INDUCED TERMINAL DIFFERENTIATION AND TUMORIGENIC SUPPRESSION IN MURINE KERATINOCYTE SOMATIC-CELL HYBRIDS

The development of malignancy has been associated with both the activa tion of oncogenes and the inactivation of tumor suppressor genes. Wher eas recent data implicate tumor suppressor genes as cell-cycle checkpo ints, the nature and timing of tumor suppressor gene inactivation duri ng multistage carcinogenesis is still largely uncharacterized. To addr ess this issue, we used a syngeneic mouse epidermal model system. By c reating somatic-cell hybrids between nontumorigenic x benign (291 x 29 1.09RAT), nontumorigenic x malignant(291 x 291.05RAT and 291 x 291.03R AT), benign x malignant (291.09RAT x 291.03RAT) and malignant x malign ant (291.03RAT x 291.05RAT) clones, multiple tumor suppressor activiti es were detected. Most importantly, we demonstrated the first example of the complete suppression of benign papillomas in vivo, thus implica ting tumor suppressor gene activity loss as an early event in skin car cinogenesis. In addition, the carcinoma phenotype was suppressed in vi vo by nontumorigenic, benign, and heterologous malignant keratinocytes . The somatic-cell hybrids expressed the differentiation-specific kera tins, K1 and K10, in response to high extracellular calcium concentrat ions (1.4 mM) in vitro. All of the hybrids had fewer local metastases than did the parental lines, and when tumor formation was not suppress ed, the resulting tumors were highly differentiated. Polymerase chain reaction analysis of the neomycin-resistance gene at nontumorigenic in jection sites indicated an absence of injected hybrids, and subsequent analyses failed to detect nontumorigenic 291 cells 1 wk after transpl antation. These data demonstrate that distinct tumor suppressor gene a ctivities are lost at discrete stages during multistage carcinogenesis and are consistent with the hypothesis that tumor suppression can occ ur through induction of terminal differentiation. (C) 1995 Wiley-Liss Inc.