Cancer and stem cells share the ability to silence tumor suppressors. We focused on Lefty, which encodes one of the most abundant tumor suppressors in embryonic stem (ES) cells and is not expressed in somatic cancer cells. We found that transforming growth factor β (TGF-β) induced demethylation of the Lefty B cytosine-phosphate-guanine (CpG) island and increased Lefty expression (10-200 times) in human pancreatic cancer cells and human liver cancer cells (PLC/PRF/5 and HLF). Expression of Cripto, another important factor in Nodal-Lefty signaling, was not increased after adding TGF-β. We generated reprogrammed cancer cells that revealed high expression of immature marker proteins, high proliferation, and the potential to express morphological patterns of ectoderm, mesoderm, and endoderm, suggesting that these cells may have cancer stem cell-like phenotypes. We investigated Lefty and found that reprogrammed human liver cancer cells (induced pluripotent cancer cells) displayed a much lower ability to express Lefty, although less Lefty B CpG methylation was also observed. We also found that a MEK inhibitor dramatically enhanced Lefty expression in human pancreatic cancers with mutated ras, whereas Lefty B CpG methylation was not decreased. These observations indicate that despite the demethylation of DNA strands in promoter regions of pluripotency-associated genes, including Lefty gene, Lefty expression was not induced well in reprogrammed cells. Of note was the fact that Lefty is abundantly expressed in human ES cells but not in induced pluripotent stem (iPS) cells. We thus think that reprogrammed cancer cells share the mechanism for expression of Lefty with iPS cells. This shared mechanism may contribute to the cancerous transformation of iPS cells.
Keywords: DNA methylation; Ras-MAPK signaling; TGF-β.