The x(c)(-) cystine/glutamate antiporter (xCT) as a potential target for therapy of cancer: yet another cytotoxic anticancer approach?

Unraveling the Potential Role of Glutathione in Multiple Forms of Cell Death in Cancer Therapy

Glutathione is the principal intracellular antioxidant buffer against oxidative stress and mainly exists in the forms of reduced glutathione (GSH) and oxidized glutathione (GSSG). The processes of glutathione synthesis, transport, utilization, and metabolism are tightly controlled to maintain intracellular glutathione homeostasis and redox balance. As for cancer cells, they exhibit a greater ROS level than normal cells in order to meet the enhanced metabolism and vicious proliferation; meanwhile, they also have to develop an increased antioxidant defense system to cope with the higher oxidant state. Growing numbers of studies have implicated that altering the glutathione antioxidant system is associated with multiple forms of programmed cell death in cancer cells. In this review, we firstly focus on glutathione homeostasis from the perspectives of glutathione synthesis, distribution, transportation, and metabolism. Then, we discuss the function of glutathione in the antioxidant process. Afterwards, we also summarize the recent advance in the understanding of the mechanism by which glutathione plays a key role in multiple forms of programmed cell death, including apoptosis, necroptosis, ferroptosis, and autophagy. Finally, we highlight the glutathione-targeting therapeutic approaches toward cancers. A comprehensive review on the glutathione homeostasis and the role of glutathione depletion in programmed cell death provide insight into the redox-based research concerning cancer therapeutics.

Memantine ameliorates pulmonary inflammation in a mice model of COPD induced by cigarette smoke combined with LPS

An enhanced chronic inflammatory response in the airways has been regarded as a critical characteristic of chronic obstructive pulmonary disease (COPD). Memantine, an N-methyl-d-aspartate (NMDA) receptors antagonist, has been reported to alleviate lung inflammation. In this study, we investigated the effect and mechanism of memantine on the COPD model induced by cigarette smoke (CS) combined with LPS. Mice and RAW264.7 cells were treated with LPS in the presence or absence of CS. We performed H&E staining to analysis the lung histopathological characteristics. Cytokines (IL-6, TNF-α, and IFN-γ) levels in bronchoalveolar lavage fluid (BALF), lung tissue homogenates and RAW264.7 cell culture medium were determined. Glutamate levels in plasma and culture medium of RAW264.7 were determined. The intracellular Ca2+ flux in RAW264.7 cells was measured by fluo-3 AM staining. The protein levels of NR-1, xCT, ERK1/2, and AKT signaling in the lung tissue and cells were investigated. The result showed that CS and LPS stimulation caused inflammation response, a significant increase in the release of cytokines, including TNF-α, IL-6, and IFN-γ, the elevated release of glutamate and protein levels of NR-1 and xCT, increased Ca2+ influx, and the activation of the ERK1/2 pathway in vitro and in vivo. The above effects of CS and LPS stimulation could be significantly attenuated by memantine treatment. In conclusion, memantine can effectively ameliorate pulmonary inflammation in CS + LPS-induced COPD in mice via reducing NR-1 and xCT expression, glutamate release, Ca2+ influx, and the phosphorylation of Erk1/2. We provided a possible mechanism by which memantine ameliorates COPD in mice.

Temozolomide toxicity operates in a xCT/SLC7a11 dependent manner and is fostered by ferroptosis

The glutamate exchanger xCT (SLC7a11) is causally linked with the malignancy grade of brain tumors and represents a key player in glutamate, cystine and glutathione metabolism. Although blocking xCT is not cytotoxic for brain tumors, xCT inhibition disrupts the neurodegenerative and microenvironment-toxifying activity of gliomas. Here, we report on the use of various xCT inhibitors as single modal drugs and in combination with the autophagy-inducing standard chemotherapeutic agent temozolomide (Temodal/Temcad®, TMZ). xCT overexpressing cells (xCT^OE) are more resistant to the FDA and EMA approved drug sulfasalazine (Azulfidine/Salazopyrin/Sulazine®, SAS) and RNAi-mediated xCT knock down (xCT^KD) in gliomas increases the susceptibility towards SAS in rodent gliomas. In human gliomas, challenged xCT expression had no impact on SAS-induced cytotoxicity. Noteworthy, other xCT inhibitors such as erastin and sorafenib showed enhanced efficacy on xCT^KD gliomas. In contrast, cytotoxic action of TMZ operates independently from xCT expression levels on rodent gliomas. Human glioma cells with silenced xCT expression display higher vulnerability towards TMZ alone as well as towards combined TMZ and SAS. Hence, we tested the partial xCT blockers and ferroptosis inducing agents erastin and sorafenib (Nexavar®, FDA and EMA-approved drug for lung cancer). Noteworthy, xCT^OE gliomas withstand erastin and sorafenib-induced cell death in a concentration-dependent manner, whereas siRNA-mediated xCT knock down increased susceptibility towards erastin and sorafenib. TMZ efficacy can be potentiated when combined with erastin, however not by sorafenib. Moreover, gliomas with high xCT expression are more vulnerable towards combinatorial treatment with erastin-temozolomide. These results disclose that ferroptosis inducers are valid compounds for potentiating the frontline therapeutic agent temozolomide in a multitoxic approach.

Sulfasalazine impacts on ferroptotic cell death and alleviates the tumor microenvironment and glioma-induced brain edema

The glutamate transporter xCT (SCL7a11, system Xc-, SXC) is an emerging key player in glutamate/cysteine/glutathione homeostasis in the brain and in cancer. xCT expression correlates with the grade of malignancy. Here, we report on the use of the U.S. Food and Drug Administration and EMA-approved xCT inhibitor, sulfasalazine (SAS) in gliomas. SAS does not affect cell viability in gliomas at concentrations below 200 μM. At higher concentrations SAS becomes gliomatoxic. Mechanistically SAS inhibits xCT and induces ferroptotic cell death in glioma cells. There is no evidence for impact on autophagic flux following SAS application. However, SAS can potentiate the efficacy of the standard chemotherapeutic and autophagy-inducing agent temozolomide (Temcat, Temodal or Temodar®). We also investigated SAS in non-transformed cellular constituents of the brain. Neurons and brain tissue are almost non-responding to SAS whereas isolated astrocytes are less sensitive towards SAS toxicity compared to gliomas. In vivo SAS treatment does not affect experimental tumor growth and treated animals revealed comparable tumor volume as untreated controls. However, SAS treatment resulted in reduced glioma-derived edema and, hence, total tumor volume burden as revealed by T2-weighted magnetic resonance imaging. Altogether, we show that SAS can be utilized for targeting the glutamate antiporter xCT activity as a tumor microenvironment-normalizing drug, while crucial cytotoxic effects in brain tumors are minor.

Excitatory amino acid glutamate: role in peripheral nociceptive transduction and inflammation in experimental and clinical osteoarthritis

Although a large proportion of patients with osteoarthritis (OA) show inflammation in their affected joints, the pathological role of inflammation in the development and progression of OA has yet to be clarified. Glutamate is considered an excitatory amino acid (EAA) neurotransmitter in the mammalian central nervous system (CNS). There are cellular membrane glutamate receptors and transporters for signal input modulation and termination as well as vesicular glutamate transporters (VGLUTs) for signal output through exocytotic release. Glutamate been shown to mediate intercellular communications in bone cells in a manner similar to synaptic transmission within the CNS. Glutamate-mediated events may also contribute to the pathogenesis and ongoing processes of peripheral nociceptive transduction and inflammation of experimental arthritis models as well as human arthritic conditions. This review will discuss the differential roles of glutamate signaling and blockade in peripheral neuronal and non-neuronal joint tissues, including bone remodeling systems and their potentials to impact OA-related inflammation and progression. This will serve to identify several potential targets to direct novel therapies for OA. Future studies will further elucidate the role of glutamate in the development and progression of OA, as well as its association with the clinical features of the disease.

Histone deacetylases inhibition by SAHA/Vorinostat normalizes the glioma microenvironment via xCT equilibration

Malignant gliomas are characterized by neurodegenerative actions leading to the destruction of surrounding brain parenchyma. The disturbance in glutamate homeostasis caused by increased expression of the glutamate transporter xCT plays a key role in glioma progression. We demonstrate that the HDAC-inhibitor SAHA specifically inhibits the xCT-transporter expression. Thereby, tumor cell stress is engendered, marked by increase in ROS. Moreover, SAHA dependent xCT-reduction correlates with the inhibition of ATF4-expression, a factor known to foster xCT expression. Since xCT/system Xc- is pivotal for the brain tumor microenvironment, normalization of this system is a key in the management of malignant gliomas. To date, the problem lay in the inability to specifically target xCT due to the ubiquitous expression of the xCT-transporter—i.e. in non-cancerously transformed cells too—as well as its essential role in physiological CNS processes. Here, we show xCT-transporter equilibration through SAHA is specific for malignant brain tumors whereas SAHA does not affect the physiological xCT levels in healthy brain parenchyma. Our data indicate that SAHA operates on gliomas specifically via normalizing xCT expression which in consequence leads to reduced extracellular glutamate levels. This in turn causes a marked reduction in neuronal cell death and normalized tumor microenvironment.

Early intraarticular injection of hyaluronic acid attenuates osteoarthritis progression in anterior cruciate ligament-transected rats

SUBJECT

Hyaluronic acid (HA) is widely used to relieve the symptoms of osteoarthritis (OA). An association of reduction of glutamate content with the synovial fluid of OA rats was reported previously.

DESIGN

Anterior cruciate ligament transaction (ACLT) was performed on one knee in male Wistar rats, the other knee was assigned to sham control and HA or saline was injected intraarticularly into the ACLT knee from week 3 to week 7. Knee dialysate was collected for amino acid measurement at week 20. Morphology and histopathology of the femoral medial condyles and synovium were examined and evaluated using Mankin and synovitis scores.

RESULTS

HA injection provided better cartilage (3.38 ± 0.03 vs. 5.45 ± 0.0.02) and synovial condition (3 ± 0.02 vs. 6.03 ± 0.02) than saline controls. Moreover, HA injection reduced the concentration of glutamates in knee dialysates compared to saline controls (1.11 ± 0.14-folds and 2.21 ± 0.19-folds of the sham-operated knee, respectively). Cystine/glutamate antiporter system [Formula: see text] expression was significantly downregulated in the saline group, but not in the HA group (0.32 ± 0.08-folds and 0.71 ± 0.10-folds of the sham-operated knee, respectively).

CONCLUSION

Early intraarticular injection of HA attenuates the progression of cartilage destruction in the ACLT knee, and the downregulation of the cystine/glutamate antiporter system [Formula: see text] was accompanied by the progression of OA.

Inhibition of xc⁻ transporter-mediated cystine uptake by sulfasalazine analogs

A series of sulfasalazine analogs were synthesized and tested for their ability to block cystine-glutamate antiporter system xc⁻ using L-[(14)C]cystine as a substrate. Replacement of sulfasalazine's diazo group with an alkyne group led to an equally potent inhibitor, 2-hydroxy-5-((4-(N-pyridin-2-ylsulfamoyl)phenyl)ethynyl)benzoic acid 6. Our SAR studies also revealed that the carboxylate group of sulfasalazine is essential for its inhibitory activity while the phenolic hydroxyl group is dispensable. Truncated analogs lacking an N-pyridin-2-ylsulfamoyl moiety were less potent than sulfasalazine, but may serve as more tractable templates because of their low molecular weight by applying a variety of fragment growing approaches. Given that sulfasalazine is rapidly metabolized through cleavage of the diazo bond, these analogs may possess a more desirable pharmacological profile as system xc- blockers, in particular, for in vivo studies.

Dissection of mitogenic and neurodegenerative actions of cystine and glutamate in malignant gliomas

Malignant glioma represents one of the most aggressive and lethal human neoplasias. A hallmark of gliomas is their rapid proliferation and destruction of vital brain tissue, a process in which excessive glutamate release by glioma cells takes center stage. Pharmacologic antagonism with glutamate signaling through ionotropic glutamate receptors attenuates glioma progression in vivo, indicating that glutamate release by glioma cells is a prerequisite for rapid glioma growth. Glutamate has been suggested to promote glioma cell proliferation in an autocrine or paracrine manner, in particular by activation of the (RS)-α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid hydrate (AMPA) subtype of glutamate receptors. Here, we dissect the effects of glutamate secretion on glioma progression. Glioma cells release glutamate through the amino-acid antiporter system X(c)(-), a process that is mechanistically linked with cystine incorporation. We show that disrupting glutamate secretion by interfering with the system X(c)(-) activity attenuates glioma cell proliferation solely cystine dependently, whereas glutamate itself does not augment glioma cell growth in vitro. Neither AMPA receptor agonism nor antagonism affects glioma growth in vitro. On a molecular level, AMPA insensitivity is concordant with a pronounced transcriptional downregulation of AMPA receptor subunits or overexpression of the fully edited GluR2 subunit, both of which block receptor activity. Strikingly, AMPA receptor inhibition in tumor-implanted brain slices resulted in markedly reduced tumor progression associated with alleviated neuronal cell death, suggesting that the ability of glutamate to promote glioma progression strictly requires the tumor microenvironment. Concerning a potential pharmacotherapy, targeting system X(c)(-) activity disrupts two major pathophysiological properties of glioma cells, that is, the induction of excitotoxic neuronal cell death and incorporation of cystine required for rapid proliferation.

Pharmacogenomic approach reveals a role for the x(c)- cystine/glutamate antiporter in growth and celastrol resistance of glioma cell lines

The x(c)(-) cystine/glutamate antiporter has been implicated in GSH-based chemoresistance because it mediates cellular uptake of cystine/cysteine for sustenance of intracellular GSH levels. Celastrol, isolated from a Chinese medicinal herb, is a novel heat shock protein 90 (Hsp90) inhibitor with potent anticancer activity against glioma in vitro and in vivo. In search of correlations between growth-inhibitory potency of celastrol in NCI-60 cell lines and microarray expression profiles of most known transporters, we found that expression of SLC7A11, the gene encoding the light chain subunit of x(c)(-), showed a strong negative correlation with celastrol activity. This novel gene-drug correlation was validated. In celastrol-resistant glioma cells that highly expressed SLC7A11, sensitivity to celastrol was consistently increased via treatment with x(c)(-) inhibitors, including glutamate, (S)-4-carboxyphenylglycine, sulfasalazine, and SLC7A11 small interfering RNA. The GSH synthesis inhibitor, buthionine sulfoximine, also increased celastrol sensitivity, whereas the GSH booster, N-acetylcysteine, suppressed its cytotoxicity. Furthermore, the glioma cell lines were dependent on x(c)(-)-mediated cystine uptake for viability, because cystine omission from the culture medium resulted in cell death and treatment with sulfasalazine depleted GSH levels and inhibited their growth. Combined treatment of glioma cells with sulfasalazine and celastrol led to chemosensitization, as suggested by increased celastrol-induced cell cycle arrest, apoptosis, and down-regulation of the Hsp90 client protein, epidermal growth factor receptor. These results indicate that the x(c)(-) transporter provides a useful target for glioma therapy. x(c)(-) inhibitors such as sulfasalazine, a Food and Drug Administration-approved drug, may be effective both as an anticancer drug and as an agent for sensitizing gliomas to celastrol.