LOSS OF TUMORIGENICITY IN A METHOTREXATE-RESISTANT HUMAN LEUKEMIA-CELL LINE

We have studied the tumorigenicity of a CCRF-CEM-derived cell line (CE M/MTX-3) resistant to MTX. Eight of nine mice inoculated with drug-sen sitive CEM cells developed tumors within 5 weeks, but 16 weeks after i noculation with CEM/MTX-3 cells, none of nine mice developed tumors. W e were unable to detect dihydrofolate reductase gene overexpression, a mplification, or rearrangement in CEM/MTX-3 cells. Instead, the resist ance in CEM/MTX-3 cells appeared to be due largely to decreased methot rexate accumulation. Because tumorigenicity could have been related to intracellular folate levels, we cultured CEM and CEM/MTX-3 cells in f olate-rich and folate-deprived media. When inoculated in mice, CEM cel ls cultured in either medium rapidly formed tumors. As before, CEM/MTX -3 cells grown in either medium did not, suggesting that factors other than low folate levels contributed to the inability of CEM/MTX-3 cell s to form tumors. Cytogenetic analysis revealed that the CEM/MTX-3 kar yotype contained a unique and complex translocation marker chromosome that was not observed in the CEM cell line and which involved chromoso mal breakpoints at bands 11p14, 22p11, and 22p13. Although biochemical mechanisms are not yet delineated, this remodeled chromosome could be related to the loss of tumorigenicity in CEM/MTX-3 cells.