Metformin Reduces Thyroid Cancer Tumor Growth in the Metastatic Niche of Bone by Inhibiting Osteoblastic RANKL Productions

Background: Metformin has antitumoral actions in human cancers, including the thyroid, while its effects on metastatic lesions are unclear. Patients with bone metastasis (BM) from thyroid cancers have poor survival. Because metformin inhibits the activation of osteoclasts, which has essential roles in BM, the aim of this study was to investigate the therapeutic effects of metformin on thyroid cancer BM and osteoclast activation in the bone microenvironment. Methods: The anaplastic thyroid cancer (ATC) cell lines FRO and SW1736 were used to test the antitumoral effect of metformin in vitro and in vivo. A murine model of BM was established by intratibial injection of cancer cells. To mimic the BM microenvironment, osteoblasts were treated with conditioned media from the FRO (FRO-CM) and SW1736 (SW1736-CM) cells. Thyroid cancer patients with or without BM were recruited, and the serum receptor activator of nuclear factor kappa-B ligand (RANKL) levels was measured. Results: Metformin treatment significantly reduced the viabilities of the FRO and SW1736 cells in vitro and the tumor growth of SW1736 in vivo. In the murine model of BM, metformin delayed tumor growth in the bone and decreased the numbers of tartrate-resistant acid phosphatase-positive osteoclasts on the bone surface with reduced RANKL in the bone marrow. Furthermore, FRO- or SW1736-CM significantly increased the osteoblastic RANKL productions and activated osteoclast differentiation in whole marrow cultures, which were blocked by metformin treatment. Among 67 thyroid cancer patients, the serum RANKL levels were significantly increased in BM patients compared with patients with lung-only metastasis or no distant metastasis. In addition, the interleukin-6 superfamily in the FRO- or SW1736-CM stimulated STAT3 phosphorylation, which was inhibited by gp130 blocking. Metformin treatment decreased the FRO- or SW1736-CM-induced STAT3 phosphorylation by AMPK phosphorylation. Metformin also inhibited the FRO- or SW1736-CM-induced osteoclastic differentiation of bone marrow-derived monocyte/macrophage by RANK/c-Fos/NFATC1 signaling. Conclusions: In the microenvironment of BM, metformin effectively reduced ATC tumor growth by inhibiting cancer cell viability, blocking cancer cell-induced osteoblastic RANKL production, which further activated osteoclastogenesis, and directly reduced osteoclast differentiation. These multifactorial actions of metformin suggest that it has potential therapeutic effects in thyroid cancer BM.