Ceramide-induced c-myc downregulation involves p2|WAFl increase and rbhypophosphorylation in human fibroblasts

Ceramide is a lipid mediator intraceUularly generated by the extracellular stimuli-induced sphingomyelin hydrolisis (TNFcc, FASL.ILlβ) or by the ceramide-sinthase activation. The ceramide role in apoptosis induction has been supported by several reports, whereas increasing interest is coming on its antiproliferative action. Here we describe a novel biochemical mechanism for ceramide to generate growth arrest in human diploid cells. In this pathway, the rise of intracellular ceramide, through the induction of p2iWAFij inhibits the cyclin Dl-dependent kinase activity and likely causes Rb hypophosphoryjation As consequence an increased association between pRb and E2F into a trascriptionally inactive complex is observed, with downregulation of the cmyc proto-oncogene. Indeed, c-myc contains two E2F sites in its regulatory region and the distal site was found crucial for c-myc expression. At the same time is also crucial the role of an appropiate c-myc activation for cell proliferation.