Atlantic salmon HNF-3/forkhead: cDNA sequence, evolution, expression, and functional analysis

Roles of hepatocyte nuclear factors (HNF) in the regulation of reproduction in teleosts

Hepatocyte nuclear factor (HNF) families are composed of liver-enriched transcription factors and upstream regulators of many liver-specific genes. HNF are involved in liver-specific gene expression, metabolism, development, cell growth and many cellular functions in the body. HNF genes can be activated or influenced by several hormones and insulin-like growth factors (IGF), and different combinations of the four HNF factors form a network in controlling the expression of liver-specific or liver-enriched genes. The functions of these factors and their interactions within the gonads of bony fishes, however, are not well understood, and the related literature is scant. Recently, several members of the HNF families have been detected in teleost gonads together with their downstream genes (IGF-I and IGF-II), suggesting that these HNF could be upregulated in vitro by steroid hormones. Thus, the hormone-HNF-IGF-gonad interaction may be an alternative axis in the reproductive mechanism that acts in concert with the conventional hypothalamus-pituitary-gonad pathway. This may help the early development and maturation of the gonad or gamete, sexual maturity or reversion and spawning-regulating mechanisms among fishes to be understood.

Steroid hormones (17β-estradiol and hydrocortisone) upregulate hepatocyte nuclear factor (HNF)-3β and insulin-like growth factors I and II expression in the gonads of tilapia (Oreochromis mossambicus) in vitro

Hepatocyte nuclear factors (HNF-1alpha, -1beta and -3beta) and insulin-like growth factors (IGF-I and -II), which are involved in liver-specific gene expression, metabolism, development and cell growth, have been found in the gonads of tilapia (Oreochromis mossambicus). However, the functions of these factors and how they interact within the gonads of bony fish are not understood. In the present study, we provided experimental evidence that the expression of HNF-3beta in the gonads of tilapia, but not HNF-1alpha and -1beta, was affected in vitro by 17beta-estradiol and hydrocortisone. Immunohistochemical staining confirmed that tilapia HNF-3beta was mainly found in the nuclei of hepatocytes, the follicular granulosa cells of the ovaries, and the interstitial cells of the testes of adult tilapia. Further data were gathered at various steroid concentrations (0.1, 1, 10, 100, and 1000 nM) over various culture intervals (6, 12, 18, 24, 30, and 36 h) and subjected to semi-quantitative RT-PCR analysis. The expression of downstream genes (IGF-I and -II) followed the same temporal patterns as HNF-3beta, albeit at decreased levels for 30 and 36 h culture intervals. Both hormones upregulated HNF-3beta mRNA expression at concentrations of 0.1-10 nM, and reached optimal physiological concentrations for induction of IGFs at 1-10 nM. The identity of the PCR fragments was concurrently verified by sequencing and PCR-Southern hybridization. We inferred that HNF-3beta and IGFs may play a regulatory role in tilapia gonads during oocyte maturation and spermatogenesis.

Characterization of the seabass pancreatic alpha-amylase gene and promoter

Seabass (Lates calcarifer) pancreatic alpha-amylase gene was cloned and characterized. The alpha-amylase cDNA has 1620 bp and the deduced polypeptide has 522 amino acids. Southern blot indicated that there are two gene copies in the seabass genome. Sequence analysis showed that except for the loss of an intron in seabass, the coding region and the exon/intron boundaries are highly homologous to those of mammalian amylases. However, the promoter regions are distinctively divergent. To investigate the seabass amylase promoter, a series of deletion mutants was generated and fused to the luciferase reporter gene, followed by studies of their functional activity in rat AR42J cell line. Besides identifying several potential regulatory elements that have been previously identified in the human and mouse pancreatic amylase promoter, we have identified a glucocorticoid response element (GRE). However, while the human and mouse pancreatic amylase promoters are highly homologous between nucleotide -160 and transcription start site where GRE is located, the 5' promoter deletion mutants revealed that the GRE of the seabass amylase promoter was located far upstream -947 to -776 bp of the promoter. Site-directed mutagenesis of the putative GRE and electrophoretic mobility shift assays (EMSA) confirmed that this region was responsible for dexamethasone induction. However, no functional PTF-1 binding site, which is responsible for pancreas-specific transcription in higher vertebrates, was identified in seabass amylase promoter. Instead a Hepatocyte Nuclear Factor 3 binding site was found to modulate the amylase promoter expression. The evolutionary significance of this divergence in promoter regulation between seabass and mammals requires further studies.

Hepatocyte nuclear factors-1alpha, -1beta, and -3beta expressed in the gonad of tilapia (Oreochromis mossambicus)

Hepatocyte nuclear factors (HNFs) are upstream regulators of many liver-specific genes and are involved in many cellular functions in the body, but their existence, expression, and function in gonads are still poorly understood. Here we report on the first cloning of partial cDNAs of HNF-1alpha and -1beta and full HNF-3beta cDNA from a tilapia (Oreochromis mossambicus) liver cDNA library. The deduced amino acid sequence of tilapia HNF-3beta has a 90 to 96% identity with those of other fishes (dwarf gourami, medaka, and zebrafish), 74% with mammals (human, rat, and mouse), and 82% with Xenopus. RT-PCR detected IGF-I and -II and HNF-1alpha, -1beta, and -3beta in both liver and gonads and the identity of the PCR fragments was confirmed by PCR hybridization. Immunoprecipitation and Western blotting also detected all three HNF proteins in both liver and gonads. Expression of HNFs in the gonads of the tilapia suggests that multi-HNFs may form a cascade to regulate gonadal physiology in the bony fish.