Role of transforming growth factor (TGF)-beta Type I and TGF-beta type II receptors in the TGF-beta1-regulated gene expression in pituitary prolactin-secreting lactotropes.

Transforming growth factor beta1 (TGF-beta1) inhibits pituitary lactotrope proliferation and secretion of PRL in an autocrine/paracrine manner. In this study, the role of TGF-beta1 type I (TbetaR-I) and TGF-beta type II (TbetaR-II) receptors in TGF-beta1-regulated gene expression in lactotropes was determined using anterior pituitary cells known to be responsive to TGF-beta1 growth inhibition and using a transformed PR1 cell line known to be nonresponsive to TGF-beta1 growth inhibition. Treatment with TGF-beta1 inhibited cell proliferation and decreased PRL mRNA levels in anterior pituitary cells, but in PR-1 cells, the treatment caused only decreased PRL mRNA levels. Affinity labeling of TGF-beta binding proteins indicated that anterior pituitary cells contain several TGF-beta-binding protein complexes, including the 65 kDa size TbetaR-I and 95 kDa size TbetaR-II. In the PR1 cells, the major complex found was similar to the 65 kDa size of TbetaR-I. Immunocytochemistry identified TbetaR-I and TbetaR-II receptor proteins in lactotropes but detected primarily TbetaR-I receptor protein in PR1 cells. RT-PCR detection of TbetaR-I and TbetaR-II mRNA identified both receptor mRNA transcripts in anterior pituitary cells and in PR1 cells but the levels of TbetaR-II and TbetaR-I mRNA transcripts in PR1 cells was much lower than that in anterior pituitary cells. Determination of the TGF-beta1 gene responses in PR1 cells following TbetaR-I and TbetaR-II gene transfection indicated that PR1 cells transactivate transcription of the TGF-beta-responsive p3TP-Lux reporter in the absence of cotransfected TbetaR-II receptor. The introduction of the TbetaR-II receptor alone or in combination with TbetaR-I confer ligand-independent reporter transactivation in these cells. When only TbetaR-I was introduced along with reporter, a ligand-dependent transactivation was observed. These data suggest for the first time that the TGF-beta1-mediated transcriptional activation response can be distinguished from the growth response in lactotropes. Furthermore, the TGF-beta1 gene-transcription response is less dependent on TbetaR-II receptor expression than is the TGF-beta1 growth-inhibitory response.