Suppression of malignancy in human cancer cells: issues and challenges.

Analysis of the many, sometimes seemingly contradictory, reports on the partial suppression of malignancy in highly unstable rodent intraspecies and rodent--human hybrid cells emphasizes the limitations of this approach to the analysis of the basic nature of malignancy, especially in naturally occurring human cancers. During the past 5 years, Stanbridge and then Klinger reported complete suppression, not elimination, of malignancy [defined as capacity to produce progressively growing tumors in athymic (nude) mice] in stable hybrids of different human cancer cells with normal human fibroblasts or with differentiating epithelial keratinocytes and, importantly, also in stable hybrids of two parental cancers of different somatic cell origin. The nontumorigenic human hybrid cells are not rejected by some nonthymic immune mechanism of nude mice and survive in vascularized foci; the initial multiplication of these cells is stopped by some unknown proliferation controlling substance(s) to which their malignant parent(s) do not respond. The heritable properties of infinite multiplication in vitro, loss of contact inhibition, etc. remained in the nontumorigenic hybrids but, remarkably, the in vitro production of alpha human choriogonadotropin by HeLa cells was suppressed along with tumorigenicity and reappeared in the tumorigenic revertants. If it is assumed that human cancers of different somatic cell origin are caused by a loss of different specific regulatory genes, as the most recent data reviewed here suggest, the challenge is to determine in molecular terms what those missing genes are, how they function, and whether it may be possible to restore to the cancer cells what they have lost.