Expression of a P-glycoprotein gene is inducible in a multidrug-resistant human leukemia cell line.

A human T lymphoblastoid CCRF-CEM cell line exhibiting cross resistance to a variety of drugs was selected with increasing doses of actinomycin D. A subline, designated CCRF ACTD400+, was permanently cultured in the presence of 400 ng/ml Actinomycin D for several months. Using a fragment of the human mdr1 cDNA we found high expression of a 5 kb mRNA species which was not detectable in the sensitive parental CCRF-CEM cell line. The extent of the mdr-mRNA expression in resistant cells, however, depended on the presence or absence of actinomycin D in the culture medium: when the inhibitor was omitted, the expression decreased to about 60% after one month. In reverse, the steady state level of the P-glycoprotein mRNA increased about 2.5-fold within 72 h after the original dose of the drug was added again. In further experiments we recorded the actinomycin D or adriamycin dose response curves of the variously treated sublines by evaluation of [3H]uridine or [3H]thymidine incorporation, respectively, into acid insoluble material. Consistently, the drug sensitivity of the respective macromolecular synthesis was found to decrease with increasing mdr-mRNA levels.