TRANSFORMING-GROWTH-FACTOR BETA-1 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 t ested the hypothesis that this mammotrope-derived inhibitor is transfo rming 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 cul tures of anterior pituitary cells transfected with a rat PRL-luc const ruct. Measurement of luciferase activity by luminometry of extracts re vealed that administration of TGF beta 1, over a range of doses shown by others to be secreted by cultures of pituitary cells, caused a sign ificant (P < 0.05) suppression of PRL gene expression. In contrast, im munoremoval 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 P RL-luc construct; exposed them to TGF beta 1, its neutralizing antibod y, 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 exe rted 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.