RUNX2 promotes breast cancer bone metastasis by increasing integrin α5-mediated colonization

Highlights

• High RUNX2 expression increases the risk of bone metastasis in LN-negative breast cancer patients.

• ITGA5 is a transcriptional target of RUNX2.

• RUNX2 facilitates cancer cell bone colonization in an ITGA5-dependent manner.

Abstract

Runt-related transcription factor 2 (RUNX2) is regarded as an important contributor to breast cancer bone metastasis. However, previous studies did not provide direct clinical evidence for a role of RUNX2 in bone-specific metastasis in breast cancer, and the mechanism of RUNX2 in cancer cell recruitment and adhesion to the bone remains unclear. In this study, we showed that RUNX2 expression is positively correlated with the risk of bone-specific metastasis in lymph node-negative breast cancer patients. Then, we identified ITGA5 as a transcriptional target of RUNX2 from multiple candidate genes encoding adhesion molecules or chemokine receptors. We further provided experimental and clinical evidence that RUNX2, in an integrin α5-dependent manner, promotes the attraction and adhesion of breast cancer cells to the bone and confers cancer cell survival and bone colonization advantages. Overall, our findings clarify an adhesion-dependent mechanism of RUNX2 for the osteotropism and bone colonization of breast cancer cells and implicate RUNX2 and integrin α5 as potential molecular markers for the prediction of bone metastasis and therapeutic targets for the treatment of breast cancer bone metastasis.

Introduction

Bone is the preferred site for breast cancer metastasis. Almost 65–75% of patients with advanced breast cancer develop bone metastases, which predominantly cause osteolytic lesions [1]. Runt-related transcription factor 2 (RUNX2), a critical transcription factor for osteogenic lineage commitment and bone formation, activates skeletal gene expression by binding to osteoblast-specific cis-acting element 2 (OSE2) ACCACAxA (x represents any base) [2], [3], [4]. RUNX2 has been found to be highly expressed in breast cancer cell lines with high bone metastatic potential [5], [6], [7]. The in vitro and in vivo evidence demonstrates that RUNX2 is an important contributor to breast cancer bone metastasis [7], [8] due to its contribution to the acquisition of invasive and motility capabilities [4], [6], [9] and the formation of osteolytic lesions [10], [11], [12] in the progression of bone metastasis. However, current studies still lack direct clinical evidence of the promotion of bone-specific metastasis of breast cancer by RUNX2, and the mechanism of RUNX2 in breast cancer cell recruitment and adhesion to the bone remains unclear.

Integrins (ITGs) are transmembrane adhesion receptors composed of noncovalently linked α and β heterodimeric subunits, and they mediate the adhesion of cells to extracellular matrix (ECM) and adhesion-dependent cell growth [13], [14]. Integrin α5β3 has been observed to be overexpressed in bone-residing breast cancer metastases compared with corresponding primary breast cancer [15], [16]. The integrin α5β3 expressed on the surface of breast cancer cells anchors the cancer cells to the bone by binding the tripeptide Arg-Gly-Asp (RGD) motif in the bone matrix proteins [13], [14], which is associated with bone tropism [16], [17], [18], [19] and bone resorption [20]. Integrin α5β3 has been considered as a therapeutic target for breast cancer bone metastases [13], [14], and its peptidomimetic antagonists have been developed to inhibit bone metastasis [20], [21], [22]. Based on the fact that there are potential OSE2 motifs in the promoters of both ITGA5 and ITGB3 genes coding the integrin α5 and β3 subunits, we hypothesized that integrin α5β3 may be transcriptionally regulated by RUNX2 and mediates RUNX2-driven bone tropism, attachment to the bone, and the adhesion-dependent survival advantage of breast cancer cells in the bone microenvironment.

Here, we present clinical evidence for the role of RUNX2 in breast cancer bone metastasis. We further identify ITGA5 as a novel RUNX2 transcriptional target and demonstrate that RUNX2 facilitates cancer cell recruitment and colonization in the bone in an ITGA5-dependent manner.