Downregulation of miR-29c-3p is associated with a poor prognosis in patients with laryngeal squamous cell carcinoma

Laryngeal squamous cell carcinoma (LSCC), as the most common histopathological type of laryngeal cancer, is the second-most common malignancy of the head and neck [1, 2].

According to studies, almost 3650 people died from LSCC worldwide in 2012, concomitantly with the increasing incidence and mortality of LSCC year-by-year [3]. At present, surgery, radiotherapy and chemotherapy are the main treatments for LSCC, but these treatments cannot meet the requirements for reducing the morbidity and mortality of LSCC. Moreover, the 5-year survival rate of advanced patients was less than 50%, mainly due to metastasis and recurrence [4]. Most patients already have advanced stage LSCC because of the lack of Early diagnosis. In addition, accumulating studies have proved miRs can serve as potential diagnostic biomarkers for laryngeal cancer [5]. Therefore, exploring novel biomarkers for the early stage diagnosis, prognosis and treatment of LSCC patients has been in high demand.

MiRNAs, 19 to 24 nucleotides in length, are endogenous noncoding single-stranded small RNA molecules that exist in many organisms and regulate gene expression at the posttranscriptional level by binding the 3′-untranslated region (3′-UTR) of target mRNAs through complete or incomplete complementary seed sequences and assembling in RNA-induced silencing complex (RISC) [6‐8]. Evidently, miRNAs play important roles in all tumor-related processes, including proliferation, differentiation, apoptosis, metastasis, invasion and angiogenesis in LSCC.

MiR-29c-3p, as a tumor suppressor in the miRNAs family [9], has been demonstrated to be downregulated in many solid tumors, such as nasopharyngeal carcinoma [10], gastric cancer [11], hepatocellular carcinoma [12], gallbladder cancer [13], esophageal cancer [14], breast cancer [15], colon cancer [16] and head and neck cancers [17], which also revealed that lower expression of miR-29c was associated with lymph node metastasis, tumor differentiation, TNM stage and poor prognosis. Study has revealed that miR-29s  group were frequently downregulated in head and neck squamous cell carcinoma (HNSCC), such as tongue squamous cell carcinoma and hypopharyngeal squamous cell carcinoma, which can normally inhibit cancer cell migration and invasion through their regulated pathways [18]. Besides, numerous studies have showed that miR-29c can inhibit the proliferation, invasion and metastasis of tumors and promote apoptosis by regulating a variety of oncogenes, cell pathways, cell cycle and epithelial to mesenchymal transition (EMT) [7, 19‐21]. In contrast, decreased expression of miR-29c was associated with a poor prognosis [22] and can even be used as a novel marker for cancer metastasis [17], drug treatment [7] and prognosis [13]. A recent study has demonstrated that miRNA arrays may contribute to the early diagnosis of LSCC, as well as the detection of clinical stage and prognosis [5]. Ayaz et al. found miR-331-3p, 603, 1303, 660-5p and 212-3p were not expressed in healthy individuals and patients with any other diseases but in LSCC, which suggessted that they could serve as novel non-invasive biomarkers for diagnosis of LSCC [23]. However, the role of miR-29c in LSCC is still unclear and lacks any relevant studies.

In the present study, we determined and compared the expression of miR-29c in LSCC tissues and adjacent normal laryngeal tissues. Furthermore, we evaluated the relationships between miR-29c expression and the clinical parameters and the survival of LSCC patients and revealed the prognostic factors of LSCC via multivariate Cox hazard regression analysis.

To date, the pathogenesis of laryngeal cancer is still unclear and the resistance to chemotherapy or radiotherapy account for frequent recurrent rate and high mortality. Therefore, it is vital to explore the molecular mechanisms and novel drug therapeutic targets of LSCC. In recent years, miRNAs have become a research hotspot because they are promising for use as biomarkers and drug therapeutic targets for various cancer diseases [24‐26].

It was confirmed that among the expression profiles of 738 miRNAs in LSCC patients and healthy controls, 17 were upregulated and 9 were downregulated [5], but did not provide information about miR-29c. Our study first provided evidence that miR-29c was significantly downregulated in LSCC and played a critical role in cancer occurrence, which is consistent with the study on HNSCC [17].

In addition, smoking and drinking are considered to be risk factors in upper respiratory tract cancer (UADTC). It was reported that cigarettes can induce a large number of DNA mutations, leading to more DNA damage and making the risk of laryngeal cancer higher [27‐29]. Our results also indicated that low level expression of miR-29c correlates with smoking index≥400, which implied that miR-29c might be involved in the pathomechanism of cigarettes inducing laryngeal cancer. Studies have also proven that overexpression of miR-29c inhibited the growth and metastasis of tumor cells and arrested tumor cells in the G1 phase, suggesting that miR-29c might be closely related to proliferation, cell cycle and apoptosis [30]. Our results demonstrated that the expression of miR-29c in tumors < 3 cm was higher than that in tumors ≥3 cm, which meant that miR-29c might regulate the proliferation and apoptosis of laryngeal carcinoma cells and further affect tumor size.

Furthermore, our results also indicated that the expression level of miR-29c in glottic laryngeal carcinoma was significantly higher than that in supraglottic and subglottic laryngeal carcinoma, and the subglottic laryngeal carcinoma had the lowest miR-29c expression. This may be associated with the fact that most glottic laryngeal cancer patients were diagnosed in the early stage of laryngeal cancer progression, when the expression level of miR-29c was also higher than that in advanced progression. Therefore, miR-29c might play a significant role in regulating the progression of laryngeal cancer.

Previous studies identified that miR-29c inhibited tumor invasion and metastasis by regulating different target genes and signaling pathways, such as MMP2 [31] and the MAPK pathway [13]. It has been emphasized that parecoxib is a potential drug for the treatment of digestive tract tumors, after a study found that parecoxib inhibited glioblastoma cell proliferation, migration and invasion by upregulating miR-29c [21]. In our study, we had demonstrated that miR-29c expression in laryngeal carcinoma patients without lymph node metastasis was significantly higher than that in patients with lymph node metastasis. Furthermore, a low level of miR-29c correlated with LSCC clinical stages, indicating that miR-29c might participate in laryngeal cancer cell migration and invasion, and increased miR-29c expression might be a novel treatment for laryngeal carcinoma.

Notably, Nygren et al [32] showed that a downregulation of miR-29c was associated with poor prognosis in breast cancer patients. Our present study demonstrated that low expression level of miR-29c in laryngeal cancer tissues significantly correlated with shorter survival time. By the multivariate Cox hazard regression analysis, we found that the miR-29c expression level was an independent prognostic positive factor for LSCC. A similar result was also found in pancreatic cancer patients [31].

In the future, we will pay more attention to the regulated mechanism of miR-29c in laryngeal cancer cell migration and invasion and explore a novel drug treatment for laryngeal cancer patients by increasing miR-29c expression.

To our knowledge, the present study first reports that miR-29c-3p is downregulated in LSCC and low expression of miR-29c-3p is an independent predictor of poor prognosis for patients with LSCC. As a tumor suppressor, miR-29c may be a useful biomarker for tumor progression, early diagnosis and prognosis assessment. In the future, miR-29c may be a therapeutic drug target for laryngeal cancer.