MyoD is a transcriptional activator required for muscle-specific gene expression. Expression of exogenous MyoD in numerous terminally differentiated cell lineages ( neurons, adipocytes, skin cells, chondrocytes adn others) redirect their fates towards the skeletal muscle phenotype. Furthermore, MyoD - and the related Myf-5 protein- is essential for the formation of skeletal muscles in the animal. In order to activate transcription, MyoD requires the assistance of other interacting partners such as the p300/CBP and PCAf coactivators. We have previously reported that p300 and PCAF are nodal coactivators of the muscle specific transcription factor MyoD. MyoD is acetylated by PCAF and this results in an increased DNA-binding and transcriptional activity. Acetylation and deacetylation are in a dynamic equilibrium and we have now begun to explore the role of the deacetylase HDAC-1 in controlling muscle differentiation. Data generated in my laboratory indicate that the deacetylase HDAC-1 counteracts the ability of MyoD to convert naive fibroblasts to muscle cells and impedes differentiation of cultured mouse myoblasts. The molecular basis of this phenomenon correlates with the ability of HDAC-1 to physically interact and deacetylate both MyoD and histones surrounding chromatin MyoD-binding sites in undifferentiated myoblasts. Upon induction of cellular differentiation, the tumor suppressor protein pRb is dephosphorylated, engages HDAC-1 and blocks transcription of genes controlled by the transcription factor E2F and required for G1 progression. During this process, HDAC-1 is physically displaced from MyoD to pRB allowing acetylation of MyoD by the acetyltrasferases PCAF. PRb is essential for muscle differentiation as indicated by the fact that muscle cells derived from pRb-/- nullizygous mice fail to differentiate. Using fibroblasts derived from pRb-/- animals and transduced with a retrovirus coding for MyoD, we have documented that reintroduction of exogenous pRb abrogates the interaction of MyoD with HDAC-1. Consequently, these cells can now withdraw from the cell cycle and start expressing markers of terminal muscle differentiation such as skeletal myosin heavy chain (MHC) and muscle creatine kinase (MCK). Our data support a model in which shuttling of HDAC-1 from MyoD to pRb regulates transcription and promotes cellular differentiation. The results of this research are now in press in Molecular Cell. We have further evaluated the effects of global acetylation during muscle differentiation by employing the specific deacetylase inhibitor trichostatin A (TSA). Our results indicate that mouse myoblasts exposed to TSA display a hypertrophic response as judged by an increase d expression of MHC, multinucleation and increased fiber diameter. In collaboration with Dr. Eric Hoffman's laboratory at the Children National Hospital. Washington D.C., we are now identifying the molecular targets of TSA using genome-wide expression profiling.
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