Glutathione transferases: substrates, inihibitors and pro-drugs in cancer and neurodegenerative diseases

Abstract

Glutathione transferase classical GSH conjugation activity plays a critical role in cellular detoxification against xenobiotics and noxious compounds as well as against oxidative stress. However, this feature is also exploited by cancer cells to acquire drug resistance and improve their survival. As a result, various members of the family were found overexpressed in a number of different cancers. Moreover several GST polymorphisms, ranging from null phenotypes to point mutations, were detected in members of the family and found to correlate with the onset of neuro-degenerative diseases. In the last decades, a great deal of research aimed at clarifying the role played by GSTs in drug resistance, at developing inhibitors to counteract this activity but also at exploiting GSTs for prodrugs specific activation in cancer cells. Here we summarize some of the most important achievements reached in this lively area of research.

Fig. 1. Structure of a representative GST.

The structure of human GSTP1-1 in complex with GSH and the inhibitor NBDHEX is shown (pdb code 3GUS). The N-terminal thioredoxin-like domain is coloured in pink while the all-helical C-terminal domain is coloured in cyan. The G-site is occupied by a GSH molecule while the H-site is occupied by a NBDHEX molecule which are shown in sticks

Fig. 2. Overview of enzymatic biotransformation of xenobiotics.

Harmful molecules may diffuse across the plasma-membrane and, inside cells, they may be targeted by the enzymes of the so-called Phase I metabolism. Main ones belong to the cytochrome P450 family, comprising several enzymes catalyzing different reactions including hydroxylation—the major reaction involved—oxidation and reduction. In the subsequent Phase II metabolism, the main role is played by GSTs that catalyze the conjugation of Phase I-modified xenobiotics to endogenous GSH. The conjugate obtained is then actively transported out of the cell by different transmembrane efflux pumps (Phase III). Some compounds may enter Phase II metabolism directly

Fig. 3. GST substrates.

a First two steps of the metabolism of aflatoxin B1; b detoxification pathway of 4-hydroxynonenal involving GST; c GST-catalyzed conjugation of acrolein and subsequent trasnformation steps d enzymatic inactivation of cisplatin catalyzed by GSTs; e first step of the metabolic pathway of busulfan; f enzymatic inactivation of dichoroacetate leadind to glyoxylate

Fig. 4. Role of GSTP1-1 in JNK signaling pathway.

Monomeric GSTP1 protects tumoral cells from apoptosis by inhibition of the JNK signaling pathway through the formation of a GSTP1-JNK-cJun complex that hampers c-Jun phosphorilation. Under stress conditions, GSTP1 may dissociate from the complex and dimerize, thus enabling JNK to phosphorilate c-Jun. This event may also be triggered by the GST inhibitor NBDHEX which binds GSTP1 and induces its release from the complex

Fig. 5. GST inhibitors.

a GST-catalyzed conjugation of ethacrynic acid; b A conjugate of ethacrynic acid and glucosamine (EAG) reacts with GSH and inhibits GSTs; c Ethacraplatin is a Pt(IV)-complex compound which contains two ethacrynic acid moieties. When exposed to GSTs, this compound inhibits the enzyme and liberates cisplatin; d NBDHEX and its MC3181 derivative are GST inhibitors that bind the enzyme H-site and are conjugated by GSH leading to the formation of a stable σ complex; e Auranofin structure and its inhibitory effect on GSTs; f Ezatiostat (TLK199) is a GSH analogue that exerts its inhibitory effects on GSTs through G-site binding; g Piperlongumine is hydrolized inside cells and forms a conjugate with GSH that exerts inhibition of GST acitivity

Fig. 6. GST prodrugs.

a Bioconversion of canfosfamide in an active alkilating metabolite played by GSTP1-1; b synthesis and bioconversion played by GST of a metformin sulfonamide prodrug; c GSH conjugation with nitric oxide releasing agents to form diazeniumdiolate anion and subsequent release of two NO molecules; d Release of active doxorubicin by GST sulfomidase activity played on nitrobenzenesulfonyl analogues