Regulation of angiotensin II binding sites in neuronal cultures by protein kinase C

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Abstract
The radioligand binding of 125I-angiotensin II (ANG II) and calcium phospholipid-dependent protein kinase C (PKC) activity were measured to study the specificity and mechanisms of PKC involvement in the regulation of ANG II-specific binding site expression in neuronal cultures prepared from the brains of 1-day-old rats. Previously, PKC-activating phorbol esters were shown to increase the specific binding of 125I-ANG II in neuronal cultures. However, phorbol esters have many biological effects, which may nonspecifically act to increase 125I-ANG II-specific binding. In the present study, mezerein and teleocidin A, two activators of PKC that are chemically unrelated to phorbol esters, increased the specific binding of 125I-ANG II in a dose- and time-dependent manner with 50% effective dose (ED50) values of 32 and 79 nM, respectively. The PKC antagonist H-7 dose dependently inhibited phorbol 12-myristate 13-acetate (TPA)-stimulated increases in 125I-ANG II binding, whereas downregulation of PKC activity by chronic phorbol ester incubations of 24 and 48 h prevented TPA-stimulated increases in 125I-ANG II-specific binding. TPA (0.8 microM), mezerein (0.76 microM), and teleocidin A (0.5 microM) all caused a rapid translocation of PKC activity from the cytosol to the particulate fraction by 15 min. Temporally, the maximal stimulation of PKC translocation by mezerein, teleocidin A, and TPA preceded their ability to stimulate maximal 125I-ANG II-specific binding. Taken together, these results suggest that PKC is directly involved in the stimulation of ANG II-specific binding site expression and that translocation of PKC is a prerequisite for the increased expression of ANG II binding sites.