2A). As expected, messenger RNA levels of Ink4a and Arf were 2.1-fold and 8.0-fold higher in freshly purified Bmi1−/− Dlk+ cells than in wild-type Dlk+ cells, respectively. Colonies derived from Bmi1−/− Dlk+ cells also showed increased (5.8-fold greater) expression compared to the wild-type colonies. To determine whether Bmi1 is involved in transcriptional regulation of the Ink4a/Arf locus, we performed chromatin immunoprecipitation (ChIP) assays using wild-type selleck inhibitor Dlk+ cells. ChIP
assays demonstrated the binding of Bmi1 to the Ink4a/Arf locus and increased levels of monoubiquitinylated histone H2A (H2Aub1) (Fig. 2B). To understand the role of the Ink4a and Arf genes in hepatic stem cells, we next analyzed Ink4a/Arf−/− Dlk+ cells in culture. In clear contrast with Bmi1−/− Dlk+ cells, Ink4a/Arf−/− Dlk+ cells showed pronounced growth activity in culture. The number of large colonies (consisting of more than 100 cells) derived from Ink4a/Arf−/− Dlk+ cells was significantly increased compared to that derived from wild-type Dlk+ cells (Fig. 3A,B). By day 14 of culture, Ink4a/Arf−/− Dlk+ cells gave rise to distinctly abnormal and large colonies compared to wild-type Dlk+ cells (Fig. 3B,C). More learn more than 95% of large colonies from Ink4a/Arf−/−
Dlk+ cells further expanded beyond day 21 of culture, although wild-type colonies barely maintained their growth activity (Fig. 3A). Immunocytochemical analyses showed an increase in the proportion and number of Alb+CK7+ bipotent cells in colonies derived from Ink4a/Arf−/− Dlk+ cells, particularly in their central area (Fig. 3C). The absolute number of bipotent cells in large colonies derived from wild-type and Ink4a/Arf−/− Dlk+ cells at day 7 of culture was 8.2 ± 2.3 versus 22.7 ± 4.6 (P < 0.05) (Fig. 3D). Flow cytometric analyses revealed that the percentage of Dlk+ cells in wild-type colonies
was 0.9% ± 0.2% at day 7 and 0.5% ± 0.1% at day 14 of culture, although that 4-Aminobutyrate aminotransferase in Ink4a/Arf−/− colonies was 8.6% ± 0.7% and 4.5% ± 0.3%, respectively (Fig. 3E). These findings indicate the enhanced self-renewal capability of hepatic stem cells on the loss of Ink4a/Arf expression. Of note, messenger RNA expression of Bmi1 was comparable between wild-type and Ink4a/Arf−/− Dlk+ cells (data not shown). As expected, but importantly, the ability of wild-type Dlk+ cells to propagate colonies was extremely compromised by cotransduction with Ink4a and Arf retroviruses. Immunocytochemical analyses and flow cytometric analyses showed that the Dlk+ fraction and bipotent cells were significantly reduced in culture (Supporting Fig. 4). We previously reported that forced expression of Bmi1 enhances the self-renewal capacity of hepatic stem/progenitor cells and eventually induces their transformation.