RNA interference inhibition of matrix metalloproteinase-1 prevents melanoma metastasis by reducing tumor collagenase activity and angiogenesis

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.