Novobiocin-induced VP-16 accumulation and MRP expression in human leukemiaand ovarian carcinoma cells

We have previously reported that novobiocin potentiates the cytotoxic activ ity of etoposide (VP-16) and teniposide (VM-26) in a number of experimental tumor cell lines by inhibition of the efflux of the epipodophyllotoxins by an ATP-requiring trans porter. In leukemia cells from 12/19 patients and i n ovarian carcinoma cells from 2/4 patients, novobiocin, in a concentration range of 150-1000 mu M, increased the intracellular accumulation of VP-16 by 30-250% by inhibiting its efflux, Novobiocin did not significantly incre ase the intracellular concentration of VP-16 in human mononuclear bone marr ow cells fi om two individuals with normal bone marrow, suggesting that it might be possible to selectively modulate the intracellular accumulation of the epipodophyllotoxin in tumor cells relative to normal hematopoietic tis sue. Previous findings from our laboratory have provided evidence that the membrane transporter for VP-16 which is inhibited by novobiocin is distinct from the P-glycoprotein. The expression of MRP, measured by immunoblotting , was variable in novobiocin-responsive and non-responsive leukemia cells, indicating that no direct relationship existed between the modulatory activ ity of novobiocin on the transport of VP-16 and the expression of the MRP g ene. The findings indicate that the novobiocin-sensitive VP-16 transporter is (i) present in high frequency in leukemia and ovarian carcinoma cells, a nd (ii) probably not the P-glycoprotein or MRP.