Phenotypic transformation of clonal myogenic cells to cells resembling chondrocytes.

Abstract

The nicotinamide analogue 6-aminonicotinamide and dibutyryl 3':5'-cAMP inhibit myogenesis in a clonal rat cell line from skeletal muscle. Both reagents produce a similar morphological response in the cells, and stimulate collagen and glycosaminoglycan synthesis. These data suggest that 6-aminonicotinamide and dibutyryl cAMP induce a phenotypic transformation of myogenic ceNls to cells that share many characteristics with chondrocytes. Clonal skeletal muscle myogenic cell lines divide as mononucleate cells and fuse to form multinucleate myotubes when the cells become confluent (1). Clonal analysis has demonstrated that all of the cells within a clone can form myotubes (1). The following events are temporally associated with myotube formation: (i) increases in the specific activities of myokinase and creatine kinase (2), (ii) an increase in the rate of myosin heavy chain synthesis (3), (iii) the appearance of nicotinic acetylcholine receptors (4), and (iv) a well-defined sequence of electrophysiological development (4, 5). In addition, myotubes are capable of localizing acetylcholine sensitivity to the point of nerve contact (6) and receiving functional synaptic input from spinal cord neurons (7).In contrast to the unique developmental pathway displayed by the myogenic cell cultures, mesenchymal cells in, for example, chick limb buds are capable of differentiating into either chondrocytes or muscle; at later stages of development they are able to give rise to only one cell type (8). The phenotype expressed by embryonic chick mesodermal cells can also be influenced by reagents that alter their intermediary metabolism. For example, nicotinamide analogues such as 3-acetylpyridine (3AP) and 6-aminonicotinamide (6AN) specifically alter the sequence of mesodermal differentiation in chick embryos (9). These in vivo experiments have been extended to a chick cell culture system, where 3AP and 6AN potentiate some aspects of the chondrocytic phenotype in limb bud mesoderm (10, 11). Both in vvo and in vitro, nicotinamide (NA) relieves the teratogenic effects of SAP and 6AN. These observations, along with the fact that NAD+ levels increase during myogenesis (12), support the hypothesis that niacin metabolism may have a regulatory role in the differentiation of mesodermal cells (9, 10). In addition, an involvement of cyclic nucleotides in mesodermal differentiation has been proposed (13, 14). Here we ask whether or not the phenotype of a cell line that is normally myogenic can be changed by altering the NAD+ and cyclic nucleotide metabolism of the cell. Assuming that a chondrocyte is the mesodermally derived cell that synthesizes the majority of the principal cartilage components, collagen and the glycosaminoglycans (15)

Effects of Some Inhibitors on Myogenic Differentiation of Avian Myoblasts Transformed with Rous Sarcoma Virus.

“…Cholesterol (1 mg/ml), N,N'-hexamethylenebisacetamide (HMBA, 9 mM), and methylamine (5 mM)were purchased from Nakalai Tesque (Kyoto, Japan). Lysolecithin (150 //g/ml), trifluoper- 32,38,39,42,43,46,47). Onthe other hand, it has been suggested that there are some differences between avian and mammalian myogenic differentiation (15, 40), hence we first examined whether or not these inhibitors block the fusion of QM-RSV cells at 41°C.…”