title: Okadaic acid induces apoptosis through double-stranded RNA-dependent protein kinase/eukaryotic initiation factor-2alpha pathway in human osteoblastic MG63 cells.
abstract: Double-stranded RNA-dependent protein kinase (PKR) is a participant in the cellular antiviral response and phosphorylates the alpha-subunit of eukaryotic translation initiation factor 2alpha (eIF-2alpha) to block protein synthesis. Treatment of human osteosarcoma cell line MG63 cells with a serine and threonine protein phosphatase inhibitor, okadaic acid, at the concentration of 100 nM, but not at 20 nM, induced apoptosis. To investigate the functional relationship between phosphatases and apoptosis, we examined the phosphorylation levels of PKR and eIF-2alpha by Western blot analysis. During treatment of cells with it at the higher concentration (100 nM), okadaic acid increased the level of phosphorylated PKR in MG63 cells, this kinase phosphorylating eIF-2alpha. However, at the lower concentration (20 nM), okadaic acid did not affect the level of phosphorylated PKR. In the cells treated with 100 nM okadaic acid, activation of NF-kappaB also occurred. Even though inhibition of translation occurred simultaneously in MG63 cells, the expression of pro-apoptotic proteins Fas and Bax was not affected by 100 nM okadaic acid in these cells. We concluded that the inhibition of translation decreased anti-apoptotic protein expression, thus resulting in apoptosis. Our results also suggest that the inhibition of the protein phosphatase activity by okadaic acid induced apoptosis in MG63 cells through PKR and eIF-2alpha.
authors.affiliation: Department of Anatomy, School of Dentistry, The University of Tokushima, Kuramoto, Tokushima 770-8504, Japan. morimoto@dent.tokushima-u.ac.jp
authors.forename: Hiroyuki
authors.initials: H
authors.lastname: Morimoto
authors.forename: Hirohiko
authors.initials: H
authors.lastname: Okamura
authors.forename: Kaya
authors.initials: K
authors.lastname: Yoshida
authors.forename: Seiichiro
authors.initials: S
authors.lastname: Kitamura
authors.forename: Tatsuji
authors.initials: T
authors.lastname: Haneji
mesh_terms: D017209:Apoptosis; D015153:Blotting, Western; D045744:Cell Line, Tumor; D004247:DNA / Q000378:metabolism; D004305:Dose-Response Relationship, Drug; D004587:Electrophoresis, Agar Gel; D004591:Electrophoresis, Polyacrylamide Gel; D015852:Eukaryotic Initiation Factor-2 / Q000378:metabolism*; D006801:Humans; D008858:Microscopy, Phase-Contrast; D016328:NF-kappa B / Q000378:metabolism; D019319:Okadaic Acid / Q000494:pharmacology*; D010006:Osteoblasts / Q000378:metabolism; D010766:Phosphorylation; D014176:Protein Biosynthesis; D011720:Pyrazoles / Q000378:metabolism; D011743:Pyrimidines / Q000378:metabolism; D013997:Time Factors; D019892:eIF-2 Kinase / Q000378:metabolism*
chemical_list: C108123:4-amino-5-(4-methylphenyl)-7-(tert-butyl)pyrazolo(3,4-d)pyrimidine; C412373:AG 1879; D015852:Eukaryotic Initiation Factor-2; D016328:NF-kappa B; D011720:Pyrazoles; D011743:Pyrimidines; D019319:Okadaic Acid; D004247:DNA; D019892:eIF-2 Kinase