Multifactorial mechanisms of drug resistance in tumor cell populations selected for resistance to specific chemotherapeutic agents

Differential and kinetic effects of cell cycle inhibitors on neoplastic and primary astrocytes

Alterations in cell cycle regulation underlie the unrestricted growth of neoplastic astrocytes. Chemotherapeutic interventions of gliomas have poor prognostic outcomes due to drug resistance and drug toxicity. Here, we examined the in vitro growth kinetics of C6 glioma (C6G) cells and primary astrocytes and their responses to 2 phase-specific inhibitors, lovastatin and hydroxyurea. C6G cells demonstrated a shorter G₁ phase and an earlier peak of DNA synthesis in S phase than primary astrocytes. As C6G cells and primary astrocytes re-entered the cell cycle in the presence of lovastatin or hydroxyurea, they exhibited different sensitivities to the inhibitory effects of these agents, as measured by [³H]-thymidine incorporation. Compared to primary astrocytes, C6G cells were more sensitive to lovastatin, but less sensitive to hydroxyurea. Studies using 2 different paradigms of exposure uncovered dramatic differences in the kinetics of DNA synthesis inhibition by these 2 agents in C6G cells and primary astrocytes. One notable difference was the ability of C6G cells to more easily recover from the inhibitory effects of hydroxyurea following short exposure. Our results provide insight into C6 glioma drug resistance as well as the inhibitory effects of these 2 phase-specific inhibitors and their chemotherapeutic potential.

Induction of apoptosis in p53-deficient L1210 cells by an I-κ-B-α-inhibitor (Bay 11-7085) via a NF-κ-B-independent mechanism

The effects of Bay 11-7085, an inhibitor of I-kappa-B-alpha kinase, were compared in the wild-type and deoxyadenosine-resistant mouse leukemia cell lines. At higher concentrations, Bay 11-7085 caused apoptosis and necrosis in the two cell lines. However, at much lower concentrations of Bay 11-7085, the deoxyadenosine-resistant cells became much more apoptotic than the parental wild-type L1210 cells. Under these conditions (low drug concentrations), the level of apoptotic cells far exceeded the fraction of necrotic cells. The apoptotic effects of Bay 11-7085 on the deoxyadenosine-resistant cells was both time- and concentration-dependent. Caspase-3 activity was activated in the Bay 11-7085-treated cells. The molecular basis for the difference in the apoptotic response between the wild-type and deoxyadenosine-resistant L1210 cells is not defined at this time, but these cell lines may provide a comparative model system in which differences in the cells that lead to apoptotic or necrotic responses to drugs can be defined and used in development of appropriate drugs for clinical use.

mRNA expression profiles for the response of human tumor cell lines to the antimalarial drugs artesunate, arteether, and artemether

The antimalarial artemisinin derivatives artesunate (ART), arteether (ARE), and artemether (ARM) reveal remarkable antineoplastic activity. In the present investigation, we identified mRNA expression profiles associated with the response of tumor cells to ART, ARE, and ARM. We performed correlation and hierarchical cluster analyses of inhibition concentration 50% (IC(50)) values and basal mRNA expression levels of 464 genes deposited in the database of the National Cancer Institute, USA. Correlating IC(50) values of ART, ARE, and ARM and of 16 established antineoplastic drugs revealed that the artemisinin derivatives could not be assigned with a known class of drugs with defined mode(s) of action. The basal mRNA expression of 208 out of 464 genes (45%) correlated significantly with IC(50) values of at least one artemisinin derivative. These genes were from different classes (drug resistance genes, DNA damage and repair genes, apoptosis-regulating genes, proliferation-associated genes, oncogenes, tumor suppressor genes and cytokines). We identified two different gene clusters by hierarchical cluster analysis. One cluster contained predominately genes significantly correlated to all three artemisinin derivatives. This overlapping set of genes points to common molecular mechanisms of tumor inhibition by all three drugs in which genes affecting cellular proliferation may play an important role. The second cluster contained genes differentially associated with the response of artemisinin derivatives to cancer cells. The number of correlating drug resistance genes in this cluster increased in the order ART<ARE<ARM and was paralleled by increasing IC(50) values of the three drugs in the same order. The higher activity of ART in comparison to ARE and ARM may, thus, be explained by a lower number of drug resistance genes affecting ARTs action. The present analysis is a starting point for the generation of hypotheses on candidate genes and for a more detailed dissection of the functional role of individual genes for the activity of artemisinin derivatives in tumor cells.

Lack of competition of substrates for P-glycoprotein in MCF-7 breast cancer cells overexpressing MDR1

The MCF-7/Adr cells overexpress MDR-1 which contributes to the drug-resistant phenotype. Our studies show: 1. The retention of daunomycin in the MCF-7/Adr cells relates to a temperature-dependent and energy-dependent process. 2. The MCF-7/Adr cells retain less rhodamine-123 than the parental MCF-7 cells. 3. The MCF-7/Adr cells retain less daunomycin than the parental MCF-7 cells as measured by mean fluorescence or radioactive daunomycin. 4. Cyclosporin A and verapamil effectively block the effluxes of rhodamine-123 and daunomycin from the MCF-7/Adr cells. 5. On short-term incubation, 2-deoxyglucose lowers the NTP levels to a greater extent than sodium azide, showing the importance of glycolysis in the MCF-7 cell lines. 6. Although the MCF-7/Adr cells show cross-resistance to VP-16, actinomycin D and vinblastine, these drugs do not compete with daunomycin for the efflux pump. 7. These data suggest that either there must be multiple MDR-1 pumps that differ in substrate specificity or that there are distinct substrate sites on MDR-1.