Loss of tumorigenicity in a methotrexate-resistant human leukemia cell line

We have studied the tumorigenicity of a CCRF-CEM-derived cell line (CEM/MTX-3) resistant to MTX. Eight of nine mice inoculated with drug-sensitive CEM cells developed tumors within 5 weeks, but 16 weeks after inoculation with CEM/MTX-3 cells, none of nine mice developed tumors. We were unable to detect dihydrofolate reductase gene overexpression, amplification, or rearrangement in CEM/MTX-3 cells. Instead, the resistance in CEM/MTX-3 cells appeared to be due largely to decreased methotrexate accumulation. Because tumorigenicity could have been related to intracellular folate levels, we cultured CEM and CEM/MTX-3 cells in folate-rich and folate-deprived media. When inoculated in mice, CEM cells 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 cells to form tumors. Cytogenetic analysis revealed that the CEM/MTX-3 karyotype contained a unique and complex translocation marker chromosome that was not observed in the CEM cell line and which involved chromosomal 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.