Tumor-associated macrophages mediate the link between inflammation and cancer progression. In this paper, Klampfer and Augenlicht show that macrophage-derived soluble factors induce canonical Wnt signaling in colon cancer cells and promote their growth. Tumor cells induced the release of interleukin (IL)-1beta from macrophages, which induced phosphorylation of GSK3beta, stabilized beta-catenin, enhanced T-cell factor (TCF)-dependent gene activation and induced the expression of Wnt target genes in tumor cells. Neutralization experiments using anti-IL-1beta-specific antibodies, or silencing of IL-1beta in THP1 macrophages, showed that IL-1beta was required for macrophages to induce Wnt signaling and to support the growth of tumor cells. Constitutive activation of signal transducer and activator of transcription (STAT)1 in THP1 macrophages was essential for the induction of IL-1beta and thus for the activation of beta-catenin signaling in tumor cells. Vitamin D3, an effective chemopreventive agent, interrupted this crosstalk by blocking the constitutive activation of STAT1 and the production of IL-1beta in macrophages, and therefore-in a vitamin D receptor-dependent manner-inhibited the ability of macrophages to activate Wnt signaling in colon carcinoma cells. These data therefore established that vitamin D3 exerts its chemopreventive activity by interrupting a crosstalk between tumor epithelial cells and the tumor microenvironment.