RECOMBINANT HUMAN RETINOBLASTOMA PROTEIN INHIBITS CANCER CELL-GROWTH

Aberrant expression of the tumor suppressor gene RB1 is associated wit h a variety of solid tumors and hematopoietic neoplasms. Certain cance r cell lines in which the protein encoded by RBI (p110(RB)) is absent have been reported to show decreased growth rate, clonogenicity, or tu morigenicity following insertion of a transcriptionally active RBI gen e. We asked whether these RE-deficient cells could be growth inhibited by direct exposure to purified p110(RB). We report a decrease in upta ke of tritiated thymidine by 5637 bladder carcinoma cells (RB-negative ) when purified recombinant p110(RB) is added to culture media. Intern alization of the protein by cells and translocation to the nucleus are demonstrated by immunohistochemistry, FACS, and detection of radiolab eled protein in subcellular fractions. Next, we chose a well-described leukemia cell culture model to investigate the potential effect of re combinant p110(RB) in clinical disease. We observed dose-related decre ases in cell number or colony formation in vitro in 8 of 20 acute myel ogenous leukemia samples, 7 of which did show endogenous p110(RB) dete ctable by immunohistochemistry. Histological appearance following expo sure to p110(RB) shows cytoplasmic vacuolization and nuclear lobulatio n of degenerating cells. We conclude that purified p110(RB) added to c ulture media is internalized by cells, translocated to the nucleus, an d exerts a growth-inhibitory effect on certain cancer cell types.