MMP9 and melanoma: TRAF6 promotes the invasion and metastasis of melanoma, glioblastoma, gastric cancer and other tumors by overexpressing MMP-9, leading to malignant transformation of tumors [14, 58, 59] (Fig. 3) and down-regulation of TRAF6 in A549 lung adenocarcinoma cells effectively reduces MMP9 expression [60].