The role of differentiation in the suppression of tumorigenicity in human cell hybrids.

HeLax human keratinocyte hybrid cell populations behave like transformed cells in culture but do not form progressive tumors when inoculated into athymic nude mice. In this respect they behave like HeLa/fibroblast hybrids (Stanbridge et al., 1982). However, the small nodules that do form occasionally have the appearance of moderate to highly differentiated squamous-cell carcinomas. These observations suggest that differentiation might be the signal suppressing growth of human hybrid cells in vivo. Differentiation occurs rapidly and is not site-specific. Reconstitution of HeLa/keratinocyte hybrid cells in culture from differentiated nodules gave rise to a line of stem cells, capable of renewed proliferation and differentiation, and a line which lost the ability to differentiate and formed large, progressive tumors when reinjected. These tumors are anaplastic carcinomas that resemble tumors formed both by HeLa and tumorigenic segregant HeLa/fibroblast hybrid cells. Morphological evidence indicates that non-tumorigenic HeLa/fibroblast hybrid cells also respond to differentiation signals, whereas the tumorigenic segregants do not. Furthermore, the nontumorigenic hybrid-cells in the nude mouse take on the "phenotypic signature" of the normal parental cell irrespective of the tumorigenic parental cell. Efforts to identify factors in vitro mimicking these differentiation signals have so far failed, but such factors may be important elements in the regulation of proliferation in vivo.