Collagenase Activity and Angiogenesis Prevents Melanoma Metastasis by Reducing Tumor RNA Interference Inhibition of Matrix Metalloproteinase-1

Melanoma incidence is increasing worldwide, and metastatic melanoma is almost completely resistant to every known therapy. New approaches to treating melanoma are urgently needed, and a greater understanding of the biology of melanoma invasion and metastasis will aid in their creation. A high proportion of invasive melanomas have a constitutively active Raf/mitogen-activated protein kinase/extracellular signal-regulated kinase (MEK/ERK) signaling cascade; however, the downstream effectors of ERK signaling that contribute to melanoma invasion and metastasis are unknown. ERK signaling drives the production of the interstitial collagenase matrix metalloproteinase-1 (MMP-1), which is expressed specifically by invasive melanomas. Using short hairpin RNAs (shRNA) to knock down MMP-1 expression in a human melanoma cell line, we investigated the role of MMP-1 in melanoma metastasis in a xenograft model. Knockdown of MMP-1 had no effect on primary tumor growth, but reduction of MMP-1 expression significantly decreased the ability of the melanoma to metastasize from the orthotopic site in the dermis to the lung. Mechanistically, tumor cells expressing MMP-1 shRNAs had diminished collagenase activity, which is required for tumor cell invasion. Additionally, attenuation of MMP-1 expression reduced angiogenesis. These results show, for the first time, that targeted inhibition of MMP-1, a single effector of the Raf/MEK/ERK signaling cascade, prevents the progression of melanoma from a primary to metastatic tumor and, as such, may represent a useful therapeutic tool in controlling this disease. [Cancer Res 2007;67(22):10849–58]