Effect of myogenic determination on tumorigenicity of chemically transformed 10T1/2 cells

Transforming agents have been postulated to interfere with cellular differentiation programs, thus causing uncontrolled growth. Inducing transformed cells to differentiate can result in loss of the transformed phenotype since many end‐stage differentiated cells are unable to divide. We attempted to bypass or suppress the tumorigenic phenotype of 3‐methylcholanthrene (MCA)‐transformed 10T1/2 cells (MCA CI 15C1) by induction of myogenic determination. MCA CI 15C1 cells were either treated with the hypomethylating drug 5‐azacytidine (5‐aza‐CR) or were transfected with the muscle determination gene MyoD1, both of which induce a myogenic phenotype in 10T1/2 cells. Colonies containing myoblast‐like cells were isolated and examined. Muscle markers were detected both in 5‐aza‐CR‐treated and in MyoD1‐transfected myogenic clones by immunofluorescence and northern analyses. The myogenic clones did not show decreased tumorigenicities relative to that of the parental cells upon subcutaneous injection in nude mice. Some of the resulting tumors, however, were classified as rhabdomyosarcomas rather than fibrosarcomas. Although induction of myogenic determination was not sufficient to abolish the tumorigenic phenotype of MCA CI 15C1 cells, several tumors showed decreased levels of MyoD1 mRNA, suggesting that growth in vivo either selected for or caused decreased determination gene expression.