Increased levels of forkhead box M1B transcription factor in transgenic mouse hepatocytes prevent age-related proliferation defects in regenerating liver

Abstract

The forkhead box (Fox) family of transcription factors share homology in the winged helix͞forkhead DNA-binding domain and play important roles in regulating cellular proliferation, differentiation, longevity, and cellular transformation. Forkhead box M1B (FoxM1B) is a ubiquitously expressed member of the Fox transcription factor family whose expression is restricted to proliferating cells and that mediates hepatocyte entry into DNA synthesis and mitosis during liver regeneration. Recent cDNA microarray studies indicated that age-related defects in cellular proliferation are associated with diminished expression of the FoxM1B transcription factor. Here, we show that increased levels of FoxM1B in regenerating liver of old transgenic mice restore the sharp peaks in hepatocyte DNA replication and mitosis that are the hallmarks of young regenerating mouse liver. Restoration of the young regenerating liver phenotype is associated with increased expression of numerous cell cycle regulatory genes that include cyclin D1, cyclin A2, cyclin F, cyclin B1, cyclin B2, Cdc25B, and p55cdc. Cotransfection assays in the human hepatoma HepG2 cell line demonstrated that FoxM1B protein stimulated expression of both the cyclin B1 and cyclin D1 promoters, suggesting that these cyclin genes are a direct FoxM1B transcriptional target. These results suggest that FoxM1B controls the transcriptional network of genes that are essential for cell division and exit from mitosis. Our results indicate that reduced expression of the FoxM1B transcription factor contributes to the decline in cellular proliferation observed in the aging process.T he mammalian liver is one of the few adult organs capable of completely regenerating itself in response to injury through the release of growth factors that stimulate reentry of terminally differentiated hepatocytes into the cell cycle (1-3). Liver regeneration induced by a two-thirds partial hepatectomy (PHx) results in synchronous induction of sharp peaks in hepatocyte DNA replication (S phase) and mitosis (M phase), which requires participation of the IL-6-signaling pathway (3-5). The forkhead box (Fox) family of transcription factors (6) shares homology in the winged-helix DNA-binding domain (7), and its members play important roles in regulating transcription of genes involved in cellular proliferation, differentiation, metabolic homeostasis, longevity, and cellular transformation (8-18). The mammalian (human) Fox family member FoxM1B (previously known as HFH-11B or trident) is a ubiquitously expressed transcription factor restricted to proliferating cells of the mouse embryo (including liver) and is essential for embryonic development (19), but its expression diminishes during postnatal cellular differentiation (20). In regenerating liver, FoxM1B expression is reactivated before DNA replication (S phase) and sustained throughout the period of hepatocyte proliferation (20). Liver regeneration studies with transgenic mice, in which the transthyretin (TTR) promoter functioned to prematurely express FoxM1...

Myocardium defects and ventricular hypoplasia in mice homozygous null for the Forkhead Box m1 transcription factor

“…These results suggest that Foxm1 may regulate other genes essential for cardiomyocyte entry into mitosis during heart morphogenesis. In this study, we found that Foxm1 Ϫ/Ϫ hearts display reduced expression of Cdc25B phosphatase, a result consistent with diminished levels of Cdc25B in proliferating Foxm1 Ϫ/Ϫ hepatocytes following a partial hepatoctomy (Wang et al, 2001). In addition, increased nuclear levels of Cdk inhibitor p21 may not only prevent activation of Cdk2, but p21 can also complex with and inhibit activity of M-phase promoting Cdk1 protein (Op De Beeck et al, 2001;Patel et al, 2002;Dash and El-Deiry, 2005).…”

“…3I), we observed an 80% reduction in the expression of Cdc25B phosphatase (Fig. 4), a known transcriptional target for Foxm1 protein (Wang et al, 2001). Because Cdc25B is essential for activation of Cdk1/ cyclin B complex during M-phase progression (Borgne and Meijer, 1996), diminished Cdc25B levels may cause delayed cardiomyocyte entry into mitosis in Foxm1 Ϫ/Ϫ hearts, causing polyploid phenotype.…”

“…Previous studies demonstrated that Foxm1 directly regulates Cdc25B and p21 Cip1 genes in hepatocytes and mouse embryonic fibroblasts (Wang et al, 2001;Krupczak-Hollis et al, 2004;Wang et al, 2005). However, little is known about cardiomyocyte-specific Foxm1 targets.…”

Global gene expression profile of normal and regenerating liver in young and old mice

“…According to previous results (Bucher et al 1964;Fry et al 1984;Wang et al 2001), while a strong proliferative response, as detected by BrdU incorporation, was observed in young mice (Fig. 2a, b), (LI of 52.5 %±8.8), liver regeneration was severely impaired in old mice; indeed, virtually, no hepatocyte nuclei were BrdU-positive at the same time point (LI of 1.5 %±0.5).…”

“…Indeed, while in young rats, DNA synthesis peaks at 24 h after PH, in old animals, the S phase peak is reached at 36 h. Different explanation has been proposed to justify this proliferation defect observed in aged rodents. Among these, Wang et al (2001) showed that aging process is associated with reduced expression of the cell cycle gene Forkhead Box M1 (Foxm1). Foxm1 has been suggested to be essential in regulating expression of genes involved in cell proliferation since increased levels of this transcription factor stimulate expression of cyclin D1, A2, B1, and B2 and are sufficient to restore cell cycle by promoting gene expression and to potentiate hepatocyte proliferation in aged mice.…”