Targeting IL-6 and RANKL signaling inhibits prostate cancer growth in bone.

In prostate cancer metastases to bone, cancer cell-derived cytokines stimulate RANKL expression by cells of the osteoblast lineage, which in turn activates osteoclastic bone resorption. However, it is unclear whether cells of the osteoblast lineage signal back to prostate cancer cells, and if so, whether such direct cross-talk can be targeted therapeutically. Using the human prostate cancer cell line, PC3, we identified two novel signalling pathways acting between cells of the osteoblast lineage and cancer cells. First, exposure to RANKL stimulated the expression and release of IL-6 by PC3 cells in vitro (which is known to promote RANKL expression by osteoblasts). Second, treatment of PC3 cells with IL-6 increased the expression of RANK, the cognate receptor of RANKL, and enhanced the RANKL-induced release of IL-6 by PC3 cells. Third, targeted disruption of IL-6 signaling with tocilizumab, a clinically available antibody against the human IL-6 receptor, inhibited skeletal tumor growth in vivo and reduced serum RANKL levels as well as RANK expression by PC3-derived bone tumors. Similar effects were achieved when RANK expression was knocked down in PC3 cells. In contrast, disruption of IL-6 or RANK/RANKL signalling had no effect on PC3 tumor growth in soft tissues, indicating that these signalling pathways act specifically within the bone microenvironment. In conclusion, prostate cancer cells and cells of the osteoblast lineage communicate via two inter-dependent signaling pathways, which through auto-amplification strongly enhance metastatic prostate cancer growth in bone. Both pathways may be targeted for effective therapeutic intervention.

Direct crosstalk between cancer and osteoblast lineage cells fuels metastatic growth in bone via auto-amplification of IL-6 and RANKL signaling pathways.

The bone microenvironment and its modification by cancer and host cell interactions is a key driver of skeletal metastatic growth. Interleukin-6 (IL-6) stimulates receptor activator of NF-κB ligand (RANKL) expression in bone cells, and serum IL-6 levels are associated with poor clinical outcomes in cancer patients. We investigated the effects of RANKL on cancer cells and the role of tumor-derived IL-6 within the bone microenvironment. Using human breast cancer cell lines to induce tumors in the bone of immune-deficient mice, we first determined whether RANKL released by cells of the osteoblast lineage directly promotes IL-6 expression by cancer cells in vitro and in vivo. We then disrupted of IL-6 signaling in vivo either via knockdown of IL-6 in tumor cells or through treatment with specific anti-human or anti-mouse IL-6 receptor antibodies to investigate the tumor effect. Finally, we tested the effect of RANK knockdown in cancer cells on cancer growth. We demonstrate that osteoblast lineage-derived RANKL upregulates secretion of IL-6 by breast cancers in vivo and in vitro. IL-6, in turn, induces expression of RANK by cancer cells, which sensitizes the tumor to RANKL and significantly enhances cancer IL-6 release. Disruption in vivo of this auto-amplifying crosstalk by knockdown of IL-6 or RANK in cancer cells, or via treatment with anti-IL-6 receptor antibodies, significantly reduces tumor growth in bone but not in soft tissues. RANKL and IL-6 mediate direct paracrine-autocrine signaling between cells of the osteoblast lineage and cancer cells, significantly enhancing the growth of metastatic breast cancers within bone.