Introduction
Lung cancer (LC) is the most common cause of cancer death worldwide; 1.6 million people die from this disease each year. It is estimated that in 2035, there will be 3 million annual deaths [
1]. LC is categorized into two main histological groups: small cell lung carcinoma (SCLC) and non-SCLC (NSCLC), accounting for 15% and 85% of all lung cancers, respectively. NSCLCs are usually subcategorized into lung adenocarcinoma (LUAD), lung squamous cell carcinoma (lSCC), and large cell lung carcinoma (LCLC). It can be then deduced that LC is a heterogeneous disease, so accurate diagnosis is vital to provide the most appropriate treatment. In this sense, important advances in the treatment of LC have been achieved in the past decade. However, the survival rates for LC remain yet low since 75% of cases are identified in advanced stages. Since an early diagnosis through biomarkers is key to survival [
2], new ways are needed to identify clinical biomarkers that could accelerate more accurate diagnosis, prognosis, and monitoring the evolution of the disease.
Spurred on by these challenges, studies based on the different types of RNA molecules, such as mRNA, sncRNA (miRNA and piRNA) and lncRNA, are becoming a profitable strategy for identifying biomarkers [
3‐
5]. These molecules include miRNAs, small non-coding RNAs between 21 and 23 nucleotides in length, which have been shown to participate in gene regulation [
6]. miRNAs can regulate transcription factors, tumor suppressor genes, and other regulatory elements that promote or inhibit cancer proliferation [
7]. Moreover, miRNA–gene interactions have been reported to play an essential role in carcinogenesis. For example, miR-21, cluster miR-17-92 and miR-221/222 have been shown to act as oncogenes in lung tumor progression, while let-7 families, miR-34 and miR-200 act predominantly as tumor suppressors [
8,
9]. In addition, some critical miRNAs for the diagnosis and treatment of cancer have recently been identified, coming from the analysis of circulating tumor cells, circulating miRNAs or miRNAs encapsulated in secreted microvesicles, such as exosomes [
10‐
12]. A recent work implied that the breast cancer exosomes carrying miR-21 were more strongly associated with bone metastasis than with non-metastatic cancer or other metastatic sites [
13].
So, the main purpose of the present study is to expand the list of biomarkers useful to diagnose LC in liquid biopsies, based on the miRNA–mRNA interaction between tumor and healthy tissue. The candidate miRNAs have been validated in larger cohorts and liquid biopsy databases, confirming their diagnostic value. Moreover, the candidates hsa-miR-130-3p, hsa-miR-182-5p and hsa-miR-203-3p appear to come from exosomes secreted by tumor cells.
Discussion
The present work is based on the differential expression changes between miRNA and mRNA in lSCC and LUAD. From the point of view of gene dysregulation, the total number of gDEs in lSCC (3667) is 23.3% higher than in LUAD (2818). This same trend is maintained when we observe the total number of miDEs, where the percentage in lSCC (360) is 77.3% higher than in LUAD (202). These profiles could be indicating a higher level of gene dysregulation in lSCC compared to LUAD, which could be caused by external factors such as tobacco, is in agreement with the evidence that shows that LUAD is the most frequent lung cancer among the non-smokers, and therefore less exposed to carcinogenic substances [
27,
28]. On the other hand, miDE–gDE correlation, as well as gene interaction studies, verify that 15 miDE–gDEs pairs in lSCC and 4 pairs in LUAD interact significantly with each other, forming a specific interaction network for lSCC and another for LUAD. Thus, these interactions correspond to 19 unique miRNAs that repress 35 unique mRNAs, of which some of them are related to angiogenesis and cell migration (Tables
1 and
2).
Validation of the differential expression in larger cohorts ruled out only hsa-miR-149-5p, hsa-miR-1246, hsa-miR-448, which showed that the integration of mRNA and miRNA expression data generates very robust results, even from very small cohorts, such as those used in this work.
The expression in tumor tissue of hsa-miR-369-3p, common to both types of cancer, seems to be involved in different functional processes depending on the tumor type. In LUAD this miRNA seems down-regulated the expression of the mRNAs (BMPR2, RASSF8), meanwhile, in lSCC seems down-regulate the expression of the mRNAs (ATP8A1, IDI1, LAMP3, NECAB1, PDE4D, PREX2). However, the lack of detection in extracellular fluids of hsa-miR-369-3p, together with hsa-miR-196a-5p, hsa-miR31-5p and hsa-miR-376c-3p has led us to discard them as possible non-invasive biomarkers, although they could be candidates in tumor tissue biopsies.
The hsa-miR-449a, hsa-miR-212-3p, hsa-miR-124-3p, hsa-miR-1304-3p, hsa-miR-137, hsa-miR-196b-5p, hsa-miR-377-3p and hsa-miR-656-3p are candidate circulating miRNA detection biomarkers for LUAD and lSCC. Among them, hsa-miR-449a could be specific for LUAD as it is not significantly expressed in tissue in lSCC, while hsa-miR-124-3p would be specific for lSCC as it is not described as miDE in LUAD. The candidates hsa-miR-377-3p and hsa-miR-656-3p present high levels of detection in plasma, but not in bronchoalveolar lavage, suggests differential secretion by cell type in tumor development.
Possible biomarkers hsa-miR-203-3p, hsa-miR-130b-3p and hsa-miR-182-5p show special interest when they are specifically identified in exosomes secreted by tumor tissue, both in LUAD and in the lSCC (Fig.
6).
Analyzing their relationship as possible biomarkers secreted in exosomes by tumor tissue, hsa-miR-203a-3p has been described as negatively regulated in both gastric cancer and gastric cancer cell lines, showing that its overexpression reduces the proliferation of cancer cells [
29]. However, our results show the opposite behavior, as it is strongly detected in exosomes secreted by the tumor tissue. Furthermore, this same miRNA has already been proposed as a biomarker for the detection of lung cancer from plasma using qPCR [
30].
In bladder cancer, it has been shown that hsa-miR-130b-3p could inhibit the expression of the
PTEN gene, that which promotes proliferation, migration, invasion and rearrangement of the cytoskeleton by activating the pathway signaling
PI3K; these same authors propose that the inhibition of this miRNA could induce cell apostosis [
31]. Likewise, this miRNA has already been described as an NSCLC oncogene [
32]. These evidences corroborate the importance of hsa-miR-130b-3p as a possible diagnostic biomarker specify in lung cancer.
The hsa-miR-182-5p should be highlighted due to its high level of detection in bronchoalveolar lavage, serum, and exosomes. Furthermore, accumulating evidence indicates that dysregulation of hsa-miR-182-5p can serve as diagnostic and pronostic biomarkers for some cancers. The literature reported the functionality of hsa-miR-182-5p in different types of cancer, such as renal cell carcinoma and liver cancer [
33,
34], although it is not yet clear in lung cancer. In this sense, a recent study [
35] shows that this miRNA is overexpressed in tumor tissue and in the peripheral blood of patients with NSCLC; in addition, and according to the same authors, its inhibition suppresses cell proliferation and increases apoptosis in NSCLC cell culture.
Last, we cannot consider the miDE detectable only in bronchoalveolar lavage as non-invasive biomarkers, since the technique itself is invasive.
In conclusion, a miDE–gDE interaction network specific for lSCC and another specific for LUAD are presented, which work as active regulators of gene expression. These interaction network allowed the identification of 11 candidate miRNAs for biomarkers in non-invasive lung cancer biopsy (Table
4), of which hsa-miR-449a would be specific for LUAD, while hsa-miR-124-3p would be specific to lSCC. On the other hand, the possible candidates hsa-miR-130b-3p, hsa-miR-203a-3p and hsa-miR-182-5p have a high detection rate in exosomes secreted by lung tissue, and seem to have a clear relationship with tumor development, so its in-depth study could shed light on the processes of tumor growth and development. Following these results, we propose that
in silico studies, based on mRNA and miRNA expression data, may constitute a useful tool in the identification of new non-invasive biomarkers that help in the early diagnosis and prognosis of cancer lung.
Publisher's Note
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.