Abstract 425: Role of MLF1 in Cardiomyocyte Cell Cycle Regulation

The neonatal mammalian heart is capable of significant regeneration after cardiac injury. However, that ability is lost in the early postnatal period, coinciding with the development of cardiomyocyte cell cycle arrest. Myeloid leukemia factor 1 (MLF1) is a protein expressed in hematopoietic cells, skeletal muscle and cardiac muscle. In hematopoietic cells, MLF1 is thought to be a negative cell cycle regulator, possibly acting through a p53-dependent mechanism. However, the role of MLF1 in the cardiovascular system is not well understood. Here, we describe MLF1 as a regulator of cardiomyocyte cell cycle progression. We first examined the expression pattern of MLF1 in neonatal mouse heart. MLF1 protein specifically expresses in the nucleus, and the expression level increases from P1 to P7 as revealed by quantitative PCR and Western Blot. To elucidate a potential role of Mlf1 in cardiomyocyte proliferation, we silenced Mlf1 in neonatal rat ventricle myocytes (NRVMs) which showed increased cardiomyocyte proliferation by staining with a mitosis marker phospho-Histone 3 (pH3). Then we generated an inducible cardiac-specific Mlf1 knockout mouse model and assessed for the prolongation of the proliferative window by echocardiography, cell size analysis, and immunostaining for cell cycle markers. Staining the Mlf1 KO hearts with proliferation markers, pH3 to determine mitosis and Aurora B kinase to determine cytokinesis, indicated that deletion of Mlf1 resulted in increased cardiomyocyte proliferation. It also showed improvement in left ventricular systolic function following myocardial infarction by echocardiography. In contrast, overexpression of MLF1 in heart showed decreased cardiac function as quantified by ejection fraction. These results identify MLF1 as a negative cell cycle regulator of cardiomyocyte proliferation.