Endocrine Abstracts

Because the sodium iodide symporter, NIS, may be artificially expressed in cells to introduce ablative doses of radioactive iodide, it may be of clinical use. NIS is normally found in the basolateral membrane of thyrocytes and its expression and activity are known to be regulated by TSH. We have used primary cultures of human thyrocytes to examine other regulators of NIS activity. NIS activity was measured as uptake of ¹²⁵I into cells in the presence of the thyroperoxidase inhibitor, methimazole. Pan activation of protein kinase C (PKC) with tetradecanoylphorbol acetate resulted in rapid loss of NIS activity (42.8 plus/minus 8.9% SEM p0.01)within 3 hours, continuing to an 88.7 (plus/minus 2.3% SEM p0.001) inhibition at 24 hours. This correlated with down-regulation of several PKC isoenzymes including that of PKC delta. Specific activation of this PKC isoenzyme with Bistratene A resulted in marked (302 plus/minus 135% SEM p0.01 n=4) stimulation of NIS activity. This increase was sustained for 24 hours and there was no down regulation of PKC delta over this period. Immunoprecipitates of PKC delta using a specific antisera (Santa Cruz) were found to contain full length, 70 kDa NIS when examined by Western blotting (monoclonal antibody to NIS kindly donated by Dr JC Morris, Mayo Clinic). Kinase assays using PKC delta immunoprecipitates showed that NIS was phosphorylated by PKC delta. We conclude that

NIS is a substrate for PKC delta and phosphorylations mediated by this PKC isoenzyme increase NIS activity.