System xc-, the cystine/glutamate exchanger, is a membrane transporter that plays a critical role in the antioxidant response of cells. Recent work has shown that System xc- localizes to the plasma membrane during oxidative stress, allowing for increased activity to support production of glutathione. In this study, we used site-directed mutagenesis to examine the role of C-terminal lysine residues (K422, K472, K473) of xCT (SLC7A11) in regulating System xc-. We observed that K473R exhibits loss of transporter activity and membrane localization and is 7.5 kD lower in molecular weight, suggesting that K473 regulates System xc- trafficking and is modified under basal conditions. After ruling out ubiquitination and neddylation, we demonstrated that unlike WT xCT, K473R lacks N- and O-glycosylation and is sequestered in the endoplasmic reticulum. Next we demonstrated that K473Q, a constitutively acetylated lysine mimic, also exhibits loss of transporter activity, decreased membrane expression, and a 4 kD decrease in molecular weight, however, it is N- and O-glycosylated and localized to the endoplasmic reticulum and Golgi. These results suggest that acetylation and deacetylation of K473 in the endoplasmic reticulum and Golgi, respectively, serves to regulate the progression of the transporter through the biosynthetic pathway.

Objective: Pancreatic carcinoma (PANC) is one of the important aggressive cancers, with deficiency in effective therapeutics. Studies have unveiled that miR-139-5p expression is significantly downregulated in other types of cancers. However, the functions and mechanisms of miR-139-5p in PANC remain unclear. Methods: Bioinformatic analysis was performed to analyze the differentially expressed genes in the TCGA database. PANC cell line with overexpressed miR-139-5p and Solute Carrier Family 7, Member 11 (SLC7A11) was established, and has been used to detect cell proliferation, migration, invasion and colony formation in PANC. Subsequently, bioinformatic analysis and luciferase assay were performed to confirm that SLC7A11 was a target gene of miR-139-5p. Xenograft mouse model was used to investigate the role of miR-139-5p in PANC tumorigenicity. Results: Through bioinformatic analysis, miR-139-5p was predicted to regulate phosphatidylinositol signaling pathway by targeting SLC7A11. MiR-139-5p was found to be lowly expressed in PANC tissues, while SLC7A11 was highly expressed. Low expression of miR-139-5p and high expression of SLC7A11 were positively associated with poor clinical outcomes. PANC cell proliferation, migration, and invasion could be inhibited by miR-139-5p overexpression and could be promoted by SLC7A11 overexpression. MiR-139-5p could regulate the protein expression level of PI3K and Akt associated with phosphatidylinositol signaling pathway could be by inhibiting the expression of SLC7A11. MiR-139-5p overexpression could suppress PANC tumor growth and the expression of SLC7A11, p-PI3K, p-Akt in tumor tissues. Therefore, the inhibiting effect of miR-139-5p to PANC cell proliferation, invasion and migration, at least, was partly due to its inhibiting effect on SLC7A11 expression. Conclusion: These results demonstrated a novel role of miR-139-5p/SLC7A11 in PANC and provided potential prognostic predictors for PANC patients.

The amino acid transporters SLC38A5 and SLC7A11 are upregulated in triple-negative breast cancer (TNBC). SLC38A5 transports glutamine, methionine, glycine and serine, and therefore activates mTOR signaling and induces epigenetic modifications. SLC7A11 transports cystine and increases cellular levels of glutathione, which protects against oxidative stress and lipid peroxidation via glutathione peroxidase, a seleno (Se)-enzyme. The primary source of Se is dietary Se-methionine (Se-Met). Since SLC38A5 transports methionine, we examined its role in Se-Met uptake in TNBC cells. We found that SLC38A5 interacts with methionine and Se-Met with comparable affinity. We also examined the influence of Se-Met on Nrf2 in TNBC cells. Se-Met activated Nrf2 and induced expression of Nrf2-target genes, including SLC7A11. Our previous work discovered niclosamide, an antiparasitic drug, as a potent inhibitor of SLC38A5. Here we found SLC7A11 to be inhibited by niclosamide with an IC50 value in the range of 0.1-0.2 M. In addition to the direct inhibition of SLC38A5 and SLC7A11, pretreatment of TNBC cells with niclosamide reduced the expression of both transporters. Niclosamide decreased glutathione levels, inhibited proliferation, suppressed GPX4 expression, increased lipid peroxidation, and induced ferroptosis in TNBC cells. It also significantly reduced the growth of the TNBC cell line MB231 in mouse xenografts.

Mutations in p53 and KRAS are seen in most cases of colon cancer. The impact of these mutations on signaling pathways related to cancer growth has been studied in depth, but relatively less is known on their effects on amino acid transporters in cancer cells. This represents a significant knowledge gap because amino acid nutrition in cancer cells profoundly influences macropinocytosis and ferroptosis, two processes with opposing effects on tumor growth. Here we used isogenic colon cancer cell lines to investigate the effects of p53 loss and KRAS activation on two amino acid transporters relevant to macropinocytosis (SLC38A5) and ferroptosis (SLC7A11). Our studies show that the predominant effect of p53 loss is to induce SLC7A11 with resultant potentiation of antioxidant machinery and protection of cancer cells from ferroptosis whereas KRAS activation induces not only SLC7A11 but also SLC38A5, thus offering protection from ferroptosis as well as improving amino acid nutrition in cancer cells via accelerated macropinocytosis. Niclosamide, an FDA-approved anti-helminthic, blocks the functions of SLC7A11 and SLC38A5, thus inducing ferroptosis and suppressing macropinocytosis with resultant effective reversal of tumor-promoting actions of oncogenic mutations in p53 and KRAS. These findings underscore the potential of this drug in colon cancer treatment.