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Jun inhibits myogenic differentiation
H Y Su1, T J Bos, F S Monteclaro
1Department of Microbiology, University of Southern California School of Medicine, Los Angeles 90033.
Abstract:
Myoblasts from skeletal muscle of chicken or Japanese quail embryos were infected with avian sarcoma virus 17 (ASV-17), a retrovirus carrying the jun oncogene. At high multiplicities of infection ASV-17-induced morphologic transformation inhibited fusion of myoblasts into myotubes and stimulated extended replication. The expression of the muscle-specific proteins desmin, myosin and creatine phosphokinase was inhibited in ASV-17-infected cultures. Immunofluorescent staining detected strong expression of the ASV-17 Gag-Jun fusion protein in the nuclei of infected mononuclear myoblasts, but Gag-Jun was not detectable in multinucleated myotubes that occurred in clonal populations of ASV-17-infected quail myoblasts. This result suggests that the nuclear expression of viral jun and myogenic differentiation are mutually exclusive events. A mutant of ASV-17, ts jun-1, is partly temperature-sensitive in its ability to transform chicken embryo fibroblasts. At the non-permissive temperature of 41.5 degrees C, multinucleated myotubes readily formed in ts jun-1-infected myoblast cultures and expressed muscle-specific proteins detectable by immunofluorescent staining. These myotubes also showed strong immunofluorescent staining for Gag-Jun in the cell nuclei. The nuclear expression of a Jun protein that is defective in its transforming function appears therefore to be compatible with myogenesis. Several retroviral constructs carrying various viral and cellular jun inserts, as well as jun deletion mutants and recombinants between c-jun and v-jun, were tested for their effect on myogenic differentiation. There was an approximate correlation between the ability of a construct to transform chicken embryo fibroblasts and its effectiveness in interfering with myogenic differentiation. We conclude that the expression of an oncogenic jun gene in myoblasts strongly inhibits myogenic differentiation, and that a highly transforming Jun protein cannot be expressed in the nuclei of differentiating myotubes, while the presence of transformation-defective variants of Jun is compatible with differentiation.
Insights
Avian sarcoma virus 17 (ASV-17) infection inhibits muscle cell differentiation by oncogenic jun gene expression. Transformation-defective Jun proteins, however, are compatible with myogenesis, indicating oncogenic potential is key to inhibition.
Area of Science:
- Molecular Biology
- Oncology
- Developmental Biology
Background:
- Myogenesis, the process of muscle cell differentiation, is crucial for skeletal muscle development.
- Oncogenes, such as the jun oncogene, can disrupt normal cellular processes.
- Avian sarcoma virus 17 (ASV-17) is a retrovirus that carries the jun oncogene.
Purpose of the Study:
- To investigate the effect of avian sarcoma virus 17 (ASV-17) infection and jun oncogene expression on myoblast differentiation.
- To determine the relationship between viral jun expression, cellular transformation, and myogenic differentiation.
Main Methods:
- Infection of chicken and quail myoblasts with ASV-17 and its mutants.
- Assessment of myoblast fusion, replication, and expression of muscle-specific proteins (desmin, myosin, creatine phosphokinase).
- Immunofluorescent staining to detect Gag-Jun fusion protein localization in myoblasts and myotubes.
Main Results:
- High-multiplicity ASV-17 infection inhibited myoblast fusion and muscle-specific protein expression, while promoting replication.
- Nuclear expression of the ASV-17 Gag-Jun fusion protein was detected in mononuclear myoblasts but not in multinucleated myotubes, suggesting mutual exclusivity.
- A temperature-sensitive mutant (ts jun-1) with defective transforming ability allowed myotube formation and muscle protein expression, with nuclear Gag-Jun presence, indicating transformation-defective Jun is compatible with myogenesis.
- Correlation observed between the transforming ability of viral jun constructs and their inhibitory effect on myogenic differentiation.
Conclusions:
- Oncogenic expression of the jun gene in myoblasts strongly inhibits myogenic differentiation.
- Highly transforming Jun proteins are incompatible with the nuclei of differentiating myotubes.
- Transformation-defective Jun variants can be expressed in the nuclei of differentiating myotubes, supporting myogenesis.