Cadmium-induced multistep transformation of cultured Indian muntjac skin fibroblasts.

During the past five years we have made a series of cadmium-transformed and resistant fibroblast cell lines by continuous low-level exposure to cadmium. In the present paper we describe the use of four of these lines with varying degrees of transformation to investigate the multistep nature of cadmium carcinogenesis. These include: (a) M cell, an immortal but nontransformed muntjac skin fibroblast line; (b) CCR5, a morphologically transformed and cadmium-resistant line derived from M cells after 20-months continuous exposure to small step-wise increases in cadmium; (c) SCR5, a tumorigenic line derived by selection (in the absence of cadmium) of rapidly growing CCR5 agar colonies; (d) T1, a line derived from an SCR5 tumour growing in a nude mouse. We have compared the morphological characteristics of the four cell lines using light and electron microscopy and evaluated their ability to grow in liquid culture, soft agar and nude mice. We have also examined the changes which have occurred in their cytoskeletons and extracellular matrices using fluorescent antibodies to actin, tubulin and fibronectin and related these to the strength of their cell-cell and cell-substrate attachments and to their levels of transformation and tumorigenesis. We have shown that, while some changes occur in a single step (e.g. intracellular cytoskeletal changes), others are gradual (e.g. changes in extracellular matrix, focus formation and ability to grow in soft agar). We conclude that continuous exposure to low levels of cadmium can initiate growth and structural changes which subsequently lead to cell transformation and tumorigenesis on the removal of cadmium. Though change with cadmium was slow, many of the transformed characteristics are similar to those reported for viral and chemically transformed cells.