After ligation with transferrin (Tf), transferrin receptor (TfR) is aggregated on cell surface in clathrin coated pits and then internalized into the cell. In the present work, the mechanism of (Tf/TfR) and the role of Ceramide in this mechanism were studied. After Tf/TfR ligation, Ceramide level on plasma membrane outer leaflet was temporarily elevated as clarified by immunocytochemical detection of cell surface Ceramide in human "jurkat" T lymphoma cell line. Ceramide elevation was confirmed by Diacylgelycerol kinase assay for biochemical Ceramide measurement. The fast and brief Cer elevation was the result of acid sphingomyelinase activity, and no detection of neutral sphingomyelinase activity was noticed. Sphingomyelin synthase activity was obviously decreased in the same time frame of Ceramide generation, to maintain the elevated level of Ceramide. Inhibition of Ceramide generation by Imipramine, an acid sphingomyelinase inhibitor, or neutralization of the generated Ceramide by anti-Ceramide mAb lead to miss-internalization of transferrin, instead of being clathrin mediated it becomes, most probably, raft mediated as indicated by altered Tf recycling and cell fractionation studies. Miss-internalization of transferrin leads to abrogation of cell growth and finally apoptotic cell death.
Abbreviations: ASMase – Acid Sphingomyelinase; Cer – Ceramide; DGK - Diacylgelycerol kinase; NSMase – Neutral Sphingomyelinase; SM – Sphingomyelin; SMS – Sphingomyelin synthase; Tf – Transferrin; TfR – Transferrin receptor.