Membrane glycoprotein changes associated with anthracycline resistance in HL-60 cells

Tunicamycin depresses P-glycoprotein glycosylation without an effect on its membrane localization and drug efflux activity in L1210 cells

P-glycoprotein (P-gp), also known as ABCB1, is a member of the ABC transporter family of proteins. P-gp is an ATP-dependent drug efflux pump that is localized to the plasma membrane of mammalian cells and confers multidrug resistance in neoplastic cells. P-gp is a 140-kDa polypeptide that is glycosylated to a final molecular weight of 170 kDa. Our experimental model used two variants of L1210 cells in which overexpression of P-gp was achieved: either by adaptation of parental cells (S) to vincristine (R) or by transfection with the human gene encoding P-gp (T). R and T cells were found to differ from S cells in transglycosylation reactions in our recent studies. The effects of tunicamycin on glycosylation, drug efflux activity and cellular localization of P-gp in R and T cells were examined in the present study. Treatment with tunicamycin caused less concentration-dependent cellular damage to R and T cells compared with S cells. Tunicamycin inhibited P-gp N-glycosylation in both of the P-gp-positive cells. However, tunicamycin treatment did not alter either the P-gp cellular localization to the plasma membrane or the P-gp transport activity. The present paper brings evidence that independently on the mode of P-gp expression (selection with drugs or transfection with a gene encoding P-gp) in L1210 cells, tunicamycin induces inhibition of N-glycosylation of this protein, without altering its function as plasma membrane drug efflux pump.

Novel mechanisms of drug resistance in leukemia

A key issue in the treatment of acute leukemia is the development of resistance to chemotherapeutic drugs. Several mechanisms may account for this phenomenon, including failure of the cell to undergo apoptosis in response to chemotherapy, or failure of the drug to reach and/or affect its intracellular target. This review focuses on the latter mechanism, and on intracellular drug transport resistance mechanisms in particular. Expression of the ATP-binding cassette (ABC) transporter P-glycoprotein (Pgp) has generally been reported to correlate with prognosis in acute myeloid leukemia (AML). Additionally, but more controversial, expression of the ABC transporter multidrug resistance protein (MRP) and the vault-transporter lung resistance protein (LRP) have been correlated with outcome in AML. Despite these findings, functional efflux assays indicate the presence of non-Pgp, non-MRP transporters in AML. Recently, a novel ABC transporter, breast cancer resistance protein (BCRP) was cloned and sequenced in our laboratory. Transfection and overexpression of BCRP in drug-sensitive cells confers drug-resistance to the cells. BCRP is a half-transporter, and may homodimerize or form heterodimers (with a yet unknown half-transporter) to produce an active transport complex. Relatively high expression of BCRP mRNA is observed in approximately 30% of AML cases, suggesting a potential role for this new transporter in drug resistance in leukemia.

Tunicamycin potentiates drug cytotoxicity and vincristine retention in multidrug resistant cell lines

Tunicamycin (TM), an inhibitor of glycoprotein processing, was investigated for its potential to reverse the multiple drug resistance (MDR) phenotype. When TM was added in vitro to drug-resistant NIH-3T3-MDR and KB-8-5-11 cells, they developed an increased sensitivity to doxorubicin, epirubicin, vincristine and colchicine. Similarly, the sensitivity of NIH-3T3-MDR cells to cisplatin was also enhanced by TM. In the presence of TM, drug-sensitive NIH-3T3-parental cells exhibited greater susceptibility to the toxic effects of doxorubicin, epirubicin, vincristine (marginally significant), and colchicine, but not to cisplatin. Tunicamycin-treated drug-sensitive KB-3-1 cells showed an increased response to vincristine, but not to the other anticancer drugs. Pretreatment with TM inhibited glycoprotein synthesis in all the cell lines. Neither prior exposure to, nor co-incubation with TM, influenced the uptake of vincristine (VCR) in the various cell lines. However, NIH-3T3-MDR cells accumulated less VCR than their drug-sensitive controls and also exhibited reduced efflux of the drug when treated with TM. There were no significant differences in the levels of intracellular VCR uptake between drug-sensitive KB-3-1 and KB-8-5-11 cells. Tunicamycin increased intracellular VCR retention in KB-8-5-11 and NIH-3T3-MDR cells, but not in NIH-3T3-parental cells. However, drug-sensitive KB-3-1 cells expressed reduced VCR retention in response to TM exposure, indicating that correlations between VCR toxicity and its retention in the presence of TM should be made with caution. The results suggest that the enhancement of intracellular VCR retention in MDR cells lines caused by TM is likely to be the result of inhibition of VCR efflux. Inhibition of glycoprotein synthesis during TM exposure may account for the changes in VCR efflux and retention observed in the MDR cell lines. The enhancement of cisplatin cytotoxicity in NIH-3T3-MDR cells after exposure to TM is an interesting observation, since it is generally believed that agents which modify the MDR phenotype do not show a sensitising effect to cisplatin. These findings may have applications in the reversal of drug resistance.

Non-P-glycoprotein multidrug resistance in cell lines which are defective in the cellular accumulation of drug

Non-Pgp mdr related to a defect in drug accumulation has now been documented in a number of different cell lines exposed to certain cytotoxic agents. In studies conducted thus far most isolates have been obtained after selection in either adriamycin or mitoxantrone. The work in this area is in its early stages and very little is known about the molecular events which contribute to this mode of drug resistance. At the present time no protein with drug binding properties comparable to Pgp has been identified in non-Pgp mdr isolates. Evidence based on the finding that all isolates do not respond in the same way to reversal agents such as verapamil suggests the possibility that more than one mechanism may exist for non-Pgp mdr. Future studies may thus reveal that cells contain a multiplicity of genes which upon transcriptional activation can function to alter drug transport processes and thus contribute to the development of mdr. Identifying and characterizing these genes will be important since they may function in transport systems of normal cells. The exact identify of proteins which contribute to non-Pgp mdr remains to be determined. One protein designated P190 has been found to be overexpressed in cell lines of human promyelocytic leukemia, lung and adenocarcinoma treated with adriamycin. The protein also is increased in some clinical samples from patients undergoing chemotherapy. P190 which has a minor sequence homology with Pgp can bind ATP and may thus contribute to the energy dependent drug efflux systems found in cells containing this protein. Transfection studies with a P190 cDNA should determine whether this protein actually contributes to drug resistance. Many other protein changes have been detected in non-Pgp mdr cells but the importance of these in resistance also remains to be determined. In some systems a particular protein change can be identified in multiple independent isolates suggesting a correlation between the development of resistance and the presence of this cellular alteration. Experiments conducted thus far on the mechanism of non-Pgp mdr are intriguing. Studies utilizing fluorescence microscopy to follow the fate of daunomycin suggests that the drug passes to the interior of the cell and eventually localizes in the Golgi apparatus. Drug located at this site may move directly into an efflux pathway for rapid extrusion from the cell. Evidence also indicates that as drug leaves the Golgi some may be sequestered into other organelles such as lysosomes or mitochondria.(ABSTRACT TRUNCATED AT 250 WORDS)