EVIDENCE THAT PHOSPHORYLATION EVENTS PARTICIPATE IN THYROID-HORMONE ACTION

Using both a protein phosphatase inhibitor, okadaic acid (OA), and a protein kinase inhibitor, H7, to modify phosphorylation events in the cell, we investigated the effects of these agents on transcriptional activation via exogenous rat thyroid hormone receptor (TR) isoforms in transiently transfected cells and endogenous TRs. CV-1 cells were transiently cotransfected with expression plasmids encoding either the rat TR alpha(1) or TR beta(1), and luciferase reporter plasmids containing either the synthetic DR4 or the chick lysozyme F2 thyroid hormone response elements (TREs). For both receptor isoforms, there was an enhancement of transcriptional activity after incubation with 5 nM T-3 for 24 h compared to hypothyroid levels. There was little change in transcriptional activation in the presence of 25 nM OA alone; however, for both TR isoforms and both TREs studied, OA augmented the stimulatory effects of T-3 For the F2 TRE, transcriptional activation via TR alpha(1) increased from 19- to 35-fold, and that via TR beta(1) increased from 6- to 10-fold in the presence of T-3 and OA competed to that with T-3 alone. Similar results were found for the DR4 TRE. OA enhanced transcriptional activation by T-3 in a dose-dependent manner. Increasing concentrations of OA (0, 5, 25, and 50 nM) further increased relative luciferase activity from 11-fold in the absence of OA to 45-fold in the presence of 50 nM OA. The protein kinase inhibitor, H7, caused no change in the transcriptional activity of the reporter plasmids via TR alpha(1) in the absence of T-3, but completely blocked transcriptional activation by T-3 for both the DR4 and the F2 TREs. H7 also blocked stimulation of endogenous GH and inhibition of endogenous TR beta(2) mRNAs by T-3 in GH(3) cells. These results indicate that phosphorylation events in the cell play an important role in transcriptional activation via both TR isoforms.