RESUMEN
Although many preclinical studies have implicated ß3 integrin receptors (αvß3 and αIIbß3) in cancer progression, ß3 inhibitors have shown only modest efficacy in patients with advanced solid tumours. The limited efficacy of ß3 inhibitors in patients could arise from our incomplete understanding of the precise function of ß3 integrin and, consequently, inappropriate clinical application. Data from animal studies are conflicting and indicate heterogeneity with respect to the relative contributions of ß3-expressing tumour and stromal cell populations in different cancers. Here we aimed to clarify the function and relative contributions to metastasis of tumour versus stromal ß3 integrin in clinically relevant models of spontaneous breast cancer metastasis, with particular emphasis on bone metastasis. We show that stable down-regulation of tumour ß3 integrin dramatically impairs spontaneous (but not experimental) metastasis to bone and lung without affecting primary tumour growth in the mammary gland. Unexpectedly, and in contrast to subcutaneous tumours, orthotopic tumour vascularity, growth and spontaneous metastasis were not altered in mice null for ß3 integrin. Tumour ß3 integrin promoted migration, protease expression and trans-endothelial migration in vitro and increased vascular dissemination in vivo, but was not necessary for bone colonization in experimental metastasis assays. We conclude that tumour, rather than stromal, ß3 expression is essential and is required early for efficient spontaneous breast cancer metastasis to bone and soft tissues. Accordingly, differential gene expression analysis in cohorts of breast cancer patients showed a strong association between high ß3 expression, early metastasis and shorter disease-free survival in patients with oestrogen receptor-negative tumours. We propose that ß3 inhibitors may be more efficacious if used in a neoadjuvant setting, rather than after metastases are established.