G protein-coupled receptor 56-mediated lung metastasis mechanisms in osteosarcoma LM8 cells

Osteosarcoma has a poor prognosis and frequently metastasizes to the lungs. This study examined the role of transglutaminase 2 (TGM2), a ligand for adhesion G protein-coupled receptor 56 (GPR56), in lung metastasis of LM8 mouse osteosarcoma cells. Gβγ‑dependent ERK signaling mediated by GPR56 and the adhesion of GPR56-expressing LM8^MOCK cells and GPR56‑deficient LM8^LOW cells were analyzed using siRNA, western blotting, and cell staining. TGM2 and Ki67 expression in metastatic lung lesions was assessed by transplantation and immunohistochemistry. TGM2 enhanced endogenous ERK signaling in LM8^MOCK cells. In the absence of TGM2, ERK signaling increased in LM8^LOW cells but remained unchanged in LM8^HIGH cells compared with LM8^MOCK cells. In the presence of TGM2, ERK activity in LM8^LOW cells resembled LM8^MOCK, whereas it was decreased in LM8^HIGH cells. LM8^LOW cells showed reduced adhesion, whereas LM8^HIGH cells did not differ from LM8^MOCK cells. Proliferation did not differ between LM8^LOW cells and LM8^HIGH cells. TGM2 was co‑expressed in CD31 + endothelial cells, and Ki67‑positive LM8 cells were observed along bronchi. These findings suggest that GPR56 promotes lung metastasis through TGM2‑mediated adhesion to pulmonary endothelial cells and enhanced ERK‑dependent proliferative signaling.