We show that pharmacological inactivation of GSTP over a sus
We show that pharmacological inactivation of GSTP1 over a sustained period does not show any observable toxicity, and not only prevents breast tumor growth but even slows established breast tumor growth in mice. A highly potent GSTP1 inhibitor, ezatiostat (developed by Telik Inc.) has passed phase II clinical trials in patients for the treatment of myelodysplastic syndrome, indicating that GSTP1 inhibitors are likely to be well tolerated in humans (Mahadevan and Sutton, 2015). Beyond the many previously reported biochemical and therapeutic roles of GSTP1 (Grek et al., 2013, Tew and Townsend, 2011, Townsend and Tew, 2003), our study suggests that GSTP1 inhibitors may also be promising stand-alone therapeutics for TNBCs. Our study also underscores the utility of using reactivity-based chemoproteomic platforms coupled with functional metabolomic approaches to identify novel metabolic drivers and pathways underlying breast cancer malignancy.
Significance TNBCs and stem-like breast cancers that have undergone EMT have been linked to breast cancer malignancy and poor prognosis. While these breast cancer cell types are particularly dangerous, there are few to no therapeutic strategies that specifically target these types of cells. Here, we have used chemoproteomic profiling platforms to identify GSTP1 as a novel potential therapeutic target for TNBCs. We show that GSTP1 interacts with GAPDH to activate its activity and that GSTP1 inactivation impairs glycolytic metabolism after the GAPDH step in glycolysis to impair ATP generation, nucleotide and fatty ceramide metabolism, and oncogenic signaling pathways. We show that GSTP1 inhibitors show selective killing of TNBC cells over non-TNBC or non-transformed mammary epithelial cells and that long-term GSTP1 inhibition in mice does not cause overt toxicity or weight loss. Taking our findings together, we show that GSTP1 inhibitors may potentially be novel therapeutic agents to specifically target TNBCs.
Introduction GSTs (E.C. 126.96.36.199) are a multigene superfamily of enzymes that have been classified into eight distinct gene families coding for seven cytosolic isoforms (alpha, mu, pi, theta, omega, kappa and zeta) and one microsomal form. GSTP1-1, or GST pi, is expressed in most human tissues except in adult liver and in cancer cell lines like the Burkitt lymphoma Raji, HepG2 hepatoma and MCF7 breast cancer cells . Its subunits are between 23 and 28kDa and its gene has a length of 2.8kbp with 7 exons and 6 introns. Studies of human GSTP1-1 promoter show that the necessary regulatory elements are situated in the region −80 to −8, containing AP-1 and Sp1 sites . GSTs play an important role in detoxification by catalysing the conjugation of electrophilic compounds such as xenobiotic drugs, toxins and carcinogens ,  to glutathione (GSH) allowing the drug to be exported from the cell through the GS-X pump in an ATP-dependent way. However, GSTs also allow the development of resistance to chemotherapy and elevated levels of GSTP1-1 mRNA are found in cell lines resistant to a range of anticancer drugs. Indeed, MCF7, an estrogen-receptor positive breast cancer cell line, was found to develop a resistance to ethacrynic acid , doxorubicin and benzopyrene  when transfected with GSTP1-1 and multidrug resistant protein 1. Ovarian cancer cell lines overexpressing GSTP1-1 are resistant to doxorubicin or taxol . COS cells become resistant to doxorubicin  and CHO cells resist to cisplatine and carboplatine  after transfection with the GSTP1-1 gene. Moreover, carcinogenesis is found to be related in many cases to an overexpression of GSTs, in particular GSTP1-1. Indeed, in many human tumours, like prostate carcinoma , squamous-cell carcinoma , acute lymphoblastic leukemia  and chronic lymphoid leukemia , GSTP1-1 is overexpressed, even though in the corresponding normal tissues the protein is either absent or expressed at very low levels. GSTP1-1 can thus be used as a valuable prognostic tool in sarcoma  or gastric carcinoma .