Novel drug targets can be identified by differential analysis of RNA transcripts isolated from cancer cell lines and tissues. We have extended this approach by analyzing differences in gene expression resulting from the drug treatment of transformed and nontransformed cells. A mouse mammary epithelial cell line (C57MG), which conditionally expresses the Wnt-1 proto-oncogene, was left untreated or treated with retinoic acid in the presence or absence of Wnt-1 expression. The experiment was performed in triplicate, and RNA extracted from the four samples was analyzed by hybridization to over 12,000 unique oligonucleotide probe sets. Reproducible alterations in gene expression that occurred in response to retinoic acid, Wnt-1, or retinoic acid plus Wnt-1 relative to untreated cells were identified. Greater attention was given to genes encoding cell surface antigens that were selectively up-regulated by the combination of Wnt-1 and retinoic acid. These genes included the tumor necrosis factor family 4-1BB ligand, ephrin B1, stra6, autotaxin, and ISLR. Administration of retinoic acid to mice bearing tumors driven by activation of the Wnt-1/beta-catenin pathway resulted in increased expression of stra6 in the tumors but not in normal tissue. In principal, the therapeutic index of antibodies directed against these antigens should be enhanced by co-administration of retinoic acid.

Genetic defects in the Wnt-1 signaling pathway contribute to human tumor progression and are especially prevalent in colorectal cancer. We screened mouse C57MG cells to isolate mRNAs induced by Wnt-1 and identified Stra6, an mRNA known to be up-regulated by retinoic acid. Up-regulation of Stra6 mRNA was also observed in hyperplastic mammary tissue and mammary gland tumors from transgenic mice expressing Wnt-1 and in human tumors that frequently harbor defects in Wnt-1 signaling. Stimulation of C57MG cells with retinoic acid plus Wnt-1 resulted in expression of Stra6 transcript to levels greatly exceeding that observed with either stimulus alone. This synergy could be explained in part by the up-regulation of retinoic acid receptor-gamma that was observed in response to Wnt-1 signaling. Accordingly, treatment of human colorectal cancer cell lines with retinoic acid resulted in the up-regulation of Stra6 mRNA and accumulation of Stra6 protein at the cell membrane. The data support a model in which Wnt-1 signaling synergizes with retinoids to activate retinoic acid receptor-gamma-responsive genes in human cancers.

The Wnt gene family encodes proteins that serve key roles in differentiation and development. Wnt proteins interact with seven transmembrane receptors of the Frizzled family and activate a signaling pathway leading to the nucleus. A primary biochemical effect of Wnt-1 signaling is the stabilization of cytoplasmic (beta)-catenin which, in association with transcription factors of the Lef/tcf family, regulates gene expression. The recent identification of a new class of secreted proteins with similarity to the extracellular, ligand-binding domain of Frizzled proteins, soluble Frizzled related proteins (sFRP), suggested that additional mechanisms could regulate Wnt signaling. Here we demonstrate that FrzA, a sFRP that is highly expressed in vascular endothelium and a variety of epithelium, specifically binds to Wnt-1 protein, but not Wnt-5a protein, and modulates Wnt-1 signaling. FrzA associated with Wnt-1 either when expressed in the same cell or when soluble FrzA was incubated with Wnt-1-expressing cells. FrzA efficiently inhibited the Wnt-1 mediated increase in cytoplasmic (beta)-catenin levels as well as the Wnt-1 induction of transcription from a Lef/tcf reporter gene. The effects of FrzA on (beta)-catenin levels could be demonstrated when co-expressed with Wnt-1 or when individual cells expressing FrzA and Wnt-1 were co-cultured. These data demonstrate the existence of a negative regulatory mechanism mediated by the selective binding of FrzA to Wnt-1 protein.

The beta-catenin signal transduction pathway, which can be activated by secreted Wnt proteins, plays a key role in normal embryonic development and in malignant transformation of the mammary gland and colon. Here we demonstrate, for the first time, that Wnt and beta-catenin signaling also function in cells of the vasculature. RT-PCR analysis showed that primary endothelial and smooth muscle cell cultures, of both mouse and human origin, express members of the Wnt and Wnt receptor (Frizzled) gene families. Transfection of an expression vector for Wnt-1 into primary endothelial cells increased both the free pool of beta-catenin and the transcription from a Lef/tcf-dependent reporter gene construct. Expression of Wnt-1, but not Wnt-5a, also stimulated proliferation of primary endothelial cell cultures. These data show that Wnt and Frizzled proteins can regulate signal transduction, via beta-catenin, in endothelial cells. These findings suggest that Wnt signaling may feature in normal differentiation of the vasculature as well as in pathological settings where endothelial and smooth muscle proliferation is disturbed.