COORDINATE CHANGES IN EXPRESSION OF PROTECTIVE GENES IN DRUG-RESISTANT CELLS

Maintenance of cellular homeostasis is a critical survival trait when cells are exposed to electrophilic chemicals. Because conjugation and elimination of these toxins is dependent upon sequential and coordinat ed metabolic pathways, acquired resistance through a gradual adaptive response would rarely be expected to be the consequence of changes in one gene product. Human HT29 colon cancer cells chronically exposed to EA have acquired resistance to the drug. Commensurate with resistance , EA is more effectively conjugated to GSH and effluxed from the resis tant cells. Using directed and random (differential display) approache s, a number of detoxification and/or protective gene products have bee n shown to be expressed at elevated levels. These include gamma-GCS (s imilar to 3-fold), GST-pi (similar to 3-fold), MRP (similar to 3-fold) , NQO1 (similar to 3-fold), DDH (20-fold), and SSP 3521, a transcripti onal regulator (similar to 3-fold). Multiple mechanisms contribute to these increases, including enhanced transcriptional rate and prolonged mRNA and protein half lives. Further indications for the involvement of transcriptional regulators is found in HL60 adriamycin-resistant ce lls which overexpress MRP, GST-pi and gamma-GCS and also have 15-20-fo ld more DNA-dependent protein kinase. It is possible that this enzyme serves as an early stress response gene which may activate downstream transcription factors. Intriguingly, the catalytic subunit of DNA-depe ndent protein kinase has a high avidity for [S-35]azidophenacyl-GSH. H igh levels of GSH conjugates indicate cell stress and it would seem re asonable to speculate that DNA-dependent protein kinase may serve as a receiver and transmitter of signals which contribute to drug resistan ce and maintain cell viability. (C) 1997 Elsevier Science Ireland Ltd. All rights reserved.