Hormonal regulation of thyroid peroxidase in normal and transformed rat thyroid cells

The hormonal induction of thyroid peroxidase (TPO) mRNA is studied in the functional rat thyroid cell line FRTL-5 and compared to the induction of thyroglobulin (TG) mRNA and I- uptake. TPO and TG mRNAs are regulated by TSH and by insulin-like growth factor I (IGF-I) and/or insulin. However, while TPO is more sensitive to TSH regulation (5- to 6-fold increase vs. 2- to 3-fold increase by IGF-I), TSH and IGF-I are equally potent in increasing TG mRNA levels (3- to 4-fold). Regulation of I- uptake appears to be different: thus TSH greatly (15-fold) increases I- uptake, while IGF-I or insulin are completely ineffective. TPO and TG mRNAs and I- transport display different sensitivity to transformation of rat thyroid cells. Thus, when another differentiated rat thyroid cell line, the PC cells, are transformed by human c-myc (PC myc), TPO and TG mRNAs are both present at normal levels, while I- uptake is slightly decreased; in the PC cells transformed by polyomavirus middle-T-antigen (PC PyMLV) TPO mRNA is undetectable and I- uptake is greatly decreased, while TG mRNA is present at normal levels. All three differentiated functions are switched off in PC cells transformed by the cooperation of c-myc and polyomavirus middle-T-antigen (PC myc + PyMLV).     

In vitro and in vivo regulation of thyrotropin receptor mRNA levels in dog and human thyroid cells.

Regulation of thyrotropin (TSH) receptor (TSHr) mRNA accumulation as compared with two other thyroid differentiation markers (thyroglobulin and thyroperoxidase (TPO] has been investigated by Northern blot. In dogs in vivo, chronic stimulation of the thyroid TSHr mRNA although it increased the levels of thyroglobulin and TPO mRNA. In dogs treated with thyroxin, the quiescent thyroids expressed normal levels of TSHr and TPO mRNA but depressed levels of thyroglobulin mRNA. In primary cultures of dog thyrocytes, dedifferentiation of the cells by treatment with epidermal growth factor or 12-O-tetradecanoylphorbol-13-acetate led to decreased TSHr mRNA levels and nearly abolished thyroglobulin and TPO gene expression. However, TSHr mRNA was always present, compatible with the fact that these cells, when treated by TSH, reexpress differentiation. Treatment of the cells with TSH or forskolin transiently increased the TSHr mRNA level after 20 h, an effect inhibited by cycloheximide. This up-regulation was confirmed at the protein level: forskolin-treated cells showed an enhanced cAMP response to TSH and an increased binding of labeled TSH to their membranes. Long term TSH treatment led to a slight down-regulation of TSHr mRNA in dog thyrocytes, but in human thyroid cells no marked down-regulation was observed.     

Pairs of cyclic AMP analogs, that are specifically synergistic for type I and type II cAMP-dependent protein kinases, mimic thyrotropin effects on the function, differentiation expression and mitogenesis of dog thyroid cells

The role of the two different isozymes of the cAMP-dependent protein kinase is still unclear. We have investigated the potential roles for each isozyme in dog thyroid cells, a model in which the function, expression of differentiation and proliferation are positively regulated by thyrotropin acting through cyclic AMP. The dog thyroid contains both type I and type II cAMP-dependent protein kinases. These isozymes were selectively activated in vitro by type-I-directed and type-II-directed analog pairs. In thyroid slices, both type-I directed and type II-directed analog pairs synergistically activated thyroid hormone synthesis, as measured by incorporation of 131I into proteins and thyroid hormone secretion as determined by the release of butanol-extractable 131I. In primary cultures of dog thyroid cells both isozyme-directed analog pairs synergistically enhanced iodide trapping, a marker of differentiation, and DNA synthesis, as measured by the percentage of cells incorporating [3H]thymidine into their nuclei. However, DNA synthesis was more sensitive to type-I-directed pairs. The results demonstrate that both cAMP-dependent protein kinase isozymes can mediate the action of cAMP on function, differentiation expression and cell proliferation in dog thyroid cells.