Loss of RUNX3 expression promotes cancer‐associated bone destruction by regulating CCL5, CCL19 and CXCL11 in non‐small cell lung cancer

Non-small cell lung cancer (NSCLC) frequently metastasizes to bone, which is associated with significant morbidity and a dismal prognosis. RUNX3 functions as a tumour suppressor in lung cancer and loss of expression occurs more frequently in invasive lung adenocarcinoma than in pre-invasive lesions. Here, we show that RUNX3 and RUNX3-regulated chemokines are linked to NSCLC-mediated bone resorption. Notably, the receptor activator of nuclear factor-κB ligand (RANKL)/osteoprotegerin (OPG) ratio, an index of osteoclastogenic stimulation, was significantly increased in human osteoblastic cells treated with conditioned media derived from RUNX3-knockdown NSCLC cells. We aimed to identify RUNX3-regulated factors that modify the osteoblastic RANKL/OPG ratio and found that RUNX3 knockdown led to CCL5 up-regulation and down-regulation of CCL19 and CXCL11 in NSCLC cells. Tumour size was noticeably increased and more severe osteolytic lesions were induced in the calvaria and tibiae of mice that received RUNX3-knockdown cells. In response to RUNX3 knockdown, serum and tissue levels of CCL5 increased, whereas CCL19 and CXCL11 decreased. Furthermore, CCL5 increased the proliferation, migration, and invasion of lung cancer cells in a dose-dependent manner; however, CCL19 and CXCL11 did not show any significant effects. The RANKL/OPG ratio in osteoblastic cells was increased by CCL5 but reduced by CCL19 and CXCL11. CCL5 promoted osteoclast differentiation, but CCL19 and CXCL11 reduced osteoclastogenesis in RANKL-treated bone marrow macrophages. These findings suggest that RUNX3 and related chemokines are useful markers for the prediction and/or treatment of NSCLC-induced bone destruction.