Functional conservation of interactions between a homeodomain cofactor and a mammalian FTZ-F1 homologue

Prox1 Suppresses the Proliferation of Breast Cancer Cells via Direct Inhibition of c-Myc Gene Expression

Breast cancer is one of the most lethal malignancies in women worldwide and is characterized by rapid growth and low survival rates, despite advances in tumor biology and therapies. Novel therapeutic approaches require new insights into the molecular mechanisms of malignant transformation and progression. To this end, here, we identified Prox1 as a negative regulator of proliferation and tumor-related metabolism in breast cancer. In particular, we showed that breast tumors from human patients exhibited reduced levels of Prox1 expression, while high expression levels of Prox1 were associated with a favorable prognosis in breast cancer patients. Moreover, we experimentally demonstrated that Prox1 was sufficient to strongly suppress proliferation, migration, and the Warburg effect in human breast cancer cells without inducing apoptosis. Most importantly, over-expression of Prox1 inhibited breast tumor growth in vivo in both heterotopic and orthotopic xenograft mouse models. The anti-tumorigenic effect of Prox1 was mediated by the direct repression of c-Myc transcription and its downstream target genes. Consistently, c-Myc over-expression from an artificial promoter that was not targeted by Prox1 reversed Prox1's anti-tumor effects. These findings suggest that Prox1 has a tumor suppressive role via direct transcriptional regulation of c-Myc, making it a promising therapeutic gene for breast cancer.

A novel variant of NR5A1, p.R350W implicates potential interactions with unknown co-factors or ligands

INTRODUCTION

NR5A1 and NR5A2 belong to an orphan nuclear receptor group, and approximately 60% of their amino acid sequences are conserved. Transcriptional regulation of NR5A receptors depends on interactions with co-factors or unidentified ligands.

PURPOSE AND METHODS

We employed in vitro and in silico analysis for elucidating the pathophysiology of a novel variant in the ligand-binding domain of NR5A1, p.R350W which was identified from a 46,XY patient with atypical genitalia.

RESULTS

In the study, [1] reporter assays demonstrated that R350 is essential for NR5A1; [2] 3D model analysis predicted that R350 interacted with endogenous ligands or unknown cofactors rather than stabilizing the structure; [3] R350 is not conserved in NR5A2 but is specifically required for NR5A1; and [4] none of the 22 known missense variants of the ligand binding domain satisfied all the previous conditions [1]-[3], suggesting the unique role of R350 in NR5A1.

CONCLUSION

Our data suggest that NR5A1 has unidentified endogenous ligands or co-activators that selectively potentiate the transcriptional function of NR5A1 in vivo.

Cyclin D1 controls development of cerebellar granule cell progenitors through phosphorylation and stabilization of ATOH1

During development, neural progenitors are in proliferative and immature states; however, the molecular machinery that cooperatively controls both states remains elusive. Here, we report that cyclin D1 (CCND1) directly regulates both proliferative and immature states of cerebellar granule cell progenitors (GCPs). CCND1 not only accelerates cell cycle but also upregulates ATOH1 protein, an essential transcription factor that maintains GCPs in an immature state. In cooperation with CDK4, CCND1 directly phosphorylates S309 of ATOH1, which inhibits additional phosphorylation at S328 and consequently prevents S328 phosphorylation-dependent ATOH1 degradation. Additionally, PROX1 downregulates Ccnd1 expression by histone deacetylation of Ccnd1 promoter in GCPs, leading to cell cycle exit and differentiation. Moreover, WNT signaling upregulates PROX1 expression in GCPs. These findings suggest that WNT-PROX1-CCND1-ATOH1 signaling cascade cooperatively controls proliferative and immature states of GCPs. We revealed that the expression and phosphorylation levels of these molecules dynamically change during cerebellar development, which are suggested to determine appropriate differentiation rates from GCPs to GCs at distinct developmental stages. This study contributes to understanding the regulatory mechanism of GCPs as well as neural progenitors.

Prox1 inhibits neurite outgrowth during central nervous system development

During central nervous system (CNS) development, proper and timely induction of neurite elongation is critical for generating functional, mature neurons, and neuronal networks. Despite the wealth of information on the action of extracellular cues, little is known about the intrinsic gene regulatory factors that control this developmental decision. Here, we report the identification of Prox1, a homeobox transcription factor, as a key player in inhibiting neurite elongation. Although Prox1 promotes acquisition of early neuronal identity and is expressed in nascent post-mitotic neurons, it is heavily down-regulated in the majority of terminally differentiated neurons, indicating a regulatory role in delaying neurite outgrowth in newly formed neurons. Consistently, we show that Prox1 is sufficient to inhibit neurite extension in mouse and human neuroblastoma cell lines. More importantly, Prox1 overexpression suppresses neurite elongation in primary neuronal cultures as well as in the developing mouse brain, while Prox1 knock-down promotes neurite outgrowth. Mechanistically, RNA-Seq analysis reveals that Prox1 affects critical pathways for neuronal maturation and neurite extension. Interestingly, Prox1 strongly inhibits many components of Ca2+ signaling pathway, an important mediator of neurite extension and neuronal maturation. In accordance, Prox1 represses Ca2+ entry upon KCl-mediated depolarization and reduces CREB phosphorylation. These observations suggest that Prox1 acts as a potent suppressor of neurite outgrowth by inhibiting Ca2+ signaling pathway. This action may provide the appropriate time window for nascent neurons to find the correct position in the CNS prior to initiation of neurites and axon elongation.

Ras Pathways on Prox1 and Lymphangiogenesis: Insights for Therapeutics

Over the past couple of decades, lymphatics research has accelerated and gained a much-needed recognition in pathophysiology. As the lymphatic system plays heavy roles in interstitial fluid drainage, immune surveillance and lipid absorption, the ablation or excessive growth of this vasculature could be associated with many complications, from lymphedema to metastasis. Despite their growing importance in cancer, few anti-lymphangiogenic therapies exist today, as they have yet to pass phase 3 clinical trials and acquire FDA approval. As such, many studies are being done to better define the signaling pathways that govern lymphangiogenesis, in hopes of developing new therapeutic approaches to inhibit or stimulate this process. This review will cover our current understanding of the Ras signaling pathways and their interactions with Prox1, the master transcriptional switch involved in specifying lymphatic endothelial cell fate and lymphangiogenesis, in hopes of providing insights to lymphangiogenesis-based therapies.

The Orphan Nuclear Receptors Steroidogenic Factor-1 and Liver Receptor Homolog-1: Structure, Regulation, and Essential Roles in Mammalian Reproduction

Nuclear receptors are intracellular proteins that act as transcription factors. Proteins with classic nuclear receptor domain structure lacking identified signaling ligands are designated orphan nuclear receptors. Two of these, steroidogenic factor-1 (NR5A1, also known as SF-1) and liver receptor homolog-1 (NR5A2, also known as LRH-1), bind to the same DNA sequences, with different and nonoverlapping effects on targets. Endogenous regulation of both is achieved predominantly by cofactor interactions. SF-1 is expressed primarily in steroidogenic tissues, LRH-1 in tissues of endodermal origin and the gonads. Both receptors modulate cholesterol homeostasis, steroidogenesis, tissue-specific cell proliferation, and stem cell pluripotency. LRH-1 is essential for development beyond gastrulation and SF-1 for genesis of the adrenal, sexual differentiation, and Leydig cell function. Ovary-specific depletion of SF-1 disrupts follicle development, while LRH-1 depletion prevents ovulation, cumulus expansion, and luteinization. Uterine depletion of LRH-1 compromises decidualization and pregnancy. In humans, SF-1 is present in endometriotic tissue, where it regulates estrogen synthesis. SF-1 is underexpressed in ovarian cancer cells and overexpressed in Leydig cell tumors. In breast cancer cells, proliferation, migration and invasion, and chemotherapy resistance are regulated by LRH-1. In conclusion, the NR5A orphan nuclear receptors are nonredundant factors that are crucial regulators of a panoply of biological processes, across multiple reproductive tissues.

The role of prospero homeobox 1 (PROX1) expression in follicular thyroid carcinoma cells

The prospero homeobox 1 (Prox1) transcription factor is a key player during embryogenesis and lymphangiogenesis. Altered Prox1 expression has been found in a variety of human cancers, including papillary thyroid carcinoma (PTC). Interestingly, Prox1 may exert tumor suppressive or tumor promoting effect, depending on the tissue context. In this study, we have analyzed Prox1 expression in normal and malignant human thyroid carcinoma cell lines. Moreover, we determined the effect of Prox1 silencing and overexpression on the cellular processes associated with the metastatic potential of tumor cells: proliferation, migration, invasion, apoptosis and anchorage-independent growth, in the follicular thyroid carcinoma (FTC) FTC-133 cell line. We found that Prox1 expression was significantly higher in FTC-derived cells than in PTC-derived cells and normal thyroid, and it was associated with the PI3K/Akt signaling pathway. In the FTC-133 cells, it was associated with cell invasive potential, motility and wound closure capacities, but not with proliferation or apoptosis. Modifying Prox1 expression also induced substantial changes in the cytoskeleton structure and cell morphology. In conclusion, we have shown that Prox1 plays an important role in the development of FTC and that its suppression prevents, whereas its overexpression promotes, the malignant behavior of thyroid follicular cancer cells.

An HDAC3-PROX1 corepressor module acts on HNF4α to control hepatic triglycerides

The histone deacetylase HDAC3 is a critical mediator of hepatic lipid metabolism, and liver-specific deletion of HDAC3 leads to fatty liver. To elucidate the underlying mechanism, here we report a method of cross-linking followed by mass spectrometry to define a high-confidence HDAC3 interactome in vivo that includes the canonical NCoR-HDAC3 complex as well as Prospero-related homeobox 1 protein (PROX1). HDAC3 and PROX1 co-localize extensively on the mouse liver genome, and are co-recruited by hepatocyte nuclear factor 4α (HNF4α). The HDAC3-PROX1 module controls the expression of a gene program regulating lipid homeostasis, and hepatic-specific ablation of either component increases triglyceride content in liver. These findings underscore the importance of specific combinations of transcription factors and coregulators in the fine tuning of organismal metabolism.HDAC3 is a critical mediator of hepatic lipid metabolism and its loss leads to fatty liver. Here, the authors characterize the liver HDAC3 interactome in vivo, provide evidence that HDAC3 interacts with PROX1, and show that HDAC3 and PROX1 control expression of genes regulating lipid homeostasis.

Prox1 represses IL-2 gene expression by interacting with NFAT2

Interleukin-2 (IL-2) is critical for T lymphocyte activation and regulated by many transcriptional factors. Prospero-related homeobox 1 (Prox1) is a multifunctional transcription factor, which can work as either a transcriptional activator or repressor depending on the cellular and developmental environment. We previously reported the Prox1 expression in T cells, raising the possibility of Prox1 involvement in the regulation of T cell function and IL-2 production. Here we demonstrated that the Prox1 expression in CD4+ T cells was downregulated by T cell receptor (TCR) activation. Overexpression of Prox1 attenuated IL-2 production, while knockdown of endogenous Prox1 by small interfering RNA increased IL-2 expression. Mechanistically, we showed that Prox1 inhibited the IL-2 promoter activity, and associated with the minimal IL-2 promoter. Prox1 repressed the nuclear factor of activated T cells 2 (NFAT2)-dependent transactivation of IL-2 gene by physically binding to NFAT2. The N-terminal region of Prox1 was essential for the binding and repression. In summary, our findings established Prox1 as a negative regulator in IL-2 gene expression through the direct interaction with NFAT2.

Nuclear receptor NR5A2 controls neural stem cell fate decisions during development

The enormous complexity of mammalian central nervous system (CNS) is generated by highly synchronized actions of diverse factors and signalling molecules in neural stem/progenitor cells (NSCs). However, the molecular mechanisms that integrate extrinsic and intrinsic signals to control proliferation versus differentiation decisions of NSCs are not well-understood. Here we identify nuclear receptor NR5A2 as a central node in these regulatory networks and key player in neural development. Overexpression and loss-of-function experiments in primary NSCs and mouse embryos suggest that NR5A2 synchronizes cell-cycle exit with induction of neurogenesis and inhibition of astrogliogenesis by direct regulatory effects on Ink4/Arf locus, Prox1, a downstream target of proneural genes, as well as Notch1 and JAK/STAT signalling pathways. Upstream of NR5a2, proneural genes, as well as Notch1 and JAK/STAT pathways control NR5a2 endogenous expression. Collectively, these observations render NR5A2 a critical regulator of neural development and target gene for NSC-based treatments of CNS-related diseases.

The molecular signals regulating the decision of neural stem cells (NSC) to proliferate versus differentiate are unclear. Here, the authors identify the nuclear receptor NR5A2 as coordinating cell-cycle exit with differentiation of NSCs via direct actions on Ink4, Prox1, Notch1 and JAK/STAT.

Role of Nuclear Receptors in Central Nervous System Development and Associated Diseases

The nuclear hormone receptor (NHR) superfamily is composed of a wide range of receptors involved in a myriad of important biological processes, including development, growth, metabolism, and maintenance. Regulation of such wide variety of functions requires a complex system of gene regulation that includes interaction with transcription factors, chromatin-modifying complex, and the proper recognition of ligands. NHRs are able to coordinate the expression of genes in numerous pathways simultaneously. This review focuses on the role of nuclear receptors in the central nervous system and, in particular, their role in regulating the proper development and function of the brain and the eye. In addition, the review highlights the impact of mutations in NHRs on a spectrum of human diseases from autism to retinal degeneration.

Prospero-related homeobox 1 (Prox1) at the crossroads of diverse pathways during adult neural fate specification

Over the last decades, adult neurogenesis in the central nervous system (CNS) has emerged as a fundamental process underlying physiology and disease. Recent evidence indicates that the homeobox transcription factor Prox1 is a critical intrinsic regulator of neurogenesis in the embryonic CNS and adult dentate gyrus (DG) of the hippocampus, acting in multiple ways and instructed by extrinsic cues and intrinsic factors. In the embryonic CNS, Prox1 is mechanistically involved in the regulation of proliferation vs. differentiation decisions of neural stem cells (NSCs), promoting cell cycle exit and neuronal differentiation, while inhibiting astrogliogenesis. During the complex differentiation events in adult hippocampal neurogenesis, Prox1 is required for maintenance of intermediate progenitors (IPs), differentiation and maturation of glutamatergic interneurons, as well as specification of DG cell identity over CA3 pyramidal fate. The mechanism by which Prox1 exerts multiple functions involves distinct signaling pathways currently not fully highlighted. In this mini-review, we thoroughly discuss the Prox1-dependent phenotypes and molecular pathways in adult neurogenesis in relation to different upstream signaling cues and cell fate determinants. In addition, we discuss the possibility that Prox1 may act as a cross-talk point between diverse signaling cascades to achieve specific outcomes during adult neurogenesis.

Prox1 regulates Olig2 expression to modulate binary fate decisions in spinal cord neurons

Specification of spinal cord neurons depends on gene regulation networks that impose distinct fates in neural progenitor cells (NPCs). Olig2 is a key transcription factor in these networks by inducing motor neuron (MN) specification and inhibiting interneuron identity. Despite the critical role of Olig2 in nervous system development and cancer progression, the upstream molecular mechanisms that control Olig2 gene transcription are not well understood. Here we demonstrate that Prox1, a transcription repressor and downstream target of proneural genes, suppresses Olig2 expression and therefore controls ventral spinal cord patterning. In particular, Prox1 is strongly expressed in V2 interneuron progenitors and largely excluded from Olig2+ MN progenitors (pMN). Gain- and loss-of-function studies in mouse NPCs and chick neural tube show that Prox1 is sufficient and necessary for the suppression of Olig2 expression and proper control of MN versus V2 interneuron identity. Mechanistically, Prox1 interacts with the regulatory elements of Olig2 gene locus in vivo and it is critical for proper Olig2 transcription regulation. Specifically, chromatin immunoprecipitation analysis in the mouse neural tube showed that endogenous Prox1 directly binds to the proximal promoter of the Olig2 gene locus, as well as to the K23 enhancer, which drives Olig2 expression in the pMN domain. Moreover, plasmid-based transcriptional assays in mouse NPCs suggest that Prox1 suppresses the activity of Olig2 gene promoter and K23 enhancer. These observations indicate that Prox1 controls binary fate decisions between MNs and V2 interneurons in NPCs via direct repression of Olig2 gene regulatory elements.

PROX1 promotes metabolic adaptation and fuels outgrowth of Wnt(high) metastatic colon cancer cells

The Wnt pathway is abnormally activated in the majority of colorectal cancers, and significant knowledge has been gained in understanding its role in tumor initiation. However, the mechanisms of metastatic outgrowth in colorectal cancer remain a major challenge. We report that autophagy-dependent metabolic adaptation and survival of metastatic colorectal cancer cells is regulated by the target of oncogenic Wnt signaling, homeobox transcription factor PROX1, expressed by a subpopulation of colon cancer progenitor/stem cells. We identify direct PROX1 target genes and show that repression of a pro-apoptotic member of the BCL2 family, BCL2L15, is important for survival of PROX1(+) cells under metabolic stress. PROX1 inactivation after the establishment of metastases prevented further growth of lesions. Furthermore, autophagy inhibition efficiently targeted metastatic PROX1(+) cells, suggesting a potential therapeutic approach. These data identify PROX1 as a key regulator of the transcriptional network contributing to metastases outgrowth in colorectal cancer.

Hepatic glucose sensing and integrative pathways in the liver

The hepatic glucose-sensing system is a functional network of enzymes and transcription factors that is critical for the maintenance of energy homeostasis and systemic glycemia. Here we review the recent literature on its components and metabolic actions. Glucokinase (GCK) is generally considered as the initial postprandial glucose-sensing component, which acts as the gatekeeper for hepatic glucose metabolism and provides metabolites that activate the transcription factor carbohydrate response element binding protein (ChREBP). Recently, liver receptor homolog 1 (LRH-1) has emerged as an upstream regulator of the central GCK-ChREBP axis, with a critical role in the integration of hepatic intermediary metabolism in response to glucose. Evidence is also accumulating that O-linked β-N-acetylglucosaminylation (O-GlcNAcylation) and acetylation can act as glucose-sensitive modifications that may contribute to hepatic glucose sensing by targeting regulatory proteins and the epigenome. Further elucidation of the components and functional roles of the hepatic glucose-sensing system may contribute to the future treatment of liver diseases associated with deregulated glucose sensors.

Dedifferentiation of neurons precedes tumor formation in Lola mutants

The ability to reprogram differentiated cells into a pluripotent state has revealed that the differentiated state is plastic and reversible. It is evident, therefore, that mechanisms must be in place to maintain cells in a differentiated state. Transcription factors that specify neuronal characteristics have been well studied, but less is known about the mechanisms that prevent neurons from dedifferentiating to a multipotent, stem cell-like state. Here, we identify Lola as a transcription factor that is required to maintain neurons in a differentiated state. We show that Lola represses neural stem cell genes and cell-cycle genes in postmitotic neurons. In lola mutants, neurons dedifferentiate, turn on neural stem cell genes, and begin to divide, forming tumors. Thus, neurons rather than stem cells or intermediate progenitors are the tumor-initiating cells in lola mutants.

Transcription factor PROX1: its role in development and cancer

The homeobox gene PROX1 is critical for organ development during embryogenesis. The Drosophila homologue, known as prospero has been shown to act as a tumor suppressor by controlling asymmetric cell division of neuroblasts. Likewise, alterations in PROX1 expression and function are associated with a number of human cancers including hematological malignancies, carcinomas of the pancreas, liver and the biliary system, sporadic breast cancer, Kaposiform hemangioendothelioma, colon cancer, and brain tumors. PROX1 is involved in cancer development and progression and has been ascribed both tumor suppressive and oncogenic properties in a variety of different cancer types. However, the exact mechanisms through which PROX1 regulates proliferation, migration, and invasion of cancer cells are by large unknown. This review provides an update on the role of PROX1 in organ development and on its emerging functions in cancer, with special emphasis on the central nervous system and glial brain tumors.

Prospero-related homeobox 1 (Prox1) functions as a novel modulator of retinoic acid-related orphan receptors α- and γ-mediated transactivation

In this study, we identify Prospero-related homeobox 1 (Prox1) as a novel co-repressor of the retinoic acid-related orphan receptors, RORα and RORγ. Prox1 interacts directly with RORγ and RORα and negatively regulates their transcriptional activity. The AF2 domain of RORs is essential for the interaction, whereas Prox1 interacts with RORs through either its 28 amino acids N-terminal region or its C-terminal prospero-like domain. RORγ antagonists stabilize the interaction between RORγ and Prox1. The homeodomain and the interaction through the prospero-like domain of Prox1 are critical for its repression of ROR transcriptional activity. Chromatin immunoprecipitation analysis demonstrated that in liver, Prox1 is recruited to the ROR response element sites of the clock genes, brain and muscle Arnt-like protein 1 (Bmal1), neuronal PAS domain protein 2 (Npas2) and cryptochrome 1 (Cry1), as part of the same complex as RORs. Knockdown of Prox1 by siRNAs in human hepatoma Huh-7 cells increased the expression of RORγ and several ROR-target genes, along with increased histone acetylation at these ROR response element sites. Chromatin immunoprecipitation sequencing analysis suggests that Prox1 is a potential ROR target gene in liver, which is supported by the regulation of the rhythmic expression of Prox1 by RORγ. Our data suggest that Prox1 is part of a feedback loop that negatively regulates the transcriptional control of clock and metabolic networks by RORs.

The new era of the lymphatic system: no longer secondary to the blood vascular system

The blood and lymphatic systems are the two major circulatory systems in our body. Although the blood system has been studied extensively, the lymphatic system has received much less scientific and medical attention because of its elusive morphology and mysterious pathophysiology. However, a series of landmark discoveries made in the past decade has begun to change the previous misconception of the lymphatic system to be secondary to the more essential blood vascular system. In this article, we review the current understanding of the development and pathology of the lymphatic system. We hope to convince readers that the lymphatic system is no less essential than the blood circulatory system for human health and well-being.

Prox1 directly interacts with LSD1 and recruits the LSD1/NuRD complex to epigenetically co-repress CYP7A1 transcription

Cholesterol 7α-hydroxylase (CYP7A1) catalyzes the first and rate-limiting step in the classical pathway of bile acids synthesis in liver and is crucial for maintaining lipid homeostasis. Hepatocyte nuclear factor 4α (HNF4α) and α₁-fetoprotein transcription factor (FTF) are two major transcription factors driving CYP7A1 promoter activity in hepatocytes. Previous researches have shown that Prospero-related homeobox (Prox1) directly interacts with both HNF4α and FTF and potently co-represses CYP7A1 transcription and bile acid synthesis through unidentified mechanisms. In this work, mechanisms involved in Prox1-mediated co-repression were explored by identifying Prox1-associated proteins using immunoprecipitation followed by mass spectrometry (IP-MS) methodology. Multiple components of the epigenetically repressive lysine-specific demethylase 1 (LSD1)/nucleosome remodeling and histone deacetylase (NuRD) complex, most notably LSD1 and histone deacetylase 2 (HDAC2), were found to be associated with Prox1 and GST pulldown assay demonstrated that Prox1 directly interacts with LSD1. Sequential chromatin immunoprecipitation (ChIP) assays showed that Prox1 co-localizes with HNF4α, LSD1 and HDAC2 on CYP7A1 promoter in HepG2 cells. Furthermore, by using ChIP assay on HepG2 cells with endogenous Prox1 knocked down by RNA interference, Prox1 was shown to recruit LSD1 and HDAC2 onto CYP7A1 promoter and cause increased H3K4 demethylation. Finally, bile acids treatment of HepG2 cells, which significantly repressed CYP7A1 transcription, resulted in increased Prox1 and LSD1/NuRD complex occupancy on CYP7A1 promoter with a concurrent increase in H3K4 demethylation and H3/H4 deacetylation. These results showed that Prox1 interacts with LSD1 to recruit the repressive LSD1/NuRD complex to CYP7A1 promoter and co-represses transcription through epigenetic mechanisms. In addition, such Prox1-mediated epigenetic repression is involved in the physiologically essential negative feedback inhibition of CYP7A1 transcription by bile acids.

Lymphatic reprogramming by Kaposi sarcoma herpes virus promotes the oncogenic activity of the virus-encoded G-protein-coupled receptor

Kaposi sarcoma, the most common cancer in HIV-positive individuals, is caused by endothelial transformation mediated by the Kaposi sarcoma herpes virus (KSHV)-encoded G-protein-coupled receptor (vGPCR). Infection of blood vascular endothelial cells (BEC) by KSHV reactivates an otherwise silenced embryonic program of lymphatic differentiation. Thus, Kaposi sarcoma tumors express numerous lymphatic endothelial cell (LEC) signature genes. A key unanswered question is how lymphatic reprogramming by the virus promotes tumorigenesis leading to Kaposi sarcoma formation. In this study, we present evidence that this process creates an environment needed to license the oncogenic activity of vGPCR. We found that the G-protein regulator RGS4 is an inhibitor of vGPCR that is expressed in BECs, but not in LECs. RGS4 was downregulated by the master regulator of LEC differentiation PROX1, which is upregulated by KSHV and directs KSHV-induced lymphatic reprogramming. Moreover, we found that KSHV upregulates the nuclear receptor LRH1, which physically interacts with PROX1 and synergizes with it to mediate repression of RGS4 expression. Mechanistic investigations revealed that RGS4 reduced vGPCR-enhanced cell proliferation, migration, VEGF expression, and Akt activation and suppressed tumor formation induced by vGPCR. Our findings resolve long-standing questions about the pathologic impact of KSHV-induced reprogramming of host cell identity, and they offer biologic and mechanistic insights supporting the hypothesis that a lymphatic microenvironment is more favorable for Kaposi sarcoma tumorigenesis.

Interrenal organogenesis in the zebrafish model

In recent years, many genes that participate in the specification, differentiation and steroidogenesis of the interrenal organ, the teleostean homologue of the adrenal cortex, have been identified and characterized in zebrafish. In-depth studies of these genes have helped to delineate the morphogenetic steps of interrenal organ formation, as well as some of the molecular and cellular mechanisms that govern these processes. The co-development of interrenal tissue with the embryonic kidney (pronephros), surrounding endothelium and invading chromaffin cells has been analyzed, by virtue of the amenability of zebrafish embryos to a variety of genetic, developmental and histological approaches. Moreover, zebrafish embryos can be subject to molecular as well as biochemical assays for the unraveling of the transcriptional regulation program underlying interrenal development. To this end, the key mechanisms that control organogenesis and steroidogenesis of the zebrafish interrenal gland have been shown to resemble those in mammals, justifying the future utilization of zebrafish model for discovering novel genes associated with adrenal development and disease.

Epistatic rescue of Nkx2.5 adult cardiac conduction disease phenotypes by prospero-related homeobox protein 1 and HDAC3

RATIONALE

Nkx2.5 is one of the most widely studied cardiac-specific transcription factors, conserved from flies to man, with multiple essential roles in both the developing and adult heart. Specific dominant mutations in NKX2.5 have been identified in adult congenital heart disease patients presenting with conduction system anomalies and recent genome-wide association studies implicate the NKX2.5 locus, as causative for lethal arrhythmias ("sudden cardiac death") that occur at a frequency in the population of 1 in 1000 per annum worldwide. Haploinsufficiency for Nkx2.5 in the mouse phenocopies human conduction disease pathology yet the phenotypes, described in both mouse and man, are highly pleiotropic, implicit of unknown modifiers and/or factors acting in epistasis with Nkx2.5/NKX2.5.

OBJECTIVE

To identify bone fide upstream genetic modifier(s) of Nkx2.5/NKX2.5 function and to determine epistatic effects relevant to the manifestation of NKX2.5-dependent adult congenital heart disease.

METHODS AND RESULTS

A study of cardiac function in prospero-related homeobox protein 1 (Prox1) heterozygous mice, using pressure-volume loop and micromannometry, revealed rescue of hemodynamic parameters in Nkx2.5(Cre/+); Prox1(loxP/+) animals versus Nkx2.5(Cre/+) controls. Anatomic studies, on a Cx40(EGFP) background, revealed Cre-mediated knock-down of Prox1 restored the anatomy of the atrioventricular node and His-Purkinje network both of which were severely hypoplastic in Nkx2.5(Cre/+) littermates. Steady state surface electrocardiography recordings and high-speed multiphoton imaging, to assess Ca(2+) handling, revealed atrioventricular conduction and excitation-contraction were also normalized by Prox1 haploinsufficiency, as was expression of conduction genes thought to act downstream of Nkx2.5. Chromatin immunoprecipitation on adult hearts, in combination with both gain and loss-of-function reporter assays in vitro, revealed that Prox1 recruits the corepressor HDAC3 to directly repress Nkx2.5 via a proximal upstream enhancer as a mechanism for regulating Nkx2.5 function in adult cardiac conduction.

CONCLUSIONS

Here we identify Prox1 as a direct upstream modifier of Nkx2.5 in the maintenance of the adult conduction system and rescue of Nkx2.5 conduction disease phenotypes. This study is the first example of rescue of Nkx2.5 function and establishes a model for ensuring electrophysiological function within the adult heart alongside insight into a novel Prox1-HDAC3-Nkx2.5 signaling pathway for therapeutic targeting in conduction disease.

Estrogen receptor β induces antiinflammatory and antitumorigenic networks in colon cancer cells

Several studies suggest estrogen to be protective against the development of colon cancer. Estrogen receptor β (ERβ) is the predominant estrogen receptor expressed in colorectal epithelium and is the main candidate to mediate the protective effects. We have previously shown that expression of ERβ reduces growth of colorectal cancer in xenografts. Little is known of the actions of ERβ and its effect on gene transcription in colon cancers. To dissect the processes that ERβ mediates and to investigate cell-specific mechanisms, we reexpressed ERβ in three colorectal cancer cell lines (SW480, HT29, and HCT-116) and conducted genome-wide expression studies in combination with gene-pathway analyses and cross-correlation to ERβ-chromatin-binding sites. Although induced gene regulation was cell specific, overrepresentation analysis of functional classes indicated that the same biological themes, including apoptosis, cell differentiation, and regulation of the cell cycle, were affected in all three cell lines. Novel findings include a strong ERβ-mediated down-regulation of IL-6 and downstream networks with significant implications for inflammatory mechanisms involved in colon carcinogenesis. We also discovered cross talk between the suggested nuclear receptor coregulator PROX1 and ERβ, demonstrating that ERβ both regulates and shares target genes with PROX1. The influence of ERβ on apoptosis was further explored using functional studies, which suggested an increased DNA-repair capacity. We conclude that reexpression of ERβ induces transcriptome changes that, through several parallel pathways, converge into antitumorigenic capabilities in all three cell lines. We propose that enhancing ERβ action has potential as a novel therapeutic approach for prevention and/or treatment of colon cancer.

Emerging actions of the nuclear receptor LRH-1 in the gut

Liver receptor homolog-1 (NR5A2) is a nuclear receptor originally identified in the liver and mostly known for its regulatory role in cholesterol and bile acid homeostasis. More recently, liver receptor homolog-1 has emerged as a key regulator of intestinal function, coordinating unanticipated actions, such as cell renewal and local immune function with important implications to common intestinal diseases, including colorectal cancer and inflammatory bowel disease. Unlike most of the other nuclear receptors, liver receptor homolog-1 acts as a constitutively active transcription factor to drive the transcription of its target genes. Liver receptor homolog-1 activity however is to a major extent regulated by different corepressors and posttranslational modifications, which may account for its tissue-specific functions. This review will provide an update on the molecular aspects of liver receptor homolog-1 action and focus on some emerging aspects of its function in normal and diseased gut. This article is part of a Special Issue entitled: Translating nuclear receptors from health to disease.

Prox1 regulates the notch1-mediated inhibition of neurogenesis

During development of the spinal cord, Prox1 controls the balance between proliferation and differentiation of neural progenitor cells via suppression of Notch1 gene expression.

Activation of Notch1 signaling in neural progenitor cells (NPCs) induces self-renewal and inhibits neurogenesis. Upon neuronal differentiation, NPCs overcome this inhibition, express proneural genes to induce Notch ligands, and activate Notch1 in neighboring NPCs. The molecular mechanism that coordinates Notch1 inactivation with initiation of neurogenesis remains elusive. Here, we provide evidence that Prox1, a transcription repressor and downstream target of proneural genes, counteracts Notch1 signaling via direct suppression of Notch1 gene expression. By expression studies in the developing spinal cord of chick and mouse embryo, we showed that Prox1 is limited to neuronal precursors residing between the Notch1+ NPCs and post-mitotic neurons. Physiological levels of Prox1 in this tissue are sufficient to allow binding at Notch1 promoter and they are critical for proper Notch1 transcriptional regulation in vivo. Gain-of-function studies in the chick neural tube and mouse NPCs suggest that Prox1-mediated suppression of Notch1 relieves its inhibition on neurogenesis and allows NPCs to exit the cell cycle and differentiate. Moreover, loss-of-function in the chick neural tube shows that Prox1 is necessary for suppression of Notch1 outside the ventricular zone, inhibition of active Notch signaling, down-regulation of NPC markers, and completion of neuronal differentiation program. Together these data suggest that Prox1 inhibits Notch1 gene expression to control the balance between NPC self-renewal and neuronal differentiation.

Early during development, neural progenitor cells (NPCs) can either proliferate or differentiate into neurons. Thus, generation of the correct number of neurons is governed by a tightly regulated balance between proliferation and differentiation, and disruption of this balance can result in severe developmental deficits, malformations, or cancers. Notch1 is a member of the Notch family of receptors, which make up a highly conserved cell signaling system. Notch1 signaling has been shown to inhibit NPC differentiation and to promote self-renewal, thereby allowing NPCs to divide and progressively generate the enormous number of neurons present in the central nervous system. The molecular mechanism by which NPCs overcome Notch1-mediated inhibition in order to differentiate into neurons, however, is not completely understood. In this study, we show that Prox1, a homeobox transcriptional repressor, plays a fundamental role in the switch to differentiation by suppressing the expression of Notch1 receptor, thereby preventing newly produced neuronal precursors from receiving inhibitory signals from Notch ligands present in neighboring cells. This transcriptional repression may regulate cell cycle exit and differentiation of NPCs as they migrate towards different regions and adopt their final cell fates. We suggest that Prox1 may exert its known influence on embryonic development, organ morphogenesis, and cancer through its ability to counteract Notch1 signaling.

Kaposin-B enhances the PROX1 mRNA stability during lymphatic reprogramming of vascular endothelial cells by Kaposi's sarcoma herpes virus

Kaposi's sarcoma (KS) is the most common cancer among HIV-positive patients. Histogenetic origin of KS has long been elusive due to a mixed expression of both blood and lymphatic endothelial markers in KS tumor cells. However, we and others discovered that Kaposi's sarcoma herpes virus (KSHV) induces lymphatic reprogramming of blood vascular endothelial cells by upregulating PROX1, which functions as the master regulator for lymphatic endothelial differentiation. Here, we demonstrate that the KSHV latent gene kaposin-B enhances the PROX1 mRNA stability and plays an important role in KSHV-mediated PROX1 upregulation. We found that PROX1 mRNA contains a canonical AU-rich element (ARE) in its 3′-untranslated region that promotes PROX1 mRNA turnover and that kaposin-B stimulates cytoplasmic accumulation of the ARE-binding protein HuR through activation of the p38/MK2 pathway. Moreover, HuR binds to and stabilizes PROX1 mRNA through its ARE and is necessary for KSHV-mediated PROX1 mRNA stabilization. Together, our study demonstrates that kaposin-B plays a key role in PROX1 upregulation during lymphatic reprogramming of blood vascular endothelial cells by KSHV.

Kaposi's sarcoma (KS) is the most common cancer in HIV-positive patients and KS-associated herpes virus (KSHV) was identified as its causing agent. We and others have discovered that when the virus infects endothelial cells of blood vessels, KSHV reprograms the cell type resembling endothelial cells in lymphatic vessels. Although endothelial cells of the blood vascular system and of the lymphatic system share functional similarities, the cell type-reprogramming does not occur under a normal physiological condition. Therefore, cell-fate reprogramming by the cancer-causing virus KSHV provides an important insight into the molecular mechanism for viral pathogenesis. Our current study investigates the molecular mechanism underlying the KSHV-mediated cell fate reprogramming. We identified that a KSHV latent gene kaposin-B plays an important role in KSHV-mediated regulation of PROX1 to promote PROX1 mRNA stability. This study will provide a better understanding on the tumorigenesis and pathogenesis of KS with a potential implication toward new KS therapy.

Heterogeneity and plasticity of lymphatic endothelial cells

Endothelial cells are found in most organs and tissues in our body. Despite their apparent morphological and functional similarities, endothelial cells exhibit remarkable heterogeneity and plasticity. In a strict sense, no two endothelial cells are identical in terms of their biological, immunological, functional, metabolic, morphological, and anatomical aspects. Their heterogeneity and plasticity are now known to be dependent upon and conferred by their microenvironments, arteriovenous-lymphatic cell identity, organ-specific vascular beds, fluid dynamics, vessel sizes, anatomical locations, physiological and pathological states, and more. Although abundant evidence is available to demonstrate endothelial heterogeneity in the blood vascular system, studies of heterogeneity and plasticity of lymphatic endothelial cells are limited because of the short history of lymphatic research. Nonetheless, a growing body of exciting work has begun to discover that lymphatic endothelial cells are as heterogeneous as blood vascular endothelial cells. In this article, we discuss the heterogeneity and plasticity of lymphatic endothelial cells.

miR-31 functions as a negative regulator of lymphatic vascular lineage-specific differentiation in vitro and vascular development in vivo

The lymphatic vascular system maintains tissue fluid homeostasis, helps mediate afferent immune responses, and promotes cancer metastasis. To address the role microRNAs (miRNAs) play in the development and function of the lymphatic vascular system, we defined the in vitro miRNA expression profiles of primary human lymphatic endothelial cells (LECs) and blood vascular endothelial cells (BVECs) and identified four BVEC signature and two LEC signature miRNAs. Their vascular lineage-specific expression patterns were confirmed in vivo by quantitative real-time PCR and in situ hybridization. Functional characterization of the BVEC signature miRNA miR-31 identified a novel BVEC-specific posttranscriptional regulatory mechanism that inhibits the expression of lymphatic lineage-specific transcripts in vitro. We demonstrate that suppression of lymphatic differentiation is partially mediated via direct repression of PROX1, a transcription factor that functions as a master regulator of lymphatic lineage-specific differentiation. Finally, in vivo studies of Xenopus and zebrafish demonstrated that gain of miR-31 function impaired venous sprouting and lymphatic vascular development, thus highlighting the importance of miR-31 as a negative regulator of lymphatic development. Collectively, our findings identify miR-31 is a potent regulator of vascular lineage-specific differentiation and development in vertebrates.

The nuclear hormone receptor Coup-TFII is required for the initiation and early maintenance of Prox1 expression in lymphatic endothelial cells

The homeobox gene Prox1 is crucial for mammalian lymphatic vascular development. In the absence of Prox1, lymphatic endothelial cells (LECs) are not specified. The maintenance of LEC identity also requires the constant expression of Prox1. However, the mechanisms controlling the expression of this gene in LECs remain poorly understood. The SRY-related gene Sox18 is required to induce Prox1 expression in venous LEC progenitors. Although Sox18 is also expressed in embryonic arteries, these vessels do not express Prox1, nor do they give rise to LECs. This finding suggests that some venous endothelial cell-specific factor is required for the activation of Prox1. Here we demonstrate that the nuclear hormone receptor Coup-TFII is necessary for the activation of Prox1 in embryonic veins by directly binding a conserved DNA domain in the regulatory region of Prox1. In addition, we show that the direct interaction between nuclear hormone receptors and Prox1 is also necessary for the maintenance of Prox1 expression during early stages of LEC specification and differentiation.

Prospero-related homeobox protein (Prox1) inhibits hepatitis B virus replication through repressing multiple cis regulatory elements

Hepatitis B virus (HBV) gene transcription is controlled by viral promoters and enhancers, the activities of which are regulated by a number of cellular factors as well as virally encoded proteins. Negative regulation of HBV cis-element activities by cellular factors has been reported less widely than their activation. In this study, we report that nuclear factor Prospero-related homeobox protein (Prox1) represses HBV antigen expression and genome replication in cultured hepatocytes. By using reporter-gene analysis, three of the four HBV promoters, namely the enhancer II/core promoter (ENII/Cp), preS1 promoter (Sp1) and enhancer I/X promoter, were identified as targets for Prox1-mediated repression. Mechanistic analysis then revealed that, for ENII/Cp, Prox1 serves as a corepressor of liver receptor homologue 1 (LRH-1) and downregulates LRH-1-mediated activation of ENII/Cp, whereas for Sp1, Prox1 partially represses Sp1 activity by interacting directly with hepatocyte nuclear factor 1. Identification of Prox1 as an HBV repressor will help in the understanding of detailed interactions between viral cis elements and host cellular factors and may also form the basis for new anti-HBV intervention therapeutics.

Repression of interferon-gamma expression in T cells by Prospero-related homeobox protein

Interferon-gamma (IFN-gamma) is a major proinflammatory effector and regulatory cytokine produced by activated T cells and NK cells. IFN-gamma has been shown to play pivotal roles in fundamental immunological processes such as inflammatory reactions, cell-mediated immunity and autoimmunity. A variety of human disorders have now been linked to irregular IFN-gamma expression. In order to achieve proper IFN-gamma-mediated immunological effects, IFN-gamma expression in T cells is subject to both positive and negative regulation. In this study, we report for the first time the negative regulation of IFN-gamma expression by Prospero-related Homeobox (Prox1). In Jurkat T cells and primary human CD4+ T cells, Prox1 expression decreases quickly upon T cell activation, concurrent with a dramatic increase in IFN-gamma expression. Reporter analysis and chromatin immunoprecipitation (ChIP) revealed that Prox1 associates with and inhibits the transcription activity of IFN-,gammapromoter in activated Jurkat T cells. Co-immunoprecipitation and GST pull-down assay demonstrated a direct binding between Prox1 and the nuclear receptor peroxisome proliferator-activated receptor gamma (PPPARgamma, which is also an IFN-gamma repressor in T cells. By introducing deletions and mutations into Prox1, we show that the repression of IFN-gamma promoter by Prox1 is largely dependent upon the physical interaction between Prox1 and PPPARgamma Furthermore, PPPARgammaantagonist treatment removes Prox1 from IFN-gamma promoter and attenuates repression of IFN-gamma expression by Prox1. These findings establish Prox1 as a new negative regulator of IFN-gamma expression in T cells and will aid in the understanding of IFN-gamma transcription regulation mechanisms.

PCNA interacts with Prox1 and represses its transcriptional activity

PURPOSE

Prox1 is a transcription factor which can function either as a transcriptional activator, transcriptional repressor or a transcriptional corepressor. This paper seeks to better understand the role of protein-protein interactions in this multitude of functions.

METHODS

We performed a yeast two-hybrid screen of an 11.5 day post coitum (dpc) mouse embryo cDNA library using the homeo-Prospero domain of Prox1 as bait. Computer modeling, cotransfection analysis and confocal immunolocalization were used to investigate the significance of one of the identified interactions.

RESULTS

Proliferating cell nuclear antigen (PCNA) was identified as a Prox1 interacting protein. Prox1 interactions with PCNA require the PCNA interacting protein motif (PIP box), located in the Prospero domain of Prox1. Computer modeling of this interaction identified the apparent geometry of this interface which maintains the accessibility of Prox1 to DNA. Prox1 activated the chicken betaB1-crystallin promoter in cotransfection tests as previously reported, while PCNA squelched this transcriptional activation.

CONCLUSIONS

Since PCNA is expressed in the lens epithelium where Prox1 levels are low, while chicken betaB1-crystallin expression activates in lens fibers where Prox1 expression is high and PCNA levels are low, these data suggest that Prox1-PCNA interactions may in part prevent the activation of betaB1-crystallin expression in the lens epithelium.

Prox1 physically and functionally interacts with COUP-TFII to specify lymphatic endothelial cell fate

Specification of endothelial cell (EC) fate during vascular development is controlled by distinct key regulators. While Notch plays an essential role in induction of arterial phenotypes, COUP-TFII is required to maintain the venous EC identity. Homeodomain transcription factor Prox1 functions to reprogram venous ECs to lymphatic endothelial cells (LECs). Here, we report that the venous EC fate regulator COUP-TFII is expressed in LECs throughout development and physically interacts with Prox1 to form a stable complex in various cell types including LECs. We found that COUP-TFII functions as a coregulator of Prox1 to control several lineage-specific genes including VEGFR-3, FGFR-3, and neuropilin-1 and is required along with Prox1 to maintain LEC phenotype. Together, we propose that the physical and functional interactions of the 2 proteins constitute an essential part in the program specifying LEC fate and may provide the molecular basis for the hypothesis of venous EC identity being the prerequisite for LEC specification.

Prospero-related homeobox 1 and liver receptor homolog 1 coordinately regulate long-term proliferation of murine fetal hepatoblasts

During early to late-fetal liver development, bipotential hepatoblasts proliferate and differentiate into hepatocytes and cholangiocytes. The prospero-related homeobox 1 gene (Prox1) is expressed in hepatoblasts, and the inactivation of Prox1 causes defective early liver development, in particular, faulty migration of fetal hepatoblasts. Prox1 binds to another hepatocyte-enriched transcription factor, liver receptor homolog 1 (Lrh1), and suppresses its transcriptional activity. However, the molecular mechanism by which Prox1 and Lrh1 regulate the characteristics of fetal hepatic cells remains unknown. We investigated the contribution of Prox1 and Lrh1 in early liver development. Embryonic day 13 liver-derived CD45-Ter119-Dlk+ cells were purified as fetal hepatic stem/progenitor cells, and formation of colonies derived from single cells was detected under low-density culture conditions. We found that overexpression of Prox1 using retrovirus infection induced migration and proliferation of fetal hepatic stem/progenitor cells. In contrast, overexpression of Lrh1 suppressed colony formation. Prox1 induced the long-term proliferation of fetal hepatic stem/progenitor cells, which exhibited both high proliferative activity and bipotency for differentiation. Prox1 up-regulated expression of cyclins D2, E1, and E2, whereas it suppressed expression of p16(ink4a), the cdk inhibitor. In addition, overexpression of Prox1 significantly inhibited the proximal promoter activity of p16(ink4a).

CONCLUSION

These results suggested that Prox1 and Lrh1 coordinately regulate development of hepatic stem/progenitor cells and that Prox1 induces fetal hepatocytic proliferation through the suppression of the promoter activity of p16(ink4a).

Adrenomedullin signaling is necessary for murine lymphatic vascular development

The lymphatic vascular system mediates fluid homeostasis, immune defense, and tumor metastasis. Only a handful of genes are known to affect the development of the lymphatic vasculature, and even fewer represent therapeutic targets for lymphatic diseases. Adrenomedullin (AM) is a multifunctional peptide vasodilator that transduces its effects through the calcitonin receptor-like receptor (calcrl) when the receptor is associated with a receptor activity-modifying protein (RAMP2). Here we report on the involvement of these genes in lymphangiogenesis. AM-, calcrl-, or RAMP2-null mice died mid-gestation after development of interstitial lymphedema. This conserved phenotype provided in vivo evidence that these components were required for AM signaling during embryogenesis. A conditional knockout line with loss of calcrl in endothelial cells confirmed an essential role for AM signaling in vascular development. Loss of AM signaling resulted in abnormal jugular lymphatic vessels due to reduction in lymphatic endothelial cell proliferation. Furthermore, AM caused enhanced activation of ERK signaling in human lymphatic versus blood endothelial cells, likely due to induction of CALCRL gene expression by the lymphatic transcriptional regulator Prox1. Collectively, our studies identify a class of genes involved in lymphangiogenesis that represent a pharmacologically tractable system for the treatment of lymphedema or inhibition of tumor metastasis.

Human NCU-G1 can function as a transcription factor and as a nuclear receptor co-activator

Background

Novel, uncharacterised proteins represent a challenge in biochemistry and molecular biology. In this report we present an initial functional characterization of human kidney predominant protein, NCU-G1.

Results

NCU-G1 was found to be a highly conserved nuclear protein rich in proline with a molecular weight of approximately 44 kDa. It is localized on chromosome 1 and consists of 6 exons. Analysis of the amino acid sequence revealed no known transcription activation domains or DNA binding regions, however, four nuclear receptor boxes (LXXLL), and four SH3-interaction motives in addition to numerous potential phosphorylation sites were found. Two nuclear export signals were identified, but no nuclear localization signal. In man, NCU-G1 was found to be widely expressed at the mRNA level with especially high levels detected in prostate, liver and kidney. Electrophoretic mobility shift analysis showed specific binding of NCU-G1 to an oligonucleotide representing the footprint 1 element of the human cellular retinol-binding protein 1 gene promoter. NCU-G1 was found to activate transcription from this promoter and required presence of the footprint 1 element. In transiently transfected Drosophila Schneider S2 cells, we demonstrated that NCU-G1 functions as a co-activator for ligand-activated PPAR-alpha, resulting in an increased expression of a CAT reporter gene under control of the peroxisome proliferator-activated receptor-alpha responsive acyl-CoA oxidase promoter.

Conclusion

We propose that NCU-G1 is a dual-function protein capable of functioning as a transcription factor as well as a nuclear receptor co-activator.

Gene regulation by homeobox transcription factor Prox1 in murine hepatoblasts

The homeobox transcription factor Prox1 is expressed in embryonic hepatoblasts and remains expressed in adult hepatocytes. Prox1-null mice show severe deficiencies in liver development, although the underlying mechanisms are unknown. We have studied the effects of Prox1 on the transcriptional profile of met-murine hepatocytes (MMH) obtained on embryonic day 14 (ED14). These immortalized murine hepatoblasts express numerous hepatoblast markers, but not Prox1. We have performed stable transfection with Prox1 cDNA, analyzed the transcriptome with Agilent mouse whole-genome microarrays, and validated genes by quantitative reverse transcription/polymerase chain reaction. We have observed the up-regulation of 22 genes and the down-regulation of 232 genes, by more than 12-fold. Many of these genes are involved in metabolic hepatocyte functions and may be regulated by Prox1 directly or indirectly, e.g., by the down-regulation of hepatocyte nuclear factor 4alpha. Prox1 induces the down-regulation of transcription factors that are highly expressed in neighboring endodermal organs, suggesting a function during hepatoblast commitment. Prox1 does not influence the proliferative activity of MMH but regulates genes involved in liver morphogenesis. We have observed the up-regulation of both type-IValpha3 procollagen and functionally active matrix metalloproteinase-2 (MMP-2), an observation that places Prox1 at the center of liver matrix turnover. This is consistent with MMP-2 expression in hepatoblasts during liver development and with the persistence of a basal lamina around the liver bud in Prox1-deficient mice. Our studies suggest that Prox1 is a multifunctional regulator of liver morphogenesis and of hepatocyte function and commitment.

A prospero-related homeobox gene Prox-1 is expressed during postnatal brain development as well as in the adult rodent brain

Prox-1, a prospero-related homeobox gene, is known to be an important transcription factor during embryogenesis. However, very little is known about Prox-1 expression and functions in the adult nervous system. Here we have investigated the expression pattern of Prox-1 mRNA and protein during postnatal brain development and in adult rat and mouse brains using in situ hybridization (ISH), immunohistochemistry (IHC) and Western blotting. In the developing and adult brain, we found prominent, but restricted Prox-1 mRNA expression in the dentate gyrus of the hippocampus, in some thalamic nuclei, notably in the anterior thalamus, and in the cerebellar cortex. Other brain regions, such as the hypothalamus and nuclei belonging to the midbrain, revealed a moderate level of Prox-1 mRNA expression. In developing cerebral cortex, Prox-1 mRNA was seen only in the thin layer under the pial surface postnatally, and the signal almost disappeared by the 28th postnatal day (PD). Using IHC and ISH approaches, we demonstrated rather restricted, but intense Prox-1 labeling in adult brain of both rat and mouse species. During postnatal brain development Prox-1 proteins by IHC, were below the detection limit at PD 14, while Prox-1 mRNA remained at a high level. Western blotting demonstrated the existence of two different variants of Prox-1 protein, one of which was about 20 kDa larger than ordinary size. During the first PDs, the larger variant predominated. At PD 14, neither protein variant could be detected. From PD 16 onwards the smaller variant started to predominate and by PD 30 the larger size protein had almost disappeared. The prominent but limited distribution of Prox-1 in the brain suggests its potentially important role during postnatal brain development and in adult CNS, which remains to be ascertained in future studies.

International Union of Pharmacology. LXVI. Orphan nuclear receptors

Half of the members of the nuclear receptors superfamily are so-called "orphan" receptors because the identity of their ligand, if any, is unknown. Because of their important biological roles, the study of orphan receptors has attracted much attention recently and has resulted in rapid advances that have helped in the discovery of novel signaling pathways. In this review we present the main features of orphan receptors, discuss the structure of their ligand-binding domains and their biological functions. The paradoxical existence of a pharmacology of orphan receptors, a rapidly growing and innovative field, is highlighted.

A Prospero-related homeodomain protein is a novel co-regulator of hepatocyte nuclear factor 4alpha that regulates the cholesterol 7alpha-hydroxylase gene

Prox1, an early specific marker for developing liver and pancreas in foregut endoderm has recently been shown to interact with alpha-fetoprotein transcription factor and repress cholesterol 7alpha-hydroxylase (CYP7A1) gene transcription. Using a yeast two-hybrid assay, we found that Prox1 strongly and specifically interacted with hepatocyte nuclear factor (HNF)4alpha, an important transactivator of the human CYP7A1 gene in bile acid synthesis and phosphoenolpyruvate carboxykinase (PEPCK) gene in gluconeogenesis. A real time PCR assay detected Prox1 mRNA expression in human primary hepatocytes and HepG2 cells. Reporter assay, GST pull-down, co-immunoprecipitation, and yeast two-hybrid assays identified a specific interaction between the N-terminal LXXLL motif of Prox1 and the activation function 2 domain of HNF4alpha. Prox1 strongly inhibited HNF4alpha and peroxisome proliferators-activated receptor gamma coactivator-1alpha co-activation of the CYP7A1 and PEPCK genes. Knock down of the endogenous Prox1 by small interfering RNA resulted in significant increase of CYP7A1 and PEPCK mRNA expression and the rate of bile acid synthesis in HepG2 cells. These results suggest that Prox1 is a novel co-regulator of HNF4alpha that may play a key role in the regulation of bile acid synthesis and gluconeogenesis in the liver.

Cooperative transcription activation by Nurr1 and Pitx3 induces embryonic stem cell maturation to the midbrain dopamine neuron phenotype

Midbrain dopamine (DA) neurons play a central role in the regulation of voluntary movement, and their degeneration is associated with Parkinson's disease. Cell replacement therapies, and in particular embryonic stem (ES) cell-derived DA neurons, offer a potential therapeutic venue for Parkinson's disease. We sought to identify genes that can potentiate maturation of ES cell cultures to the midbrain DA neuron phenotype. A number of transcription factors have been implicated in the development of midbrain DA neurons by expression analyses and loss-of-function knockout mouse studies, including Nurr1, Pitx3, Lmx1b, Engrailed-1, and Engrailed-2. However, none of these factors appear sufficient alone to induce the mature midbrain DA neuron phenotype in ES cell cultures in vitro, suggesting a more complex regulatory network. Here we show that Nurr1 and Pitx3 cooperatively promote terminal maturation to the midbrain DA neuron phenotype in murine and human ES cell cultures.