UGDH plays a key role in xenobiotic metabolism via the glucuronidation pathway, sugar metabolism, production of ECM precursors, and proteoglycan synthesis, which suggests that it may be a potential therapeutic target for a variety of diseases [21, 6]. Previous studies have shown that the expression and localization of UGDH is an early serum diagnostic marker for lung cancer patients, as well as a prognostic indicator [17]. UDPGA was shown to be a precursor for the synthesis of glycosaminoglycans and proteoglycans that promote the progression of invasive prostate cancer, and the UGDH content in the prostate tip serves as a new candidate biomarker [13]. The above suggests that UGDH may become a new diagnostic marker in clinical practice.
Previous studies have shown that UGDH is positively associated with the development of epirubicin resistance and regulation of the ECM in breast cancer [15]. Upregulation of UGDH has been associated with metastasis in ovarian cancer, where it regulates tumor-initiating cells affecting the tumor microenvironment [12]. And it may be related to melanoma development and progression [22]. Therefore, UGDH has a certain prognostic predictive value. Regarding common clinicopathological parameters of tumors, UGDH expression existed gender difference, pathologic T and M stage in LUAD. Past research has also shown that phosphorylation of UGDH at tyrosine 473 is associated with metastatic recurrence and poor prognosis in lung cancer patients [8]. Elevated UGDH expression correlated with male gender, vascular invasion, and histologic grade in LIHC. Investigations have shown that UGDH-mediated activation of UDPGA activates TGFβ/Smad signaling thereby promoting the migration of hepatocellular carcinoma cells [18], and it is associated with sorafenib resistance [9]. Moreover, the UGDH was correlated with age and cytogenetic risk in LAML, pathologic T stage in SKCM, age in SARC, pathologic stage and gender in KIRP, and histologic grade and pathologic T and M stage in KIRC.
UGDH expression differed between various molecular or immune subtypes of cancers, with differences in immune subtypes present in 30 cancer types and differences in molecular subtypes present in 17 cancers. Except for LAML, the cancer types with differences in UGDH expression and immune subtypes were consistent with the cancer types whose expression affected survival prognosis, suggesting that UGDH may intervene in cancer prognosis by affecting immune subtypes. Moreover, in subsequent studies, we focused on analyzing the role of UGDH expression on immunomodulatory factors and major histocompatibility complex molecules and targeting immune lymphocytes. However, it has also been pointed out that genes are aberrantly expressed in specific subtypes of cancer, and the results may not be reflected in the overall patients of that cancer. Thus, differences in gene expression do not affect cancer survival in certain cancers, whereas its varied expression in different molecular or immune subtypes might play different roles in the prognosis of various cancers [23]. Genetic mutations could induce abnormal gene expression and tumorigenesis. We found that UGDH mutation sites mainly include missense, truncating, inframe, splice, and fusion in the TCGA pan-cancer Atlas study. UGDH has undergone genetic alteration in 24 types of cancer, and the cancer types with alteration frequency greater than 2% include UCEC, CHOL, STAD, ACC, and LUAD. In most cancers, the main type of UGDH gene alteration is mutation, followed by amplification. It is worth noting that the UGDH gene abnormalities in LICH are mainly amplification.
To further explore the biological functions of UGDH, we established a PPI network and identified hub genes, and we showed the relationship between hub genes and cancer types in which UGDH expression affects the prognosis of cancer. For the hub genes obtained, GALE encodes UDP-galactose-4-epimerase. Previous studies have shown that it is associated with the differentiation grade of gastric cancer [24] promotes the proliferation and migration of glioblastoma cells [25], and is a potential marker for papillary thyroid carcinoma [26, 27]. The other hub genes belong to the UDP glucuronosyltransferase family, which encodes UDP- glucuronosyltransferase, an enzyme of the glucuronidation pathway that transforms small lipophilic molecules, such as steroids, bilirubin, hormones, and drugs, into water-soluble, excretable metabolites [28]. However, these enzymes have different preferences for substrates, and the preferred substrate of UGT1A1 is bilirubin. Substrates of UGT1A3 include estrone, 2-hydroxy estrone, and metabolites of benzo alpha-pyrene. Although UGT1A4 is more active on amines, steroids, and sapogenins. UGT1A6, UGT1A7, and UGT1A9 are active on phenols. UGT1A8 and UGT1A10 have glucuronidase activity on drugs such as coumarins [29]. Meanwhile, we performed GO and KEGG analysis of UGDH and its related genes. The most prominent BP enrichment result was cellular glucuronidation, followed by xenobiotic glucuronidation. For the MF analysis results, the top rank was glucuronosyltransferase activity, followed by UDP-glycosyltransferase activity. The CC enrichment results mainly included the endoplasmic reticulum membrane and its membrane, an integral component of membrane and intracellular membrane-bounded organelle. Whereas KEGG analysis was enriched to the most significant pathway was pentose and glucuronate interconversion, suggesting a high degree of correlation between the results of the GO enrichment analysis. It is known that UDP- glucuronosyltransferase is localized to the endoplasmic reticulum, and the active transport of the metabolite UDPGA involved in UGDH is thought to occur in hepatocytes [30], so the role of UGDH in hepatocellular carcinoma should be taken seriously.
Through CancerSEA analysis results, the expression of UGDH exhibited different functional states at the single-cell level in various cancers. The expression of UGDH is generally positively correlated with cell apoptosis, invasion, and proliferation, while negatively correlated with cell cycle. GSEA functional enrichment analysis was applied to cancer types in which UGDH expression affects overall prognosis. Analysis results were repeated more than twice and were considered common enrichment pathways. Most of the common enrichment pathways are immunologically related. Immunoregulatory interactions between a lymphoid and a nonlymphoid cell pathway participated adaptive Immune system [31]. The two pathways related to B cells analyzed are antigen activates B cell receptor BCR leading to generation of second messengers and CD22 mediated BCR regulation pathway, CD22 is an inhibitory B cell coreceptor that regulates B cell development and activation by downregulating BCR signaling through activation of protein tyrosine phosphatase-1 [32, 33]. There are also 3 pathways related to complement, namely Initial triggering of complement, complement cascade, and Creation of C4 and C2 activators. Creation of C4 and C2 activators pathway participated initial triggering of complement. Current research shows that the complement system is one of the inflammatory mechanisms activated in the tumor microenvironment, beside exerting anti-tumor mechanisms such as complement-dependent cytotoxicity and phagocytosis induced by therapeutic monoclonal antibodies, the complement system may promote immunosuppression and tumor growth and invasiveness [34, 35]. The analysis results also included Fc-gamma receptors (FCGR) activation and the Role of LAT2 Non-T cell activation linker (NTAL) lab on calcium mobilization pathway, the research suggests that FCGR are expressed on immune cells, bind to antibodies, and trigger antibody-induced cell-mediated antitumor responses when tumor-reactive antibodies are present [36]. The lipid raft resident adaptor molecules LAT1 and NTAL, also known as linkers for activation of B cells (LAB)/LAT2 are participants in the regulation of mast cell calcium responses [37]. In addition, the role of phospholipids in phagocytosis, scavenging of heme from plasma, and formation of the cornified envelope role in tumors still needs further investigation.
To observe the immune status of UGDH and pan-cancer in-depth, we assessed the impact of UGDH on immune infiltration by analyzing its correlation with immune lymphocytes and immunomodulatory factors in the 33 cancers. UGDH expression was positively correlation with T helper cells, Tcm, and Th2 cells in most cancers, and negatively correlated with pDC and Cytotoxic cells. Primary T cells become effector T cells and memory T cells after being activated. Effector T cells include cytotoxic T cells and helper T (Th) cells. T cells are activated by dendritic cells (DC) and differentiate into various subtypes of Th cells, including Th2 cells [38]. Th2 cells induce an immunosuppressive protumorigenic response. Th2 cell infiltration in the tumor microenvironment is commonly associated with poor clinical outcomes in human cancers [39]. The Research that Tcm cells conferred superior anti-tumor immunity compared with effector memory T cells and effector T cells in CAR-T therapy [40]. For other immune characteristics, UGDH preferred some correlation with the type of cancer. Thus, the role of UGDH in cancer immunity is complex, and its positive correlation with Th2 cells and negative correlation with cytotoxic cells might be associated with poor prognosis of some tumors.
Finally, we experimentally observed the expression and effect of UGDH in HCC. Compared with the normal group, the expression of UGDH was elevated in the liver tissue of HCC rats. Our previous experimental results also showed that the key genes AKR1B10 involved in the pentose and glucuronate interconversions pathway was consistent with the expression pattern of UGDH [41]. Subsequently, we knocked down the expression of UGDH in Huh7 cell lines and found that the knockdown of UGDH could inhibit the cell viability and in vitro tumourigenic ability of Huh7 cells. Of course, this study has some limits and did not explore the role of UGDH in more tumor types.