Tumor and Stem Cell Biology Mammary Tumor Regression Elicited by Wnt Signaling Inhibitor Requires IGFBP5

Wnt ligand–driven tumor growth is inhibited by the soluble Wnt inhibitor Fzd8CRD, but the mechanism through which this effect is mediated is unknown. In the MMTV-Wnt1 mouse model, regression of mammary tumors by Fzd8CRD treatment coincides with an acute and strong induction of insulin-like growth factor (IGF)– binding protein IGFBP5, an antagonist of IGF signaling that mediates involution of mammary gland in females after offspring areweaned. In this study, we show that repression of this IGF inhibitory pathway is crucial forWntdriven growth ofmammary tumors.We found that IGFBP5 regulation wasmediated by the b-catenin–dependent Wnt pathway. Wnt, in addition to IGF ligands, facilitated tumor growth by paracrine communication among tumor cells. In addition, Fzd8CRD caused precocious induction of IGFBP5 in normal mammary glands undergoing involution, implying an acceleration of the involution process by inhibition of Wnt signaling. The molecular and phenotypic parallel between tumor regression andmammary gland involution suggests thatWntdriven mammary tumors use the same growthmechanism as proliferating normal mammary glands. Cancer Res; 72(6); 1–11. 2012 AACR. Introduction Wnt signaling is an evolutionary conserved pathway that is crucial for developmental processes, stem cell maintenance, and cell fate determination. Misregulation of theWnt pathway is implicated in various humandiseases, particularly in cancers in which hyperactivation of the b-catenin–dependent canonical pathway is involved.MammalianWnt ligandwas originally identified as a protooncogene in viral induced mouse mammary gland tumors; overexpression of Wnt1 ligand under the control of the mouse mammary tumor virus (MMTV) promoter leads to hyperplasia and, ultimately, adenocarcinoma in the mammary gland (1). MMTV-Wnt1 tumors have characteristic histologic features and are composed of a variety of cell types, including distinct tumor luminal epithelial and basal/myoepithelial cells and host-derived stroma cells (2). Importantly, the existence of basal/myoepithelial cells in Wnt tumors is reminiscent of human basal–like breast cancers (3), which are often associated with a poor prognosis and the lack of effective treatment. A recent report also showed that Wnt signaling activation is enriched in basal-like breast cancers (4). Therefore, the MMTV-Wnt1 tumor is a useful model for studying breast cancer tumorigenesis as well as for therapeutic evaluation (1). Disruption of Wnt signaling by administration of a soluble frizzled8 receptor-immunoglobulin G (IgG) Fc fusion protein (Fzd8CRD) that competes for Wnt1 binding results in regression of the tumors (5, 6). While examining the events following inhibition of Wnt activity in established MMTV-Wnt1 tumors, we found that the rapid regression of Wnt1 mammary tumors caused by Fzd8CRD treatment coincided with acute and strong induction of insulin-like growth factor (IGF)-binding protein 5 (IGFBP5). IGFBP5 is a member of IGF-binding proteins (IGFBPs) and a key regulatory molecule during mammary gland involution (7). IGFBP5 is expressed throughout the mammary epithelium during puberty, but greatly reduced during lactation, and reexpressed during earlymammary gland involution before the onset of epithelial apoptosis (8). Overexpression of IGFBP5 results in an increase in apoptotic cells in the mammary glands, whereas mice with mutant IGFBP5 exhibit a decrease in apoptotic cells and a delay in mammary gland involution (8, 9). The predominant role of IGFBP5 in these processes is to bind IGF ligand and inhibit its signaling, but the functions independent of IGF signaling have also been reported (8, 10). IGF is a known growth and survival factor in mammary epithelial cells, and hyperactive IGF signaling is found in various human tumors including breast cancer (11, 12). IGF signaling employs 2 ligands, IGF-I and IGF-II, that can bind to IGF1 receptor (IGF1R) to activate downstream intracellular signaling pathways, providing cells withmitogenic and antiapoptotic signals. Wnt signaling also plays an important role in normal mammary gland development. Although the exact role of Wnt Authors' Affiliations: Departments of Cancer Targets, Physiological Chemistry, and Protein Chemistry, Genentech Inc., South San Francisco, California Note: Supplementary data for this article are available at Cancer Research Online (http://cancerres.aacrjournals.org/). Corresponding Author: Chie Sakanaka, Department of Cancer Targets, Genentech Inc., 1 DNAWay, South San Francisco, CA 94080. Phone: 650225-5310; Fax: 650-225-6440; E-mail: [email protected] doi: 10.1158/0008-5472.CAN-11-3668 2012 American Association for Cancer Research. Cancer Research www.aacrjournals.org OF1 Research. on April 7, 2017. © 2012 American Association for Cancer cancerres.aacrjournals.org Downloaded from Published OnlineFirst February 3, 2012; DOI: 10.1158/0008-5472.CAN-11-3668