Parathyroid Hormone Causes Translocation of Protein Kinase-C from Cytosol to Membranes in Rat Osteosarcoma Cells

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Abstract
PTH binds to specific receptors that are coupled to adenylate cyclase and activate cAMP-dependent protein kinase. Since it has been shown that PTH activates phospholipid inositol metabolism, we investigated whether PTH influences protein kinase-C (PKC) activity in rat osteosarcoma (ROS) cells 17/2.8 that contain a large number of PTH receptor. Incubation of ROS cells with PTH or phorbol 12-myristate 13-acetate (PMA) for 1-30 min caused a rapid and transient decrease in PKC activity in the cytosol, which was associated with a transient increase in PKC activity in the membrane fraction. After 1, 5, 15, and 30 min of incubation with PTH, cytosolic PKC activity decreased to 57%, 74%, 84%, and 93% of the control value, whereas membrane PKC activity increased to 156%, 122%, 111%, and 106% of the control value, respectively. After PMA treatment for 1, 5, 15, and 30 min, cytosolic PKC activity decreased by 81%, 74%, 63%, and 44%, whereas membrane-bound PKC activity increased by 83%, 44%, 28%, and 17%, respectively. The effects of PTH and PMA on PKC were dose dependent, with ED50 values of 0.3 nM PTH and 4 nM PMA. Chronic treatment of ROS cells for 3 days with PMA caused depletion of total PKC activity in cytosolic and membrane fractions to less than 10% of that in control cells. Conversely, chronic treatment of ROS cells with PTH did not deplete PKC. In addition, chronic treatment of ROS cells with PTH inhibited the responsiveness of PKC activity to subsequent acute PTH challenge, but not to acute PMA challenge, suggesting specific desensitization of this response by PTH. Activation of cytosolic PKC by diolein, phosphatidylserine, and calcium caused phosphorylation of many cytosolic proteins, including those having apparent mol wt of 39K, 35K, 33K, 25K, 19K, and 16K. Pretreatment of ROS cells with PTH resulted in a transient decrease in the phosphorylation of these cytosolic proteins by PKC. This decrease in cytosolic protein phosphorylation by treatment with PTH is temporally associated with PTH-stimulated translocation of PKC activity from the cytosol to the membranes. These data suggest a potential role for PKC in the mechanism of action of PTH in ROS cells.

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