HNC is one of the 10 most common cancers worldwide, with a five-year survival of about 40%, even with anti-cancer therapy (1, 17). RT is an important part of the definitive or adjuvant treatment for HNC. Resistance to RT, called radioresistance, is important in HNC treatment failure. MiRNAs are a group of small RNAs (smRNAs) 19–23 nucleotides long that regulate tumor cell growth. To date, several miRNAs have been associated with HNC diagnosis, prognosis, or therapy (18). Several miRNAs have also shown efficacy in regulating tumor radiosensitivity by modulating several pathways (19). However, the results of these studies are inconsistent between bench and clinical conditions, limiting their usefulness in the precision medicine era. Therefore, this study investigated a candidate miRNA that regulated tumor radiosensitivity in vitro and in vivo and predicted the prognosis of patients with HNC receiving RT. We identified miR-4776-5p as a prognostic biomarker in a clinical cohort of patients with HNC only receiving RT through an integrated bioinformatics approach and validated its regulation of radiosensitivity in both in vitro and in vivo studies using HNC tumor cells. Consistent with our findings, this miRNA had been previously reported in a miRNA-gene regulatory network for gastric cancer (20), suggesting that miR-4776-5p has strong potential as a novel radiosensitizer for improving the treatment outcomes of patients with HNC.
To understand the pathways regulated by miR-4775-5p, we first identified its target genes using the miRTarBase database and several bioinformatics tools for miRNA target prediction. Sixteen genes were identified: AP1M1, ATAD3B, CLEC11A, CMTM3, COLGALT1, FBX044, KDELR1, MCTS1, MRPL17, MTCH1, PARVB, PLOD1, PRAF2, RPS27L, SH2D2A, and VKORC1. AP1M1 is an intrinsic part of the clathrin adaptor AP-1 complex. Previous studies suggested that AP1M1 was involved in the progression of liver cancer due to hepatitis B infections (21) and identified it as a biomarker related to central nervous system (CNS) metastasis from breast cancer (22). ATAD3B is a mitochondrial membrane-bound ATPase belonging to the AAA domain-containing protein 3 ATPase family that has been associated with the progression and prognosis of various cancer types, including liver (23), breast (24), and CNS (25). Global proteome analysis of the mitochondria in Raje cells also identified ATAD3B as a potential biomarker of radioresistance (26). CLEC11A is a protein-coding gene that significantly participates in the tumor microenvironment and immune cell communication and can predict protein survival in laryngeal squamous cell carcinoma (LSCC) (27). CMTM3 was associated with cell growth and migration in oral squamous cell carcinoma (28) and pancreatic cancer (29) and has roles in the tumor microenvironment and cancer immunotherapy (30). COLGALT1 was identified as a biomarker for predicting prognosis and immune responses for kidney renal clear cell carcinoma (31). KDELR1 was involved in regulating T cell homeostasis (32) and correlated with the prognosis of glioma (33). MCTS1 was identified as an oncogene in various tumors, including HNSC (34), and correlated with immune cell infiltration (35). MRPL17 was identified as a crucial factor in oral carcinogenesis in an analysis of patients with tongue cancer (36). MTCH1 upregulation was associated with cell proliferation in hepatocellular carcinoma (37). The increasing presentation of MTCH1 neoantigen after ionizing radiation has also been noted (38). PARVB was proposed as a clinically useful biomarker for adjuvant tongue squamous cell carcinoma based on its ability to increase cell migration (39). PLOD1 belongs to the procollagen-lysine, 2-oxoglutarate 5-dioxygenase gene family, which has been associated with multiple cancers, and has a significant role in LSCC development (40). PRAF2 has been associated with the occurrence and progression of several malignant tumors, including esophageal, liver, and brain cancers (41–43). RPS27 encodes an MPS1 protein and has been reported to be involved DNA repair and regulating radiation sensitivity via the MDM2-p53 and MDM2-MRN-ATM axes (44, 45). SH2D2A encodes an adaptor protein thought to function in T-cell signal transduction, and was identified as a radiation response gene in irradiated primary human fibroblast cell lines (46).
To determine the mechanisms underlying miR-4776-5p acting as a radiosensitizer, we functionally annotated the 16 genes targeted by miR-4776-5p. Eleven functioned in the DNA damage response, apoptosis, cell proliferation, signal transduction, hypoxia, VEGF pathways, mitochondria, and the immune system. The DNA damage response and apoptosis are recognized as the traditional radiation mechanisms inhibiting tumor cell proliferation (47, 48). Increasing evidence shows the functional associations between signal transduction and cellular responses (49). Hypoxia is a common feature of the tumor microenvironment associated with RT resistance (50). VEGF knockdown enhanced radiosensitivity by inhibiting autophagy (51). Cellular organelles, particularly mitochondria, have been noted to mediate the radiation response in tumors by regulating many cellular processes involved in radioresistance (52). Recent data suggest that radiation kills the local tumor and inhibits the growth of distant metastatic lesions with immunomodulatory properties (the so-called abscopal effect) (53, 54). Therefore, these functions support the suitability of miR-4776-5p as a candidate for regulating radiosensitivity and the potential for further radiobiological research.
While numerous studies have investigated the role of miRNAs in modulating the radiosensitivity of HNCs, few present consistent results between bench and clinical conditions (55). Most previous studies investigated the expression changes of miRNAs in HNC cell lines in vitro, first using microarray detection to identify candidates. Few then validated their candidate miRNAs in clinical samples (56, 57). The recent revolution in high throughput next-generation sequencing methods, which have higher sensitivities than microarray, has led to the creation of several large sequencing databases, including TCGA, with vast amounts of sequencing data and large clinical sample sizes. To our knowledge, only two studies have investigated miRNAs related to the radiosensitivity of patients with HNC in these big databases using integrative bioinformatics approaches (13, 14). Using TCGA data, a five miRNA signature-based nomogram was proposed to predict the RT response of HNSC patients based only on an in silico data analysis (13). Also using the TCGA, Inoue et al. (14) screened human papillomavirus (HPV)-negative oropharyngeal squamous cell carcinoma (OPSC), identifying miRNA-130b as a potential biomarker for the radiosensitivity of HPV-negative OPSCs. However, these two studies had some limitations. First, patients receiving RT may also receive chemotherapy or targeted therapy. It is difficult to determine the change in prognosis due to each treatment. Second, their clinical results were not validated in bench studies confirming that the result was related to radiosensitivity. Therefore, in our study, we screened the miRNAs in patients only receiving RT and validated our in silico result with radiobiological studies in vitro and in vivo. In addition, we also validated our candidate miRNA in a clinical cohort with HNSC not receiving RT. The inconsistency between the RT-only and non-RT groups indicated that miR-4776-5p regulates the radiosensitivity and RT response instead of HNC progression.
The miR-4776-5p biological validation results showed its significant impact on DNA repair efficiency and the survival of HNC cells after irradiation. Quantification of γH2AX foci numbers showed that DSB resolution was significantly impacted in irradiated cells overexpressing miR-4776-5p. Cells overexpressing miR-4776-5p had more foci at 2.5 h after RT than cells treated with miR-NC and showed inefficient DSB repair kinetics in the 24 h after RT. Our results also showed that miR-4776-5p overexpression significantly impacted the survival of HNC cells after RT due to the unrepaired DSBs and failed cell division. Our in vivo study showed that miR-4776-5p overexpression did not affect the tumorigenesis of HNSC cells but enhanced tumor radiosensitivity based on the prolonged growth inhibition of tumor cells transfected with miR-4776-5p.