Tumour promoter induces anchorage independence irreversibly

Tumour-promoting phorbol esters elicit a variety of molecular responses from cells in culture 1–3 . Phorbol esters are also active as promoters of neoplastic transformation in 10T1/2 mouse fibroblasts, previously initiated by polycyclic aromatic hydrocarbons 4 or UV 5 or X-ray irradiation 6 . As many in vivo studies of the tumour-promoting activity of phorbol esters have been carried out in mouse skin 7 , it seems desirable to use mouse epidermal cell lines to study the mechanism of tumour promotion in vitro . Mouse epidermal cell lines would be particularly useful if they responded to phorbol esters by progressing towards a neoplastic phenotype. We have previously reported the development of cell lines derived from primary mouse epidermal cultures after carcinogen or solvent exposure 8 . Some of these cell lines remained non-tumorigenic for many passages and failed to form colonies in soft agar (frequency less than 10 −4 ) 9 . We next asked whether some of these non-tumorigenic cells might respond to tumour promoters like ‘initiated’ or ‘post-initiated’ cells in vivo by progressing towards a neoplastic state 9,10 . We report here the identification of three epidermal cell lines which respond to tumour-promoting but not to non-promoting phorbol esters by irreversibly acquiring capacity to grow in soft agar. As anchorage-independent growth characterises malignant cells derived from a variety of sources including mouse epidermis 8,11 , this response to phorbol esters may be analogous to a late stage of tumour promotion in vivo .