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Pyrrolidine dithiocarbamate up-regulates the expression of the genes encoding the catalytic and regulatory subunits of gamma-glutamylcysteine synthetase and increases intracellular glutathione levels.

ABSTRACT: Time- and dose-dependent increases in the steady-state mRNA levels of the genes encoding the catalytic and regulatory subunits of the enzyme gamma-glutamylcysteine synthetase (GCS) were observed in HepG2 human hepatocarcinoma cells after exposure to pyrrolidine dithiocarbamate (PDTC). PDTC was demonstrated to manifest both antioxidant and pro-oxidant properties in HepG2 cells, as assessed by the decreased fluorescence of the redox-sensitive dye Dihydrorhodamine 123 and by the oxidation of glutathione respectively. Attempts to characterize the signalling pathway from PDTC exposure to increases in the expression of the GCS catalytic and regulatory subunit genes demonstrated that induction by PDTC could be partially blocked by treatment with the thiol agent N-acetylcysteine and by the copper chelator bathocuproine disulphonic acid. These findings suggested that the up-regulation of the two genes resulted from a PDTC-induced pro-oxidant signal, which was partially copper-dependent. In summary, these studies demonstrate that PDTC exposure elicits a cellular response in HepG2 cells, characterized by the induction of the genes encoding the two subunits of the enzyme GCS and increased de novo synthesis of the cellular protectant GSH.

SUBMITTER: Wild AC

PROVIDER: S-EPMC1220100 | biostudies-other | 1999 Mar

REPOSITORIES: biostudies-other

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Project description:gamma-Glutamylcysteine synthetase (GCS) is reported to catalyse the rate-limiting step in glutathione biosynthesis, and is a heterodimer composed of a catalytic subunit [heavy subunit (GCSh) of Mr 73000] and a regulatory subunit [light subunit (GCSl) of Mr 31000]. In the present study, we have demonstrated for the first time a potential role for GCSl in resistance towards doxorubicin and cadmium chloride. Addition of recombinant GCSl to HeLa cell extracts in vitro was found to result in an increase in GCS activity of between 2- and 3-fold. Transient transfections of COS-1 cells with the GCSl cDNA cause an increase in GCS activity of approx. 2-fold, and a small but significant (P=0.008) increase in glutathione levels from 126.9+/-4. 2 nmol/mg protein to 178.8+/-19.1 nmol/mg protein. We proceeded to make a HeLa cell line (LN73), which stably overexpresses GCSl. These cells overexpress GCSl approx. 20-fold above basal levels. LN73 was found to have a 2-fold increase in GCS activity (437.3+/-85.2 pmol/min per mg) relative to the control cell line, HL9 (213.4+/-71. 8 pmol/min per mg). In contrast with the transient transfections in COS-1 cells, stable overexpression of GCSl was found not to be associated with an increase in glutathione content. However, when the LN73 and HL9 cells were treated with the glutathione-depleting agent, diethylmaleate, the LN73 cells were found to have an enhanced ability to regenerate glutathione, compared with HL9 cells. The cell lines were treated with various anti-cancer drugs, and their cytotoxicity was examined. No obvious differences in toxicity were observed between the different cell lines following treatment with cisplatin and melphalan. The redox-cycling agent doxorubicin, however, was found to be more toxic (approx. 2-fold) to the HL9 cells than the LN73 cells. When the cells were treated with the carcinogenic transition-metal compound, cadmium chloride, LN73 cells were found to be approx. 3-fold more resistant than HL9 cells.

Project description:We previously reported that the activity of gamma-glutamylcysteine synthetase (GCS; EC 6.3.2.2), the rate-limiting enzyme in GSH synthesis, can be acutely inhibited approximately 20-40% by agonists of various signal transduction pathways in rat hepatocytes [Lu, Kuhlenkamp, Garcia-Ruiz and Kaplowitz (1991) J. Clin. Invest. 88, 260-269]. We have now examined the possibility that GCS is phosphorylated directly by activation of protein kinase A (PKA), protein kinase C (PKC) and Ca2+/calmodulin-dependent kinase II (CMK). Phosphorylation of GCS was studied using both purified rat kidney GCS and cultured rat hepatocytes by immunoprecipitating the reaction product with specific rabbit anti-(rat GCS heavy subunit) (anti-GCS-HS) antibodies. All three kinases, PKA, PKC and CMK, phosphorylated rat kidney GCS-HS in a Mg(2+)-concentration-dependent manner, with the highest degree of phosphorylation occurring at 20 mM Mg2+. The maximum incorporation of phosphate in mol/mol of GCS was 1.17 for PKA, 0.70 for PKC and 0.62 for CMK. The degree of phosphorylation was correlated with the degree of loss of GCS activity, and no additional inhibition occurred when GCS was phosphorylated by all three kinases, suggesting that the kinases phosphorylated the same site(s). Phosphoamino analysis showed that all three kinases phosphorylated serine and threonine residues. Two-dimensional phosphopeptide mapping demonstrated that all three kinases phosphorylated the same five peptides, both PKA and PKC phosphorylated two other peptides, and only PKA phosphorylated one additional peptide. Phosphorylation of GCS decreased its Vmax for cysteine and glutamate without changing its K(m). Finally, treatment of cultured rat hepatocytes with dibutyryl cAMP and phenylephrine significantly increased the phosphorylation of GCS, suggesting a potentially important physiological role. In summary, we have demonstrated that GCS is phosphorylated and suggest that phosphorylation/dephosphorylation may regulate GCS activity.

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Pyrrolidine dithiocarbamate up-regulates the expression of the genes encoding the catalytic and regulatory subunits of gamma-glutamylcysteine synthetase and increases intracellular glutathione levels.

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