Radiation induces diffusible feeder cell factor(s) that cooperate with ROCK inhibitor to conditionally reprogram and immortalize epithelial cells

Both feeder cells and Rho kinase inhibition are required for the conditional reprogramming and immortalization of human epithelial cells. In the present study, we demonstrated that the Rho kinase inhibitor Y-27632, significantly suppresses keratinocyte differentiation and extends life span in serum-containing medium but does not lead to immortalization in the absence of feeder cells. Using Transwell culture plates, we further demonstrated that physical contact between the feeder cells and keratinocytes is not required for inducing immortalization and, more importantly, that irradiation of the feeder cells is required for this induction. Consistent with these experiments, conditioned medium was shown to induce and maintain conditionally immortalized cells, which was accompanied by increased telomerase expression. The activity of conditioned medium directly correlated with radiation-induced apoptosis of the feeder cells. Thus, the induction of conditionally reprogrammed cells is mediated by a combination of Y-27632 and a diffusible factor (or factors) released by apoptotic feeder cells.