More Muscle, Less Sir

Abstract

Muscle differentiation involves several transcription factors and the coordinated actions of histone deacetylases (HDACs) and histone acetyltransferases (HATs). Sir2 is an HDAC that is activated by the oxidized form of nicotinamide adenine dinucleotide (NAD + ) and thus may serve to transmit signals that allow regulation of gene expression in response to changes in metabolic demands by detecting the redox state of the cell. Fulco et al. investigated the role of Sir2 in muscle cell differentiation. Inhibition of Sir2 pharmacologically increased expression of reporter genes containing muscle-specific promoters; whereas overexpression of Sir2 decreased reporter gene expression. Furthermore, overexpression of Sir2 inhibited muscle-specific gene expression dependent on the transcription factors MyoD or MEF2 and inhibited differentiation of cultured myoblasts or C2C12 cells. Decreased Sir2 expression by RNA interference techniques accelerated differentiation. To determine the mechanism by which Sir2 influenced gene expression, the authors performed coimmunoprecipitation experiments and determined that Sir2 interacted with the HATs GCN5 and PCAF (PCAF acetylates and activates MyoD). In transfected cells, cultured myoblasts, or differentiated muscle cultures, PCAF, Sir2, and MyoD were found in a complex and in vitro Sir2 deacetylated MyoD. Overexpressed Sir2 was also found in a complex with MyoD and PCAF that was associated with hypoacetylated chromatin in chromatin immunoprecipitation experiments. Finally, regulation of Sir2 by changes in the ratio of concentrations of NAD + to the reduced form of NAD (NADH) was explored by culturing C2C12 cells either in pyruvate to increase [NAD + ]/[NADH] ratio or in L-lactate to decrease the [NAD + ]/[NADH] ratio. As expected, increasing the ratio decreased muscle-specific gene expression, and decreasing the ratio increased expression. The importance of Sir2 in these responses was confirmed in RNAi experiments and using pharmacological inhibitors of Sir2. The [NAD + ]/[NADH] ratio changes during differentiation (decreases) and under conditions of metabolic demand, such as exercise. Thus, Sir2 may provide a mechanism for detecting the cellular redox state and altering gene expression appropriately. M. Fulco, R. L. Schiltz, S. Iezzi, M. T. King, P. Xhao, Y. Kashiwaya, E. Hoffman, R. L. Veech, V. Sartorelli, Sir2 regulates skeletal muscle differentiation as a potential sensor of the redox state. Mol. Cell 12 , 51-62 (2003). [Online Journal]