Most cancer cells generate energy through a process known as "aerobic glycolysis," characterized by high glucose uptake and glycolysis, followed by lactic acid fermentation in the cytosol, rather than the oxidative phosphorylation and citric acid cycle observed in normal cells 13. Numerous human cancers, including glioblastoma, gastric cancer, pancreatic cancer, bladder cancer, and SCNC, exhibit elevated expression levels of PFKFB4, one of the four isoenzymes of phosphofructokinase 14. PFKFB4 regulates glycolysis in cells by generating the glycolytic signaling molecule F-2,6-BP and subsequently hydrolyzing it, thus limiting tumor development 15. However, an increasing number of studies have uncovered a connection between the occurrence, growth, and prognosis of specific malignancies and the overexpression of PFKFB4 in different tumors 16. For instance, patients with low PFKFB4 expression in gastric cancer had significantly longer overall survival (OS), initial progression survival, and post-progression survival durations compared to those with high PFKFB4 expression 17.
PFKFB4 plays a pivotal role in controlling the metabolic fluxes of the glycolytic and pentose phosphate pathways (PPP), which are the primary mechanisms tumor cells utilize to metabolize glucose. Studies have shown that PFKFB4 controls the formation of reactive oxygen species (ROS) by directing glucose metabolic intermediates to the PPP in various cancer cells 16,18,19. Moreover, in prostate cancer, PFKFB4 is known to decrease the levels of fructose-2,6-bisphosphate, redirect glucose-6-phosphate to the PPP, reduce the activity of glycolytic pathways, increase nicotinamide adenine dinucleotide phosphate (NADPH) and glutathione levels, and ultimately inhibit tumor cell proliferation. This capacity of prostate cancer cells to evade oxidative stress and cell death promotes tumor growth 20.
In addition to its role in glycolysis, glycolytic enzymes are now recognized to participate in various physiological and pathological processes 21. Notably, PFKFB4 expression was significantly higher in solid breast cancers compared to healthy tissues. Researchers have found that the protein kinase PFKFB4 phosphorylates the oncogenic steroid receptor coactivator-3 (SRC-3), enhancing its transcriptional activity and promoting the development of breast cancer 22. Furthermore, PFKFB4 is involved in regulating the cell cycle, autophagy, and tumor metastasis 23. Nevertheless, few studies have investigated the relationship between PFKFB4 and COAD.
This study has identified a potential correlation between COAD and PFKFB4 expression in clinical samples. The results indicate that COAD tissues exhibit significantly higher levels of PFKFB4 expression compared to adjacent normal tissues. Furthermore, patients with COAD in the sigmoid colon had considerably greater levels of PFKFB4 expression than those with COAD in other parts of the colon. This may be linked to the fact that sigmoid colon is the most common site of COAD, which is located in the right colon and tends to be more malignant 24. However, the investigation did not find any direct relationship between PFKFB4 expression and patient age, gender, tumor differentiation, size, depth of invasion, lymph node metastasis, or distant metastasis. This could be due to the inadequate samples.
The clinical samples collected in this study demonstrated a high expression of PFKFB4 in COAD patients. Regarding the prognosis of COAD patients, the corresponding data revealed that high expression of PFKFB4 was associated with improved OS. This finding suggests that COAD patients with high PFKFB4 expression may have better survival rates after treatment, indicating a positive correlation with prognosis before disease progression.
However, intriguingly, the data also showed that PPS was worse in COAD patients with high PFKFB4 expression. This suggests that after disease progression in COAD patients, high expression of PFKFB4 may be linked to shorter survival and an unfavorable prognosis. This finding implies that high expression of PFKFB4 might play a role in promoting the late progression of COAD.
To delve deeper into this seemingly contradictory result, we first conducted gene enrichment analysis of the co-expressed genes associated with PFKFB4 using the CAMOIP database, followed by functional annotation and enrichment analysis. This analysis revealed several PFKFB4-related pathways in COAD, including up-regulation of granulocyte activation, myeloid leukocyte activation, inflammatory response, biosynthesis of amino acids, fructose and mannose metabolism, glycolysis, gluconeogenesis, and glucose metabolism. These findings suggest that COAD patients with high PFKFB4 expression may have a higher tendency to utilize the glycolytic pathway for metabolic biosynthesis, which could be associated with enhanced immunological responses and increased immune infiltration.
Proverbially, the tumor microenvironment (TME) influences the development and recurrence of tumors, and immune cells in TME have been found to either promote or prevent tumor growth 25,26. Moreover, immune infiltration, a key element of the TME, has been shown to impact tumor development and response to therapy 27. Using the TIMER2.0 and CAMOIP databases, we observed a robust correlation between PFKFB4 expression and the levels of infiltration of various immune cells, including CD8 + T cells, CD4 + T cells, regulatory T cells (Tregs), macrophages, neutrophils, dendritic cells, activated mast cells, and resting NK cells. This suggests that PFKFB4 may also be a marker for immune status and provide information for immunotherapies.
Accordingly, we speculate that the relationship between high expression of PFKFB4 and prognosis of COAD patients is complex and diverse, and it may play different roles in different stages of COAD, leading to the seemingly contradictory results between OS and PPS. This seemingly paradoxical result may be due to the complex biological function of PFKFB4 in COAD. High expression of PFKFB4 may be beneficial to inhibit the proliferation of tumor cells in the initial stage, thereby associated with better OS. However, when the disease progresses to an advanced stage, PFKFB4 may begin to play a role in promoting tumor progression, resulting in a lower prognostic value for post-progression survival. These results may involve a variety of metabolic pathways such as biosynthesis of amino acids, glycolysis, gluconeogenesis, glucose metabolism, and inflammatory response, and a variety of infiltration of immune cells in the body, such as CD8 + T cells, CD4 + T cells, Tregs, macrophages, neutrophils, dendritic cells, active mast cells, and resting NK cells. They work together to produce a complex diversity of the relationship between PFKFB4 expression and the prognosis of COAD patients.
Unfortunately, our study has some limitations. Firstly, our clinical data might be affected by sampling errors due to the limited sample size. Secondly, although we used the TCGA database for additional analysis to address the issue of inadequate sample size, the amount of PFKFB4 gene samples in COAD remains insufficient. Therefore, more research is needed to validate our findings and explore further the molecular linkages, clinical applications, and mechanisms of PFKFB4 in COAD.