Current clinical treatments for osteosarcoma are limited by disease recurrence and primary or secondary chemoresistance. Cancer stem-like cells have been proposed to facilitate the initiation, progression, recurrence and chemoresistance of osteosarcoma. Furthermore, previous studies have reported that IL-6-STAT3 pathway is overexpressed in various types of cancer and contributes to cell proliferation, apoptosis, invasion/migration, chemoresistance and modulation of stemness features.
To examined the effect of cinobufagin on cancer progression and modulation of stemness features in osteosarcoma, and investigated the molecular mechanisms underlying such effects.
Human osteosarcoma cell lines U2OS/MG-63 were recruited in this study. Cell proliferation, migration, and invasion were determined by MTT assay, colony formation assay,wound healing assay, and cell invasion assay respectively. Its effect on stemness was assessed by flow cytometry and mammosphere formation. The protein expression levels of related proteins were detected by Western blot. The xenograft model, immunofluorescence staining and immunohistochemistry were used to determine the effect of cinobufagin on tumorigenicity in vivo experiment.
We found that cinobufagin suppressed the viability of U2OS/MG-63 spheroids/parent cells in a time-and dose-dependent manner. Notably, cinobufagin had no effect on the viability of hFOB 1.19 cells. Moreover, cinobufagin induced apoptosis, increased the width of wounds, reduced invasive osteosarcoma spheroids/parent cell numbers and reduced EMT phenotype and OPN levels in U2OS/MG-63 spheroids as well as U2OS/MG-63 parent cells lines. Noticeablely, we found that OPN levels were higher in spheroids group than that in parent cells. In addition, cinobufagin ameliorated the proportion of CD133-positive cells, the size of spheroids and Nanog, Sox-2 and Oct3/4 protein levels. Our in vivo experiments showed that cinobufagin consistently reduced tumor volume,the expressions of OPN, Sox-2, Oct3/4, Nanog and p-STAT3 by the immuno histochemistry staining as well as CD133 expression in tumor tissues by immunofluorescence analysis. From a mechanistic point of view, cinobufagin was shown to inhibit IL-6-OPN-STAT3 signaling pathway. Exogenous IL-6/OE-OPN/overexpression STAT3 attenuated the induction of cinobufagin-mediated apoptosis and the suppression of stemness properties respectively.
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