POSTTRANSCRIPTIONAL DOWN-REGULATION OF RAS ONCOGENE EXPRESSION BY INHIBITORS OF CELLULAR GLUTATHIONE

Alterations in intracellular glutathione (GSH) content are known to af fect intrinsic responses to ionizing radiation. More recently, it beca me apparent that radiation responses may depend also on the expression of specific oncogenes, including ras. These findings, suggesting a po ssible link between GSH and ras, led us to examine the effect of vario us GSH modulators on ras expression. Treatment of c-Ha-ras-transformed NIH 3T3 cells with L-buthionine S'R'-sulfoximine, dimethylfumarate, o r -1,3-bis(trans-4-hydroxycyclohexyl)-N'-nitrosourea resulted in dose- and time-dependent reduction in ras mRNA steady-state levels followed by a decrease in ras-encoded p21 protein production. The effect on ra s correlated with the extent of GSH decline, was common to different m embers of the ras family, and was independent of the mode of oncogene activation or cell phenotype. Indeed, similar drug effects were observ ed with murine cells in which overexpression of the c-Ha-ras proto-onc ogene was due to transcriptional activation (PR4, nontumorigenic) or g ene amplification (NIH 136, tumorigenic) and with malignant cells expr essing a mutated Ha-ras (RS504). Moreover, N-ras, EJras, and Ki-ras in human tumor cells were similarly affected. Molecular analysis reveale d a significant decrease in ras mRNA half-life in cells subjected to G SH inhibition, an effect that required de novo protein synthesis, but there was no change in the rate of gene transcription. These results i ndicate that pharmacological manipulation of cellular GSH content can down-regulate ras expression at the posttranscriptional level by desta bilizing ras transcripts. The potential clinical implications are disc ussed.