Tumour promoter induces anchorage independence irreversibly

Abstract

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 hydrocarbons⁴ or UV⁵ or X-ray irradiation⁶. As many in vivo studies of the tumour-promoting activity of phorbol esters have been carried out in mouse skin⁷, 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 exposure⁸. 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.