Osteosarcoma is the most commonly occurring bone cancer in children and adolescents. Unfortunately, treatment failures are common due to the development of chemoresistance, for which the underlying molecular mechanisms remain unclear. In this study, we implicate the DNA-binding protein HMGB1, which also exerts immunoregulatory effects in its secreted form, in the development of drug resistance in osteosarcoma. Anticancer agents including doxorubicin, cisplatin, and methotrexate each induced HMGB1 upregulation in human osteosarcoma cells, and RNA interference-mediated knockdown of HMGB1 restored the chemosensitivity of osteosarcoma cells in vivo and in vitro. Mechanistic investigation revealed that HMGB1 increased drug resistance by inducing autophagy, an intracellular self-defense mechanism known to confer drug resistance. We found that HMGB1 bound to the autophagy regulator Beclin1 and regulated the formation of the Beclin1-PI3KC3 [PI3KC3, phosphatidylinositol 3-kinase class 3] complex that facilitates autophagic progression. In addition, we found that interaction between HMGB1 and Beclin1 relied upon the autophagic complex ULK1-mAtg13-FIP200. Therefore, through its role as a regulator of autophagy, HMGB1 is a critical factor in the development of chemoresistance, and it offers a novel target for improving osteosarcoma therapy. Cancer Res; 72(1); 230-8. Ó2011 AACR.

Due to the emergence of adjuvant and neoadjuvant chemotherapy, the survival rate has been greatly improved in osteosarcoma (OS) patients with localized disease. However, this survival rate has remained unchanged over the past 30 years, and the long-term survival rate for OS patients with metastatic or recurrent disease remains poor. To a certain extent, the reason behind this may be ascribed to the chemoresistance to anti-OS therapy. Chemoresistance in OS appears to be mediated by numerous mechanisms, which include decreased intracellular drug accumulation, drug inactivation, enhanced DNA repair, perturbations in signal transduction pathways, apoptosis- and autophagy-related chemoresistance, microRNA (miRNA) dysregulation and cancer stem cell (CSC)-mediated drug resistance. In addition, methods employed to circumvent these resistance mechanism have been shown to be effective in the treatment of OS. However, almost all the current studies on the mechanisms of chemoresistance in OS are in their infancy. Further studies are required to focus on the following aspects: i) Improving the delivery of efficacy through novel delivery patterns; ii) improving the understanding of the signal transduction pathways that regulate the proliferation and growth of OS cells; iii) elucidating the signaling pathways of autophagy and its association with apoptosis in OS cells; iv) utilizing high-throughput miRNA expression analysis to identify miRNAs associated with chemoresistance in OS; and v) identifying the role that CSCs play in tumor metastasis and in-depth study of the mechanism of chemoresistance in the CSCs of OS.

Osteosarcoma (OS) is the most common primary malignancy of bone. There is a critical need to identify the events that lead to the poorly understood mechanism of OS development and metastasis. The goal of this investigation is to identify and characterize a novel marker of OS progression. We have established and characterized a highly metastatic OS subline that is derived from the less metastatic human MG63 line through serial passages in nude mice via intratibial injections. Microarray analysis of the parental MG63, the highly metastatic MG63.2 subline, as well as the corresponding primary tumors and pulmonary metastases revealed insulin-like growth factor binding protein 5 (IGFBP5) to be one of the significantly downregulated genes in the metastatic subline. Confirmatory quantitative RT-PCR on 20 genes of interest demonstrated IGFBP5 to be the most differentially expressed and was therefore chosen to be one of the genes for further investigation. Adenoviral mediated overexpression and knockdown of IGFBP5 in the MG63 and MG63.2 cell lines, as well as other OS lines (143B and MNNG/HOS) that are independent of our MG63 lines, were employed to examine the role of IGFBP5. We found that overexpression of IGFBP5 inhibited in vitro cell proliferation, migration and invasion of OS cells. Additionally, IGFBP5 overexpression promoted apoptosis and cell cycle arrest in the G1 phase. In an orthotopic xenograft animal model, overexpression of IGFBP5 inhibited OS tumor growth and pulmonary metastases. Conversely, siRNA-mediated knockdown of IGFBP5 promoted OS tumor growth and pulmonary metastases in vivo. Immunohistochemical staining of patient-matched primary and metastatic OS samples demonstrated decreased IGFBP5 expression in the metastases. These results suggest 1) a role for IGFBP5 as a novel marker that has an important role in the pathogenesis of OS, and 2) that the loss of IGFBP5 function may contribute to more metastatic phenotypes in OS.

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Recently, emerging evidence has demonstrated that metastasis-associated lung adenocarcinoma transcript 1 (MALAT1), a long non-coding RNAs (lncRNAs), contributes to the initiation and development of tumors, including osteosarcoma (OS). Multiple studies have suggested an oncogenic role of MALAT1 and high-mobility group protein B1 (HMGB1) in OS tumorigenesis and metastasis, but the effects and mechanisms are not unanimous. Here, we showed that MALAT1 and HMGB1 were significantly increased in human OS cell lines and knockdown of MALAT1 reduced HMGB1 expression. By using online tools, we screen out 2 candidate miRNAs, miR-142-3p and miR-129-5p which may be associated with both MALAT1 and HMGB1. Luciferase reporter assay revealed a direct interaction between the 2 miRNAs and MALAT1, respectively, via a putative binding site within MALAT1. Meanwhile, both the 2 miRNAs could bind to HMGB1 3'-untranslated region (3'-UTR) and regulate HMGB1 expression. Moreover, knockdown of MALAT1 decreased HMGB1 expression, inhibited OS cell growth and promoted apoptosis, while miR-142-3p and miR-129-5p inhibitor partly restored the inhibitory effect of MALAT1 knockdown on HMGB1 expression, OS cell growth and the promotion of apoptosis. In OS tissues, the expression of MALAT1 and HMGB1 was upregulated while the expression of miR-142-3p and miR-129-5p was downregulated. Together, our results support a MALAT1/miR-142-3p/miR-129-5p/HMGB1 axis in OS cell proliferation and tumor progression. MALAT1 promoted OS cell growth through inhibition of miR-142-3p or miR-129-5p and by targeting HMGB1.

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