Development of Drug Resistance to Gallium Nitrate through Modulation of Cellular Iron Uptake1

We have shown that transferrin-gallium (Tf-Ga) blocks DNA synthe sis through inhibition of cellular iron incorporation and a diminution in the activity of the iron-dependent M2 subunit of ribonucleotide reductase. To examine the mechanisms of drug resistance to gallium, we developed a subline of HL60 cells (R cells) which is 29-fold more resistant to growth inhibition by gallium nitrate than the parent line (S cells). R cells displayed a 2.5-fold increase in transferrin (Tf) receptor expression, without a change in receptor affinity for Tf. The uptake and release of 67Ga were similar for both S and R cells. The uptake of 59Fe-Tf by S cells was inhibited by gallium nitrate over 24-48 h of incubation. In contrast, "Fe-Tf uptake by R cells, although initially inhibited by gallium nitrate at 24 h, was no longer inhibited at 48 h of incubation. 59FeQ3 uptake by R cells was significantly greater than that of S cells, regardless of the time in culture. Despite the increase in 59Feuptake by R cells, the ferritin content of these cells was lower than that of S cells. The ribonucleotide reductase electron spin resonance signal of R cells was comparable to that of S cells. R cells were not cross-resistant to Adriamycin, vincristine, c/s-platinum or hydroxyurea. Resistance to gallium nitrate in this subline of III .6(1cells results primarily from the ability of cells to overcome the gallium-induced block in iron incorporation. In addition, intracellular iron in R cells appears to traffic preferentially to a non-ferritin compartment.