Targeting SLC1A5 and SLC3A2/SLC7A5 as a Potential Strategy to Strengthen Anti-Tumor Immunity in the Tumor Microenvironment

Abstract

Cancer cells are metabolically vigorous and are superior in the uptake of nutrients and in the release of the tumor microenvironment (TME)-specific metabolites. They create an acidic, hypoxic, and nutrient-depleted TME that makes it difficult for the cytotoxic immune cells to adapt to the metabolically hostile environment. Since a robust metabolism in immune cells is required for optimal anti-tumor effector functions, the challenges caused by the TME result in severe defects in the invasion and destruction of the established tumors. There have been many recent developments in NK and T cell-mediated immunotherapy, such as engineering them to express chimeric antigen receptors (CARs) to enhance tumor-recognition and infiltration. However, to defeat the tumor and overcome the limitations of the TME, it is essential to fortify these novel therapies by improving the metabolism of the immune cells. One potential strategy to enhance the metabolic fitness of immune cells is to upregulate the expression of nutrient transporters, specifically glucose and amino acid transporters. In particular, the amino acid transporters SLC1A5 and SLC7A5 as well as the ancillary subunit SLC3A2, which are required for efficient uptake of glutamine and leucine respectively, could strengthen the metabolic capabilities and effector functions of tumor-directed CAR-NK and T cells. In addition to enabling the influx and efflux of essential amino acids through the plasma membrane and within subcellular compartments such as the lysosome and the mitochondria, accumulating evidence has demonstrated that the amino acid transporters participate in sensing amino acid levels and thereby activate mTORC1, a master metabolic regulator that promotes cell metabolism, and induce the expression of c-Myc, a transcription factor essential for cell growth and proliferation. In this review, we discuss the regulatory pathways of these amino acid transporters and how we can take advantage of these processes to strengthen immunotherapy against cancer.

Keywords: SLC1A5; SLC3A2; SLC7A5; anti-tumor immunity; immunometabolism; natural killer cells; nutrient transporters; tumor microenvironment.

Figure 1

(A) Positive and (B) negative molecular regulations of SLC1A5, SLC3A2, and SLC7A5. SLC3A2, a type II membrane protein, dimerizes with the nutrient transporter SLC7A5 to allow their localization to the plasma membrane. SLC1A5, the sodium dependent antiporter exchanges neutral amino acids (NAA) such as threonine, asparagine or serine for glutamine and the SLC3A2/SLC7A5 heterodimer exchanges glutamine for essential amino acids (EAAs), most importantly L-leucine. While the mTOR signaling pathway regulates the expression of SLC1A5, SLC3A2, and SLC7A5, these proteins can activate mTOR in a feed-forward mechanism through the influx of essential amino acids (EAAs), especially leucine. (1) mTORC1 regulates the translation and stability of the mRNA encoding the transcription factor ATF4, which in turn controls the transcription of SLC1A5, SLC3A2, and SLC7A5. (2) mTORC1 and the influx of EAAs regulate the expression of the transcription factor c-Myc, which binds the promoters of SLC1A5 and SLC7A5 genes and upregulates their expression. (3) HIF-2α, a transcription factor that is activated in response to hypoxic conditions, binds to the promoter of SLC1A5 and SLC7A5 and activates their transcription. (4) YAP1 and TAZ bind to the promoter of SLC7A5 and activate its transcription. (5) The DNA- and RNA-binding protein YBX3 enhances the stability of SLC7A5 and SLC3A2 transcripts by binding directly to their 3’ UTRs and prevents their degradation. (6) Under nutrient stress, LLGL2 forms a trimeric complex with SLC7A5 and a regulator of membrane fusion, YKT6, allowing for the surface localization of SLC7A5 and thereby promoting cellular proliferation. (7) MARCH1 and MARCH8 ubiquitin ligases lead to the direct ubiquitination of SLC3A2 and its degradation by endosomes and lysosomes.

Figure 2

Potential strategies to utilize SLC1A5, SLC3A2, and SLC7A5 to enhance the metabolic fitness of NK and T cells and thereby strengthen anti-tumor immunotherapy. CAR-NK/T cells exhibit enhanced tumor recognition but are often metabolically disadvantaged in the TME. Upregulation of the SLC transporters in CAR-NK/T cells using the proposed tactics could enhance their function, survival, and persistence in vivo. Possible strategies are (1) to genetically engineer CAR- NK/T cells to overexpress these transporters, (2) to overexpress specific proteins that increase the expression of SLCs, (3) to knockdown genes that decrease the expression of SLCs, and (4) to engineer CAR-NK/T cells to co-express cytokines, specifically IL-15, IL-12, or IL-18.