Abstract LB-240: Smad7 blocks melanoma invasion by suppressing n-cadherin cleavage and preserving heterotypic cell-cell interactions in vivo.
Authors: DiVito Kyle A. , Trabosh Valerie , Chen You-shin , Javelaud Delphine , Mauviel Alain , Albanese Chris , Simbulan-Rosenthal Cynthia M. , Rosenthal Dean S. 
Published: February 2011

The number of TGF-ß-related proteins involved in non-canonical signaling continues to grow. Examples include receptor-Smads directing micro-RNA processing, as well as inhibitory-Smads, such as Smad7, influencing cell adhesion. Our past work has shown Smad7 overexpression blocks MMP secretion and subsequent tumorigenesis using a metastatic melanoma model. Employing live-animal fluorescent imaging and in vivo human skin grafts, we have found 1205Lu metastatic melanoma cells expressing Smad7 position themselves proximal to the epidermal-dermal junction and appear stationary within the dermis, while Smad7-deficient controls readily invade. Localization of Smad7-expressing cells within the graft correlated with absences in tumorigenesis, invasion and metastasis. Smad7-deficient controls formed dense, ulcerative tumors indicative of the aggressive nature of the cell line. Cell adhesion assays efficiently reproduced the graft and identified a strong interaction (p&lt;0.001) between Smad7-expressing cells and primary dermal fibroblasts, suggestive of the formation of heterotypic cell-cell interactions. Smad7-deficient cells and fibroblasts were incapable of heterotypic interaction as no adhesions were observed.

We explored the mechanism behind the inhibited tumorigenesis and found Smad7 stabilized the cell adhesion-related proteins -catenin and N-cadherin. Immunoprecipitation revealed Smad7 directly bound -catenin and permitted a significantly increased association with membranous N-cadherin. More importantly, -catenin linkage via Smad7 resulted in loss of N-cadherin processing, exhibited by the absence of unique N-cadherin cleavage products seen only in Smad7-deficient controls. Additionally, N-cadherin skin graft immunohistochemistry showed preserved N-cadherin expression at the surface of both Smad7-expressing cells and dermal fibroblasts indicating that stable cell-cell interactions via N-cadherin are maintained in vivo. Conversely, both Smad7-deficient cells and tumor-associated fibroblasts lacked N-cadherin expression, while interestingly, fibroblasts beyond the tumor border retained N-cadherin.

Previous work has established that stable cell adhesions via cadherins are detrimental to cell migration. We propose Smad7 functions to revert the invasive phenotype of these metastatic melanoma cells by stabilizing heterotypic cell-cell interactions between primary dermal fibroblasts and Smad7-expressing 1205Lu's, a mechanism not previously reported to occur in response to Smad7. We find these cell-cell interactions are mediated through N-cadherin and, in cooperation with loss of MMP 2 and 9, result in mitigated tumorigenesis.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr LB-240.