Activation of Notch1 signaling in stromal fibroblasts inhibits melanoma growth by upregulating WISP-1.

The tumor microenvironment is emerging as an important target for cancer therapy. Fibroblasts (Fbs) within the tumor stroma are critically involved in promoting tumor growth and angiogenesis through secretion of soluble factors, synthesis of extracellular matrix and direct cell-cell interaction. In this work, we aim to alter the biological activity of stromal Fbs by modulating the Notch1 signaling pathway. We show that Fbs engineered to constitutively activate the Notch1 pathway significantly inhibit melanoma growth and tumor angiogenesis. We determine that the inhibitory effect of 'Notch-engineered' Fbs is mediated by increased secretion of Wnt-induced secreted protein-1 (WISP-1) as the effects of Notch1 activation in Fbs are reversed by shRNA-mediated blockade of WISP-1. When 'Notch-engineered' Fbs are co-grafted with melanoma cells in SCID mice, shRNA-mediated blockade of WISP-1 reverses the tumor-suppressive phenotype of the 'Notch-engineered' Fbs, significantly increases melanoma growth and tumor angiogenesis. Consistent with these findings, supplement of recombinant WISP-1 protein inhibits melanoma cell growth in vitro. In addition, WISP-1 is modestly expressed in melanoma-activated Fbs but highly expressed in inactivated Fbs. Evaluation of human melanoma skin biopsies indicates that expression of WISP-1 is significantly lower in melanoma nests and surrounding areas filled with infiltrated immune cells than in the adjacent dermis unaffected by the melanoma. Overall, our study shows that constitutive activation of the Notch1 pathway confers Fbs with a suppressive phenotype to melanoma growth, partially through WISP-1. Thus, targeting tumor stromal Fbs by activating Notch signaling and/or increasing WISP-1 may represent a novel therapeutic approach to combat melanoma.