N-cadherin in osteolineage cells restrains breast cancer cell growth via inhibition of a PI3K-dependent, Tgf-β1-driven feed-forward loop

Tumor growth and metastases are affected by interactions between tumor and microenvironment cells. We have reported the presence of Sp7 -positive cells with an osteogenic signature in primary mouse breast cancer, where they stimulate tumor growth; and genetic ablation of Cdh 2 (encoding N-cadherin) in these cells enhances their pro-tumorigenic action. To study the molecular mechanisms of this biologic system, we used MC3T3 cells, phenotypically similar to tumor associated osteolineage cells. Ablation of Cdh2 in MC3T3 cells enhances PI3K-Akt-β-catenin signaling in response to transforming growth factor-β1 (Tgf-β1), resulting in increased production of Tgf-β1. Interference with PI3K activity is mediated by N-cadherin binding to PI3K components, p85α and p100, resulting in reduced activation of PI3K-Akt-β-catenin signaling. Downstream, Cdh2 ablation enhances Tgf-β1-induced binding of Sp1 and Lef-1 to the Tgfb1 promoter, leading to increased promoter activity and enhanced Tgf-β1 production. This is associated with miR-21 up-regulation and decreased expression of Pten, a PI3K inhibitor. MC3T3 cells promote growth of breast cancer cells (BCC) when co-cultured in vitro or co-injected in mouse mammary fat pad, and Cdh2 -deficiency enhances this pro-tumorigenic effect. Notably, genetic ablation of the Tgf-β1 receptor subunit, Tgfbr1, in BCC abrogates the pro-tumorigenic action of MC3T3 cells and its enhancement by Cdh2 ablation. Finally, Sp7-driven Tgfbr1 ablation in mice also reduces the growth of BCC in the mammary fat pad. Thus, autocrine Tgf-β1 production via PI3K-Akt-β-catenin signaling is a key mechanism by which osteolineage cells promote BCC growth, an action restrained by Ncad via interference with PI3K components.