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Human tumor cells resistant to verapamil
K R Huber1, W F Schmidt, B al-Assaad
1Children's Cancer Research Laboratory, Department of Pediatrics, School of Medicine, University of South Carolina, Columbia 29208.
Abstract:
The efficacy of the calcium channel blocker verapamil for enhancing at low concentrations the cytotoxicity of unrelated antineoplastic drugs and for inhibiting at high concentrations cell proliferation has stimulated interest in the underlying mechanisms of these two diverse effects. We have selected two human brain tumor cell lines (a TE671 medulloblastoma and a A172 glioma line) for resistance against 100 uM verapamil to aid in the elucidation of the mechanism of verapamil's antiproliferative effect. Our first experiments on the selected TE671 medulloblastoma cells show that, in the presence of 100 uM verapamil, these cells grow at a rate similar to that observed for the sensitive cells in the absence of verapamil. This resistant clone continues to exhibit resistance toward verapamil for at least three days after the verapamil has been removed from the growth medium. In contrast to the sensitive cells, the resistant cells show only slight cell cycle phase alterations after removal of verapamil from the growth medium. This, together with an unchanged c-myc gene expression after removal of verapamil, indicates a stable phenotypic alteration that is responsible for the exhibited resistance toward the antiproliferative effects of the drug. Experiments designed to elucidate the mechanism of resistance showed that these cells are not cross-resistant to the antineoplastic drugs vincristine and adriamycin. Also, the resistance is not accompanied by increased amounts of the 170-180 kDa P-glycoprotein that has been implicated in resistance phenomena of cancer cells towards antineoplastic drugs.
Insights
Verapamil resistance in medulloblastoma cells stems from a stable phenotypic alteration, not P-glycoprotein. This resistance impacts cell proliferation and persists even after drug removal.
Area of Science:
- Pharmacology
- Cancer Biology
- Molecular Biology
Background:
- Verapamil exhibits dual effects: enhancing chemotherapy at low doses and inhibiting cell proliferation at high doses.
- The mechanisms behind verapamil's diverse effects, particularly its antiproliferative action, require further elucidation.
- Human brain tumor cell lines were utilized to investigate verapamil's antiproliferative mechanism.
Purpose of the Study:
- To investigate the mechanisms underlying verapamil's antiproliferative effect.
- To develop and characterize verapamil-resistant human brain tumor cell lines.
- To understand the nature of the stable phenotypic alteration conferring verapamil resistance.
Main Methods:
- Selection of TE671 medulloblastoma and A172 glioma cell lines for resistance to 100 uM verapamil.
- Assessment of cell growth rates in the presence and absence of verapamil.
- Evaluation of cell cycle phase alterations and c-myc gene expression.
- Testing for cross-resistance to vincristine and adriamycin.
- Analysis of P-glycoprotein expression.
Main Results:
- Selected TE671 cells demonstrated stable resistance to verapamil's antiproliferative effects, maintaining growth similar to sensitive cells without verapamil.
- Resistance persisted for at least three days post-verapamil removal, with minimal cell cycle alterations.
- c-myc gene expression remained unchanged after verapamil removal, suggesting a stable phenotypic modification.
- Resistant cells were not cross-resistant to vincristine or adriamycin.
- No increased expression of P-glycoprotein (170-180 kDa) was observed in resistant cells.
Conclusions:
- A stable phenotypic alteration, not P-glycoprotein upregulation, is responsible for verapamil resistance in TE671 medulloblastoma cells.
- The findings provide insights into the mechanisms of verapamil resistance and potential therapeutic strategies.
- Further research is warranted to fully characterize the molecular basis of this stable resistance phenotype.