One third of non-small-cell lung cancer (NSCLC) patients develop bone metastasis and die within a year. Tumor growth in the bone microenvironment results in excessive bone resorption−osteolytic bone lesions, which releases many cytokines and growth factors stored in the bone matrix, such as transforming growth factor-β (TGF-β), which further stimulates metastatic tumor growth in the bone. It is not known how TGF-β signaling in resident cells of the bone microenvironment effect NSCLC-induced bone lesions.

We found that blocking TGF-β signaling in cells of the myeloid lineage, LysMCre/Tgfbr2 knockout (KO), but not in cells of the mesenchymal lineage, Col1αcre/Tgfbr2 KO, decreases osteolytic bone lesion development of H1993 or H1975 NSCLC cells in a tibial-injected mouse model. In determining the mechanism by which LysMCre/Tgfbr2 KO decreases NSCLC bone lesion development, we demonstrated that basic-fibroblast growth factor (bFGF) of the mouse origin was significantly decreased in NSCLC injected tibiae of LysMCre/Tgfbr2 KO mice relative to controls. Exogenous bFGF partially rescues the reduced NSCLC bone lesions in these LysMCre/Tgfbr2 KO mice. bFGF is expressed by cells of the mesenchymal lineage, such as osteoblasts. However, bFGF has no effect on H1993 NSCLC cell proliferation. These data suggest that the decreases in NSCLC-induced bone lesions by loss of TGF-β signaling in myeloid lineage cells are dependent on the effects of bFGF on osteoblasts and osteoclasts. We therefore hypothesize that the regulation of bFGF expression in osteoblasts is indirect through a secreted myeloid-specific TGF-β signaling downstream target, collagen triple-helix repeat-containing 1 (Cthrc1).

Cthrc1 is a TGF-β target gene and clastokine, which are proteins secreted by osteoclasts that couple osteoblasts. Cthrc1 is also a known biomarker for NSCLC progression and promotes NSCLC cell proliferation and invasion. We found Cthrc1 was significantly decreased in H1993-injected tibiae of LysMCre/Tgfbr2 KO mice relative to controls, and stimulates H1993 proliferation in a dose-dependent manner in vitro. Additionally, Cthrc1 is known to stimulate bone formation. However, the effect of Cthrc1 on osteoblasts is not known in the context of NSCLC bone metastasis. From our in vitro studies, we found that Cthrc1 has no significant effect on osteoblast differentiation, but significantly increases bFGF gene expression in MC3T3 cells. bFGF inhibits osteoblast differentiation, but stimulates osteoclast differentiation. We therefore conclude that myeloid-specific TGF-β signaling stimulates osteoclast Cthrc1 secretion, which promotes NSCLC tumor cell proliferation. Meanwhile, Cthrc1 increases bFGF expression from osteoblasts to inhibit osteoblast, but stimulate osteoclast differentiation. Taken together, these data suggests Cthrc1 is a potential novel target for effectively inhibiting NSCLC bone metastasis.

Citation Format: Paul G. Daft, Xiangqi Meng, Alexandra Vander Ark, Austin Meadows, Xiaohong Li. Cthrc1 mediated by myeloid-specific TGF-β signaling stimulates NSCLC bone metastasis. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 1565.