Abstract 4006: Soluble ALK4 extracellular domain antagonizes oncogenic Cripto signaling and inhibits tumor growth

Cripto is a small GPI-anchored glycoprotein that plays key roles during normal development and in tumorigenesis. Cripto is expressed at aberrantly high levels in human tumors and blocking its oncogenic effects on activin/Nodal/TGF-beta, Erk/MAPK and PI3K/Akt pathways represents a promising therapeutic strategy. Cripto functions as a Nodal co-receptor and binds directly to the extracellular domain (ECD) of the activin/Nodal type I receptor ALK4. We have shown that a kinase-deleted ALK4 mutant deficient in activin binding (ALK4-L75A) retains Cripto binding and can block Cripto effects on activin and Nodal signaling. We hypothesized that soluble ALK4-L75A ECD proteins would similarly bind Cripto and disrupt oncogenic Cripto function. To test this, we generated two soluble constructs incorporating the ALK4-L75A ECD, one with a C-terminal His tag (sALK4-L75A-His) and the other consisting of an ALK4-L75A ECD-Fc fusion with an N-terminal FLAG tag (sALK4-L75A-Fc). We expressed the sALK4-L75A-His and sALK4-L75A-Fc proteins in 293T cells and then purified them from conditioned media using Ni-NTA agarose and Protein-A agarose, respectively. The sALK4-L75A-Fc protein dose-dependently inhibited 125I-Cripto binding to 293T cells transfected with full length ALK4. In addition, sALK4-L75A-Fc blocked 125I-Cripto binding to untransfected human mammary epithelial MCF10A cells. In accordance with these binding data, sALK4-L75A-His inhibited Cripto-dependent Nodal signaling in 293T cells and sALK4-L75A-Fc inhibited Nodal-induced Smad2 phosphorylation in human NCCIT cells. sALK4-L75A-His also blocked Cripto-induced PI3K signaling while sALK4-L75A-Fc inhibited Cripto-induced MCF10A cell proliferation. Importantly, we have shown that sALK4-L75A-Fc can also inhibit the growth of tumor xenografts derived from NCCIT cells at doses where no toxicity was observed. Thus, we provide the first evidence that soluble forms of ALK4 inhibit oncogenic Cripto signaling. The efficacy of a soluble ALK4 receptor as an antitumor agent points to the utility of Cripto antagonists in the treatment of human cancer. 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 4006.