HNSCC is a malignant tumor derived from the mucosal epithelium, which has increased morbidity and mortality annually and is a serious human life and health risk. However, there is still a shortage of clinically useful and specific drugs for the treatment of HNSCC, and conventional treatment regimens have not achieved satisfactory results, with an effectiveness rate of approximately 50%[21]. Thus, it is urgent to elucidate the molecular mechanisms of miR-15b-5p in HNSCC and identify new reliable biological markers or therapeutic targets improve its clinical efficacy.
MiR-15b is up- or down-regulated in different cancers, where it either promotes or inhibits cancer, making it a potential biomarker for predicting disease prognosis. As the main product of miR-15b, miR-15b-5p has similar biological characteristics. A previous report demonstrated miR-15b-5p promotes prostate cancer progression and predicts tumor recurrence by targeting RECK[22]. Furthermore, miR-15b-5p is highly expressed in OSCC and laryngeal carcinoma, promotes tumor growth and metastasis, and is closely related to poor prognosis[23, 24]. Because miR-15b-5p role in HNSCC is unclear, we analyzed the expression and clinical characteristics of miR-15b-5p in HNSCC and attempted to identify the signaling pathways, target genes, and mechanism of action of miR-15b-5p using bioinformatics and evaluate its prognostic value in HNSCC.
Our GO enrichment analysis demonstrated that miR-15b-5p key genes were highly enriched in extracellular matrix genes. The extracellular matrix is essential for cell proliferation, differentiation, migration, and apoptosis, and its dysfunction can potentially lead to tumorigenesis[25]. KEGG enrichment analysis indicated that the retinol metabolism was an important signaling pathway in HNSCC, in which miR-15b-5p played a key role. Retinoids regulate cellular proliferation and differentiation[26] and have preventive and therapeutic effects on carcinogen-induced tumors[27]. The chemopreventive effects of retinoids on head and neck cancers have been reported in the literature[28], that attracted our interest in the retinol metabolism pathway. Hence, RDH12, UGT1A10, and UGT1A7 were identified as potential target genes for studying the mechanisms of HNSCC.
Promoter hypomethylation is a favorable factor for carcinogenesis associated with cancer gene activation, and changes in promoter methylation negatively correlate with mRNA expression levels[29–31]. This study revealed that promoter methylation levels of UGT1A10 and UGT1A7 were much down-regulated in HNSCC. Based on these results, high miR-15b-5p expression and oncogene activation in HNSCC may be associated with low methylation of the UGT1A10 and UGT1A7 promoters. Spearman analysis results suggested that RDH12 expression was inversely proportional to miR-15b-5p in HNSCC; nevertheless, UGT1A7 was directly proportional to miR-15b-5p. In summary, miR-15b-5p may regulate HNSCC development by targeting RDH12 and UGT1A7, which are involved in retinol metabolism.
Retinol is derived from vitamin A and vitamin A procarotenoids[32] and provides precursors for the synthesis of active retinoids[33], and it has been shown to inhibit tumor growth[34].Retinol metabolism regulates biological processes including cell development, differentiation, immune function, and energy metabolism. The dysregulation of its metabolism may upset homeostasis in the body and promote disease development[35]. RDH12 is a novel short-chain dehydrogenase/reductase involved in the conversion of retinaldehyde to retinol[36, 37] and catalyzes the redox reactions of all-trans- and cis-retinoic acid[38, 39]. Mujammami et al.[40] found the downregulation of RDH12 expression and reduced retinoic acid synthesis in patients with endometrial cancer (EC) with type 2 diabetes resulted in abnormal cell proliferation and angiogenesis, indicating a poor EC prognosis. The expression of RDH12 is downregulated in cervical squamous cell carcinoma (CSCC), and its expression is inversely proportional to the tumor size and depth of cervical infiltration[41]. In addition, RDH12 influences the prognosis of gastric cancer by regulating the tumor metabolic microenvironment and acts as a biomarker for metabolic therapy in cancer patients[42]. In our study, RDH12 expression was downregulated in HNSCC, which may affect retinoic acid synthesis through the retinol metabolism pathway, leading to abnormal cell proliferation and differentiation that promotes the development of HNSCC.
UDP-glucuronosyltransferases (UGTs) are mainly involved in glucuronidation reactions that metabolize and remove endogenous and exogenous carcinogens, including steroid hormones, bile acids, bilirubin, and environmental carcinogens such as tobacco-specific nitrosamines and benzo(A)pyrene[43, 44]. Increasing evidence suggests that UGT1A7 is associated with cancer development and its genetic polymorphisms can influence the susceptibility of individuals to diverse cancers[44, 45], like hepatocellular carcinoma[46], proximal digestive tract cancer[47] and orolaryngeal cancer[48]. The expression of UGT genes in cancer is closely related to the OS of patients, which is a promising biomarker and therapeutic target for predicting cancer development and prognosis[49]. Survival analysis suggested that high UGT1A7 expression in patients had longer DFS in HNSCC, indicating that high UGT1A7 expression is a favorable factor for a good DFS prognosis. Nevertheless, UGT1A7 was expressed at lower levels in HNSCC tissues, suggesting a shorter DFS and poor prognosis for patients with HNSCC. Hence, miR-15b-5p may be involved in regulating retinol metabolism by targeting UGT1A7, thereby affecting the prognosis of HNSCC. Therefore, miR-15b-5p is a novel biomarker for predicting the prognosis and tumor progression of patients with HNSCC and offers a theoretical underpinning for therapeutic targets in HNSCC.
This study had some limitations. First, our study was on the basis of database minding, with differential miRNAs only analyzed between HNSCC and non-cancerous tissues and lacked the evaluation of other samples, such as blood. Second, only two cell types were selected to verify miR-15b-5p expression in HNSCC. Finally, the clinical case data obtained were mainly in English, omitting data provided in other languages, and the sample size for target gene validation was relatively small, which could have easily caused errors. Therefore, the importance of miR-15b-5p in clinical diagnosis and prognosis should be assessed using more comprehensive and larger samples. The molecular role of the target genes in HNSCC also requires further confirmation with more functional experiments.