Inhibitory control of prolactin and Pit-1 gene promoters by dopamine. Dual signaling pathways required for D2 receptor-regulated expression of the prolactin gene

Nicotine acts directly on pituitary GH3 cells to inhibit prolactin promoter activity

We have employed the GH3 rat pituitary cell line to investigate whether nicotine can regulate prolactin (PRL) gene expression. Nicotine strongly inhibited (45%) transient expression of a construct containing the first 187 base-pairs of the rat PRL promoter cloned upstream of the chloramphenicol acetyl transferase (CAT) gene. This implies that nicotine acts directly on the GH3 cells to inhibit transcription directed by the PRL promoter. Expression of a control reporter construct containing the CAT gene under the control of the RSV promoter was not affected by exposure of the cells to nicotine, demonstrating that the effect of nicotine is promoter-specific. The inhibition by nicotine of PRL promoter activity was not blocked by hexamethonium, suggesting that this effect of nicotine may be mediated by a novel type of nicotine receptor previously described in frog pituitary cells. Nicotine was also observed to yield a concentration-dependent inhibition of the stimulation by thyrotrophin-releasing hormone (TRH) of PRL promoter activity, implying that nicotine can also interfere with hormonal regulation of the PRL gene. These results suggest that the reduced serum PRL levels that result from smoking may originate in part from decreased transcription of the PRL gene resulting from a direct effect of nicotine on pituitary PRL-secreting cells.

Changes in the ultrastructural distribution of prolactin and growth hormone mRNAs in pituitary cells of female rats after estrogen and bromocriptine treatment, studied using in situ hybridization with biotinylated oligonucleotide probes

The expression and distribution of prolactin (PRL) mRNA and their alterations induced by estrogen and bromocriptine were investigated using non-radioisotopic in situ hybridization (ISH) at the electron microscopic (EM) level. Our EM-ISH studies using biotinylated oligonucleotide probes showed that estrogen induced whirling changes of the rough endoplasmic reticulum (RER) of female rat PRL cells and increased transcription of PRL genes located on the polysomes of the whirling RER. The presence of mammosomatotroph cells in the rat pituitary gland was also verified in our EM-ISH studies. After bromocriptine administration, PRL cells contained many secretory granules due to the inhibition of secretion. Pre- and post-embedding EM-ISH and northern hybridization studies revealed that bromocriptine induced the distorted, vesiculated, and dilated RER, and also the suppressed PRL mRNA expression. The activity of protein kinase C (PKC), which mediates PRL gene expression, tended to be elevated by estrogen and suppressed by bromocriptine. Therefore, it is considered that the ultrastructural and quantitative changes in PRL mRNA expression evoked by estrogen and bromocriptine may be mediated by the intracellular signal transduction system, including PKC.

Neuroendocrine-specific expression of the human prohormone convertase 1 gene. Hormonal regulation of transcription through distinct cAMP response elements

Prohormone convertases are involved in the tissue-specific endoproteolytic processing of prohormones and neuropeptide precursors within the secretory pathway. In the present study, we have isolated genomic clones comprising the 5'-terminal region of the human prohormone convertase 1 (PC1) gene and identified and characterized the PC1 promoter region. We found multiple transcription start sites located within a 15-base pair region, 205 base pairs upstream of the translation start codon. The promoter region is not G+C-rich and does not contain a canonical TATA box nor a CAAT box. Transient expression assays with a set of human PC1 gene fragments containing progressive 5' deletions demonstrate that the proximal promoter region is capable of directing high levels of neuroendocrine-specific expression of reporter gene constructs. In addition, the proximal promoter region confers both basal and hormone-regulated promoter activity. Site-specific mutagenesis experiments demonstrate that two closely spaced cAMP response elements within the proximal promoter region direct cAMP-mediated hormonal regulation of transcription of the PC1 gene.

Calcitonin inhibits prolactin gene transcription in rat pituitary cells

Our recent studies have shown that calcitonin (CT)-like immunoreactive peptide is synthesized and released from cultured rat anterior pituitary (AP) cells, and may serve as a paracrine inhibitor of PRL release. The present studies investigated effects of CT on basal and TRH-induced PRL mRNA levels in rat AP and rat pituitary tumor GH(3) cells. CT attenuated steady-state PRL mRNA levels in a dose-dependent fashion in primary rat AP and GH(3) cells. The kinetics of CT action suggests that 100NM: CT caused a significant decline after 3 h, and the inhibition was sustained at least until the longest tested incubation period of 30 h. Results from nuclear run-on assays suggest that 100 nM CT decreased the rate of PRL gene transcription by 80% after 30 min of incubation. CT did not affect PRL mRNA levels in Ca(2+)-depleted GH(3) cells but dramatically decreased them in Ca(2+)-repleted cells. Bay K 8644 induced increase in PRL mRNA levels of Ca(2+)-repleted GH(3) cells and CT did not affect this increase. These results suggest that CT rapidly and selectively inhibits PRL gene transcription in primary AP and GH(3) cells, and support a possibility that CT-induced attenuation of PRL mRNA may involve cytoplasmic Ca(2+).

The D2 receptor: blocked transcription in GH3 cells and cellular pathways employed by D2A to regulate prolactin promoter activity

Although the GH3 line of somatolactotropic rat pituitary cells has proven useful for many regulation studies, the absence of functional D2 receptors on these cells long prevented their use in studies of dopaminergic action. However, it is now possible to employ GH3 cells expressing recombinant D2 receptors for such investigations. We have investigated both the level at which expression of functional D2 receptors in GH3 cells is blocked, and the cellular pathways employed by the major pituitary D2 receptor isoform, D2A, to inhibit prolactin (PRL) gene transcription. In run-off transcription assays with nuclei from either parental GH3 cells or a GH3 cell line stably expressing a D2A expression vector, Pit-1 gene transcription was detectable in either cell line, but only the latter cell line yielded detectable D2 receptor transcription, implying that the block in D2 receptor expression by GH3 cells is transcriptional. Further investigations employed GH3 cells transiently co-transfected with a D2A expression vector plus a rat PRL promoter construct (-1957)PRL-CAT. Pertussis toxin blocked repression by quinpirole, a D2 agonist, of PRL-CAT activity, demonstrating that this action is mediated by a pertussis toxin-sensitive G protein. The observations that neither of two agents expected to raise intracellular Ca2+, Bay K8644 or thyrotropin-releasing hormone, prevented quinpirole repression of PRL-CAT activity, and that the repressive effects on this construct of quinpirole and the Ca2+ channel antagonist were independent, suggested that regulation of intracellular Ca2+ levels does not play a major role in D2A-mediated repression of the PRL promoter.(ABSTRACT TRUNCATED AT 250 WORDS)

A dopamine-responsive domain in the N-terminal sequence of Pit-1. Transcriptional inhibition in endocrine cell types

The POU transcription factor Pit-1 activates the prolactin gene in pituitary lactotrophs and may integrate responses of the gene to external signals. To study the role of Pit-1 in dopaminergic inhibition of the prolactin gene, we transiently transfected Pit-1 and dopamine D2 receptor vectors into a series of heterologous cell lines and examined dopamine regulation of the prolactin gene promoter. Regulation was Pit-1-dependent in all cell lines tested. Moreover, dopamine responsiveness was cell type-specific: stimulatory in fibroblasts (COS-7) and muscle-type cells (P19/Me2SO-induced) and inhibitory in pancreatic endocrine (RIN, InR1-G9) and neural-like (P19/retinoic acid-induced) cells. Because dopaminergic responses in Pit-1-transfected RIN cells paralleled those in pituitary GH4 cells, the islet cell line was used to test for sequences in Pit-1 that mediate negative hormone signals. Dopamine responsiveness of the Pit-1 transactivation domain (residues 8-80) was examined using a chimeric LexA construct. LxPit-1, LxSp1, and Lx-glucocorticoid receptor fusions all activated basal transcription, but only LxPit-1 was regulated by dopamine. Regulatory responses of LxPit-1 and full-length Pit-1 were quantitatively similar. In addition, gain-of-function G alpha mutants that inhibit Pit-1-dependent promoters in GH4 cells also suppressed selectively Pit-1- or LxPit-1-dependent promoters in RIN cells. This demonstrates that Pit-1 can function as a specific target for distinct inhibitory G protein signals. Interestingly, Pit-1 sequences N-terminal to the DNA-binding POU domain appear to be sufficient in mediating regulation by these pathways.

Uptake of 1-methyl-4-phenylpyridinium and dopamine in the mouse brain cell nuclei

The neurotoxin N-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) causes, via its metabolite 1-methyl-4-phenylpyridinium (MPP+), parkinsonism in humans, monkeys, and mice but not in rats. When incubated with mouse brain homogenates, [3H]-MPP+ is recovered in relatively large concentrations in the brain cell nucleus. Although isolated cell nuclei from rat and mouse brain contain uptake systems for dopamine (DA), only brain cell nuclei from mice avidly take up [3H]MPP+. This nuclear uptake is ATP dependent and can be blocked by ouabain and N-ethylmaleimide. It is not, however, affected by neuronal and vesicular blockers such as benztropine, mazindol, and reserpine. Selective uptake of MPP+ into brain cell nuclei may provide a new avenue for future investigation into the complex modes of action of the neurotoxin MPTP.

Binding of a 100-kDa ubiquitous factor to the human prolactin promoter is required for its basal and hormone-regulated activity

cAMP strongly stimulates the activity of the human prolactin (hPRL) promoter. We have previously shown that two types of cis-element are required for this cAMP regulation; binding sites for the pituitary-specific factor Pit-1, and the sequence spanning nucleotides -115 to -85 (named sequence A). Sequence A contains the TGACG motif found in the consensus sequence of the cAMP-responsive element (CRE). In this study, we show that a mutation in the TGACG motif of sequence A strongly reduces not only the cAMP regulation but also the Ca2+ regulation and basal activity of the hPRL promoter. Furthermore, gel-shift assays indicate that the mutation prevents binding of a ubiquitous factor which is not the CRE-binding protein. Southwestern experiments suggest that this ubiquitous factor's molecular mass is approximately 100 kDa. We conclude that binding of a 100-kDa ubiquitous factor to sequence A is required for full basal and hormonal regulation of hPRL-promoter activity.