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Modulation of the expression of a multidrug resistance gene (mdr-1/P-glycoprotein) by differentiating agents
L A Mickley1, S E Bates, N D Richert
1Medicine Branch, National Cancer Institute, Bethesda, Maryland 20892.
Abstract:
Acquired resistance to multiple natural products in vitro is mediated by P-glycoprotein (Pgp). Expression of this protein has been demonstrated in some normal tissues and in tumor samples obtained from both untreated and treated patients. In situ hybridizations with RNA probes have demonstrated higher levels of expression of mdr-1/Pgp in well-differentiated tumors and in well-differentiated areas in tumors with mixed histologies. Expression of mdr-1/Pgp in human colon carcinoma cell lines was increased by the differentiating agents sodium butyrate, dimethyl sulfoxide, and dimethylformamide. In the SW-620 cell line addition of sodium butyrate resulted in a rapid induction of mdr-1/Pgp mRNA that was sustained for the duration of the exposure. The levels of P-glycoprotein were measured by immunoblotting and were also increased. Similar results were obtained in three other cell lines including the HCT-15 line. This induction occurred without alterations in nuclease sensitivity. Discontinuation of sodium butyrate was followed by a rapid fall in the levels of mRNA. The levels of P-glycoprotein returned to normal with a half-life of about 24 h. In spite of a 20-25-fold increase in the level of mdr-1/Pgp mRNA and P-glycoprotein, the SW-620 cell line did not demonstrate increased resistance to doxorubicin and vinblastine or decreased accumulation of vinblastine. In contrast, in the HCT-15 cell line, a 5-fold increase of mdr-1/Pgp was accompanied by a comparable fall in vinblastine accumulation which was reversed by verapamil. In the SW-620 cell line, the induced protein could be photolabeled using [3H]azidopine. Expression of mdr-1/Pgp appears to correlate with the degree of differentiation. However, its induction is not always accompanied by expression of the multidrug-resistance phenotype.
Insights
Acquired resistance to multiple drugs is linked to P-glycoprotein (Pgp). While Pgp expression correlates with tumor differentiation, its induction doesn't always confer multidrug resistance. Further research is needed to understand this complex relationship.
Area of Science:
- Molecular Biology
- Cancer Research
- Pharmacology
Background:
- P-glycoprotein (Pgp) mediates acquired resistance to natural products in vitro.
- Pgp expression is observed in normal tissues and various tumor samples.
- Higher Pgp expression is noted in well-differentiated tumors.
Purpose of the Study:
- To investigate the regulation of P-glycoprotein (Pgp) expression in human colon carcinoma cell lines.
- To determine the correlation between Pgp induction and the multidrug-resistance phenotype.
- To explore the role of differentiating agents in modulating Pgp levels.
Main Methods:
- In situ hybridization to assess mdr-1/Pgp mRNA levels.
- Immunoblotting to quantify P-glycoprotein levels.
- Treatment of cell lines with differentiating agents like sodium butyrate.
- Drug accumulation assays and photolabeling studies.
Main Results:
- Differentiating agents (sodium butyrate, DMSO, DMF) increased mdr-1/Pgp expression in colon carcinoma cell lines.
- Sodium butyrate rapidly induced mdr-1/Pgp mRNA and protein in SW-620 cells.
- Pgp induction correlated with tumor differentiation but not always with multidrug resistance.
- HCT-15 cells showed increased Pgp and decreased vinblastine accumulation, unlike SW-620 cells.
Conclusions:
- Pgp expression is regulated by differentiating agents in colon cancer cells.
- While Pgp induction correlates with differentiation, it does not consistently lead to a multidrug-resistance phenotype.
- The functional consequence of Pgp induction varies between cell lines, suggesting complex regulatory mechanisms.