Abstract
The Forkhead Box m1 (Foxm1) transcription factor is expressed in cardiomyocytes and cardiac endothelial cells during heart development. In this study, we used a novel Foxm1 -/- mouse line to demonstrate that Foxm1-deletion causes ventricular hypoplasia and diminished DNA replication and mitosis in developing cardioniyocytes. Proliferation defects in Foxm1 -/- hearts were associated with a reduced expression of Cdk1-activator Cdc25B phosphatase and NFATc3 transcription factor, and with abnormal nuclear accumulation of the Cdk-inhibitor p21 Cip1 protein. Depletion of Foxm1 levels by siRNA caused altered expression of these genes in cultured HL-1 cardiomyocytes. Endothelial-specific deletion of the Foxm1 fl/fl allele in Tie2-Cre Foxm1 fl/fl embryos did not influence heart development and cardiomyocyte proliferation. Foxm1 protein binds to the -9,259/-9,288-bp region of the endogenous mouse NFATc3 promoter, indicating that Foxm1 is a transcriptional activator of the NFATc3 gene. Foxinl regulates expression of genes essential for the proliferation of cardioniyocytes during heart development. © 2007 Wiley-Liss, Inc.