Pituitary lactotrope expresses transforming growth factor β (TGFβ) type II receptor mRNA and protein and contains 125I-TGFβ1 binding sites

Transforming growth factor β1 (TGF β1) has recently been shown to be produced in the prolactin (PRL)-secreting lactotropes of the pituitary gland. TGFβ1 inhibits lactotropic secretion and proliferation, and the production of TGFβ1 in lactotropes is reduced during lactotropic growth following estrogen treatment in ovariectomized rats. In many estrogen-responsive tissues, TGFβ1 has been shown to exert its effect by binding to TGFβ1 type II receptors (TβR II) at the cell surface. In this study, we sought to ascertain whether TβR II is involved in TGFβ1 action on lactotropes by determining the changes of TβR II mRNA and protein levels and specific 125 I-TGFβ1 binding sites on the lactotropes during estrogen-induced proliferation of lactotropes in Fischer 344 rats. Double immunohistochemical procedures were employed to identify immunoreactive TβR II in PRL-reactive cells. The majority of TβR II-reactive cells in the anterior pituitary were observed to be lactotropes. Dual immunohistochemistry and in situ hybridization procedures also indicated that lactotropes were the major cell types containing TβR II mRNA hybrids. Both the levels of immunoreactive TβR II protein and in situ TβR II mRNA hybrids in the pituitary were significantly decreased in ovariectomized rats after 15 days of estrogen treatment. Determination of 125 I-TGFβ1 binding sites in lactotropes by double immunohistochemistry and receptor autoradiography also revealed specific binding sites of 125 I-TGFβ1 in lactotropes in the anterior pituitary. 125 I-TGF β1 binding in the anterior pituitary was also reduced following estrogen treatment in ovariectomized rats. These data suggest that down-regulation of TβR II may be an important mechanism of estrogen action on lactotropic cell growth and PRL secretion, and further support the notion that TGFβ1 controls lactotropic function by autocrine/paracrine mechanisms.