Transforming growth factor β1 is a paracrine inhibitor of prolactin gene expression

We have shown previously that rat mammotropes produce an activity that suppresses PRL gene expression by neighboring mammotropes. Here, we tested the hypothesis that this mammotrope-derived inhibitor is transforming growth factor-beta 1 (TGF beta 1). To this end, we pursued a two-pronged strategy wherein we added exogenous TGF beta 1 to primary cultures of anterior pituitary cells transfected with a rat PRL-luc construct. Measurement of luciferase activity by luminometry of extracts revealed that administration of TGF beta 1, over a range of doses shown by others to be secreted by cultures of pituitary cells, caused a significant (P < 0.05) suppression of PRL gene expression. In contrast, immunoremoval of secreted TGF beta 1 led to an elevation of PRL promoter-driven reporter activity in these cultures. In a subsequent study, we repeated these experiments with a single cell model in an attempt to determine the demographics of the cellular responses. Accordingly, we transfected (via microinjection) individual mammotropes with the rat PRL-luc construct; exposed them to TGF beta 1, its neutralizing antibody, or respective controls; and then assessed PRL gene expression in "real-time" by quantification. of photons emitted by the living cells after exposure to the substrate luciferin. Our results revealed that 1) TGF beta 1 inhibited PRL gene expression in all mammotrope studied; 2) only a subgroup of mammotropes (similar to 23%) was relieved of TGF beta 1 inhibition by antibody treatment; and 3) the growth factor exerted its inhibitory effect via a paracrine, as opposed to an autocrine, mechanism. These findings identify TGF beta 1 as the paracrine agent that exerts a tonic inhibitory influence over PRL gene expression in mammotropes.