Modulatory function of protein kinase C in the activation of ornithine decarboxylase and in cAMP production in rat osteoblasts

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Abstract
The effect of activation of protein kinase C on stimulation of ornithine decar‐boxylase (ODC) activity and cAMP production was studied in fetal rat osteoblasts. Both phorbol 12‐myristate, 13‐acetate (PMA), an activator of protein kinase C, and 4α‐phorbol, ineffective in activating protein kinase C, failed to stimulate ODC activity and cAMP production. We tested the effect of protein kinase C on stimulation of ODC activity by parathyroid hormone (PTH) and forskolin. In contrast to PTH‐stimulated ODC activity, which was not affected by PMA, forskolin‐stimulated (1 and 10 μM) ODC activity was dose dependently reduced. PMA (400 nM) reduced both 1 and 10 μM forskolin‐stimulated ODC activity to the same level, ∼ 3 nmol CO2/mg protein, which suggests a controlling role of protein kinase C in forskolin‐stimulated ODC activity. The study of the effect of protein kinase C on PTH‐ and forskolin‐stimulated cAMP production also revealed differences between PTH and forskolin. When PMA was added simultaneously with PTH (4 and 20 nM) or forskolin (1 and 10 μM) the PTH‐stimulated cAMP production was dose‐dependently potentiated by PMA, whereas forskolin‐stimulated cAMP production was not affected. However, both PTH‐ and forskolin‐stimulated cAMP production was dose‐dependently augmented when PMA was added 3 min prior to PTH or forskolin. With increasing preincubation periods (up to 24 h) with PMA instead of a potentiation an inhibition was observed. This inhibition is not due to PTH receptor desensitization, although, on basis of the present results desensitization can not completely be excluded. In all cases 4α‐phorbol was without effect. The present results show that protein kinase C modulates stimulation of ODC activity and cAMP production in fetal rat osteoblasts. The modulation of both ODC activity and cAMP production appears to be dependent on the nature of the stimulator. The present data suggest a role for protein kinase C in limiting the cAMP‐mediated stimulation of ODC activity in these cells. Furthermore, it is suggested that protein kinase C can interfere at more than one site in the cAMP‐generating system.

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