Characterization of the precursors of alpha and beta subunits of follitropin following cell-free translation of rat and ovine pituitary mRNAs

Cloning of the Elk TSH .BETA.-Subunit cDNA and Seasonal Expression of the Pituitary Glycoprotein Hormone Genes

We report the elk (Cervus elaphus) thyroid stimulating hormone (TSH) beta-subunit cDNA cloning, nucleotide and deduced amino acid sequences. The TSH beta-subunit cDNA was obtained by RT-PCR of polyadenylated pituitary RNA. The deduced elk TSH beta-subunit peptide chain shares between 93 to 99% sequence similarities with the reported TSH beta-subunit of a sub-set of related species. The TSH beta-subunit gene is expressed in the elk pituitary gland as a mature transcript of approximately 600 bases, which corresponds to the size of the mRNA expressed in the sheep pituitary gland. Seasonal expression of the pituitary gonadotropin genes was investigated by Northern blot analyses. Samples of elk pituitary glands collected during the breeding season showed elevated steady state levels of common alpha-subunit and FSH and LH beta-subunit gene expression, consistent with the seasonal reproductive cycling of this species. Samples collected before the breeding season demonstrated decreased expression of the gonadotropin genes. TSH, which is not directly tied to reproduction, had similar levels of expression, regardless of the animal's reproductive status.

Cloning and DNA sequence analysis of the cDNA for the precursor of porcine follicle stimulating hormone (FSH) beta subunit

A cDNA library of porcine anterior pituitary was constructed in an expression vector lambda gt11. The cDNAs encoding the porcine follicle stimulating hormone (pFSH) beta subunit were obtained using anti-human FSH beta antiserum or synthetic oligonucleotide. The nucleotide sequence of the pFSH beta subunit cDNA clone (929 base) has been determined. The cDNA contained a part of the signal sequence (six amino acids) and the whole amino acid sequence of the mature molecule (109 amino acids). The predicted protein sequence was different from that of the amino acid sequence analysis in ten residues and had an extended carboxyl terminus. Northern analysis showed that the length of the pFSH greater than subunit mRNA was about 1.8 kb.

Demonstration of high molecular weight forms of inhibin in bovine follicular fluid (bFF) by using monoclonal antibodies to bFF 32K inhibin

Monoclonal antibodies specifically recognizing each 20K and 13K subunit of bovine follicular fluid (bFF) 32K inhibin have been prepared. By immunoblotting procedures using these antibodies, we have demonstrated in bFF at least six inhibin forms, the apparent molecular weights of which are estimated to be 120K, 108K, 88K, 65K, 55K and 32K daltons, respectively. Amongst them 65K, 55K and 32K inhibin forms comprise two polypeptide subunits linked by disulfide bridge(s). In these inhibin forms a polypeptide of 13K daltons, a shorter subunit of bFF 32K inhibin, is attached by disulfide linkage(s) to polypeptides of 57K, 44K and 20K daltons, which are immunologically related to a larger subunit of the 32K inhibin, to yield 65K, 55K and 32K inhibins, respectively. On the other hand the higher molecular weight forms, 120K, 108K and 88K inhibins, are composed of three polypeptide subunits. In these forms a polypeptide of 62K daltons, immunologically related to the shorter subunit of bFF 32K inhibin, is attached by disulfide bridge(s) as the third component to the respective smaller inhibin forms which are composed of two subunits. These findings suggest that the complex formation of the subunit precursors may be extremely important and that restricted proteolytic cleavages following the complex formation may yield such divergent forms of inhibin in bFF.

The effect of castration and androgen replacement on rat pituitary follicle stimulating hormone investigated by reversed-phase high performance liquid chromatography

Rat follicle stimulating hormone (rFSH) was analysed in extracts of rat anterior pituitary using a newly developed separation method of reversed-phase high performance liquid chromatography (RP-HPLC) and post-column detection of immunoreactivity (rFSH-IR) by radioimmunoassay (RIA) or immunoprecipitation with a specific anti-rFSH antibody. Good separation of rFSH was achieved on a Waters Radial-Peak 8MBC18 cartridge with a linear (60 min) gradient from 10-60% acetonitrile at constant 0.1% trifluoroacetic acid concentration. The elution profiles of radioiodinated purified rFSH [(125I)rFSH] as well as extracts of adult male and female rat pituitaries contained two main immunoreactive components (peaks 1 and 2) which eluted at 16-17 min and 21 min respectively. In the (125I)rFSH elution profile, the peaks of immunobinding corresponded to two major peaks of radioactivity. The male pituitary showed greater binding to anti-rFSH-serum of peak 1 and 2 eluate compared to female pituitary. At day 20 after orchidectomy, the amount of immunoreactive rFSH (rFSH-IR) increased in both peaks (by 30% in peak 1 and 180% in peak 2); however, the rFSH-IR level in peak 2 appeared to be more variable compared with that in peak 1. Subcutaneous implantation of 5 mg or 15 mg of dihydrotestosterone (DHT) at the time of orchidectomy prevented the post-castration elevation of rFSH-IR at 10 and 20 days after surgery. The precise nature and physiological significance of the two elution peaks are not clear at this time; however, only peak 2 showed immunoreactivity with anti-rLH-alpha-subunit antibody (AFP 87713681) and the elution time of peak 2 coincided with that of (125I) rLH-alpha subunit (AFP-7264 B).(ABSTRACT TRUNCATED AT 250 WORDS)

Complementary DNA sequences of ovarian follicular fluid inhibin show precursor structure and homology with transforming growth factor-beta

Inhibin, a specific and potent polypeptide inhibitor of the secretion of follicle-stimulating hormone (FSH), of gonadal origin and thus a potential contraceptive, may constitute a missing link in the mechanism controlling the differential secretion of the pituitary gonadotropins. Inhibin-like bioactivity has been reported in various fluids and extracts of testis and in ovarian follicular fluid. Although there have been several attempts to purify inhibin from seminal plasma, purification from follicular fluid has been more successful (refs 14-16; for review see ref. 17). We have previously isolated two forms (A and B) of inhibin from porcine follicular fluid. Each form comprised two dissimilar subunits of relative molecular mass (Mr) 18,000 (18K, referred to here as the alpha-subunit) and 14K (the beta-subunit), crosslinked by one or more disulphide bridge(s). Forms A and B differ in the N-terminal sequence of their 14K subunit. Preliminary structural characterization of porcine and bovine ovarian inhibins shows that they have similar properties. Here, we have used the N-terminal amino-acid sequence data on the subunits of each inhibin to identify cloned complementary DNAs encoding the biosynthetic precursors and report that inhibins are the product of a gene family that also includes transforming growth factor-beta (TGF-beta) and whose structural organization is similar to that of pituitary and placental glycoprotein hormones.

Minimal quantities of FSH beta subunit are released by rat anterior pituitary cells in primary culture

FSH beta in the medium and extracts of cultured rat anterior pituitary cells was quantitated with a heterologous radioimmunoassay utilizing anti-rFSH beta and hFSH beta as the standard and tracer. After 4 days in culture, extracts were prepared from washed cells, cells incubated for 6 h or 24 h in fresh medium or cells incubated for 6 h in the presence of 10(-7) M GnRH. FSH beta/rFSH molar ratios were approximately 0.05, 0.04, 0.21 and 0.35, respectively. The elevation in FSH beta/rFSH molar ratios in 24 h basal and GnRH-treated cultures was due in part to an increase in intracellular FSH beta. Both unstimulated and GnRH-treated cultures contained non-detectable quantities of FSH beta in the medium (less than 0.063 ng/100 000 cells; FSH beta/rFSH molar ratio less than 0.024). Thus, it appears that cultured rat anterior pituitary cells contain a small amount of uncombined FSH beta but that minimal quantities are released. Furthermore, GnRH may increase intracellular quantities of FSH beta but does not induce its release.

Further characterization of the porcine LH subunits precursors in the translation products of pituitary RNAs

We have shown that the precursors of porcine LH alpha and beta subunits have similar apparent molecular weights (Biochem. Biophys. Res. Comm., 1984, 118, 254). The aim of the present study was to characterize physiochemical features which permit to distinguish these precursors from each other. We report here that these precursors can be clearly distinguished by both their tryptic digests analyzed by one-dimension electrophoresis and their behavior in two-dimension electrophoresis whose first dimension was made in a non equilibrium pH gradient (2D-NEPHGE). The 2D-NEPHGE method developed in this study appears also useful to estimate simultaneously the translation activities of pituitary hormone mRNAs without a previous immunoprecipitation.

Effect of gonadectomy on pituitary levels of mRNA encoding gonadotropin subunits and secretion of luteinizing hormone

Using cell-free translation of pituitary mRNAs we have investigated how, following gonadectomy in rats, the translational capacity of the specific messages encoding precursors to gonadotropin subunits alpha, LH-beta and FSH-beta increases with time. In parallel, serum LH was assayed in order to compare release and synthesis patterns. We observed a rapid rise in the rate of synthesis of all three precursors, with a significant increase already detectable 4 days after gonadectomy, and a plateau reached after 21 days. The kinetics were similar in both males and females, but maximal translational values for alpha-subunit were slightly higher in males. During the same time period, serum LH rapidly increased in the males, while in the females the rise of circulating LH was somewhat delayed. Although no direct correlation seems to exist between synthesis and release processes of gonadotropins, it is evident from our previous findings and the present data that both phenomena are dependent on gonadal steroids. In this respect, estradiol has been shown to regulate negatively, via different routes, the synthesis as well as the secretion of pituitary gonadotropins.

Quantitation of LH subunits released by rat anterior pituitary cells in primary culture

Immunoreactive rLH, rLH alpha, rLH beta and rFSH in the cell extracts and medium of rat anterior pituitary cell cultures were measured after gel filtration on Sephadex G-100 Superfine. Cell extracts contained uncombined rLH alpha and rLH beta present as approximately 40% and 4%, respectively, of native rLH on a molar basis. Unstimulated cultures appeared to release a large excess of rLH alpha (approximately 2560% of rLH on a molar basis) as well as a minimum of uncombined rLH beta. Stimulation of cultures with physiological (LHRH) or non-physiological agents (A23187, elevated K+) increased the absolute but not the relative quantities of uncombined subunits released relative to those present intracellularly. Thus, rat anterior pituitary cell cultures appear to produce and release an excess of free alpha subunit as well as a minimum of uncombined rLH beta. However, one cannot be certain whether the uncombined rLH beta represents a pool of 'free' rLH beta or simply results from the dissociation of rLH.

Orchiectomy in young rats results in differential regulation of follicle-stimulating hormone and luteinizing hormone content

While it is generally accepted that GnRH stimulates release of pituitary gonadotropins, it is not clear what regulates synthesis. The orchiectomized immature rat, with sustained high plasma levels of LH and FSH, provides an opportunity to study how gonadotropin biosynthesis responds to loss of the gonad. We have measured plasma and pituitary LH and FSH in castrate and sham operated rats after orchiectomy at 15, 30, 45, and 60 days of age. Plasma FSH and LH concentrations by RIA were markedly elevated in castrates within one to three days after castration, and they remained elevated in all groups. By contrast, pituitary content measurements revealed differences between the two gonadotropins: while LH content in castrates consistently exceeded that in controls, FSH content in castrates was lower. Pituitary LH excess was evident by seven to ten days after castration. The pituitary FSH deficit in younger animals was similarly apparent by seven to ten days. In the older groups, however, FSH content decreased as early as three days, but returned toward normal by 21 days. In orchiectomized young rats, pituitary LH is elevated, but FSH content is depressed. This discrepancy is delayed, but more marked, in younger rats. In view of sustained high plasma levels after castration, our findings imply differential regulation of synthesis, processing, or storage of the two gonadotropins.

In vitro translation of distinct mRNAs coding for the precursors of porcine LH subunits

Conditions are described to characterize and estimate the precursors of porcine LH alpha and beta subunits and indirectly their specific mRNAs. Poly(A) RNAs extracted from castrated male pig anterior pituitaries were translated in a wheat-germ system in the presence of [35S] cysteine and [35S] methionine. The translation products were precipitated by antisera directed against reduced and carboxymethylated LH alpha and beta subunits and analyzed by high resolution electrophoresis. It is shown that the precursors of pLH alpha and beta subunits are located in two distinct congruent to 15 K proteins and represent--on the basis of the incorporation of the [35S] labeled aminoacids into proteins--congruent to 0.12% and 0.05% respectively of the total translation products. It is suggested that in the pig, as in other species, the LH alpha and beta subunits are encoded by two distinct mRNAs, and at variance with other species the leader sequence of LH alpha mRNA is longer than that of LH beta mRNA.

Estradiol regulates mRNAs encoding precursors to rat lutropin (LH) and follitropin (FSH) subunits

Anterior pituitary mRNA was prepared using a microscaled method and translated in a wheat-germ cell-free system in the presence of [35S] labeled cysteine and methionine. Translation products, immunologically related to LH beta, FSH beta and the common subunit alpha, were isolated as precursors with antisera to denatured subunits and characterized by SDS-polyacrylamide gel electrophoresis and fluorography. The radioactive bands were excised from the gel and counted for quantitative evaluation. Our data show that translation of pituitary mRNAs from ovariectomized (ovx) rats results in precursor levels increased by 10 fold for alpha and 14 fold for LH beta as compared to the levels in normal rats. The increase in FSH beta precursor was impossible to evaluate as a specific immunoprecipitation product was undetectable in the case of normal rats. Estradiol, but not progesterone, administered in vivo to ovariectomized rats, reversed the stimulatory effect of ovariectomy on the expression of mRNAs coding for gonadotropin subunit precursors 48 h after injection. These results suggest that estradiol, but not progesterone, negatively regulates the synthesis of the pituitary gonadotropins in the female rats via changes in specific mRNA levels. This hormonal control probably occurs at the transcriptional level.

Biosynthesis of gonadotropin releasing hormone (GnRH): present status

Although the mechanism of GnRH synthesis has not yet been elucidated, many experimental data argue in favour of a ribosomal pathway resulting in the biosynthesis of a precursor with a size larger than the decapeptide. The existence of high mol. wt (HMW) immunoreactive forms of GnRH in hypothalamic extracts has been reported but without any evidence that they are precursors of the hypothalamic hormone. Some results obtained with immunocytological methods also suggest that at least a part of immunoreactive GnRH in perikarya might be extended on one or both its terminal ends. In our laboratory, we first investigated the process of GnRH biosynthesis in vivo by infusing tritiated amino acids into the third ventricle of rats. The results, along with the demonstration of the existence of HMW GnRH, support a ribosomal mechanism of GnRH synthesis. Also, preliminary data from our laboratory demonstrate that poly (A+) enriched RNA preparations from rat hypothalami contain compounds capable of hybridizing to a synthetic oligodeoxynucleotide probe which is complementary to a part of the GnRH sequence.