Evans-Anderson Heather J, Alfieri Christina M, Yutzey Katherine E
Division of Molecular Cardiovascular Biology, Cincinnati Children's Hospital Medical Center, Ohio 45229, USA.
Circ Res. 2008 Mar 28;102(6):686-94. doi: 10.1161/CIRCRESAHA.107.163428. Epub 2008 Jan 24.
Cardiomyocytes actively proliferate during embryogenesis and withdraw from the cell cycle during neonatal stages. FOXO (Forkhead O) transcription factors are a direct target of phosphatidylinositol-3 kinase/AKT signaling in skeletal and smooth muscle and regulate expression of the Cip/Kip family of cyclin kinase inhibitors in other cell types; however, the interaction of phosphatidylinositol-3 kinase/AKT signaling, FOXO transcription factors, and cyclin kinase inhibitor expression has not been reported for the developing heart. Here, we show that FOXO1 and FOXO3 are expressed in the developing myocardium concomitant with increased cyclin kinase inhibitor expression from embryonic to neonatal stages. Cell culture studies show that embryonic cardiomyocytes are responsive to insulin-like growth factor 1 stimulation, which results in the induction of the phosphatidylinositol-3 kinase/AKT pathway, cytoplasmic localization of FOXO proteins, and increased myocyte proliferation. Likewise, adenoviral-mediated expression of AKT promotes cardiomyocyte proliferation and cytoplasmic localization of FOXO. In contrast, increased expression of FOXO1 negatively affects myocyte proliferation. In vivo myocyte-specific transgenic expression of FOXO1 during heart development causes embryonic lethality at embryonic day 10.5 because of severe myocardial defects that coincide with premature activation of p21(cip1), p27(kip1), and p57(kip2) and decreased myocyte proliferation. Transgenic expression of dominant negative FOXO1 in cardiomyocytes does not obviously affect heart development at embryonic day 10.5, but results in abnormal morphology of the myocardium by embryonic day 18.5 along with decreased cyclin kinase inhibitor expression and increased myocyte proliferation. These data support FOXO transcription factors as negative regulators of cardiomyocyte proliferation and promoters of neonatal cell cycle withdrawal during heart development.
心肌细胞在胚胎发育过程中积极增殖，并在新生儿阶段退出细胞周期。FOXO（叉头框O）转录因子是骨骼肌和平滑肌中磷脂酰肌醇-3激酶/AKT信号通路的直接靶点，并在其他细胞类型中调节细胞周期蛋白激酶抑制剂Cip/Kip家族的表达；然而，在发育中的心脏中，磷脂酰肌醇-3激酶/AKT信号通路、FOXO转录因子和细胞周期蛋白激酶抑制剂表达之间的相互作用尚未见报道。在此，我们表明，从胚胎期到新生儿期，FOXO1和FOXO3在发育中的心肌中表达，同时细胞周期蛋白激酶抑制剂的表达增加。细胞培养研究表明，胚胎心肌细胞对胰岛素样生长因子1刺激有反应，这导致磷脂酰肌醇-3激酶/AKT通路的诱导、FOXO蛋白的细胞质定位以及心肌细胞增殖增加。同样，腺病毒介导的AKT表达促进心肌细胞增殖和FOXO的细胞质定位。相反，FOXO1表达增加对心肌细胞增殖有负面影响。在心脏发育过程中，体内心肌细胞特异性转基因表达FOXO1会在胚胎第10.5天导致胚胎致死，这是由于严重的心肌缺陷，与p21（cip1）、p27（kip1）和p57（kip2）的过早激活以及心肌细胞增殖减少相一致。在心肌细胞中显性负性FOXO1的转基因表达在胚胎第10.5天对心脏发育没有明显影响，但在胚胎第18.5天导致心肌形态异常，同时细胞周期蛋白激酶抑制剂表达减少，心肌细胞增殖增加。这些数据支持FOXO转录因子作为心肌细胞增殖的负调节因子以及心脏发育过程中新生儿细胞周期退出的促进因子。
