POSTTRANSCRIPTIONAL REGULATION OF MRP GS-X PUMP AND GAMMA-GLUTAMYLCYSTEINE SYNTHETASE EXPRESSION BY 1-(4-AMINO-2-METHYL-5-PYRIMIDINYL) METHYL-3-(2-CHLOROETHYL)-3-NITROSOUREA AND BY CYCLOHEXIMIDE IN HUMAN GLIOMA-CELLS/

Treatment of human glioma A172 cells with 5-pyrimidinyl)methyl-3-(2-ch loroethy-3-nitrosourea (ACNU) for 2 to 4 hr resulted in a 2- to 3-fold increase in steady-state levels of multidrug resistance-associated pr otein (MRP) and gamma-glutamylcysteine synthetase (gamma-GCS) mRNA. Nu clear run-on assays revealed a less than 0.5-fold increase in transcri ption rates of these genes under the same treatment conditions, sugges ting that posttranscriptional regulation plays an important role for t he increased mRNA levels. In the absence of ACNU, rates of MRP and gam ma-GCS mRNA degradation were similar in A172 cells as determined by in cubating cells with the RNase inhibitor, Actinomycin D. ACNU treatment s resulted in increased MRP mRNA stability. Induction of MRP and gamma -GCS mRNA by ACNU apparently did not require de novo protein synthesis as determined by the use of protein synthesis inhibitor cycloheximide (CHX). However, CHX alone could induce accumulation of gamma-GCS mRNA , also by posttranscriptional mechanism, Taken together, these results demonstrate that (i) posttranscriptional regulation is primarily invo lved in the induction of MRP and gamma-GCS expression by ACNU and CHX in human glioma cells; and (ii) despite the fact that these two genes have been reported to be frequently co-expressed, their responses to t he treatments of RNA and protein synthesis inhibitors are not the same . (C) 1997 Academic Press.