Effects of vitamin D 3 metabolites on production of prolactin and growth hormone in rat pituitary cells

Abstract

Recent animal studies and tissue-culture studies have led to the suggestion that prolactin (PRL) and growth hormone (GH) are involved in the regulation of 25-hydroxyvitamin d-1α-hydroxylase activity. This study was undertaken to investigate the effects of vitamin D on the production of PRL and GH. A clonal strain of rat-pituitary tumor cells (GH 3) that spontaneously synthesize and secrete PRL and GH was used as model system. The hormones were measured by radioimmunoassay, and the amount of hormones that accumulated in the medium during 24 h was used as a measure of prolactin. Treatment of the GH 3 cells with 1α, 25-dihydroxyvitamin D 3 (1α, 25-(OH) 2D) caused a parallel and dose-dependent decrease in the production of both PRL and GH, and the effects were significant at 10 −11 and 10 −10 M, respectively. The maximal inhibition (20–40% of controls) by 1α, 25-(OH) 2D was obtained at 10 −6 M after 6 days of treatment, and the effect was detectable after 2 days. Similar effects were observed with 25-hydroxy-vitamin D 3 (25-OHD) and the synthetic analog 1α-hydroxyvitamin D 3 (1α-OHD), but the ED 50 for these substances were more than 10 3 times higher than the corresponding concentration of 1α, 25-(OH) 2D. In contrast, there was no effect of 24,25-dihydroxyvitamin D 3 (24,25-(OH) 2D) (10 −11−10 −6M) on the production of PRL and GH. None of the vitamin D analogs affected cell growth measured as total cell protein. Equimolar concentrations (10 −7M) of 25-OHD, 1α-OHD and 24,25-(OH) 2D slightly reduced the inhibitory effect of 1α,25-(OH) 2D when used in combination. The stimulatory effects of thyroliberin (TRH) and estrogens on PRL production were partly inhibited by treatment with 1α,25-(OH) 2D. GH 3 cells pre-treated with 1α,25-(OH) 2D had similar TRH and 17β-estradiol-binding characteristics as untreated control cells. We conclude that, in GH 3 cells, 1α,25-(OH) 2D reduces the production of PRL and GH and strongly counteracts the actions of TRH and estrogens without affecting receptor affinity and concentration. Our results suggest a feed-back loop between the renal tubular cells and the anterior pituitary.