Modulation of invasive potential in different clonal subpopulations of a rat rhabdomyosarcoma cell line (BA-HAN-1) by differentiation induction.

Three clonal subpopulations (A, B, C) isolated from the same rhabdomyosarcoma of the rat and differing in their degree of spontaneous differentiation were tested for their invasive potential before and after differentiation induction with retinoic acid (RA), N-monomethylformamide (NMF) and sodium butyrate (NaBut). Invasive potential was analysed in an in vitro assay using embryonic chick heart fragments in organotypic culture. In standard culture medium, all three subpopulations were shown to be invasive, progressively replacing the chick heart fragments within 7-11 days after confrontation. After exposure to RA, NMF or NaBut, marked differences in the invasive potential of these subpopulations were, however, observed. Subpopulation C exhibited a pronounced decline in invasive potential, as evidenced by a significant decrease (P = 0.005) in the proportion of chick heart fragments with advanced stages of invasion. This response, however, was confined to the differentiation-inducing agents RA and NaBut, which had also produced a marked increase in morphological and/or biochemical differentiation (P = 0.0001). In contrast, NMF, which had only minor effects on differentiation, failed to affect the invasive potential of subpopulation C. In subpopulation B, a transient inhibition of single cell invasion became evident after exposure to RA, whereas NMF and NaBut failed to affect the invasive potential in spite of minor effects on differentiation. In the least differentiated subpopulation A, which was shown to be refractory to the differentiation-inducing effects of RA, NMF and NaBut, there was also no observation of any reduction of invasive potential. The results of our study demonstrate that differentiation-inducing agents can significantly reduce the invasive potential of malignant tumors, although marked differences of response are to be expected between the different subpopulations of a tumor.