Transforming growth factor-beta and p27 expression in pituitary cells

Transforming growth factor-beta (TGF beta) is a member of a family of growth factors that regulates differentiation and cellular proliferation in a wide variety of tissues, including the anterior pituitary gland. TGF beta regulates the expression of various proteins, including p27Kipl (p27), a cell cycle inhibitory protein. The cell types in normal rat anterior pituitary producing TGF beta1, one of the principal isoforms of TGF beta, and p27 were examined by in situ methods. The regulation of p27 messenger RNA (mRNA) and protein by TGF beta1 was also examined in cultured anterior pituitary cells. In situ hybridization, in situ reverse transcriptase PCR, and immunocytochemical staining for pituitary hormones showed that PRL, TSH, and gonadotroph cells all had a higher percentage of cells expressing TGF beta1 mRNA and p27 protein than did GH and ACTH cells. After treatment with 10(-9) M TGF beta1 in vitro for 3 days, there were significant decreases in p27 mRNA and protein levels (P < 0.05) in normal pituitary cells. The GH3 and GHRH-CL1 cell lines, which secrete PRL and GH, had undetectable p27 protein by immunocytochemical staining and immunoblotting, although the GH3 cell line had p27 mRNA detected by reverse transcriptase PCR. Analysis of [3H]thymidine uptake in cultured dissociated pituitary cells by double staining for hormones showed that only PRL cells had significant proliferative activity during a 3-day cell culture period. There was a biphasic effect of TGF beta1 on PRL cell proliferation, with marked inhibition by 10(-9) M and a slight stimulation by 10(-13) M. These results indicate that there is a differential distribution of both TGF beta1 and p27 in various anterior pituitary cell types and that TGF beta1 directly down-regulates p27 in cultured anterior pituitary cells.