Maintaining induced pluripotent stem (iPS) cells in an undifferentiated, self-renewing state during long-term cultivation is, at present, a major challenge. We previously showed that human amniotic epithelial cells (HuAECs) were able to provide a good source of feeder cells for mouse and human embryonic or spermatogonial stem cells; however, the epigenetic mechanisms have not been elucidated. In the present study, mouse embryonic fibroblasts (MEFs) and HuAECs were compared as feeder layers for the long-term culture of human iPS cells. The HuAEC feeders allowed human iPS cells to maintain a high level of alkaline phosphatase (AP) activity and to express key stem cell markers during long-term subculture whereas the MEF feeders did not,. Moreover, the HuAEC feeders significantly affected the cell cycle regulation of the iPS cells, maintaining them in the resting stage and the early stage of DNA synthesis (G0/G1 stage). Furthermore, the CpG islands of the Nanog and Oct4 promoters were hypomethylated, while the Nanog- and Oct4-specific loci exhibited higher levels of histone H3 acetylation and lower levels of H3K27 trimethylation in iPS cells cultured on HuAECs compared with those cultured on MEFs. The DNA methyltransferase 1 (DNMT1) expression in iPS cells cultured on HuAECs was shown to be lower than in those cultured on MEFs. In addition, DNMT1-silenced human iPS cells were able to maintain pluripotency over long-term culture on MEFs. In combination, these results suggest that endogenous DNMT1 expression in human iPS cells may be regulated by HuAEC feeder cells and that Nanog and Oct4 are crucial components required for the maintenance of iPS cells in an undifferentiated, proliferative state, capable of self-renewal.
Keywords: DNA methyltransferase 1; human amniotic epithelial cells; human induced pluripotent stem cells; pluripotency maintenance.