Protein phosphatases and cleavage of nucleolin in apoptosis-induced osteoblastic cells

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Protein phosphorylation and dephosphorylation has been recognized as a key mechanism in cell function. Okadaic acid is a potent inhibitor of PP1 and PP2A and induces apoptosis in human or mouse cells including osteoblasts. Nucleolin is an abundantly expressed nucleolar phosphoprotein and is located mainly in the nucleolus. The staining pattern of nucleolin in cultured osteoblastic cells is similar to that of PP1δ. Nucleolin was demonstrated to bind to PP1δ in nucleolus in human osteoblastic cells by using immunocytochemical and immunoprecipitation methods. AgNORs and nucleolin, visible as dots in the nuclei of the control osteoblastic cells, disappeared from the nuclei of the osteoblastic cells treated with okadaic acid. A major band, 110 kDa, was detected in the proteins obtained from osteoblastic cells. The level of the 110 kDa protein decreased in the apoptotic osteoblastic cells, whereas an additional band, 80 kDa, appeared and the level of this protein increased in the proteins prepared from okadaic acid-induced apoptotic osteoblastic cells. The present results indicate that PP1δ directly binds to nucleolin in the nucleolus of osteoblastic cells and that nucleolin is cleaved during apoptosis in osteoblastic cells.