Prostate cancer (PCa) is an epithelial malignancy with a high incidence that occurs in the male genitourinary system (1). In recent years, the morbidity and mortality of PCa have increased dramatically worldwide. Based on the last data, the morbidity of PCa ranks no. 1, and the mortality ranks no. 2 among male malignant tumors in 112 countries (2). In 2020, there were 1.4 million new PCa cases and 370,000 deaths globally (2). The treatment options of PCa vary based on cancer grade and stage. For example, surgery is the standard treatment for early PCa, and it can lead to a favorable prognosis. Therefore, early and accurate diagnosis is crucial for the treatment of PCa patients. Although the prostate-specific antigen (PSA) screening has made great contributions to the early diagnosis of PCa, some limitations are readily apparent. It is well established that the “diagnostic gray zone” existed in PSA screening due to the poor specificity (3). A prostate biopsy can provide an accurate diagnosis, but it is a time-consuming and expensive method that requires an experienced urologist and causes great suffering for patients. Therefore, the identification of effective and practical biomarkers for early and accurate diagnosis of PCa (especially men with a PSA of 4–10 ng/ml) is urgent and important.

AFU containing two isoforms, AFU1 and AFU2, is an enzyme that is capable of clearing the terminal α-l-fucose residues from glycoproteins (4, 5). AFU1 and AFU2 are encoded by FUCA1 gene and FUCA2 gene, respectively. Interestingly, high α-l-fucose expression has been reported to be correlated with many cancers, such as breast, thyroid, and colorectal cancers (6–10). Therefore, it is reasonable to speculate that AFU with the function of hydrolyzing α-l-fucose may imply tumor-suppressive function. Previous studies have confirmed that AFU is indeed lowly expressed in a variety of cancers, including colon cancer, colorectal cancer, and breast cancer (11–13). Other than that, low expression of AFU usually predicts a worse prognosis in cancer patients (12–14).

However, the possible correlations between AFU and PCa have not yet been explored. Consequently, the current study is conducted to investigate the relationship between AFU expression and PCa. We hope that the current research can identify a promising early diagnostic and effective prognostic biomarker for PCa.

The present study indicates, for the first time, that AFU can effectively distinguish PCa from patients with PSA levels ranging from 4 to 10 ng/ml. We find that compared with the BPH patients, the PCa patients have lower serum AFU expression and smaller values of F/T PSA, both with gray-zone PSA level. We are aware that PSA is secreted by prostate epithelial cells, and its level will be elevated in PCa and BPH (17). Therefore, it is difficult to distinguish early PCa from BPH solely dependent on the PSA expression. In line with a previous study (18), our results indicated that no meaningful difference in PSA levels was observed between BPH and PCa patients with “gray-zone PSA”. To validate and further test the diagnostic reliability of F/T PSA and AFU, the logistic regression analysis was applied. However, the results illustrated that only AFU but not F/T PSA still exhibited a robust and independent diagnostic value for PCa. Likewise, the ROC analysis indicated that the diagnostic efficiency of F/T PSA was inferior compared with that of AFU. These data indicated that the diagnostic value of F/T PSA was more vulnerable to be interfered with other factors such as age and PSA level; for this reason, F/T PSA was not a reliable indicator for PCa patients with gray-zone PSA level. After confirmation of the diagnostic value of AFU for PCa, the possible correlations between AFU expression and PCa patients’ clinicopathological varies, which were further explored. Consistent with the above conclusion, lower expression of AFU implied a worse outcome. Compared with those of the early-stage group (pT1 and pT2 stages), the levels of AFU were markedly decreased in the advanced stage group (pT3 and pT4 stages). Furthermore, the patients with lymph node metastases had lower expression of AFU than those without lymph node metastases. The expression of AFU in prostate tissue was assessed using IHC. Similarly, it indicated the AFU expression was lower in high-grade PCa in contrast to low-grade PCa.

All the above analyses were conducted for AFU protein; next, the mRNA level of AFU was further evaluated based on TCGA database. Previous studies report AFU containing two subtypes, AFU1 and AFU2, which are encoded by genes FUCA1 and FUCA2, respectively (5, 19, 20). Therefore, the relationships between FUCA1/FUCA2 expression and PCa patients’ clinicopathological characteristics were further assessed based on TCGA database. It has long been known that the incidence of malignant disease in human is rapidly increased with aging, while our result suggested that the FUCA1 level was decreased with the increase of age. Beyond that, we found significant correlations among FUCA1/FUCA2 expression and pathological grade, pathological stage, postoperative residual tumor numbers, and primary therapeutic effect. Although FUCA1 and FUCA2 both showed prognostic value for patients’ PFI, the diagnostic performance of FUCA1 is more accurate and stable than that of FUCA2. All of these results imply that FUCA1/FUCA2 may be acting in a tumor-suppressive role, and lower expression of FUCA1/FUCA2 prognosticates worse pathological results, less therapeutic effect, and shorter PFI. This finding is consistent with many previous studies, which further validate the reliability of our conclusion (12, 13, 21–23).

The KEGG analysis indicated the biological function of FUCA1 and FUCA2 mainly involved glycosylation, especially glycoprotein. Glycosylation plays an important role in the initiation and progression of human disease including infection, inflammation, metabolism, and, of course, tumors (24–26). Some well-known tumor markers such as haptoglobin and CA 19-9 are fucosylated glycoproteins (27, 28). Apart from this, several key signal proteins, like integrin, E-cadherin, TGF-β receptors, and epidermal growth factor receptor (EGFR), are glycoproteins as well, which indicates that modification of glycosylation has a complex and crucial effect on their functions (29–32). Remarkably, many studies have revealed that abnormal glycosylation can lead to tumor onset and progression (33–36). AFU encoded by FUCA1 or FUCA2 can remove the terminal fucose residues from glycans and prevents aberrant accumulation of fucose-containing glycans (5, 26). Thus, the lack of AFU, which is responsible for the degradation of glycans, causes the overexpression of glycans and may prompt tumor initiation and development.

In this study, we first showed that AFU could be an effective diagnostic marker for PCa patients who had “gray-zone PSA”. In addition, our study demonstrated that low expression of AFU portends a worse prognosis of PCa. However, some limitations that existed in the present study deserve special attention. First, our study is retrospective research, which only allows for speculation based on the available data. Second, in consideration of the longer survival time of PCa patients, the differences in survival among different subgroups are difficult to be analyzed. Third, by bioinformatics analysis, we speculated that AFU suppressed the progression of PCa via regulation of glycosylation metabolism, which awaited further experimental validation.

AFU can effectively distinguish PCa from patients with gray-zone PSA levels; and lower AFU expression predicates advanced pathological results, poor therapeutic effect, more postoperative residual tumor numbers, and worse prognosis of PCa patients.

The original contributions presented in the study are included in the article/ Supplementary Material . Further inquiries can be directed to the corresponding authors.

The studies involving human participants were reviewed and approved by the Research Ethics Committee of Qilu Hospital of Shandong University. The patients/participants provided their written informed consent to participate in this study.

Study design: GC and ZX. Data collection: CZ, JL, DL, and ZX. Writing: CZ, GX, and GC. Editing: CZ, FC, SW, and DH. All authors contributed to the article and approved the submitted version.

This work was supported by grants from the National Science Foundation of Shanghai (No. 18ZR1405800) and the Project for Key Medical Specialty Construction in Jinshan District (6th Period, Type A) (No. JSZK2019A03) to GC.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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