Stable overexpression of Smad7 in human melanoma cells impairs bone metastasis

Melanoma has a propensity to metastasize to bone, where it is exposed to high concentrations of transforming growth factor-beta (TGF-beta). Because TGF-beta promotes bone metastases from other solid tumors, such as breast cancer, we tested the role of TGF-beta in melanoma metastases to bone. 1205Lu melanoma cells, stably transfected to overexpress the natural TGF-beta/Smad signaling inhibitor Smad7, were studied in an experimental model of bone metastasis whereby tumor cells are inoculated into the left cardiac ventricle of nude mice. All mice bearing parental and mock-transfected 1205Lu cells developed osteolytic bone metastases 5 weeks post-tumor inoculation. Mice bearing 1205Lu-Smad7 tumors had significantly less osteolysis on radiographs and longer survival compared with parental and mock-transfected 1205Lu mice. To determine if the reduced bone metastases observed in mice bearing 1205Lu-Smad7 clones was due to reduced expression of TGF-beta target genes known to enhance metastases to bone from breast cancer cells, we analyzed gene expression of osteolytic factors, parathyroid hormone-related protein (PTHrP) and interleukin-11 (IL-11), the chemotactic receptor CXCR4, and osteopontin in 1205Lu cells. Quantitative reverse transcription-PCR analysis indicated that PTHrP, IL-11, CXCR4, and osteopontin mRNA steady-state levels were robustly increased in response to TGF-beta and that Smad7 and the TbetaRI small-molecule inhibitor, SB431542, prevented such induction. In addition, 1205Lu-Smad7 bone metastases expressed significantly lower levels of IL-11, connective tissue growth factor, and PTHrP. These data suggest that TGF-beta promotes osteolytic bone metastases due to melanoma by stimulating the expression of prometastatic factors via the Smad pathway. Blockade of TGF-beta signaling may be an effective treatment for melanoma metastasis to bone.