Glutathione‑degrading enzymes in the complex landscape of tumors (Review)

Abstract

Glutathione (GSH)‑degrading enzymes are essential for starting the first stages of GSH degradation. These enzymes include extracellular γ‑glutamyl transpeptidase (GGT) and intracellular GSH‑specific γ‑glutamylcyclotransferase 1 (ChaC1) and 2. These enzymes are essential for cellular activities, such as immune response, differentiation, proliferation, homeostasis regulation and programmed cell death. Tumor tissue frequently exhibits abnormal expression of GSH‑degrading enzymes, which has a key impact on the development and spread of malignancies. The present review summarizes gene and protein structure, catalytic activity and regulation of GSH‑degrading enzymes, their vital roles in tumor development (including regulation of oxidative and endoplasmic reticulum stress, control of programmed cell death, promotion of inflammation and tumorigenesis and modulation of drug resistance in tumor cells) and potential role as diagnostic biomarkers and therapeutic targets.

Keywords: ChaC1; GGT; GSH degrading enzyme; tumor.

Figure 1

Role of GSH-degrading enzymes in mammals. The classical GSH degradation pathway occurs extracellularly. Intracellular GSH, GSSG and GS-X are released from cells into the extracellular space via MRP1 transporter. GGT, located on the plasma membrane, hydrolyzes them to Cys-Gly and Glu, serving as the initial step in extracellular GSH degradation. Cys-Gly undergoes catalysis to Gly and Cys by DPEP1 or minopeptidase N. ChaC1 and ChaC2 hydrolyze GSH to Cys-Gly and Glu or their cyclized form of 5-oxo-proline directly in the cell. Among these, 5-oxo-proline is hydrolyzed to Glu by 5-oxop-rolinase. Cys-Gly, induced by cytoplasmic Cys-Gly peptidase LAP or CNDP2, is hydrolyzed to Gly and Cys, completing the degradation of GSH. ChaC1, glutathione-specific γ-glutamylcyclotransferase 1; CNDP2, carnosine dipeptidase 2; DPEP1, M19 metallopeptidase dipeptidase 1; GCL, glutamate cysteine ligase; GGT, γ-glutamyl transpeptidase; GlyT1-2, glycine transporter 1 and 2; GS, glutathione synthetase; LAP, leucyl aminopeptidase; MRP1, multidrug resistance protein 1.

Figure 2

Ribbon drawing of the human GGT1 heterodimer. The heavy and light chain subunits are shown in green and red, respectively. The binding site is displayed in pink. The GGT heterodimer has a stacked αββα-core. Image obtained from US Data Center for the Global Protein Data Bank, Sequence Annotations in 3D: 4Z9O (rcsb.org/) GGT, γ-glutamyl transpeptidase.

Figure 3

Regulation of GGT. The regulation and expression of GGT remain incompletely characterized. GGT mRNA transcription is co-triggered by multiple potential cis-reactive elements, similar to rat GGT promoters. The proximal region of the GGT promoter contains the binding sites for TRE (often called AP-1 binding elements), AP-2, and Sp1. Ras protein and its downstream oxidative stress effectors, such as ERK1/2, p38MAPK, PI3K/AKT and JNK signaling pathways, serve a key role in upregulating GGT. The P13K/AKT, ERK1/2 and p38MAPK signaling pathways activate Nrf2 via EpRE and sMaf. Nrf2 then transfers from cytoplasm to the nucleus, forming heterodimers with other proteins to bind to EpRE, participating in the upregulation of GP5 activity. Activated H-Ras is also implicated in inducing GP2 activation via downstream ERK1/2, p38MAPK and JNK. Inflammatory conditions activate the NF-κB pathway, initiating downstream TNF-α transcription and transfer to the 536 bp site of the nuclear GGT proximal promoter. The site contains a p50, TNF-α and Sp1 binding site, thereby promoting GGT expression. AP2, activator protein 2; ERK1/2, extracellular signal-regulated kinase 1/2; EpRE, electrophile response element; GGT, γ-glutamyl transpeptidase; GP2, GGT promoter 2; sMaf, small musculoaponeurotic fibrosarcoma; Sp1, specific protein 1; TRE, cis-regulatory element. Drawn with Figdraw (figdraw.com).

Figure 4

Ribbon drawing of human ChaC glutathione specific γ-glutamylcyclotransferase 1 protein. Very high confident areas are shown in dark blue, Confident areas are shown in light blue, Low confident areas are shown in yellow; very low confident areas are shown in red. The binding site is shown in green. Figure obtained from UniProt, 2018 (uniprot.org/). ChaC, glutathione-specific γ-glutamylcyclotransferase.

Figure 5

Regulation of ChaC1. Oxidative and ER stress, and viral infection induce PERK/eIF2α/ATF4/ATF3/CHOP cascade activation via UPR. ATF3 may primarily regulate basal ChaC1 expression via ATF/CRE, while ATF4 may mainly regulate stress-induced ChaC1 expression via ATF/CRE and ACM. In response to ER stress, C/EBP-β recruits ATF4 to the ChaC1 promoter, but the precise C/EBP-β response element on the ChaC1 promoter remains unclear. CARE serves a secondary role in regulating human ChaC1 transcription. Amino acid starvation induces ChaC1 expression, activating ATF4 via the GCN2/eIF2a/ATF4/ATF3 pathway. ACM, a novel-248 ATF/CRE modifier; ATF/CRE, activating transcription factor/cAMP response element; CARE, conserved-209 CEBP-ATF response element; C/EBP-β, CCAAT/enhancer binding protein β; CHOP, C/EBP homologous protein; eIF2α, eukaryotic initiation factor-2α; ER, endoplasmic reticulum; GCN2, general control nonderepressible 2; PERK, protein kinase R-like ER kinase; UPR, unfolded protein response. Figure constructed using Figdraw (figdraw.com).

Figure 6

Functions of glutathione-degrading enzymes in tumors. The functions of GGT and ChaC1/ChaC2 include regulating oxidative and ER stress and programmed cell death, promoting inflammation and cell drug resistance. ChaC1, glutathione-specific γ-glutamylcyclotransferase 1; ER, endoplasmic reticulum; GGT, γ-glutamyl transpeptidase. Figure constructed with Figdraw (figdraw.com).