THYROTROPIN REGULATION OF THYROID PEROXIDASE MESSENGER RIBONUCLEIC-ACID LEVELS IN CULTURED RAT-THYROID CELLS - EVIDENCE FOR THE INVOLVEMENT OF A NONTRANSCRIPTIONAL MECHANISM

The influence of TSH on thyroid peroxidase (TPO) gene expression was investigated in FRTL5 rat thyroid cells. Cultured in the presence of TSH, these cells express a TPO mRNA species of 3.1 kilobases. TSH withdrawal from the culture medium led to a decline in TPO mRNA levels over 24 h. In contrast, no decline in β-actin mRNA levels occurred after 24 h of incubation in TSH-free medium. TSH (1 mU/ml) added to FRTL5 cells cultured in the absence of TSH increased TPO mRNA levels 7- to 9-fold compared to levels in control cells. This effect of TSH on TPO mRNA accumulation in FRTL5 cells was time related (it was already present after 4 h and was maximal after 24 h of cell exposure to TSH), dose related (0.01 and 1 mU/ml were, respectively, the lowest and the maximally effective doses), and independent of new protein synthesis, in that it was not prevented by cycloheximide (100 μM). cAMP analogues [8-bromo-cAMP and (Bu)2cAMP] also increased TPO mRNA levels, although to a lesser degree than TSH. Run-on transcription analysis in nuclei prepared from FRTL5 cells previously cultured in the presence or absence of TSH did not reveal any difference in TPO mRNA transcripts. These results suggest that TSH regulatese the level of TPO mRNA in FRTL5 cells, in part via the second messenger cAMP and by a nontranscriptional mechanism. This TSH effect may represent a primary site of TSH action in regulating TPO bioactivity.