Tumor-Associated Antigen xCT and Mutant-p53 as Molecular Targets for New Combinatorial Antitumor Strategies

Abstract

The cystine/glutamate antiporter xCT is a tumor-associated antigen that has been newly identified in many cancer types. By participating in glutathione biosynthesis, xCT protects cancer cells from oxidative stress conditions and ferroptosis, and contributes to metabolic reprogramming, thus promoting tumor progression and chemoresistance. Moreover, xCT is overexpressed in cancer stem cells. These features render xCT a promising target for cancer therapy, as has been widely reported in the literature and in our work on its immunotargeting. Interestingly, studies on the TP53 gene have revealed that both wild-type and mutant p53 induce the post-transcriptional down modulation of xCT, contributing to ferroptosis. Moreover, APR-246, a small molecule drug that can restore wild-type p53 function in cancer cells, has been described as an indirect modulator of xCT expression in tumors with mutant p53 accumulation, and is thus a promising drug to use in combination with xCT inhibition. This review summarizes the current knowledge of xCT and its regulation by p53, with a focus on the crosstalk of these two molecules in ferroptosis, and also considers some possible combinatorial strategies that can make use of APR-246 treatment in combination with anti-xCT immunotargeting.

Keywords: APR-246; PRIMA-1; breast cancer; p53; xCT.

Figure 1

xCT function and transcriptional regulation. The cystine/glutamate antiporter xCT exports intracellular glutamate and imports extracellular cystine. Exported glutamate is involved in tumor microenvironment shaping, promoting invasiveness and metastasization. The imported cystine is reduced to cysteine and used for the biosynthesis of GSH, which is responsible for cancer cell protection from reactive oxygen species (ROS) excess due to genomic instability, altered metabolism and external insults, such as chemotherapy. ROS induce the upregulation of xCT, which destabilizes the KEAP1-NRF2 complex; NRF2 translocation into the nucleus and its binding to the ARE sequence on the SLC7A11 gene promoter lead to xCT mRNA transcription and protein production. Amino acid depletion also induces xCT upregulation through ATF4, which binds to the Amino Acid Response Element (AARE) sequences on the SLC7A11 gene promoter. The mTORC2/AKT signaling axis is another relevant pathway in xCT post-translational modulation that, when activated, inhibits xCT through Ser26 phosphorylation. This figure was created with BioRender.com.

Figure 2

p53 function and transcriptional regulation. Under homeostatic conditions (green arrows), p53 is targeted by MDM2, which induces the proteasome-mediated degradation of p53. After cell injury or stress stimuli (red arrows), p53 levels increase due to several activated pathways that converge on the inhibition of MDM2 or post-translational p53 modifications (e.g., acetylation, phosphorylation). p53 activation strengthens the thermodynamic stability of the protein and induces its homo-tetramerization, inducing the transcription of target genes that are responsible for cell cycle arrest, DNA repair, senescence, apoptosis and ferroptosis. p53 is also a direct transcriptional inducer of MDM2, establishing a negative feedback loop. The role of p53 in inhibiting xCT gene transcription, thus contributing to ferroptosis, is also indicated. This figure was created with BioRender.com.

Figure 3

A schematic explanation of the rationale for combined treatment. (a) The effects of mut-p53 on cell cycle and redox balance. mut-p53 favors cancerogenesis by inhibiting NRF2-dependent xCT transcription and consequently slightly increasing the amount of intracellular ROS, which stimulates the cell to proliferate. (b) The effects of APR-246 treatment. mut-p53 is converted to its active form (wt-p53), leading to pro-apoptotic functions and the selective induction of ROS increase via GSH depletion, although the mut-p53-dependent inhibition of xCT expression is lost. This causes cells to undergo ferroptosis. (c) The effects of the combined treatment. APR-246-dependent GSH depletion together with xCT inhibition sharply decrease GSH levels, inducing high ROS levels that overwhelm the antioxidant capacity of the cell and finally lead to ferroptotic death. This figure was created with BioRender.com.