Regulation of p57KIP2 during muscle differentiation: role of Egr1, Sp1 and DNA hypomethylation

The cdk inhibitor p57(kip2) plays a critical role in many differentiation processes by performing not only redundant but also specific functions. Compared to other cdk inhibitors, p57(kip2) shows a more restricted expression pattern during development and in adult tissues. We have previously reported that in muscle cells, p57(kip2) is induced by the myogenic factor MyoD through an indirect mechanism involving p73 proteins as intermediaries. We have also reported that p57(kip2) shows a differential responsiveness to MyoD-dependent regulation in different cell types. In this work we have further investigated the molecular mechanism by which MyoD activates p57 promoter. We show that the minimal promoter element able to confer MyoD responsiveness contains multiple Sp1 and Egr1 recognition sites and that both transcription factors are necessary for the increase in p57 RNA. We also suggest that the role of MyoD-induced p73 consists in promoting the binding of Sp1 to p57(kip2) promoter. Moreover, we show that Egr1 and Sp1 are concomitantly recruited to p57 promoter in vivo only in differentiation conditions and only in responsive cells. Bisulfite sequencing suggested a functional link between the methylation status and the differential activity of p57 promoter, both during differentiation and in distinct cell types. These results, which highlight the involvement of epigenetic factors in the regulation of p57 expression in muscle cells, could be of general relevance to explain its tissue and cell type restriction during development.