miR-33a-5p enhances the sensitivity of lung adenocarcinoma cells to celastrol by regulating mTOR signaling

MicroRNAs (miRNAs or miRs) have recently become a popular focus of cancer research due to their ability to act as oncogenes or tumor suppressors. In the present study, miR-33a-5p expression was identified to be downregulated in lung adenocarcinoma samples compared with normal, which suggested that miR-33a-5p may serve as a tumor suppressor gene. Transfection with miR-33a-5p mimics inhibited the proliferation and migration of A549 and LTEP-a-2 cells and increased cellular apoptosis. A luciferase reporter assay confirmed that miR-33a-5p targets the 3-untranslated region of the mechanistic target of rapamycin (mTOR) gene. mTOR expression was decreased in A549 and LTEP-a-2 cells treated with miR-33a-5p mimics, as well as the expression of its downstream effectors phosphorylated (p)-p70 ribosomal protein S6 kinase (p70S6K) and p-eukaryotic translation initiation factor 4E binding protein 1 (4EBP1). Following treatment with celastrol, miR-33a-5p expression was upregulated, and miR-33a-5p could enhance cellular sensitivity to celastrol. Western blot analysis revealed that the expression of mTOR, p-p70S6K and p-4EBP1 decreased following celastrol treatment. These results suggested that mTOR was involved in the mechanism by which miR-33a-5p enhanced the sensitivity of lung adenocarcinoma cells to celastrol. Furthermore, LTEP-a-2 cells were xenografted subcutaneously into nude mice, to examine the effect of celastrol and miR-33a-5p on the growth of LTEP-a-2 cells in vivo. The results demonstrated that tumor growth in the celastrol-treated or miR-33a-5p-treated group was attenuated compared with the control group. Notably, tumor growth in the combination treatment group was almost arrested after 2 weeks. In addition, celastrol upregulated the expression of miR-33a-5p, and high expression of miR-33a-5p inhibited mTOR and its downstream effectors. In summary, miR-33a-5p inhibited the proliferation of lung adenocarcinoma cells, enhanced the antitumor effect of celastrol, and improved sensitivity to celastrol by targeting mTOR in lung adenocarcinoma in vitro and in vivo.