Our data revealed that circDENND4C knockdown, miR-200b overexpression, and MMP-9 silence obviously suppressed lung cancer cell proliferation, arrested cell-cycle progression, and reduced cell migration and invasion in vitro, while promoting cell death, but circDENND4C overexpression, miR-200b inhibition, and MMP-9 overexpression significantly reversed the above results via the gain- and loss-of-function experiments. This evidence concerns the gene MMP9 and lung cancer.