2-DIMENSIONAL ELECTROPHORETIC STUDIES ON DOWN-REGULATED INTRACELLULARTRANSFERRIN IN HUMAN FIBROBLASTS IMMORTALIZED BY TREATMENT WITH EITHER 4-NITROQUINOLINE 1-OXIDE OR CO-60 GAMMA-RAYS

The immortalization of human cells is an important early step of carci nogenesis. To investigate the mechanisms of immortalization, we establ ished three immortalized human fibroblast cell lines by treatment with either 4-nitroquinoline 1-oxide (4NQO) or Co-60 gama rays. Using two- dimensional gel electrophoresis (2-D PAGE), we identified proteins whi ch are down-regulated in these immortalized cell lines. Their isoelect ric points ranged between 5.5 and 6.3, and their molecular masses were approximately 80 kDa. The proteins were also decreased in another hum an fibroblast cell line immortalized with simian virus 40 (SV40) and t wo human cervical cell lines. The proteins were present in a cytoskele tal fraction, reacted with anti-transferrin antibody and some of them were phosphorylated. The proteins were identified as transferrin, Alth ough the 2-D PAGE patterns of our proteins and serum transferrin of di fferent origin were distinct from each other, their isoelectric points and molecular weights were similar. We implicate some functions of in tracellular transferrin, other than an iron transporter, in immortaliz ation of human cells and discuss a sew aspect of transferrin metabolis m in the cells.