regulated by STAT3 and induces cardiomyocyte proliferation by juvenilizing cardiomyocytes.