Ph.D Thesis

Ph.D StudentOstrovsky Olga
SubjectInvolvement of the MAP Kinase Signal Transduction Pathways
in the Differentiation of Skeletal Muscle
DepartmentDepartment of Medicine
Supervisor ASSOCIATE PROF. Eyal Bengal


   During myogenesis, proliferating myoblasts withdraw from cell cycle and are either eliminated by apoptosis or differentiate into myotubes. Mitogen-activated protein kinase (MAPK) activity induces with onset of muscle terminal differentiation.

   We investigated the role of MAPK pathway in muscle differentiation.  Activation of MAPK by expression of an active Raf1-estrogen receptor chimera protein reduced apoptosis of myoblasts in differentiation medium. Activation of Raf1 prevented the proteolytic activation of proapoptotic caspase-9 protein. The antiapoptotic function of Raf1 correlated with accumulation of p21WAF1-protein resulting from its increased stability. Antisense expression of p21WAF1 was used to determine whether the p21WAF1-protein mediated the antiapoptotic activity of Raf1.  Reduction of p21WAF1-protein in muscle cells abolished antiapoptotic activity of MAPK. We conclude that MAPK contributes to muscle differentiation by preventing apoptosis of differentiating myoblasts, and this activity mediated by stabilization of p21WAF1-protein.

   Activating protein-1 family of transcription factors promotes cell proliferation and antagonizes muscle cell differentiation. We tested the role of c-Jun dimerization protein, JDP2, in muscle differentiation.  Endogenous expression of JDP2 is induced in myoblasts and rhabdomyosarcoma cells programmed to differentiate. Ectopic expression of JDP2 in myoblasts inhibits proliferation and induces differentiation. JDP2 reduces tumorogenic characteristics of rhabdomyosarcoma cells in tissue culture and injected to nude mice, and suppresses angiogenesis into tumors. JDP2 synergies with TPA in inducing rhabdomyosarcoma differentiation. JDP2 induces p38 MAPK activity in myoblasts and rhabdomyosarcoma cells programmed to differentiate resulting in the phosphorylation and activation of MEF2 transcription factor. Thus, JDP2 inhibits proliferation, promotes muscle differentiation and rescues myogenesis in rhabdomyosarcoma cells.