L-canavanine modulates cellular growth, chemosensitivity and P-glycoprotein substrate accumulation in cultured human tumor cell lines

L-Canavanine (L-CAV) is a naturally occurring L-arginine analog that induce s the formation of non-functional proteins in a variety of organisms. Previ ous studies have shown that L-CAV is cytotoxic for several human tumor cell lines. In this study, we have evaluated the cytotoxicity of L-CAV for both parental and multi-drug resistant (MDR) human tumor cells. We have also de termined the effect of L-CAV exposure on cellular expression and activity o f the MDR P-glycoprotein (P-gp) membrane efflux pump, and the effect of L-C AV on cellular accumulation of P-gp substrates. The effect of pre-treatment with non-cytotoxic doses of L-CAV on cellular sensitivity to ten standard antineoplastic agents was also evaluated, in order to assess the chemosensi tization potential of L-CAV. 3-(4,5-Dimethylthiazol-)2,5-diphenyl tetrazoli um bromide (MTT) cytotoxicity assays revealed that the MDR variants of huma n uterine sarcoma and leukemic cells were equally sensitive to L-CAV as com pared with their respective parental controls. Although the presence of fre e L-CAV in the uptake media did nor influence cellular accumulation of P-gp substrates, cells cultured for 72 h in 250 mu M L-CAV accumulated from 16 to 23% less P-gp substrate than untreated controls. Although L-CAV-cultured sarcoma cells accumulated 17% less doxorubicin (DOX) than untreated contro ls, they were three times more sensitive to its cytotoxic effects. L-CAV-tr eated cells were also significantly more sensitive to cisplatin, 5-fluorour acil, mitoxantrone and bleomycin than were untreated controls. Indirect imm unofluorescence revealed that 72-h exposure to as much as 1000 mu M L-CAV d id not alter cellular expression of P-gp. These studies suggest that L-CAV may be equally cytotoxic for both parental and MDR tumor cells, and that L- CAV neither induces the expression of, nor is a substrate for, P-gp. The ob servation that L-CAV pre-treatment reduces cellular accumulation of DOX, ye t sensitizes tumor cells to DOX and other DNA-targeting antineoplastic drug s, suggests a role for L-CAV as a chemosensitizer for the chemotherapy of c ancer. (C) 1998 Elsevier Science Ireland Ltd. All rights reserved.