Genetic polymorphism and function of glutathione S-transferases in tumor drug resistance

Identification and functional characterization of the human glutathione S-transferase P1 gene as a novel transcriptional target of the p53 tumor suppressor gene

The glutathione S-transferase P1 (GSTP1) is involved in multiple cellular functions, including phase II metabolism, stress response, signaling, and apoptosis. The mechanisms underlying the significantly high GSTP1 expression in many human tumors are, however, currently not well understood. We report here that the GSTP1 gene is a heretofore unrecognized downstream transcriptional target of the tumor suppressor p53. We identified a p53-binding motif comprising two consecutive half-sites located in intron 4 of the GSTP1 gene and is highly homologous to consensus p53-binding motifs in other p53-responsive genes. Using a combination of electrophoretic mobility shift assay and DNase I footprinting analyses, we showed that wild-type p53 protein binds to the GSTP1 p53 motif and luciferase reporter assays showed the motif to be transcriptionally functional in human tumor cells. In a temperature-sensitive p53-mutant cells, levels of both p21/WAF1 and GSTP1 gene transcripts increased time dependently when cells were switched from the inactive mutant state to the wild-type p53 state. Small interfering RNA-mediated reduction of p53 expression resulted in a specific decrease in GSTP1 expression and in tumor cells with mutated p53; adenovirally mediated expression of wild-type p53 increased GSTP1 expression significantly. In a panel of early-passage brain tumor cultures from patients, high levels of GSTP1 transcripts and protein were associated with wild-type p53 and, conversely, low GSTP1 levels with mutant p53. p53 expression knockdown by small interfering RNA increased cisplatin sensitivity. The ability of wild-type p53 to transcriptionally activate the human GSTP1 gene defines a novel mechanism of protecting the genome and, potentially, of tumor drug resistance.

Role of glutathione S-transferase Pi in cisplatin-induced nephrotoxicity

One of the dose-limiting toxicities of cisplatin is nephrotoxicity. Renal toxicity is localized to quiescent proximal tubule cells, where the formation of DNA-adducts cannot account for the dose-limiting toxicity. Our earlier results have shown that a glutathione conjugate of cisplatin is metabolized to a nephrotoxicant via gamma-glutamyl transpeptidase (GGT) and a cysteine S-conjugate beta-lyase. The present study was designed to evaluate the potential role of glutathione S-transferase Pi (GSTP) in the initial steps of the bioactivation of cisplatin. Wild-type mice and mice deficient in both murine GSTP genes (GstP1/P2) were treated with cisplatin. Toxicity in both male and female mice was evaluated 5 days after treatment and renal damage was most severe in wild-type male mice. Wild-type males have approximately 10-fold higher levels of GSTP expression in the liver than females, suggesting that hepatic GSTP in the wild-type males contributed to the formation of the nephrotoxic platinum-glutathione conjugate. In GstP1/P2 null mice the gender difference in toxicity was eliminated. Our data show that GSTP expression is a determinant in cisplatin-induced nephrotoxicity and its levels contribute to sex-dependent differences.

Glutathione S-transferase m1 and t1 null genotypes increase susceptibility to idiosyncratic drug-induced liver injury

Individual vulnerability to drug-induced liver injury (DILI) might result from deficiencies in the detoxification process, which determines the level of exposure to the reactive metabolite. We evaluated whether a genetically determined reduction in the ability to detoxify electrophilic compounds, such as that expected among individuals with glutathione S-transferase (GST) null genotypes, might play a role in determining the risk for DILI and its clinical expression. Genomic DNA from 154 patients (74 men, 80 women; mean age, 53 years) with a diagnosis of DILI as assessed with the Council for International Organizations of Medical Science scale and 250 sex- and age-matched healthy controls were analyzed. A multiplex polymerase chain reaction-based method was used to detect GSTM1 and GSTT1 gene deletions. Carriers of double GSTT1-M1 null genotypes had a 2.70-fold increased risk of developing DILI compared with noncarriers (odds ratio 2.70, 95% confidence interval 1.45-5.03; P = 0.003). The odds ratio for DILI patients receiving antibacterials, and NSAIDs were 3.52 (P = 0.002), and 5.61 (P = 0.001), respectively. Patients with amoxicillin-clavulanate hepatotoxicity (n = 32) had a 2.81-fold increased risk (P = 0.037). Patients classified by the combined GSTT1 and GSTM1 null genotypes did not differ with regard to the type of injury, clinical presentation, or outcome, except for the predominance of women in the combined null genotype (P < 0.001).

CONCLUSION

The double-null genotype for GSTT1 and GSTM1 might play a role in determining the susceptibility to develop DILI, as a general mechanism that occurs regardless of the type of drug involved, and predominantly in women.

Inhibition of apoptosis in prostate cancer cells by androgens is mediated through downregulation of c-Jun N-terminal kinase activation

Androgen deprivation induces the regression of prostate tumors mainly due to an increase in the apoptosis rate; however, the molecular mechanisms underlying the antiapoptotic actions of androgens are not completely understood. We have studied the antiapoptotic effects of androgens in prostate cancer cells exposed to different proapoptotic stimuli. Terminal deoxynucleotidyl transferase-mediated nick-end labeling and nuclear fragmentation analyses demonstrated that androgens protect LNCaP prostate cancer cells from apoptosis induced by thapsigargin, the phorbol ester 12-O-tetradecanoyl-13-phorbol-acetate, or UV irradiation. These three stimuli require the activation of the c-Jun N-terminal kinase (JNK) pathway to induce apoptosis and in all three cases, androgen treatment blocks JNK activation. Interestingly, okadaic acid, a phosphatase inhibitor that causes apoptosis in LNCaP cells, induces JNK activation that is also inhibited by androgens. Actinomycin D, the antiandrogen bicalutamide or specific androgen receptor (AR) knockdown by small interfering RNA all blocked the inhibition of JNK activation mediated by androgens indicating that this activity requires AR-dependent transcriptional activation. These data suggest that the crosstalk between AR and JNK pathways may have important implications in prostate cancer progression and may provide targets for the development of new therapies.