MicroRNA‑21: A potential therapeutic target in lung cancer (Review).

In this review, the role of microRNA‑21 (miRNA‑21) as an oncogene in lung cancer was investigated. Studies have shown that miRNA‑21 can promote the progression of lung cancer by targeting downstream target genes, and its expression can be modulated by transcription factors, DNA methylation or competitive endogenous RNA as an upstream regulator. This review highlights that miRNA‑21 can promote the progression of lung cancer through multiple signaling pathways, with a focus on the PI3K/AKT, MEK/ERK, TGF‑β/SMAD, Hippo, NF‑κB and STAT3 signaling pathways. Mechanistically, miRNA‑21 plays an important role in the progression of lung cancer by regulating multiple biological processes, such as proliferation, invasion, metastasis, apoptosis and angiogenesis in lung cancer cells. Higher expression of miRNA‑21 is associated with chemotherapy, radiotherapy and immune resistance in lung cancer. Targeting these molecular pathways may be a novel therapeutic strategy for treating lung cancer. Additionally, miRNA‑21 can serve as a biomarker for lung cancer diagnosis, prognosis and treatment response. This review also summarized the following: i) Current methods employed to inhibit the expression of miRNA‑21 in lung cancer, including CRISPR/Cas9 technology; ii) the application of natural anticancer agents, oligonucleotides, small molecules and miRNA sponges; and iii) the nano‑delivery systems developed for miRNA‑21 inhibitors. Finally, the advancements in research on miRNA mimics and inhibitors in clinical trials, which may promote the application of miRNA‑21 in clinical trials in lung cancer, were discussed. Given that lung cancer is a considerable public health challenge, these studies provide new ways of treating patients with lung cancer.