Differential regulation of γ-glutamylcysteine synthetase heavy and light subunit gene expression

Abstract

γ-Glutamylcysteine synthetase (GCS) is the rate-limiting enzyme in the biosynthesis of glutathione and is composed of a heavy and a light subunit. Although the heavy subunit is enzymically active alone, the light subunit plays an important regulatory role by making the holoenzyme function more efficiently. In the current study we examined whether conditions which are known to influence gene expression of the heavy subunit also influence that of the light subunit, and the mechanisms involved. Treatment of cultured rat hepatocytes with hormones such as insulin and hydrocortisone, or plating hepatocytes under low cell density increased the steady-state mRNA level of the heavy subunit only. Treatment with diethyl maleate (DEM), buthionine sulphoximine (BSO) and t-butylhydroquinone (TBH) increased the steady state mRNA level and gene transcription rates of both subunits. These treatments share in common their ability to induce oxidative stress and activate nuclear factor κB (NF-κB). Treatment with protease inhibitors 7-amino-1-chloro-3-tosylamido-2-heptanone (TLCK) or L-1-tosylamido-2-phenylethyl chloromethyl ketone (TPCK) had no influence on the basal NF-κB and GCS subunit mRNA levels, but blocked the activation of NF-κB by DEM, BSO and TBH, and the increase in GCS heavy subunit mRNA level by BSO and TBH. On the other hand, the DEM-, BSO- and TBH-induced increase in GCS light-subunit mRNA level was unaffected by TLCK and TPCK. Thus only the heavy subunit is hormonally regulated and growth sensitive, whereas both subunits are regulated by oxidative stress. Signalling through NF-κB is involved only in the oxidative-stress-mediated changes in the heavy subunit gene expression.