High level expression of Prop-1 gene in gonadotropic cell lines

Pituitary stem cells

The anterior pituitary is derived from Rathke's pouch precursors, which differentiate into specific hormone-secreting cell lineages. Sustained low postnatal and adult pituitary cell turnover is governed by stem/progenitor cells that undergo slow mitotic activity and give rise to hormone-secreting cells in response to physiological demands and feedback loops. Pituitary cell populations exhibit stem cell properties, which include stem cell marker expression, non-hormone expression, and the ability to self-renew and to potentially differentiate into any of five hormone-secreting cell lineages. Specific signaling pathways underlie differentiated pituitary cell development and regulation. Several validated pituitary stem cell models have been reported and have the potential for functional regeneration of pituitary hormone-secreting cell functions.

Pituitary development: a complex, temporal regulated process dependent on specific transcriptional factors

Pituitary organogenesis is a highly complex and tightly regulated process that depends on several transcription factors (TFs), such as PROP1, PIT1 (POU1F1), HESX1, LHX3 and LHX4. Normal pituitary development requires the temporally and spatially organised expression of TFs and interactions between different TFs, DNA and TF co-activators. Mutations in these genes result in different combinations of hypopituitarism that can be associated with structural alterations of the central nervous system, causing the congenital form of panhypopituitarism. This review aims to elucidate the complex process of pituitary organogenesis, to clarify the role of the major TFs, and to compile the lessons learned from functional studies of TF mutations in panhypopituitarism patients and TF deletions or mutations in transgenic animals.

Molecular cloning of LIM homeodomain transcription factor Lhx2 as a transcription factor of porcine follicle-stimulating hormone beta subunit (FSHβ) gene

We cloned the LIM-homeodomain protein LHX2 as a transcription factor for the porcine follicle-stimulating hormone β subunit gene (Fshβ) by the Yeast One-Hybrid Cloning System using the upstream region of -852/-746 bases (b) from the transcription start site, called Fd2, as a bait sequence. The reporter assay in LβT2 and CHO cells revealed the presence of an LHX2-responsive region other than Fd2. A potential LHX2 binding sequence was confirmed as AATTAAT containing a consensus homeodomain binding core sequence AATT by Systematic Evolution of Ligands by Exponential Enrichment analysis. DNase I footprinting demonstrated three AATTAAT sequences located at regions -835/-829, -818/-812 and -806/-800 b in the Fd2 region and 12 binding sites in the distal and proximal regions mostly containing an AATT-core sequence. RT-PCR analysis of Lhx2 expression during porcine fetal and postnatal pituitary development showed a gradual increase from fetal day (f) 40 to postnatal day (p) 8 followed by a slight decrease to p230, suggesting that LHX2 may play its role largely in the late fetal and postnatal periods. The analyses of Lhx2 expression in pituitary tumor-derived cell lines showed their expressions in cell lines including αT31, LβT2 and others. Since LHX2 was previously identified as a transcription factor for Cga and the in vitro experiments in the present study suggested that LHX2 regulated the expression of Fshβ, it is possible that LHX2 controls the synthesis of FSH at the transcription level.

The highly related LIM factors, LMO1, LMO3 and LMO4, play different roles in the regulation of the pituitary glycoprotein hormone α-subunit (αGSU) gene

LMO1, LMO3 and LMO4 were cloned from the adult porcine pituitary cDNA library. Amino acid sequences of porcine LMO1, LMO3 and LMO4 were highly conserved among mammalian species. Transfection assay of the pituitary-derived cell line LβT2 was carried out using the pituitary αGSU (glycoprotein hormone α-subunit) promoter (−1059/+12 b) fused to pSEAP2-Basic vector as a reporter gene. The results demonstrated that, whereas LMO4 showed no apparent effect, αGSU promoter activity was markedly repressed by LMO1 but activated by LMO3, indicating the different roles of the three highly homologous proteins, LMO1, LMO3 and LMO4. Knockdown assay by LMO siRNAs (small interfering RNAs) confirmed the above results for LMO1 and LMO3, whereas that by LMO4 siRNA increased the expression, indicating different modes of action. RT–PCR (reverse transcription–PCR) for total RNAs of several cell lines showed that LMO1 and LMO4 mRNAs were present ubiquitously in all cell lines, except for LMO1 in L929 cells. In contrast, LMO3 mRNA was abundant only in LβT4 and GH3 cells with only small amounts in LβT2 and MtT/S cells, indicating the cell-type-specific function of this protein. Real-time analyses of porcine pituitary ontogeny revealed that the three LMO genes are expressed during the fetal period and decline immediately afterwards, followed by a remarkably low level of LMO3 and LMO4 after birth. RT–PCR of the porcine tissues examined showed ubiquitous expression of LMO4, whereas LMO1 and LMO3 are expressed tissue specifically. Thus the present study demonstrated that three highly related LIM cofactors, LMO1, LMO3 and LMO4, have different effects on αGSU gene expression in the pituitary glands.

Activin A induces ovine follicle stimulating hormone beta using -169/-58 bp of its promoter and a simple TATA box

BACKGROUND

Activin A increases production of follicle stimulating hormone (FSH) by inducing transcription of its beta subunit (FSHB). This induction has been studied here in LbetaT2 gonadotropes using transient expression of ovine FSHBLuc (-4741 bp of ovine FSHB promoter plus exon/intron 1 linked to Luc). Several sequences between -169/-58 bp of the ovine FSHB proximal promoter are necessary for induction by activin A in LbetaT2 cells, but deletions between -4741/-752 bp decrease induction > 70% suggesting the existence of other important 5' sequences. Induction disappears if a minimal T81 thymidine kinase promoter replaces the ovine FSHB TATA box and 3' exon/intron. The study reported here was designed to determine if sequences outside -169/-58 bp are important for induction of ovine FSHB by activin A.

METHODS

Progressively longer deletions of ovine FSHBLuc were created between -4741/-195 bp. Deletions internal to this region were created also, but replaced with substitute DNA. The ovine FSHB TATA box region (-40/+3 bp) was replaced by thymidine kinase and rat prolactin minimal promoters, and substitutions were made in 3' intron/exon sequences. All constructs were tested for basal and activin A-induced expression in LbetaT2 cells.

RESULTS

Successive 5' deletions progressively lowered fold-induction by activin A from 9.5 to zero, but progressively increased basal expression. Replacing deletions with substitute DNA showed no changes in basal expression or fold-induction. Induction by activin A was supported by the minimal rat prolactin promoter (TATA box) but not the thymidine kinase promoter (no TATA box). Replacement mutations in the 3' region did not decrease induction by activin A.

CONCLUSION

The data show that specific ovine FSHB sequences 5' to -175 bp or 3' of the transcription start site are not required for induction by activin A. A minimal TATA box promoter supports induction by activin A, but the sequence between the TATA box and transcription start site seems unimportant.

Intracellular localization of porcine single-strand binding protein 2

We have previously cloned cofactor CLIM2 (Ldb1/NL1) as a binding protein for LIM homeodomain transcription factor and now seek a protein interacting with CLIM2. Ultimately, SSBP2 was cloned as CLIM2 binding protein from the adult porcine pituitary cDNA library by the Yeast Two-Hybrid System. The amino acid sequence of porcine SSBP2 shows a high identity (99%) with those of other mammalian species, man, and mouse. Using fluorescence protein-fused SSBP2 and its deletion mutants, we observed that SSBP2 overexpressed in CHO cells predominantly localizes in mitochondria. Expression of mutant SSBP2s demonstrated that the first 241 amino acid residues are responsive for the mitochondrial localization. When CLIM2 vector was co-transfected, SSBP2 changed its location to nuclei. The similar translocation was also observed when CLIM2 vector was transfected 17 h after the transfection of SSBP2 vector. The first 120 residues of SSBP2 are responsible for the nuclear localization by guidance with CLIM2. RT-PCR demonstrated that SSBP2 was expressed in the porcine pituitary from fetal 40 days to postnatal 230 days in both genders and in the variety of pituitary and non-pituitary tumor cell lines, indicating that SSBP2 is present ubiquitously and plays a universal function during fetal and postnatal pituitary development.

Molecular cloning and characterization of porcine homeodomain transcription factor Msx1

We cloned a porcine ortholog of homeodomain transcription factor Msx1 from the porcine pituitary cDNA library. The amino acid sequence of Msx1 shows high conservation among mammalian species. RT-PCR for porcine fetal and postnatal pituitaries showed that Msx1 is already expressed at early fetal day 40, decreases to a low level before birth and then remarkably decreases after birth. On the other hand, Msx1 expression was observed in all pituitary-derived cell line tested, with most in a gonadotrope lineage LbetaT4. Transfection assay demonstrated that Msx1 markedly repressed the basal Cga and Fshb gene expression, while Lhb expression was affected slightly. Taken together, Msx1 may play a role in repressing gene expression in the fetal and postnatal periods.

Identification and analysis of prophet of Pit-1-binding sites in human Pit-1 gene

Prophet of Pit-1 (Prop1) is a transcription factor that regulates Pit-1 gene expression. Because Pit-1 regulates the differentiation of pituitary cells and the expressions of GH, prolactin and TSHbeta genes, Prop1 mutation results in combined pituitary hormone deficiency in humans. However, Prop1-binding sites in human Pit-1 gene and the mechanism leading to combined pituitary hormone deficiency have remained unclear. In this study, we identified and analyzed Prop1-binding elements of the human Pit-1 gene. Prop1 stimulated the expression of the reporter plasmid containing Pit-1 gene from translation start site to -1340 dose dependently in GH3 cells. The activation by Prop1 was observed in GH3 and TtT/GF cells but not COS7, HeLa, JEG3, and HuH7 cells. Deletion analysis of Pit-1 gene showed that the Prop1-responsive elements were present within the -257-bp region. Within the -257-bp region, there are four elements similar to consensus sequence of paired-like transcription factors. Because Prop1 is a member of paired-like transcription factors, we assessed the elements. EMSA and transient transfection assay using the mutation of the elements revealed that the element from -63 to -53 (the proximal Prop1 binding element) was essential to Prop1-binding and Prop1-induced activation of Pit-1 reporter plasmid. A region at -8kb of human Pit-1 gene is similar to the distal region containing Prop1-binding elements in mouse Pit-1 gene. We showed the region functioned as an enhancer. Furthermore, chromatin immunoprecipitation assay showed that the proximal element could bind Prop1 in vivo cultured cells. Taken together, these findings indicated the novel functioning binding elements of Prop1 in human Pit-1 gene.

Cloning and characterization of porcine CArG binding factor A expression in the anterior pituitary

CArG binding factor A (CBF-A) is a transcription factor first isolated from mouse C2 myogenic cells. Several lines of evidence indicate that CBF-A is also present in the anterior pituitary lobe and participates in the process of development and cell transformation. This study was performed to clone porcine CBF-A and to investigate its roles in the porcine anterior pituitary lobe. The predicted amino acid sequence of porcine CBF-A showed a unique insertion of five TG-repeats in the N-terminal region in comparison with those of other mammals, whereas the other regions appeared to be mostly conserved including two RNA recognition motifs in the middle region. Investigation of the expression of CBF-A gene during porcine pituitary development by RT-PCR showed an exclusive and temporary decrease in expression level shortly after birth in both sexes that was gradually but insufficiently restored. The expression of fluorescence protein-fused CBF-A in CHO cells demonstrated that CBF-A is located in the nuclei. We examined whether CBF-A regulates the expression of pituitary hormone genes in CHO cells and found that CBF-A significantly stimulated the promoter activity of growth hormone and prolactin by about 2-fold but did not stimulate the LHbeta gene. The specific DNA binding ability of porcine CBF-A was examined using serial oligonucleotides, CArG box and CC(W)0-6GG (W=A or T). As a result, porcine CBF-A was shown to have a high binding affinity for double- and single-stranded CC(W)6GG but no affinity for the known sequences of the CBF-A-target genes. Accordingly, this study demonstrated that porcine CBF-A may play a role in regulating at least two pituitary hormone genes, GH and PRL.

Expression of Porcine FSH.BETA. Subunit Promoter-driven Herpes Simplex Virus Thymidine Kinase Gene in Transgenic Rats

A transgenic rat was established using the construct of porcine FSHbeta subunit promoter, the -852/+10 bp region, fused to a Herpes simplex virus thymidine kinase (HSV-TK) gene. Integration of the transgene was confirmed by PCR of tail DNA. RT-PCR of total RNAs of the pituitary, gonad, cerebellum, liver, kidney, adrenal gland, prostate, and uterus revealed that FSHbeta was only expressed in the pituitary. Analysis of the expression of reporter gene, HSV-TK, using two specific primer sets revealed that different transcripts were present in the pituitary and testis. The transcript initiated at the transcription initiation site of the porcine FSHbeta gene was detected in the pituitary, and another within the TK gene was found in the testis, indicating ectopic testis-specific expression. Immunohistochemistry of the pituitary glands of the transgenic rats for FSH and HSV-TK demonstrated that the FSH-producing cells also produced HSV-TK. The results indicated that the -852/+10 bp region of the FSHbeta promoter contains an element(s) that determines the tissue-specific expression. We succeeded in producing FSHbeta promoter-driven HSV-TK transgenic rats and were the first time to do so using an animal other than the mouse. The transgenic rats show male infertility that involves abnormal spermatogenesis. We also observed a decrease in the weight of the testis and epididymis, and both motile and living spermatozoa were absent in the epididymis. Consequently, the FSHbeta-HSV-TK transgenic rat will provide a useful model for studies on FSH function and male infertility.