Jun B Expression Is Regulated Differently by Three Mitogenic Pathways in Thyrocytes

Abstract

In dog thyrocytes in primary culture, thyrotropin (TSH), acting through cyclic AMP, induces proliferation and differentiation expression, while tetradecanoylphorbol acetate (TPA) or epidermal growth factor (EGF) induces proliferation and dedifferentiation. In this work, we have investigated the regulation of mRNA expression of the protooncogene jun B in these cells. TSH stimulated jun B expression very transiently with biphasic kinetics similar to those obtained for c- myc mRNA accumulation. Forskolin reproduced these effects, suggesting that they are, as other effects of TSH in this system, mediated by cyclic AMP. As shown by nuclear run-on experiments, jun B is regulated by the cAMP pathway at the transcriptional level. As in other cell types, EGF or TPA caused a more sustained increase in mRNA levels. In thyroid slices, in which DNA synthesis appears to be induced by the wounding process, jun B is also induced, suggesting a correlation with the proliferative status of the cell. Interestingly, two jun B mRNAs of 2.1 and 2.3 kb were induced by all the mitogenic pathways. The kinetics of their accumulation were different; i.e., TPA induced the smaller transcript with some delay after the longer one and cycloheximide induced the progressive shortening of the first appearing heavier mRNA. The 2.3-kb messenger has a longer poly(A) tail, and kinetics in the presence of actinomycin D suggested it could represent a precursor form of the 2.l-kb messenger. It is suggested that the specific kinetics of cyclic-AMP-induced accumulation of jun B mRNA could be related to the dual stimulation of differentiation and proliferation by TSH in dog thyrocytes.