SIX1 reduces the expression of PTEN via activating PI3K/AKT signal to promote cell proliferation and tumorigenesis in osteosarcoma
Introduction
Osteosarcoma is one of the most aggressive bone tumors in children and adolescents, characterized by high metastatic potential [1]. Approximately 2/5 of the patients appear with metastases at the moment of diagnosis, and only 1/5 patients could achieve long-term survival among them [2]. Even though a lot of effort has been made to improve outcome of patients with osteosarcoma, including neo-adjuvant chemotherapy and multi-modular therapies, the 5-year survival rate of patients with this disease remains poor [1,3]. Therefore, exploring the molecular mechanism of osteosarcoma occurrence and searching novel therapeutic strategies for osteosarcoma in order to improve the treatment outcomes is an urgent need.
Great achievements have been obtained in exploring the pathological mechanism of osteosarcoma development. Certain genes and signal pathways have been identified to exert key roles in osteosarcoma progression. Sineoculis homeobox homolog 1 (SIX1) is an evolutionary conserved transcription factor [4] and a critical regulator of embryonic development and implicated in tumor onset and progression [5]. SIX1 was frequently dysregulated in several cancers, such as pancreatic cancer [6], breast cancer [7], ovarian cancer [8], colorectal cancer [9,10], prostate cancer [11], as well as osteosarcoma [12,13], resulting in more aggressive and metastatic human cancer cell phenotype, and was closely associated with poor prognosis. In osteosarcoma, Liu et al. [13] verified that SIX1 was overexpressed in osteosarcoma cells and exert role in improvement of cell growth, proliferation, and migration of U2OS cells, as well as suppression of cell apoptosis. Additionally, Chao et al. [12] illuminated that SIX1 was differently expressed in cases of osteosarcomas with different clinicopathological features, showing positive correlations with Enneking stage and tumor size, which indicated poor prognosis in patients with positive SIX1 expression. But the specific molecular mechanism of SIX1 on osteosarcoma progression remains unknown.
The tumor suppressor phosphatase and tensin homolog deleted from chromosome ten (PTEN) is encoded by a 200 kb gene located on chromosome10q23, a genome region that suffers mutations or loss of heterozygosity in many human cancers [14]. Functional loss of PTEN is a major determinant, which influences a variety of cancer development, including colorectal cancer [15], gastric cancer [16], breast cancer [17] and osteosarcoma [18]. Inactivation of PTEN leads to loss of its lipid phosphatase activity and accumulation of phosphatidylinositol 3, 4, 5-trisphosphate (PIP3), and bring the activation of AKT. And PTEN/PI3K/AKT signaling pathway is primary positive or negative regulators in tumorigenesis for many solid cancers including osteosarcoma [19]. However, it’s unclear that whether SIX1 is able to promote osteosarcoma progression through regulating PTEN/PI3K/AKT signal molecules.
In the present study, we set out to characterize in more detail the role of the SIX1/PTEN/PI3K pathway in the progression of osteosarcoma. Firstly, we detected the expression level of SIX1 and PTEN in osteosarcoma tissues, blood samples and cell lines. Then, we explored the function of SIX1 on osteosarcoma cell growth, clone formation, apoptosis, migration and invasion and investigated the relationship between SIX1 and PTEN/PI3K/AKT signal molecules. Finally, we studied the tumorigenesis of SIX1/PTEN in vivo.
Section snippets
Tissues and blood samples collection
This study involving human and animals was approved by the ethics committee of Xi’an Jiaotong University Health Science Center, and have been performed in accordance with the Helsinki Declaration. A total of 25 primary osteosarcoma tissues and their paired adjacent non-tumor tissues were collected at our hospital from April 2015 to March 2016. None of these osteosarcoma patients received any radiation therapy or chemotherapy before surgery. Histological slides were reviewed by two experienced
Overexpression of SIX1 in osteosarcoma tissues and cell lines
To explore the function of SIX1 and PTEN on osteosarcoma progression, we first examined the expression level of SIX1 and PTEN in 25 paired osteosarcoma tissues. It was shown in Fig. 1A, compared with the adjacent non-tumor tissues, the expression level of SIX1 in osteosarcoma tissues were significantly up-regulated, whereas PTEN expression was decreased. Besides, we detected the mRNA level of blood SIX1 and PTEN in 25 cases osteosarcoma patients and 25 healthy people, as same as the protein
Discussion
Metastases is a common phenomenon of patients with osteosarcoma, it has been estimated that 40% of osteosarcoma patients occur metastases at initial diagnosis, whereas 25–50% of patients without metastases at initial presentation subsequently develop metastases [2]. Accordingly, molecular mechanisms responsible for osteosarcoma carcinogenesis have attracted much attention. In previous studies, gene expression profiling has been used to identify SIX1, a member of the SIX family of homeodomain
Conflict of interest
The authors declare that they have no conflict of interest.
Acknowledgment
This study was supported by self-financing.
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