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

Abstract

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 Cdh2 (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-{beta}-catenin signaling in response to transforming growth factor-{beta}1 (Tgf-{beta}1), resulting in increased production of Tgf-{beta}1. Interference with PI3K activity is mediated by N-cadherin binding to PI3K components, p85 and p100, resulting in reduced activation of PI3K-Akt-{beta}-catenin signaling. Downstream, Cdh2 ablation enhances Tgf-{beta}1-induced binding of Sp1 and Lef-1 to the Tgfb1 promoter, leading to increased promoter activity and enhanced Tgf-{beta}1 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-{beta}1 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-{beta}1 production via PI3K-Akt-{beta}-catenin signaling is a key mechanism by which osteolineage cells promote BCC growth, an action restrained by Ncad via interference with PI3K components. HighlightsO_LIIn tumor-associated osteolineage cells, N-cadherin reduces the growth of breast tumors; it also inhibits Tgf-{beta}1-activated PI3K/AKT/{beta}-catenin signaling C_LIO_LITgf-{beta}1 produced by tumor microenvironment cells stimulates breast cancer cell growth and further autocrine production of Tgf-{beta}1 C_LIO_LIThe anti-tumorigenic action of N-cadherin is mediated by a braking effect on a Tgf-{beta}1-driven, pro-tumorigenic, feed-forward cycle between microenvironment and tumor cells. C_LI