Parathyroid hormone, prostaglandin E2) and prostacyclin activate cAMP-dependent protein kinase in osteoblast- rich normal rat calvarial cells and in clonal rat osteogenic sarcoma cells of osteoblastic phenotype. The present study was undertaken to determine the activation of the enzyme in relation to cellular cAMP concentrations at increasing doses of the three hormones and also to test that the activity ratio measurement of the enzyme (ratio of the activity in the absence of cAMP to the activity in the presence of excess cAMP) was a true reflection of intracellular activation of the enzyme. With each hormone, using either normal or malignant osteoblasts, activation of the enzyme took place at hormone concentrations lower than those required to produce detectable changes in cAMP concentrations in the incubations. Stimulation of activity was abolished by addition of the heat-stable inhibitor of cAMP-dependent protein kinase, indicating that activation was of cAMP-dependent protein kinase alone. To demonstrate that protein kinase activation occurred intracellularly and not during sample preparation, charcoal was added at the time of cell disruption to absorb free cAMP. Under these conditions, no change was observed in the concentration of bovine parathyroid hormone required to cause activation of cAMP-dependent protein kinase. Finally, addition of purified cAMP-dependent protein kinase type I or type II to treated cells at the time of lysis did not result in significant activation of added isoenzyme, except at hormone concentrations sufficient to increase the total cAMP concentration of incubations. It is concluded that activity ratio measurement reflects the intracellular state of activation of cAMP-dependent protein kinase in the osteoblast-like cells treated by hormones and, furthermore, that only a fraction of the maximally generated cAMP is necessary for full enzyme activation.
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