Differential effect of age on transforming growth factor-beta 1 inhibition of prolactin gene expression versus secretion in rat anterior pituitary cells.

Abstract

Transforming growth factor-beta 1 (TGF-beta 1) synthesized in the pituitary may act as an autocrine/paracrine regulator of lactotrope function. We examined the effects of TGF-beta 1 on PRL messenger RNA (mRNA), PRL synthesis, and PRL secretion in cultured anterior pituitary (AP) cells from rats at different ages. APs excised from ovariectomized female Sprague-Dawley rats, either young(2-3 months old; average serum PRL: 9 ng/ml), middle-aged (11-12 months old; average serum PRL: 133 ng/ml), or old (24 months old; average serum PRL: 159 ng/ml), were dispersed and cultured for 5 days. Then, cells were washed and challenged with increasing doses of TGF-beta 1 (0-100 ng/ml) for 1-48 h in serum-free medium. Northern blot analysis showed an increase in basal PRL mRNA levels, and a decrease in responsiveness to TGF-beta 1 with age. TGF-beta 1 suppressed PRL mRNA in a dose- and time-dependent manner in cells from young rats. Maximum inhibition was observed at 0.5-1 ng/ml of TGF-beta 1. At 0.5 ng/ml TGF-beta 1, significant reduction in PRL mRNA was detected at 6 h, and maximum inhibition was observed at 12-48 h post TGF-beta 1 incubation. Cells from middle-aged rats were less responsive to TGF-beta 1, whereas cells from old rats did not seem to respond under our experimental conditions. In addition to its effect on PRL mRNA in young AP cells, TGF-beta 1 dose dependently inhibited the rate of PRL synthesis, as indicated by reduced [35S]methionine incorporation into immunoprecipitated PRL. Responsiveness of PRL synthesis to TGF-beta 1 inhibition also decreased with age; however, significant inhibition by TGF-beta 1 on PRL synthesis could still be observed in old AP cells. Analysis by RIA demonstrated that young AP cells produced lower levels (15 micrograms/10(6) cells.24 h) of PRL in culture medium than old AP cells (32 micrograms/10(6) cells.24 h). TGF-beta 1 decreased medium PRL levels in old AP cells as efficaciously as in young AP cells. Significant reduction in medium PRL secreted by young AP cells was observed at 3 h when changes in both PRL mRNA and PRL synthesis were not evident. Taken together, our data suggest that TGF-beta 1 affects PRL production at multiple levels. Moreover, its inhibition on PRL synthesis and mRNA expression, but not on PRL secretion, is age-related. Thus, TGF-beta 1 may play an important role in regulating lactotrope function during aging.