Increased membrane-bound protein kinase C in porcine thyroid cells following TSH exposure

Activation of protein kinase C (PKC) follows its translocation from the cytosol to an active membrane-bound form. Previously, we demonstrated that only high concentrations of partially-purified bovine thyroid-stimulating hormone (TSH) could translocate PKC in vitro when measured by histone phosphorylation. Measuring PKC by Western blotting, we assessed whether physiological concentrations of human TSH could translocate PKC in porcine thyroid cells. The known PKC stimulator, phorbol ester 12-0-tetradecanoylphorbol 13-acetate (TPA), caused maximal translocation of PKC at 100 nM with a 84.5% decrease in cytosolic PKC and a corresponding 252.1% increase in membrane-bound PKC. Although TSH did not affect cytosolic PKC, human TSH at 1, 10 and 100 microU/ml produced increases compared to control values in membrane-bound PKC of 80.3 +/- 15.8, 66.8 +/- 9.7 and 74.0 +/- 19.8%, respectively (mean +/- SE, p < 0.01 at all TSH concentrations used). These studies provide evidence that physiological concentrations of TSH increase membrane-bound PKC in vitro, a process linked to its activation. In addition, we observed qualitative differences in phorbol ester and TSH-mediated PKC activation.