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ABSTRACT

We have elucidated the action of tri-iodothyronine (T₃) on the synthesis and secretion of seven major plasma proteins in a human hepatoblastoma cell line, Hep G2, and established an in vitro experimental model of human liver cells for the study of the mechanism of the action of thyroid hormone.

Hep G2 cells cultured in serum-free medium were treated with various concentrations of T₃. During the first 24 h of T₃ treatment, accumulation of α-fetoprotein in the medium was decreased in a dose-dependent manner (10⁻¹¹-10⁻⁸ m), and the inhibitory effect was enhanced during the second 24 h of T₃ treatment. On the other hand, α₁-antitrypsin accumulation in the medium during the second 24 h of hormone treatment was decreased by T₃ (10⁻⁹–10⁻⁸ m), although no change in accumulation was observed during the first 24 h of T₃ treatment.

The newly synthesized [³⁵S]Met-labelled α₁-acid glycoprotein was increased by T₃ and reached 3·4-fold within 37 h of 10⁻⁸ m T₃ treatment. The stimulatory effect increased time-dependently (4·6fold after 61 h). In contrast, the synthesis of α-fetoprotein was reduced to half of that of the control after T₃ treatment for 37 h.

Although the content of newly synthesized [³⁵S]α₁-antitrypsin was not affected by 10⁻⁸ m T₃ treatment during 3 days of hormone treatment, the accumulation of α₁-antitrypsin in the medium decreased to 87%; in contrast, total cellular newly synthesized α₁-antitrypsin increased to 105–130% of that of the control. From these results, it is suggested that α₁-antitrypsin secretion might be suppressed by T₃ treatment. However, T₃ did not affect the accumulation of albumin, transferrin, fibronectin and α₂-macroglobulin in the medium throughout the experiments.

It was shown that T₃ has diverse control mechanisms on the synthesis and secretion of plasma proteins in Hep G2 cells: stimulation (α₁-acid glycoprotein) or inhibition (α-fetoprotein) of plasma protein synthesis, and inhibition of protein secretion (α₁-antitrypsin).

ABSTRACT

Gonadotrophin-releasing hormone (GnRH) is considered to have an important role in the control of reproduction in salmonid fish, although we do not have any direct evidence. To clarify this problem by molecular techniques, we first determined the nucleotide sequence of the mRNA encoding the precursor of salmon-type GnRH (sGnRH) from the masu salmon, Oncorhynchus masou.

The masu salmon sGnRH precursor was composed of a signal peptide, sGnRH and a GnRH-associated peptide (GAP) which was connected to sGnRH by a Gly-Lys-Arg sequence. The amino acid sequence of sGnRH and Gly-Lys-Arg were highly conserved when compared with the corresponding regions of African cichlid sGnRH and mammalian GnRH precursors. However, the GAP region was markedly divergent, with a 66% amino acid similarity to African cichlid GAP and an 8·3–15% similarity to mammalian GAPs. Northern blot analysis indicated the presence of a single mRNA species of about 600 bases in the olfactory bulb and telencephalon and in the diencephalon. The signal was more intense in the former regions.

An in-situ hybridization study further revealed that sGnRH neurones were distributed in the olfactory nerve, the ventral part of the olfactory bulb, the ventral part of the telencephalon, the lateral preoptic area and the preoptic nucleus. The sGnRH neurones were thus longitudinally scattered between the olfactory nerve and the lateral preoptic area in the rostroventral part of brain. The intensity of the hybridization signals and the size of hybridization-positive somata were much greater in the olfactory nerve and the rostral olfactory bulb than in the other regions. Preoptic sGnRH neurones were scarcely detected in immature masu salmon, whereas they were more frequently observed in maturing animals. It is possible that the olfactory and the preoptic sGnRH neurones have different physiological roles in salmonid fish.

ABSTRACT

Salmon gonadotropin-releasing hormone (sGnRH) is considered to have an important role in the control of reproduction in salmonid fish. As a basis for understanding the physiological functioning of sGnRH at the molecular level, we characterized the nucleotide sequences of two types of cDNAs encoding the precursors of sGnRH in sockeye salmon (ss), Oncorhynchus nerka, by a cloning strategy based on reverse transcription-PCR. The two types of cDNAs are referred to as ss-pro-sGnRH-I and -II, and consisted of 435 and 481 bases respectively. Both precursors are predicted to contain a signal peptide, the hormone and a GnRH-associated peptide that is attached to the hormone via a Gly-Lys-Arg sequence. The presence of two types of mRNAs hybridizing with either cDNA was confirmed by Northern blot analysis of brain RNA from sockeye salmon, masu salmon, O. masou, and rainbow trout, O. mykiss. The ss-pro-sGnRH-I cDNA had 97·2% and 82·8% overall identity with sGnRH cDNA from masu salmon and putative sGnRH cDNA deduced from the gene of the Atlantic salmon, Salmo salar respectively, whereas the ss-pro-sGnRH-II cDNA had 80·0% and 91·2% overall identity with the former and the latter respectively. The nucleotide sequences of ss-sGnRH-I and -II cDNAs showed less similarity (79·3%). These results indicated that each salmonid species possesses two differing sGnRH genes. The results of Southern blot analysis using genomic DNA extracted from individuals support this evidence in sockeye salmon, masu salmon and rainbow trout.

ABSTRACT

A cDNA clone encoding the complete tilapia (a teleost fish, Oreochromis niloticus) cytochrome P450 aromatase (P450arom) was isolated from an ovarian follicle cDNA library. The deduced amino acid sequence (522 amino acid residues) had 72·2% and 59·5% homology with rainbow trout and catfish P450arom respectively, and about 50% homology with mammalian and avian P450arom. Expression of this cDNA in COS-7 cells produced a protein that converted exogenous testosterone to estrogens. Northern blots using a tilapia P450arom cDNA fragment and Western blots using an antiserum against a tilapia P450arom polypeptide fragment revealed a single P450arom mRNA (2·6 kb) and a single protein (59 kDa) in tilapia ovarian tissue respectively. These analyses also revealed that the levels of both P450arom mRNA and protein were low in early vitellogenic follicles, increased in midvitellogenic follicles, and declined to non-detectable levels in post-vitellogenic follicles. Changes in the ability of follicles to convert exogenous testosterone to estrogens (aromatase activity) were similar to those of P450arom mRNA and protein. These observations indicated that the capacity of tilapia ovarian follicles to synthesize estradiol-17β is closely related to the contents of P450arom mRNA and protein within them.

ABSTRACT

Human thyroid hormone receptor (c-erb A protein) produced by Escherichia coli expression vector plasmid was purified sequentially using polyethylenimine precipitation of DNA, hydroxylapatite column chromatography, ammonium sulphate precipitation, Sephacryl S-300 gel filtration and mono Q-Sepharose column chromatography. These column procedures resulted in 41.3-fold purification of 3,5,3′-tri-iodo-l-thyronine (T₃) binding activity over the initial E. coli extract. Purified protein as well as crude preparation showed high-affinity binding to T₃. The c-erb A protein enriched by column purification was further purified by electroelution after electrophoresis. Rabbit antibody against the c-erb A protein was prepared and used for the Western blotting analysis. The antibody recognized c-erb A protein but not the bacterial proteins in crude E. coli extract. When partially purified rat hepatic nuclear thyroid hormone receptor was analysed, a 56kDa receptor was specifically recognized by the antibody.