Phenotypic and karyotypic transitions in the spontaneous transformation of a rat cell line

After 20-50 transfers, a rat myofibroblast line, Hmf-n, 'spontaneously' transforms to an established (immortalized) line of smaller, rapidly cycling fibroblastoid cells (tHmf-f). From these 1 degree transformants, colonies of larger, slower growing anchorage-independent (tHmf-e) cells of epithelioid phenotype emerge. Both transformants grow in low serum and low calcium media, but the tHmf-f cells are highly tumorigenic in nude mice, have diminished substrate adhesivity, and limited anchorage independence, whereas tHmf-e are less tumorigenic, firmly substrate adherent, and markedly anchorage independent. Most tHmf-f are trisomic; most tHmf-e transformants are hypodiploid, a third are tetraploid, and all have chromosomal abnormalities, but no trisomy. Hmf-n cells have polar stress fiber arrays terminating in vinculin adhesion plaques, colinear extracellular fibronectin matrices, and linear non-coincident deposits of fodrin. Microtubules (mt) and vimentin-intermediate filaments (IF) parallel the actin cables. Stress fibers of the tHmf-f are moderately reduced, their vinculin adhesion plaques and fibronectin matrices intact; fodrin is diffuse. Mts and IFs are normal and axial. Most epithelioid tHmf-e have no stress fibers, adhesion plaques, or extracellular fibronectin; instead, dense actin microfilament meshworks are attached to plasma membrane, as is fodrin. Mt and IF are radial. Both transformed phenotypes are stable over greater than 300 continuous passages. The differentiation-inducing agents DMSO, cyclic AMP, 5-azacytidine, and mezerein, were ineffective in normalizing shape or cytoskeleton of transformed Hmf, and butyrate was selectively toxic to 50% of tHmf-e. But hydrocortisone induced striking polarization, and increase in number, and alignment of stress fibers of both tHmf-f and tHmf-e. Growth, anchorage, cytoskeletal arrangements, and tumorigenic potential are not closely correlated in these stable, spontaneously transformed lines of distinct pheno- and karyotype originating from the same normal parental cell, suggesting independent acquisition of properties associated with transformation.