REGULATION OF P-GLYCOPROTEIN-1 AND P-GLYCOPROTEIN-2 GENE-EXPRESSION AND PROTEIN-ACTIVITY IN 2 MCF-7 DOX CELL-LINE SUBCLONES/

The MCF-7 doxorubicin-resistant cell line MCF-7/Dox has been used exte nsively for studies of the multidrug resistance phenomenon. Using fluo rescence-activated cell sorting (FAGS), these cells were separated int o two populations on the basis of rhodamine 123 (R123) accumulation. W e designated these as low P-glycoprotein (LP-gp) and high P-gp (HP-gp) cells on the basis of their P-gp content. Using the reverse transcrip tase - polymersase chain reaction technique controlled by homologous i nternal standards, we analysed levels of MDR1 and MDR2 mRNA in each ce ll type. LP-gp and HP-gp cells had MDR1 mRNA levels of 2.17 +/- 0.17 a nd 6.65 +/- 2.29 amol ng(-1) total RNA respectively, compared with 0.0 0088 +/- 0.00005 amol ng(-1) in wild-type MCF-7 cells (MCF-7/WT). MCF- 7/WT cells additionally contained 0.023 +/- 0.016 amol ng(-1) of MDR2 mRNA, which was unchanged in LP-gp cells, but lower than in HP-gp cell s, which contained 0.42 +/- 0.08 amol ng(-1). Both LP-gp and HP-gp cel ls contained increased copies of the MDR1 gene. However, the degree of gene amplification did not correlate with the changes in MDR1 mRNA le vels, indicating further regulatory levels of gene expression. The lev el of P-gp detected by MRK16 correlated with R123 accumulation. HP-gp cells expressed a 10-fold higher level of P-gp1 than LP-gp cells. Howe ver, there was only a 3-fold increase in MDR1 mRNA level in HP-gp cell s compared with LP-gp cells. These data suggest that some regulation o f P-gp1 expression also occurred al the post-translational level. Phos phorylation of P-gp by protein kinase C (PKC)-alpha is necessary for i ts activity. Our analysis of PKC-alpha, theta and epsilon isozyme leve ls, and subcellular distribution, shows a co-regulation of expression with P-gp, suggesting a necessary role for PKC in P-gp regulation.