ABSTRACT

The modulation of FSH secretion at the beginning and middle of the follicular phase of the cycle represents the key event in the growth and selection of the preovulatory follicle. However, the mechanisms that operate within the pituitary gland to control the increased release of FSH and its subsequent inhibition in vivo remain unclear. Treatment of ewes with bovine follicular fluid (bFF) during the luteal phase has been previously shown to suppress the plasma concentrations of FSH and, following cessation of treatment on day 11, a rebound release of FSH occurs on days 12 and 13. When luteal regression is induced on day 12, this hypersecretion of FSH results in an increase in follicle growth and ovulation rate. To investigate the mechanisms involved in the control of FSH secretion, ewes were treated with twice daily s.c. injections of 5 ml bFF on days 3–11 of the oestrous cycle and luteal regression was induced on day 12 with prostaglandin (PG). The treated ewes and their controls were then killed on day 11 (luteal), or 16 or 32h after PG and their pituitaries removed and halved. One half was analysed for gonadotrophin and gonadotrophin-releasing hormone (GnRH) receptor content. Total pituitary RNA was extracted from the other half and subjected to Northern analysis using probes for FSH-β, LH-β and common α subunit. Frequent blood samples were taken and assayed for gonadotrophins. FSH secretion was significantly (P<0.01) reduced during bFF treatment throughout the luteal phase and then significantly (P<0.01) increased after cessation of treatment, with maximum secretion being reached 18– 22h after PG, and then declining towards control values by 32h after PG. A similar pattern of LH secretion was seen after bFF treatment. Pituitary FSH content was significantly (P<0.05) reduced by bFF treatment at all stages of the cycle. No difference in the pituitary LH content was seen. The increase in GnRH receptor content after PG in the controls was delayed in the treated animals. Analysis of pituitary mRNA levels revealed that bFF treatment significantly (P<0.01) reduced FSH-β mRNA levels in the luteal phase. Increased levels of FSH-β, LH-β and α subunit mRNA were seen 16h after PG in the bFF-treated animals, at the time when FSH and LH secretion from the pituitary was near maximum. These results suggest that the rebound release of FSH after treatment with bFF (as a source of inhibin) is related to a rapid increase in FSH-β mRNA, supporting the concept that the rate of FSH release is directly related to the rate of synthesis.

ABSTRACT

While the regulation of gonadotrophin secretion by gonadotrophin-releasing hormone (GnRH) has been well documented in both rats and sheep, its role in the synthesis of gonadotrophin subunits remains unclear. We have investigated the effects of the specific inhibition of GnRH by a GnRH agonist on the expression of gonadotrophin subunit genes and the subsequent storage and release of both intact hormones and free α subunit.

Treatment with GnRH agonist for 6 weeks abolished pulsatile LH secretion, reduced plasma concentrations of FSH and prevented GnRH-induced release of LH and FSH. This was associated with a reduction of pituitary LH-β mRNA and FSH-β mRNA levels (to 5 and 30% of luteal control values respectively), but not α mRNA which was significantly increased (75% above controls). While there was a small decrease in the pituitary content of FSH (30% of controls), there was a drastic reduction in LH pituitary content (3% of controls). In contrast to the observed rise in α mRNA, there was a decrease in free α subunit in both the pituitary and plasma (to 30 and 80% of control levels).

These results suggest that, while GnRH positively regulates the expression of both gonadotrophin β-subunit genes, it can, under certain circumstances, negatively regulate α-subunit gene expression. Despite the complete absence of LH and FSH in response to GnRH, there remained a basal level of β-subunit gene expression and only a modest reduction (50%) in the plasma levels of both FSH and LH, suggesting that there is a basal secretory pathway. The dramatic reduction in LH pituitary content indicates that GnRH is required for the operation of a regulatory/storage pathway for the secretion of LH. There appears to be no similar mechanism for FSH. The LH-specific pathway is probably dependent upon the availability of LH-β subunits which subsequently plays a role in regulating α subunit by sequestering, assembling and storing the intact hormone in the presence of GnRH. Finally, in the absence of responsiveness to GnRH, the regulation of α-subunit production is not at the level of gene transcription. Inefficient translation of the mRNA or rapid degradation of the free α chain may account for the observed dramatic decrease in production of α subunit.

ABSTRACT

An efficient expression system in Escherichia coli for several biologically active insulin-like growth factor-I (IGF-I) fusion peptide analogues is described. These novel IGF-I fusion protein analogues have properties that make them very useful reagents in the investigation of IGF-I action. The analogues comprise an IGF-I sequence and the first 11 amino acids of methionyl porcine growth hormone (pGH) and include [Met¹]-pGH(1–11)-Val-Asn-IGF-I, which contains the authentic IGF-I sequence, and two analogues, [Met¹]-pGH(1–11)-Val-Asn-[Gly³]-IGF-I and [Met¹]-pGH(1–11)-Val-Asn-[Arg³]-IGF-I, where Glu3 in the human IGF-I sequence has been replaced by Gly or Arg respectively. The three peptides are referred to as Long IGF-I, Long [Gly³]-IGF-I or Long [Arg³]-IGF-I depending on the IGF-I sequence present. Production of the purified fusion peptides was aided by folding the reduced and denatured fusion peptide sequence under conditions that gave very high yields of biologically active product. Introduction of a hydrophobic N-terminal extension peptide appears to facilitate the correct folding of the IGF-I analogues compared with that obtained previously when folding normal-length IGFs. The biological activities of the IGF-I fusion peptides were compared with authentic IGF-I and the truncated analogue, des(1–3)IGF-I. In L6 rat myoblasts, all the analogues were more potent than authentic IGF-I in their abilities to stimulate protein and DNA synthesis and inhibit protein breakdown. In H35 hepatoma cells, where the IGFs act through the insulin receptor, the Long IGF-I analogues maintained a similar potency relative to IGF-I as was observed in the L6 myoblasts. The order of biological potency in cell lines secreting IGF-binding proteins (IGFBPs) into the medium was Long [Arg³]IGF-I-des(1–3)IGF-I>Long [Gly³]-IGF-I>Long IGF-I>IGF-I. In chicken embryo fibroblasts, a cell line that does not secrete detectable IGFBPs into the medium, Long [Arg³]-IGF-I, was less potent than IGF-I. Investigation of receptor and IGFBP association by these analogues reinforced our previous findings that N-terminal analogues of IGF-I show increased biological potency due to changes in the degree of their IGFBP interactions.

ABSTRACT

While the fetal pituitary synthesizes and releases prolactin, it is also produced within the utero-placental unit during pregnancy in women and has been localized in the amnion, chorion and decidua. However, it is not clear whether prolactin is synthesized within all these non-fetal pituitary tissues. We have investigated prolactin production and its gene expression using tissue culture, immunocytochemistry and in-situ hybridization techniques. Prolactin was immunolocalized not only in the decidua but also in amnion and trophoblast cells. In contrast, the in-situ hybridization results showed that silver grains, formed by specific hybridization of a prolactin cDNA probe to prolactin mRNA, were confined to decidual cells of early and term pregnancy. The results from tissue cultures correlated well with those of in-situ hybridization, that is that only the decidua made detectable prolactin, while it was undetectable in the culture medium from trophoblast tissue, irrespective of the stage of pregnancy. This study, for the first time, establishes that only decidualized cells are involved in biosynthesis of prolactin; other prolactin-containing cells in the amnion and trophoblast appear to sequester prolactin, possibly via receptors, suggesting that prolactin may play an important paracrine role within the amnion and syncitio- and cytotrophoblast of the utero-placental unit.

The interaction of IGF binding protein-2 (IGFBP-2) with IGF-I and -II has been investigated in solution using nuclear magnetic resonance (NMR) spectroscopy. Chemical shift perturbations in ¹⁵N- and ²H/¹⁵N-labelled IGF-I or -II upon binding to unlabelled thioredoxin-tagged bovine IGFBP-2 (Trx^1–279IGFBP-2) have been monitored to identify residues involved directly in the binding interaction as well as any affected by conformational changes associated with the interaction. A key step in obtaining high-quality spectra of the complexes was the use of transverse relaxation optimised spectroscopy (TROSY) methods with partially deuterated ligands. Indeed, because the effects of conformational averaging and aggregation are eliminated in IGF-I and -II bound to IGFBP-2, the spectra of the complexes are actually superior to those of the free ligands. Comparison of our results with the crystal structure of the complex between IGF-I and an N-terminal fragment of IGFBP-5 allowed identification of those residues perturbed by the C-domain of IGFBP-2. Other perturbations, such as those of Gly¹⁹ and Asp²⁰ of IGF-I (and the corresponding residues in IGF-II) – which are located in a reverse turn linking N-domain and C-domain interactive surfaces – are due to local conformational changes in the IGF-I and -II. Our results confirm that the C-domain of IGFBP-2 plays a key role in binding regions of IGF-I and -II that are also involved in binding to the type-1 IGF receptor and thereby blocking ligand binding to this receptor.