In this study, we demonstrated that double-stranded RNA-dependent protein kinase (PKR) is required for the calcification of osteoblasts via the signal transducers and activators of transcription 1 alpha(STAT1 alpha) signaling in vitro. A dominant-negative mutant PKR cDNA, in which the amino acid lysine at 296 was replaced with arginine and which does not have catalytic activity, was transfected into mouse ostcoblastic MC3T3-E1 cells, thereby, we established cells that stably expressed the PKR mutant gene (PKR-K/R). Phosphorylation of PKR was not stimulated by polyinosic-polycytidylic acid in the mutant cells. The PKR-K/R mutant cells exhibited up-regulated cell growth and had low alkaline phosphatase (ALP) activity. The PKR-K/R mutant cells were not able to form bone nodules in vitro. In the PKR-K/R mutant cells, runt-related gene 2 (Runx2)-mediated transcription decreased compared with the levels in the control cells. The expression of STAT1 alpha protein increased and the protein was translocated to the nucleus in the PKR-K/R mutant cells. When the expression of STAT1 alpha a protein in PKR Mutant cells was suppressed using RNAi, the activity of Runx2-mediated transcription recovered to the control level. Our results indicate that PKR is a stimulator of Runx2 transcription and is a negative modulator of STAT1 alpha expression. Our findings also suggest that PKR plays important roles in the differentiation and calcification of osteoblasts by modulating STAT1 alpha and/or Runx2 expression. (c) 2005 Elsevier Inc. All rights reserved.