In vivo detection of γ-glutamyl-transferase up-regulation in glioma using hyperpolarized γ-glutamyl-[1-13C]glycine

Abstract

Glutathione (GSH) is often upregulated in cancer, where it serves to mitigate oxidative stress. γ-glutamyl-transferase (GGT) is a key enzyme in GSH homeostasis, and compared to normal brain its expression is elevated in tumors, including in primary glioblastoma. GGT is therefore an attractive imaging target for detection of glioblastoma. The goal of our study was to assess the value of hyperpolarized (HP) γ-glutamyl-[1-13 C]glycine for non-invasive imaging of glioblastoma. Nude rats bearing orthotopic U87 glioblastoma and healthy controls were investigated. Imaging was performed by injecting HP γ-glutamyl-[1-13 c]glycine and acquiring dynamic 13 C data on a preclinical 3T MR scanner. The signal-to-noise (SNR) ratios of γ-glutamyl-[1-13 C]glycine and its product [1-13 c]glycine were evaluated. Comparison of control and tumor-bearing rats showed no difference in γ-glutamyl-[1-13 C]glycine SNR, pointing to similar delivery to tumor and normal brain. In contrast, [1-13 c]glycine SnR was significantly higher in tumor-bearing rats compared to controls, and in tumor regions compared to normal-appearing brain. Importantly, higher [1-13 C]glycine was associated with higher GGT expression and higher GSH levels in tumor tissue compared to normal brain. Collectively, this study demonstrates, to our knowledge for the first time, the feasibility of using HP γ-glutamyl-[1-13 c]glycine to monitor GGt expression in the brain and thus to detect glioblastoma.Redox homeostasis is essential for managing oxidative stress and ensuring cell survival. Cancer cells, in particular, are often characterized by high levels of oxidative stress. This oxidative stress is the result of reactive oxygen species (ROS) that accumulate due to a variety of factors, including rapid cell proliferation, hypoxia, metabolic reprogramming and oncogenic signaling. The tripeptide glutathione (L-γ-glutamyl-L-cysteinyl-glycine, GSH) is the most abundant, non-enzymatic antioxidant molecule present in mammalian cells and plays a crucial role in regulating tumor oxidative stress due to its role in reducing ROS 1,2 . Several tumor types, including primary glioblastomas (GBMs), are characterized by elevated GSH levels, especially under hypoxic conditions 3 . Elevated GSH levels and reduced oxidative stress have also been linked to resistance to chemotherapy in GBMs 4 .GSH import is a critical step in the maintenance of both intracellular and extracellular redox status 5,6 . Although GSH can be transported out of cells and into the extracellular environment, most cells are incapable of importing intact GSH 7 . Instead, GSH is first degraded to its constituent amino acids 7,8 and the released amino acids are then transported into the cell and used as substrates for de novo GSH synthesis 6 . Specifically, GSH degradation is initiated by cleavage of the gamma-glutamyl bond 9 via the cell surface-bound glycoprotein gamma-glutamyl transpeptidase (GGT) whose catalytic site faces the extracellular environment 7,10-12 . After removal of the glutamyl group from GSH, cysteinylg...

Imaging 6-Phosphogluconolactonase Activity in Brain Tumors In Vivo Using Hyperpolarized δ-[1-13C]gluconolactone

“…Tumor volume was evaluated as the sum of manually contoured tumor areas in each slice multiplied by slice thickness using an in-house IDL-based software (33)(34)(35). 2D EPSI data were processed using in-house Matlab codes (R2019a, Mathworks) that have previously been described (33,34) and are available from a github repository (https://github.com/ViswanathLab/ EPSI). For each voxel at every time point, spectra were analyzed after a 5 Hz line broadening by determining the area under each peak by integration.…”

“…All studies were performed under the UCSF Institutional Animal Care and Use Committee approval (IACUC Protocol No: AN170079). Orthotopic tumors were generated by intracranial implantation of~3 x 10 5 cells per rat into athymic male nu/nu rats (5 weeks old) as previously described (33,34). Tumor volume was monitored by T2-weighted MRI as described below and hyperpolarized d-[1-13 C]gluconolactone metabolism was assessed once tumors reached a volume of 0.28 ± 0.08 cm 3 .…”

In Vivo Brain GSH: MRS Methods and Clinical Applications

“…The authors detected higher levels of [1-13C] glycine in tumor-bearing rats compared with controls, and in tumor tissue compared with the normal brain. Higher [1-13C] glycine was accompanied by an enhanced GGT expression and increased GSH levels in the tumor tissue [ 117 ]. Other authors demonstrated that the mutation of IDH1 inhibits the growth of glioma cells, possibly mediating prolonged survival in the glioma.…”

Expression of gamma-glutamyltransferase 1 in glioblastoma cells confers resistance to cystine deprivation–induced ferroptosis

“…Indeed, deletion of GGT1 expression in glioblastoma cells led to the depletion of intracellular GSH under cystine deprivation, suggesting that glioblastoma cells utilize extracellular glutathione for the maintenance of intracellular GSH when they are unable to obtain extracellular cystine. It was also reported that GGT1 is highly expressed in numerous cancers, including glioblastoma (10,11). In particular, GGT1 is detected in exosomes from cancer cells, and serum exosomal GGT activity was reported to be a useful biomarker for several types of cancer (28,29).…”