Myosin isoform expression in rat rhabdomyosarcoma induced by Moloney murine sarcoma virus.

Myosin isoform expression was analyzed in experimental rhabdomyosarcoma (RMS) using monoclonal antibodies (mAbs) and immunofluorescence techniques. Tumors induced by inoculating newborn rats with Moloney murine sarcoma virus (Mo-MSV) were examined 30-90 days after birth. Nine tumors and two lymph node metastases were studied by direct, indirect, and double immunofluorescence assays using a panel of five anti-myosin mAbs. The mAb BF-45 was specifically reactive with embryonic myosin heavy chain (MHC), mAb BF-34 was specific for a neonatal MHC epitope, mAb BF-B6 was directed against an epitope present in both embryonic and neonatal MHC, and mAbs BF-F3 and BF-32 detected epitopes present in adult MHC isoforms. Anti-desmin antibodies were also used for comparison. The results of this study show that: (1) the majority of neoplastic cells stained for desmin while only a minority of neoplastic cells were labeled by anti-myosin antibodies; (2) myosin positive tumor cells contained predominantly embryonic and neonatal MHC types but rare RMS cells reacted exclusively with anti-adult myosin antibodies; and (3) adult and embryonic MHC phenotypes were occasionally detected within the same tumor cell especially in RMS with the longest latencies. Together these results would suggest that the mechanism(s) regulating MHC gene expression in skeletal muscle cells can be altered by the transforming activity of Mo-MSV.