Thyrotropin-releasing hormone and gonadotropin-releasing hormone-associated peptide modulation of [Ca2+]i in human lactotrophs

Conservation of Three-Dimensional Helix-Loop-Helix Structure through the Vertebrate Lineage Reopens the Cold Case of Gonadotropin-Releasing Hormone-Associated Peptide

GnRH-associated peptide (GAP) is the C-terminal portion of the gonadotropin-releasing hormone (GnRH) preprohormone. Although it was reported in mammals that GAP may act as a prolactin-inhibiting factor and can be co-secreted with GnRH into the hypophyseal portal blood, GAP has been practically out of the research circuit for about 20 years. Comparative studies highlighted the low conservation of GAP primary amino acid sequences among vertebrates, contributing to consider that this peptide only participates in the folding or carrying process of GnRH. Considering that the three-dimensional (3D) structure of a protein may define its function, the aim of this study was to evaluate if GAP sequences and 3D structures are conserved in the vertebrate lineage. GAP sequences from various vertebrates were retrieved from databases. Analysis of primary amino acid sequence identity and similarity, molecular phylogeny, and prediction of 3D structures were performed. Amino acid sequence comparison and phylogeny analyses confirmed the large variation of GAP sequences throughout vertebrate radiation. In contrast, prediction of the 3D structure revealed a striking conservation of the 3D structure of GAP1 (GAP associated with the hypophysiotropic type 1 GnRH), despite low amino acid sequence conservation. This GAP1 peptide presented a typical helix-loop-helix (HLH) structure in all the vertebrate species analyzed. This HLH structure could also be predicted for GAP2 in some but not all vertebrate species and in none of the GAP3 analyzed. These results allowed us to infer that selective pressures have maintained GAP1 HLH structure throughout the vertebrate lineage. The conservation of the HLH motif, known to confer biological activity to various proteins, suggests that GAP1 peptides may exert some hypophysiotropic biological functions across vertebrate radiation.

Prolactin secretion and intracellular Ca(2+) change in rat lactotroph subpopulations stimulated by thyrotropin-releasing hormone

Thyrotropin-releasing hormone (TRH) may stimulate lactotrophs to increase intracellular Ca(2+) and to secrete prolactin (PRL). In this study, PRL contents in lactotrophs were determined by the sequential cell immunoblot assay (SCIBA) and their changes in intracellular Ca(2+) was analyzed by confocal microscopy. Significant correlations were found in the corresponding parameters between TRH treatments with a recovery interval of 2 h. Measuring the PRL contents after the first TRH treatment and then determining the intracellular Ca(2+) changes after the second TRH treatment revealed four lactotroph subpopulations. Type I cells (51%) showed significant responses of both PRL secretion and intracellular Ca(2+) concentration. Type II cells (22%) increased in PRL secretion, but without changes in intracellular Ca(2+). Type III cells (17%) have increased in intracellular Ca(2+), but without changes in PRL secretion. Type IV cells (10%) did not show changes in PRL secretion and intracellular Ca(2+).

Spontaneous and agonist-induced calcium oscillations in single human nonfunctioning adenoma cells

The effects of gonadotropin-releasing hormone (GnRH) and GnRH-associated peptide (GAP) on cytosolic free calcium concentration ([Ca(2+)](i)) were investigated in 20 human nonfunctioning pituitary adenomas. We divided these tumors into three classes according to their response pattern to hypothalamic peptides. In type I adenomas (8 out of 20 adenomas), GnRH and GAP mobilized intracellular calcium ions stored in a thapsigargin (TG)-sensitive store. For the same concentration of agonist, two distinct patterns of GnRH-GAP-induced Ca(2+) mobilization were observed (1) sinusoidal oscillations, and (2) monophasic transient. The latter is followed by a protein kinase C (PKC)-dependent increase in calcium influx through L-type channels. In type II adenomas (7 out of 20 adenomas), GnRH and GAP only stimulate calcium influx through dihydropyridine-sensitive Ca(2+) channels by a PKC-dependent mechanism. TG (1 μM) did not affect [Ca(2+)](i) in these cells, suggesting that they do not possess TG-sensitive Ca(2+) pools. All the effects of GnRH and GAP were blocked by an inhibitor of phospholipase C (PLC), suggesting that they were owing to the activation of the phosphoinositide turnover. Type I and type II adenoma cells showed spontaneous Ca(2+) oscillations that were blocked by dihydropyridines and inhibition of PKC activity. GnRH and GAP had no effect on the [Ca(2+)](i) of type III adenoma cells that were also characterized by a low resting [Ca(2+)](i) and by the absence of spontaneous Ca(2+) fluctuations. K(+)-induced depolarization provoked a reduced Ca(2+) influx, whereas TG had no effect on the [Ca(2+)](i) of type III adenoma cells. The variety of [Ca(2+)](i) response patterns makes these cells a good cell model for studying calcium homeostasis in pituitary cells.

Long-term effects of calcium availability on prolactin and protein synthesis in human decidual cells

Ever since decidual cells were recognized as the source of decidual prolactin (dPRL), very few reports have dealt with the role of calcium (Ca2+) on dPRL synthesis and release. In a recent work, we described the presence of T-type Ca2+ channels in these cells, giving Ca(2+)-dependent action potentials. However, we failed to demonstrate any action of decidual cell Ca2+ modulation on acute dPRL release, but observed only long-term effects. We have now investigated these effects on decidual protein and dPRL synthesis after 24 h treatments. When Ca2+ channel blockers or EGTA (2 mM) were added to the culture medium, dPRL release and [3H] leucine incorporation into proteins decreased. Increasing external Ca2+ up to 2 mM instead of 0.8 mM or changing the external K+ concentration (30 mM instead of 5.6) had no consequence on dPRL release, whereas 2 mM of Ca2+ enhanced total protein synthesis. No toxicity was noted with these treatments. Finally a possible effect of Ca2+ modulation on dPRL synthesis was studied using [35S] methionine. The specific activity of [35S] methionine on dPRL was similar in control and treated cells (EGTA, 2 mM Ca2+, cobalt). These results support the idea that Ca2+ controls dPRL synthesis in decidual cells, acting only on general protein synthesis processes.