蛋白質リン酸化と脱リン酸化による骨形成と骨吸収

Serine and threonine protein phosphatases (PP) are divided into four categories, PP1, PP2A, PP2B, and PP2C. cDNA cloning revealed the existence of at least four isoforms of PP1 catalytic subunit in rat, termed PP1α, PP1γ1, PP1γ2, and PP1δ. Aanti-PP1δ antibody recognized a protein present in the nucleolar regions in human and mouse osteoblasts. Cellular fractionation revealed that PP1δ is a 37 kDa protein localized in the nucleolus. C23 (nucleolin) is a nucleolar phosphoprotein and located mainly in the nucleolus. Staining pattern of C23 in human osteoblasts was similar to that of the PP1δ. PP1δ and C23 were specifically stained as dots in the nucleus. The dual fluorescence images revealed that PP1δ and C23 were localized in the same regions in the nucleolus. These results indicate that PP1δ associate with C23 directly in the nucleolus and suggest that C23 is one of the candidates of PP1δ substrates.
Double-stranded RNA-dependent protein kinase (PKR) is a serine/threonine protein kinase expressed in mammalian cells. PKR is activated by double-stranded RNA (dsRNA), interferon, cytokines, stress signals, and viral infections. 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 constructed. The mutant cDNA was transfected into mouse osteoblastic MC3T3-E1 cells and preosteoclastic mouse macrophage RAW264.7 cells. Using these cells, our group demonstrated that PKR plays important roles in the differentiation and calcification of osteoblasts by modulating STAT1α and/or Runx2 expression. In this review, I also described that PKR is required for the osteoclast formation of preosteoclastic cell by fusing these cells to form multi-nucleated giant cells.