Isolation and uptake characteristics of human cell variants resistant to the antiproliferative effects of methylglyoxal bis(guanylhydrazone).

Four variants (VA2/MGBG) of the human cell line, VA2, have been isolated which are 10- to 20-fold more resistant than the parent line to the antiproliferative effects of the anticancer agent, methylglyoxal bis(guanylhydrazone) (MGBG). Drug resistance was not cytoplasmically transmitted by cytoplast cell fusion for any of the four sublines, suggesting that the genes responsible for resistance may be of nuclear rather than mitochondrial origin. Uptake properties were characterized in the VA2 cells and two of the four variant lines. Uptake of [14C]MGBG during long-term (0.5 to 28 hr) incubations was 3 to 4 times greater in the VA2 cells than in the VA2/MGBG sublines. However, during short-term (2 to 60 min) incubations, the uptake of [14C]MGBG or [3H]spermidine (which competes for MGBG uptake) was similar for all cell lines. This was further supported by kinetic data which indicated that, for [14C]MGBG uptake at 4 min, the apparent Km for all cell lines was 5.8 to 13.4 microM, and the Vmax, 44 to 53 pmol/mg/min. For [3H]spermidine uptake, the apparent Km values were approximately 1 microM and Vmax, 54 to 69 pmol/mg/min. Efflux studies performed on cells incubated for 30 min in 10 microM [14C]MGBG revealed that the two VA2/MGBG sublines released drug much more rapidly than did VA2 cells over an 8-hr period. Thus, while the variants may transport MGBG at a rate similar to VA2 cells, the drug is not accumulated to the same extent during long-term incubations, probably because of altered intracellular binding sites for MGBG. The identification of these sites may provide insight into the basis for antiproliferative action of MGBG.