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Page 2 of 11 Tran et al. J Cancer Metastasis Treat 2020;6:47 I http://dx.doi.org/10.20517/2394-4722.2020.104

Conclusion: The results demonstrate a novel ROR2 function in OS lung metastasis and may inform new treatment
strategies for OS patients.

Keywords: Osteosarcoma, lung metastasis, cell survival, lung capillary, ROR2

INTRODUCTION
Osteosarcoma (OS) is a primary malignant bone tumor that typically occurs in the long bones, with peak
[1]
incidence in adolescents and young adults . Distant metastases of OS, such as lung metastases, are hard to
control and are usually associated with poor prognosis. Recent treatment advances involving combinations
of chemotherapy, surgery, and other clinical applications have increased the survival rate of OS patients
without lung metastasis to 60%. However, the survival rate of OS patients with lung metastasis has not
[2]
improved over the past 30 years . A better understanding of the molecular mechanism behind the OS
metastatic process will facilitate the development of new therapeutic strategies for improving the outcomes
of patients with OS.

Wnt signaling plays essential roles in the malignant process of OS. The Wnt signaling transduction
cascades are classified into two pathways: canonical (β-catenin-dependent) and noncanonical (β-catenin-
[3]
independent) pathways . The factors of canonical Wnt signaling, such as Wnt3a, Wnt10, Lef1, and
[4]
β-catenin, are involved in the development and malignant progression of OS . In contrast, noncanonical
Wnt signaling is known to be transduced by binding Wnt5a to Frizzled or receptor tyrosine kinase-like
orphan receptors (RORs), and promotes OS malignancies through phosphatidylinositol-3 kinase (PI3K)/
[6]
AKT signaling and c-Jun N-terminal kinase (JNK) pathway .
[5]
Receptor tyrosine kinase-like orphan receptor 2 (ROR2) is characterized by three main domains: the
intracellular tyrosine kinase domain, extracellular Frizzled-like cysteine-rich domain, and membrane-
[7-9]
proximal Kringle domain . The extracellular domain of ROR2 is assumed to mediate protein-protein
interactions, and ROR2 functions as an alternative or coreceptor for Wnt5a, a representative noncanonical
[10]
Wnt ligand . ROR2 activates JNK pathway [11,12] and inhibits the β-catenin-dependent Wnt pathway [10-12] .
[13]
ROR2 is overexpressed in 73.8% of OS samples and is correlated with tumor metastasis , suggesting a
pivotal role in OS progression. However, the molecular mechanism of ROR2 function in OS lung metastasis
has not been completely elucidated.

The process of OS metastasis to the lungs includes migration, intravasation into the circulation, survival
in the circulation and lung capillaries, and extravasation into the lungs. In the lung capillaries, most of OS
cells die for various reasons such as mechanical stress and lack of attachment to proper extracellular matrix
(ECM). Anoikis is a type of apoptosis induced by the lack of proper ECM attachment necessary for cell
[14]
maintenance in tissues . Tumor cells gain the ability to resist anoikis and undergo the metastatic process,
[15]
prolonging their survival time and promoting migration and colonization at secondary sites . Several
[14]
mechanisms of anoikis resistance in cells of epithelial origin have been described . However, there are few
studies on anoikis resistance in cells of non-epithelial origin, such as OS cells.
We have recently established LM8 murine OS sublines with different abilities to metastasize to the lung
[16]
using an in vivo image-guided screening system . Based on the microarray data of the LM8 sublines, we
recently identified a novel lymphoid enhancer-binding factor 1 (LEF1)-cytoglobin (CYGB) axis as a part of
[16]
noncanonical Wnt signaling pathway that promotes OS cell extravasation into the lungs . In the present
study, we aimed to search for new therapeutic targets using the same techniques to develop treatment
strategies for OS lung metastases.