Analysis of myogenesis by somatic cell hybridization. II. Retention of myogenic competence and suppression of transformed properties in hybrids between differentiation competent and incompetent rat L6 myoblasts

Abstract

This study describes the characteristics of hybrids between two closely related rat myoblast lines, which differ both in the ability to express their program of differentiation and in the expression of neoplastic properties. Myogenic, nonneoplastic L6J1-S cells were hybridized with nonmyogenic, neoplastic L6J1-N1 cells. Six hybrid clones were isolated and expanded for analysis of myogenic competence, and four of these clones were also evaluated for parameters of transformation, including tumorigenicity, ability to clone in agar, and surface fibronectin. In addition to our analysis of isolated clones, we also assessed myogenic differentiation in colonies representing 226 early hybrid clones. Results of all these analyses demonstrate that the myogenic phenotype is retained and that the tumorigenic/transformed phenotype is suppressed in the hybrids. Furthermore, our results indicate that when the programs for myogenesis and neoplastic transformation are confronted within a single cell, they are expressed as mutually exclusive alternatives. In contrast to these results on myogenic X nonmyogenic L6 hybrids, it has been reported that isolated clones of A9 X L6 exhibited extinction of myogenic competence and retention of transformed properties. We have evaluated myotube formation in over 300 early hybrid clones between A9 and either diploid or subtetraploid L8 rat myoblasts. Our results demonstrate that all of these hybrid clones exhibit extinction regardless of the ploidy of the myoblast parent, and they further indicate that extinction is not a consequence of chromosome loss. These results support the conclusion that in A9 X L6 hybrids, the nonmyogenic, transformed phenotype is dominant.

Control of differentiation in heterokaryons and hybrids involving differentiation-defective myoblast variants.

“…Our results differ from two recent reports involving myogenic cell hybrids (33,34). Lawrence and Coleman found that hybrids between competent and differentiation-defective L6 myoblasts exhibited high rather than intermediate levels ofdifferentiation (33).…”

“…Lawrence and Coleman found that hybrids between competent and differentiation-defective L6 myoblasts exhibited high rather than intermediate levels ofdifferentiation (33). We believe that this is due to their use of moderately rather than severely differentiation-defective myoblasts .…”

“…WRIGHT However, the dominant behavior offibroblasts in suppressing myogenic functions in heterokaryons (10) suggests that fibroblast suppressive factors are quantitatively and probably qualitatively different from putative suppressive factors in defective myoblasts . There are numerous reports of the isolation of clones of nondifferentiating myoblasts (6,(28)(29)(30)(31)(32)(33). Although the high frequency of their phenotype has led most authors to be cautious about describing them as mutants, the mechanism responsible for their appearance has remained obscure .…”