Inhibition of STAT3 blocks protein synthesis and tumor metastasis in osteosarcoma cells

BACKGROUND

Osteosarcoma is the most common bone cancer. Despite advances, molecular mechanisms associated with osteosarcoma have not been fully understood. Hence, an effective treatment for osteosarcoma has yet to be developed. Even though signal transducer and activator of transcription3 (STAT3) has been implicated, its role in pathogenesis of osteosarcoma is not fully determined. In this study, we investigated the antitumor effect of napabucasin (NP) (BBI608), an inhibitor of STAT3 on osteosarcoma in vitro and in vivo and studied the underlying molecular mechanism.

METHODS

Cell viability, colony formation, apoptosis, tumor growth and metastasis assays were performed to examine the effect of NP on osteosarcoma in vitro and in vivo. Real-time RT-PCR, western analysis, immunofluorescence and reporter assays were used to monitor the expression and activity of proteins and underlying molecular pathways. Protein synthesis, co-immunoprecipitation and CAP binding assays were carried out to understand NP-mediated mechanism of actions in osteosarcoma cells.

RESULTS

Our results show that NP treatment decreases cell viability and induces apoptosis in several osteosarcoma cell lines. NP treatment suppresses both expression and phosphorylation of STAT3 in addition to blocking STAT3-mediated transcription and downstream target proteins in osteosarcoma cells. Furthermore, NP inhibits protein synthesis through regulation of the eukaryotic initiation factor 4E (eIF4E) and eIF4E-binding protein 1 (4E-BP1). NP also inhibits the progression of osteosarcoma tumors and metastasis in vivo in an orthotopic tibial model of osteosarcoma.

CONCLUSIONS

Taken together, our investigation reveals that NP acts through a novel mechanism and inhibits osteosarcoma growth and metastasis, and could be investigated clinically for treating osteosarcoma patients alone or in combination with other drugs.

Inhibitions of mTORC1 and 4EBP-1 are key events orchestrated by Rottlerin in SK-Mel-28 cell killing

Earlier studies demonstrated that Rottlerin exerts a time- and dose-dependent antiproliferative effect on SK-Mel-28 melanoma cells during 24 h of treatment, but cytotoxicity due to cell death began only after a 48 h exposure. In the current study, in order to identify the type of cell death in this cell line, which is notoriously refractory to most anticancer therapies, and to clarify the underlying mechanisms of this delayed outcome, we searched for apoptotic, necrotic/necroptotic and autophagic traits in Rottlerin-exposed cells. Although SK-Mel-28 cells are both apoptosis and autophagy competent, Western blotting analysis, caspase activity assay, nuclear imaging and the effects of autophagy, apoptosis and necroptosis inhibitors, indicated that Rottlerin cytotoxicity was due to none of the aforementioned death mechanisms. Nevertheless, in growth arrested cells, the death did occur after a prolonged treatment and most likely ensued from the observed blockage of protein synthesis that reached levels expected to be incompatible with cell survival. From a mechanistic point of view, we ascribed this effect to the documented inhibition of mTORC1 activity; mTORC1 inhibition on the one hand led to a not deadly, rather protective autophagic response but, on the other hand caused a near complete arrest of protein synthesis. Interestingly, no cytotoxicity was found towards normal skin fibroblasts, which only resulted mildly growth arrested by the drug.