3.1 Differential expression of PFKFB3 in normal tissues and various cancers
In order to explore the normal expression of PFKFB3 in various organs of the human body, the analysis results from the Human Protein Atlas database showed that the expression of PFKFB3 was higher in human lung, female reproductive system, kidney, and pancreas tissues than in other tissues (Fig. 1A), indicating its crucial role in maintaining the normal physiological function of these organs. To explore the changes in PFKFB3 expression in various tumor tissues and its possible pathological role, the TIMER2.0 database was conducted to analyze the differential expression of PFKFB3 between TCGA cancers and normal controls. Except for some cancers for which normal tissue data were not available, significant differences in PFKFB3 expression were detected between normal tissues and the other 15 cancer types. Results showed that PFKFB3 expression was high in some types of cancer tissues, including cholangial carcinoma (CHOL), COAD, HNSC, KIRC, PAAD, STAD, and THCA, and decreased in bladder urothelial carcinoma (BLCA), BRCA, diffuse large B-cell lymphoma (DLBC), kidney chromophobe (KICH), KIRP, liver hepatocellular carcinoma (LIHC), LUAD, LUSC, PRAD, and thymoma (THYM) cancers (Fig. 1B left, all p < 0.05).
Analysis of the GEPIA2 database revealed that PFKFB3 expression was higher in CHOL and PAAD cases, but lower in BRCA, diffuse large B-cell lymphoma (DLBC), KICH, KIRP, and thymoma (THYM) tumors. However, there was no statistically significant difference between the other tumors and the corresponding normal tissues (Fig. 1B, right and C).
To confirm the alteration of PFKFB3 expression at the protein level, the immunohistochemistry (IHC) analysis retrieved from the HPA was performed and was consistent with the gene transcript levels shown in Fig. 1. That is, PFKFB3 staining intensity was higher in COAD, TGCT, THCA, and CESC tumor samples but lower in KIRP, BRCA, PRAD, LUAD, and LUSC tumors than in the corresponding normal tissues (Fig. 2).
3.2 PFKFB3 expression is associated with the staging of tumors
To explore the relationship between PFKFB3 expression and pathological stages in different cancers, results from the CPTAC (Fig. 3A) and GEPIA2 (Fig. 3B) databases revealed a significant positive correlation between PFKFB3 expression and pathological stages of COAD, STAD, THCA, CESC, TGCT, and PAAD tumors, whereas there was a negative correlation between PFKFB3 and pathological stages of KIRP and ovarian serous cystadenocarcinoma (OV) (Fig. 3A, * p < 0.05). These results indicated that PFKFB3 played an important role in the progression of various tumors.
3.3 The impact of PFKFB3 expression on the survival rate
To explore whether the expression of PFKFB3 is correlated with the survival rate of patients with different tumors. The GEO and TCGA databases were used to investigate the association between PFKFB3 expression and survival prognosis in various cancer cases. The results in Fig. 4A show that high expression of PFKFB3 in ACC, KIRP, and STAD tumors and lower expression of PFKFB3 in KIRC tumors are closely related to unfavorable OS prognosis of patients. DFS represents the interval between surgery and the first diagnosis of any type of relapse or death (28). Results from Fig. 4B indicated that high expression of PFKFB3 was correlated with poor DFS time in ACC, COAD, SARC, and UVM cancers, while lower expression of PFKFB3 was correlated with poor DFS in KIRC patients. Collectively, these results revealed that the different expression patterns of PFKFB3 were significantly related to the prognosis and survival of patients in a cancer-dependent manner.
In order to further clarify the correlation between PFKFB3 expression and various survival outcomes, the Cox analysis was performed using the R packages “survival” and “forestplot” to illustrate the pan-cancer relationship between PFKFB3 association and survival. The results in Fig. 4C demonstrate that PFKFB3 expression was significantly correlated with OS in patients with KIPA, LIHC, KIRP, CESC, and ACC cancers (all p < 0.05), and was also correlated with DFS in patients with KIRP, CESC, ACC, and KIPAN (all p < 0.05). In addition, PFKFB3 expression was closely correlated with DFI in ACC patients (all p < 0.05) and correlated with PFI in ACC, UVM, KIPAN, SARC, and COAD patients (all p < or = 0.05). These results implied that PFKFB3 is significantly associated with the survival and prognosis of patients with a variety of tumors, indicating that PFKFB3 is expected to be a prognostic marker and potential therapeutic target of tumors.
3.5 PFKFB3 promoted immune infiltration among various cancers
The poor prognosis and survival rate of tumors are closely related to their migration and invasion ability. Therefore, we performed correlation analysis between PFKFB3 expression and tumor invasion and migration ability or immune infiltrating cell types. It has been reported that high expression of PFKFB3 is associated with tumor invasion and metastasis (7, 29–32). However, most of these studies were limited to laboratory studies, and the tumor types were relatively limited. In this study, we determined the immune invasion scores for 10,179 tumor samples out of 44 tumor types using biological information analysis, and then calculated the Pearson's correlation coefficient between PFKFB3 and immune infiltration scores in each tumor using the CorR. test function of the R software package Psych (version 2.1.6) to determine significantly correlated immune infiltration scores. A significant positive correlation was found between the expression of PFKFB3 and immune infiltration in 31 cancer species (Fig, 5A), for instance, glioblastoma and low-grade glioma (GBMLGG) (N = 656, p < 0.001), lower-grade glioma (N = 504, p < 0.001), BRCA (N = 1077, p < 0.001), LUAD (N = 500, p < 0.001), esophageal carcinoma (ESCA) (N = 181, p < 0.05), STES (N = 569, p < 0.05), SARC (N = 258, p < 0.01), KIRP (N = 285, p < 0.001), KIPAN (N = 878, p < 0.001), COAD (N = 282, p < 0.001), rectum adenocarcinoma (READ) (N = 373, p < 0.001), PRAD (N = 495, p < 0.001), STAD (N = 388, p < 0.001), HNSC (N = 517, p < 0.001), KIRC (N = 528, p < 0.001), LUSC (N = 491, p < 0.001), THYM (N = 118, p < 0.001), LIHC (N = 363, p < 0.001), skin cutaneous melanoma (SKCM) (N = 452, p < 0.05), BLCA (N = 405, p < 0.001), TCGA THCA (N = 503, p < 0.001), TARGET - neuroblastoma (N = 153, p < 0.001), mesothelioma (MESO) (N = 85, p < 0.001), TCGA - READ (N = 91, p < 0.001), OV (N = 416, p < 0.01), UVM (N = 79, p < 0.05), PAAD (N = 177, p < 0.001), acute myeloid leukemia (LAML) (N = 214, p < 0.001), PCPG (N = 177, p < 0.001), DLBC (N = 46, p < 0.05), KICH (N = 65, p < 0.001). In summary, the present results confirmed that high expression of PFKFB3 significantly promoted the invasion of multiple tumors, except LAML and SKCM. Therefore, controlling the high expression of PFKFB3 is expected to be an effective strategy for inhibiting tumor invasion.
According to the method described in section 2.5, we obtained six types of immune cell (B cell, CD4+ T cell, CD8+ T cell, Neutrophil, Macrophage and Dendritic cell) infiltration scores of 9,405 tumor samples out of 38 tumor types. Pearson's correlation coefficient between the PFKFB3 gene and immune cell infiltration scores for each tumor was calculated using the CorR. test function of the R software package Psych (version 2.1.6). As shown in Fig. 5B showed that the expression of PFKFB3 was significantly correlated with the infiltration of various immune cells in 38 cancer species, which might be a key factor leading to the poor prognosis of tumor survival.
3.6 Increased phosphoPFKFB3 was detected in certain cancers
Studies have shown that the phosphorylation of PFKFB3 is potentially associated with the progression of cancer and angiogenesis (4, 33–35) and remains a feature of various cancers (36). Hence, we compared the phosphorylation levels of PFKFB3 protein between primary tumor tissues and their normal controls using the CPTAC dataset (Fig. 6A). PFKFB3 phosphorylation was significantly increased in four types of cancers (LUAD, KIRC, PAAD, and HNSC) compared to the corresponding normal samples. The phosphor-PFKFB3 in GBM cells was also higher than that in the normal group, but the difference was not statistically significant.
3.7 Correlation between PFKFB3 expression and tumor stemness or RNA modification
Cancer progression is characterized by the progressive loss of the differentiated phenotype and acquisition of progenitor and stem cell-like features (24). The study of the correlation between PFKFB3 expression and the stemness of tumor cells remains an important step in the functional exploration of PFKFB3. Thus, we calculated Pearson correlations between PFKFB3 expression levels and tumor stemness in each tumor. The results illustrated that there was a significant correlation between the stemness of 14 tumors and PFKFB3 expression, including a significant positive correlation in 2 tumors, i.e., THYM (N = 119, p < 0.001), and THCA (N = 499, p < 0.001), and a significant inverse association in 12 tumors, including GBMLGG (N = 558, p < 0.05), LUAD (N = 451, p < 0.01), READ (N = 358, p < 0.01), BRCA (N = 774, p < 0.05), ESCA (N = 179, p < 0.05), STAD (N = 369, p < 0.05), LIHC (N = 366, p < 0.001), PAAD (N = 156, p < 0.001), TGCT (N = 147, p < 0.01), PCPG (N = 176, p < 0.05), BLCA (N = 403, p < 0.001), and CHOL (N = 36, p < 0.05) (Fig. 6B).
Specific chemical modifications of various biomolecules occur during tumorigenesis, and these modifications are effective in regulating molecular functions. Currently, many enzymes responsible for regulating protein and DNA modifications are targets for cancer therapy. However, RNA modifications (including m1A, m5C, m6A) are now becoming a new frontier in this field (37). Increasing evidence suggests that RNA modification pathways are improperly regulated in human cancers, suggesting that it is an ideal target for cancer therapy (38). The key enzymes or binding proteins involved in RNA modification, named as writers, erasers, and readers, catalyze, eliminate, or recognize these modifications after translation (39). Therefore, in this study, we also explored the correlation between PFKFB3 expression in various tumors and the types of modifying enzymes in the process of RNA modification to understand the pathological functions of PFKFB3 and its association with tumor progression. The results shown in Fig. 6C revealed that the high expression of enzymes involved in RNA modification was significantly associated with PFKFB3 expression in almost 40 types of cancers.
3.8 PFKFB3 involved multiple protein interactions and molecular functions
The GeneMANIA database was used to create a protein-protein interaction (PPI) network for PFKFB3 (Fig. 7A). GO and KEGG analyses were performed to further investigate the function of PFKFB3 and its closely associated genes. The ADP metabolic process, pyruvate biosynthetic process, ATP production from ADP, and glycolytic process were the most abundant biological processes (BP) involved in the gene set according to GO analysis. Intramolecular transferase activity, intramolecular transferase activity, phosphotransferases, sugar-phosphatase activity, and carbohydrate phosphatase activity were connected to the enriched molecular functions (Fig. 7B). Furthermore, the glucagon signaling pathway, central carbon metabolism in cancer, and glycolysis/gluconeogenesis were the main enriched pathways according to the KEGG analysis (Fig. 7C). Therefore, the dysregulation of PFKFB3 and its related genes in the tumor environment may significantly alter various cellular metabolic processes, suggesting that PFKFB3 remains a vital factor involved in Warburg effect in tumors, and further promoting the progression of tumors.
3.9 Functional enrichment directed PFKFB3 into the pathways of kinase regulator activity
To explore the genes correlated with PFKFB3 expression in most tumors, the top 20 genes with the most similar expression pattern to PFKFB3 in a variety of tumors were detected based on GEPIA (Fig. 8A). These results suggest that they may act synergistically to promote the occurrence and progression of tumors. KEGG analysis showed that most of these genes were enriched in kinase regulator activity pathways (Fig. 8B). In addition, the expression of PFKFB3 in all tumors was positively correlated with each gene (Fig. 8C).
3.10 High mutation of PFKFB3 was detected in cancers
Mutations in PFKFB3 in various cancers were analyzed using the cBioPortal platform. A total of 39,532 patients from the PanCancer Studies database were analyzed. Amplification of PFKFB3 accounted for the largest proportion of all mutation types, of which bladder squamous cell carcinoma, ovarian epithelial tumor, breast invasive ductal carcinoma, and tubular STAD had the highest occurrence rates of 20%, 20%, 15.08%, and 12.15%, respectively (Fig. 9).