We studied the involvement of interferon-regulated, PKR on 2-ME–mediated actions in human osteosarcoma cells. Our results show that PKR is activated by 2-ME treatment and is necessary for 2-ME–mediated induction of osteosarcoma cell death.
Osteosarcoma is the most common primary bone tumor and most frequently develops during adolescence. 2-Methoxyestradiol (2-ME), a metabolite of 17β-estradiol, induces interferon gene expression and apoptosis in human osteosarcoma cells. In this report, we studied the role of interferon-regulated double-stranded (ds)RNA-dependent protein kinase (PKR) protein on 2-ME–mediated cell death in human osteosarcoma cells.
Materials and Methods
Western blot analyses were used to measure PKR protein and phosphorylation levels. Cell survival and apoptosis assays were measured using trypan blue exclusion and Hoechst dye methods, respectively. A transient transfection protocol was used to express the dominant negative PKR mutants.
Results and Conclusions
PKR was increased in 2-ME–treated MG63 cells, whereas 17β-estradiol, 4-hydroxyestradiol, and 16α-hydroxyestradiol, which do not induce cell death, had no effect on PKR protein levels. Also, 2-ME treatment induced PKR kinase activity as indicated by increased autophosphorylation and phosphorylation of the endogenous substrate, eukaryotic initiation factor (eIF)-2α. dsRNA poly (I).poly (C), an activator of PKR protein, increased cell death when osteosarcoma cells were treated with a submaximal concentration of 2-ME. In contrast, a serine-threonine kinase inhibitor SB203580 and a specific PKR inhibitor 2-aminopurine (2-AP) blocked the 2-ME–induced cell death in MG63 cells. A dominant negative PKR mutant protein conferred resistance to 2-ME–induced cell death to MG63 osteosarcoma and 2-ME–mediated PKR regulation did not require interferon gene expression. PKR protein is activated in cell free extracts by 2-ME treatment, resulting in autophosphorylation and in the phosphorylation of the substrate eIF-2α. We conclude from these results that PKR is regulated by 2-ME independently of interferon and is essential for 2-ME–mediated cell death in MG63 osteosarcoma cells.