Generation of somatic cell hybrids for the production of biologically active factors that stimulate proliferation of other cells

.  Objective : Some normal somatic cells in culture divide a limited number of times before entering a non‐dividing state called replicative senescence and fusion of normal cells with immortal cells claimed to produce hybrid cells of limited proliferation. We reinvestigated the proliferative capacity of hybrid cells between normal cell and immortal cell. Materials and Methods : Normal pig fibroblast cells and cells of immortal mouse fibroblast cell line F7, a derivative of GM05267, were fused by polyethylene glycol treatment and subsequently the fused cells were cultured in a selective medium containing hypoxanthine‐aminopterin‐thymidine in order to enrich the hybrid cells. The hybrid cells were then monitored for chromosome content and proliferation. Results : Cytogenetic analysis revealed that the hybrid cells contained polyploidy chromosomes derived from normal pig fibroblasts. These hybrid cells exhibit no sign of replicative senescence after more than 190 population doublings in vitro. Instead, these hybrid cells have an accelerated growth and proliferate even in the complete absence of glutamine. In addition, these hybrids produce biologically active factors in the conditioned media, which not only can accelerate their own proliferation but also can reinitiate mitotic activity in the senescent–like normal fibroblast cells. Conclusions : Our results question the validity of cellular senescence as a dominant trait. Additionally, the generation of hybrid cells using the specific mouse cell line can be applied to the generation of hybrids with other normal cell types and can be used to produce tissue‐specific growth‐factor(s) to extend the lifespan and/or improve the proliferation of various normal cells, including adult stem cells.