Expression of macrophage inhibitory cytokine‐1 in prostate cancer bone metastases induces osteoclast activation and weight loss

Abstract

BACKGROUND Macrophage inhibitory cytokine‐1 (MIC‐1) belongs to the bone morphogenic protein/transforming growth factor‐β (BMP/TGF‐β) superfamily. Serum MIC‐1 concentrations are elevated in patients with advanced prostate cancer. The effects of MIC‐1 on prostate cancer bone metastases are unknown. METHODS In vitro effects of MIC‐1 on osteoblast differentiation and activity were analyzed with alkaline phosphatase and mineralization assays; osteoclast numbers were counted microscopically. MIC‐1 effects on TLR9 expression were studied with Western blotting. Human Du‐145 prostate cancer cells were stably transfected with a cDNA encoding for mature MIC‐1 or with an empty vector. The in vivo growth characteristics of the characterized cells were studied with the intra‐tibial model of bone metastasis. Tumor associated bone changes were viewed with X‐rays, histology, and histomorphometry. Bone formation was assayed by measuring serum PINP. RESULTS MIC‐1 induced osteoblast differentiation and activity and osteoclast formation in vitro. These effects were independent of TLR9 expression, which was promoted by MIC‐1. Both MIC‐1 and control tumors induced mixed sclerotic/lytic bone lesions, but MIC‐1 increased the osteolytic component of tumors. Osteoclast formation at the tumor–bone interface was significantly higher in the MIC‐1 tumors, whereas bone formation was significantly higher in the control mice. At sacrifice, the mice bearing MIC‐1 tumors were significantly lighter with significantly smaller tumors. CONCLUSIONS MIC‐1 up‐regulates TLR9 expression in various cells. MIC‐1 stimulates both osteoblast and osteoclast differentiation in vitro, independently of TLR9. MIC‐1 over‐expressing prostate cancer cells that grow in bone induce osteoclast formation and cachexia. Prostate 69:652–661, 2009.