Loss of growth control and differentiation in the fu-1 variant of the L8 line of rat myoblasts.

A nonfusing variant, fu-1, of the L8 line of rat myoblasts was isolated and characterized with respect to its growth in vitro and developmental properties. Comparative analyses of density-dependent inhibition of growth, serum requirements, cell adhesiveness, colony formation in soft agar, and hexose transport in L8 and fu-1 cells support the conclusion that the fu-1 cells are transformed. In addition, fu-1, but not L8, cells promote the development of tumors in athymic nude mice. fu-1 cells also do not make increased levels of creatinekinase (ATP:creatine N-phosphotransferase, EC 2.7.3.2) or myosin and they express an endogenous type-C virus. Both L8 and fu-1 cells express myokinase (ATP:AMP phosphotransferase, EC 2.7.4.3) activities in single cells. In contrast to fu-1 cells, the parent L8 line has increased creatine kinase and myosin after fusion and spontaneously contracts; expression of an endogenous virus could not be detected in these cells. These results suggest that loss of the ability to differentiate normally is associated with the loss of the normal control of cell division of myoblasts grown in vitro and in vivo.