Metabolic changes are a hallmark of cancer, supporting the rapid proliferation and survival of cancer under various stress conditions. Some metabolic activities have important detoxification functions, which prevent excessive accumulation of toxic metabolic intermediates. There is a "kitchen sink" model in cancer cells, where detoxification enzymes are needed only when the content of toxic metabolites in the cell increases. Therefore, targeting metabolic pathways that are necessary for cancer cell survival but dispensable in normal cells has important therapeutic potential. But identifying these metabolic pathways remains a challenge.
Fig. 1 Kitchen sink model. (Lee, et al., 2020)
On October 25, 2023, the team of Professor Dohoon Kim of the University of Massachusetts published an article in Nature titled "Disruption of sugar nucleotide clearance is a therapeutic vulnerability of cancer cells". The study reported a new idea for treating cancer by targeting the Sugar Nucleotide Biosynthesis pathway, which targets only cancer cells and does not affect normal cells.
The Enzyme UXS1 is a detoxifier that is only required in cells with high expression of UGDH. UXS1 not only removes UDPGA, a product of UGDH, but also inhibits UDPGA production by negatively feedbacking UGDH. Excessive UDPGA can disrupt the morphology and function of the Golgi apparatus, hinder the translocation of surface receptors such as EGFR to the plasma membrane, and thus reduce the cell's signal transduction ability. UGDH is expressed at elevated levels in several cancers, including lung adenocarcinoma, and its expression is further enhanced during the selection of chemotherapy resistance. Therefore, these cancer cells selectively rely on UXS1 to detoxify UDPGA, revealing that targeting UXS1 can be used to treat cancers with high UGDH expression.
First, to find metabolic pathways that are essential for cancer cell survival but dispensable in normal cells, the researchers used the Cancer cell line-dependent DEPMAP database to mine metabolic enzymes of varying importance that are required for the survival of some cancer cell lines but not others. Based on the requirement for cell survival, candidate detoxification enzymes were identified in cells that produce high levels of substrates, i.e., enzyme E2 is only required in cells that highly express enzyme E1, and E2 is required to clear toxic metabolites produced by E1. The results showed that for UXS1, one of the essential enzymes, its immediately upstream gene UGDH was the gene most associated with UXS1 demand. UGDH produces UDPGA, which acts on the substrate of the glycosylation reaction and binds to various foreign substances to excrete them from the cell. UXS1 can convert UDPGA into the sugar nucleotide xylose. Therefore, the researchers speculated that UDPGA is a toxic metabolite, and only cells that express high levels of UGDH and produce a large amount of UDPGA will rely on UXS1 for detoxification. To confirm this hypothesis, the researchers knocked out UXS1 (UXS1 KO) in 19 cancer cell lines of different tissue origins and different UGDH mRNA expression levels, and found that knocking out UXS1 was only harmful to cell lines expressing high levels of UGDH, and UXS1 KO was targeted. The researchers also constructed a doxycycline (dox)-induced UXS1 knockout cell line (iKO), and the results showed that knocking out UXS1 led to cell cycle defects and apoptosis. The above studies show that UXS1 knockout only selectively affects cancer cells with high UGDH expression levels, so the researchers used the enzyme UXS1 as a candidate antidote.
Next, the researchers analyzed two potential mechanisms of UXS1 KO toxicity. First, the accumulation of UXS1 substrates may be toxic; second, the loss of UXS1 downstream products may be harmful. To this end, the researchers found that UXS1 knockout caused an approximately 70-fold accumulation of UDPGA in the cells in a time-dependent manner, while several other UDP sugars were not substantially affected. Tracking with labeled [U-13C] glucose confirmed that UXS1 KO caused a complete abolition of UDP-xylose formation. To test the kitchen sink model, the researchers disrupted UGDH to prevent the production of UDPGA, which completely protected A549 and DLD1 cells from the toxic effects of UXS1 knockout. Similarly, treatment with 4MU, a drug that depletes UDPGA, also rescued these cells from UXS1 knockout toxicity. Overexpression of UGDH made cells with low UGDH expression and insensitive to UXS1 KO toxicity sensitive. In UXS1 KO cells, the rate of UDPGA production was greatly increased with the loss of UDP-xylose production, which means that the activity of UGDH was increased. The UDPGA accumulation observed in UXS1 KO cells is a result of the synergistic effect of the lack of UDPGA clearance by UXS1 and higher UGDH activity. The above studies indicate that UXS1 can prevent the accumulation of toxic UDPGA.
Fig. 2 UXS1 is required to detoxify the UDPGA produced by UGDH. (Doshi, et al., 2023)
Next, the researchers explored the mechanism of UDPGA accumulation on cytotoxicity after UXS1 loss. UXS1 KO induced the expression of genes related to Golgi function, as well as downregulated cell cycle and DNA damage repair response genes. UXS1 loss significantly altered the morphology of the Golgi, resulting in abnormal diffusion of cis, trans, and intermediate components of the Golgi to multiple regions throughout the cell. Depletion of UDPGA using 4MU or UGDH KO rescued the Golgi morphology in UXS1 KO cells, indicating that the change in Golgi morphology was a result of UDPGA accumulation. Analysis of the N-linked and O-linked glycosylation profiles of A549 cells after UXS1 loss revealed an overall change in the Glycosylation pattern. The researchers examined various cell surface receptors, EGFR, CD44, FGFR1, FGFR4, and IGF1R, for glycosylation defects and found that UXS1 loss led to increased gel migration of all these receptors, indicating defective glycosylation, suggesting that UXS1 loss-induced Golgi dysfunction hinders the normal maturation of important cell surface glycoproteins. Knockout of UXS1 resulted in the loss of EGFR on the cell membrane and a decrease in overall expression levels. In addition, the researchers also tested the ability of UXS1 iKO cells to respond to the EGF mitogen. Cells lacking UXS1 cannot respond correctly to EGF and cannot autophosphorylate EGFR. The above studies show that the accumulation of UDPGA after UXS1 loss changes the structure and function of the Golgi apparatus.
Finally, the researchers explored the effect of UXS1 loss on tumors in vivo. UXS1 KO in three subcutaneous xenograft models was induced using dox. For two UGDH-high-expressing cell lines, A549 and H460, UXS1 loss led to tumor growth retardation and significantly prolonged mouse survival. In the UGDH-low-expressing cell line HT1080, UXS1 loss had no effect on tumor growth and overall survival. The above data show that targeting UXS1 has tumor therapeutic potential, and its effect depends on the high UGDH status of the tumor.
By comparing tumors and adjacent normal tissues, the researchers observed elevated UGDH in lung and breast cancer, indicating that targeting UXS1 would damage cancer cells without harming normal cells, and low UGDH levels in non-cancerous cell lines also made them insensitive to UXS1 KO. Increased UGDH expression is associated with increased resistance to many drugs, including doxorubicin, paclitaxel, and gemcitabine. Therefore, the researchers studied whether exposure to chemotherapeutic drugs induces UGDH and found that resistant cell lines showed high expression of UGDH and increased sensitivity to UXS1 KO compared with the original cell lines. In the A549 xenograft model, chemotherapy induced high expression of UGDH in tumors and synergized with UXS1 iKO to inhibit tumor growth. The above studies show that chemotherapy-resistant subpopulations of cancer cells or cancer cells treated with chemotherapeutic drugs can induce high expression of UGDH, and targeting UXS1 may be harmful to these cancer cells and can be used as a cancer treatment target.
This study found that UXS1 clearance of UDPGA is a key condition for some cells to maintain Golgi homeostasis, and only cells with high expression of UGDH need this detoxification function. Compared with healthy cells, UGDH expression is elevated in multiple types of cancer cells, and only in these cancer cells, disruption of UXS1 can lead to abnormal Golgi morphology and glycosylation defects, leading to cell death. Based on this, UXS1 can be used as a new selective cancer therapeutic target, but the therapeutic effect may vary between different tumor types and individual patients, and the safety and effectiveness of clinical applications need to be further evaluated. In addition, this study also revealed an unexpected connection between the sugar nucleotide metabolic pathway and the regulation of signal transduction processes: Golgi dysfunction caused by UXS1 damage and UDPGA accumulation also impairs the maturation of cell surface Glycoproteins. The importance of this "loophole" in the sugar nucleotide metabolic pathway to cancer treatment also suggests its application prospects in downregulating extracellular signal transduction.
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