Functional analysis of the murine Pax8 promoter reveals autoregulation and the presence of a novel thyroid-specific DNA-binding activity

PAX8 modulates the tumor microenvironment of high grade serous ovarian cancer through changes in the secretome

High grade serous ovarian cancer (HGSC) arises from the fimbriated end of the fallopian tube epithelium (FTE), and in some cases, the ovarian surface epithelium (OSE). PAX8 is a commonly used biomarker for HGSC and is expressed in ∼90% of HGSC. Although the OSE does not express PAX8, murine models of HGSC derived from the OSE acquire PAX8, suggesting that it is not only a marker of Müllerian origin, but also an essential part of cancer progression, potentially from both the OSE and FTE. Previously, we have shown that PAX8 loss in HGSC cells causes tumor cell death and reduces cell migration and invasion. Herein, secretome analysis was performed in PAX8 deleted cells and we identified a reduction of the extracellular matrix (ECM) components, collagen and fibronectin. Immunoblotting and immunofluorescence in PAX8 deleted HGSC cells further validated the results from the secretome analysis. PAX8 loss reduced the amount of secreted TGFbeta, a cytokine that plays a crucial role in remodelling the tumor microenvironment. Furthermore, PAX8 loss reduced the integrity of 3D spheroids and caused a reduction of ECM proteins fibronectin and collagen in 3D cultures. Due to the ubiquitous nature of PAX8 in HGSC, regardless of cell origin, and the association of its reduced expression with decreasing tumor burden, a PAX8 inhibitor could be a promising drug target against various types of HGSC. To accomplish this, we generated a murine oviductal epithelial (MOE) cell line stably expressing PAX8 promoter-luciferase. Using this cell line, we performed a screening assay with a library of FDA-approved drugs (Prestwick Library) and quantitatively assessed these compounds for their inhibition of PAX8. We identified two hits: losartan and captropril, both inhibitors of the renin-angiotensin pathway that inhibit PAX8 expression and function. Overall, this study validates PAX8 as a regulator of ECM deposition in the tumor microenvironment.

PBRM1 loss in kidney cancer unbalances the proximal tubule master transcription factor hub to repress proximal tubule differentiation

PBRM1, a subunit of the PBAF coactivator complex that transcription factors use to activate target genes, is genetically inactivated in almost all clear cell renal cell cancers (RCCs). Using unbiased proteomic analyses, we find that PAX8, a master transcription factor driver of proximal tubule epithelial fates, recruits PBRM1/PBAF. Reverse analyses of the PAX8 interactome confirm recruitment specifically of PBRM1/PBAF and not functionally similar BAF. More conspicuous in the PAX8 hub in RCC cells, however, are corepressors, which functionally oppose coactivators. Accordingly, key PAX8 target genes are repressed in RCC versus normal kidneys, with the loss of histone lysine-27 acetylation, but intact lysine-4 trimethylation, activation marks. Re-introduction of PBRM1, or depletion of opposing corepressors using siRNA or drugs, redress coregulator imbalance and release RCC cells to terminal epithelial fates. These mechanisms thus explain RCC resemblance to the proximal tubule lineage but with suppression of the late-epithelial program that normally terminates lineage-precursor proliferation.

Gu et al. identify that transcription factor PAX8 needs the PBRM1/PBAF coactivator to activate proximal tubule genes. PBRM1 mutation/deletion thus explains the resemblance of clear cell kidney cancer to proximal tubule tissue but with suppressed terminal epithelial markers. This oncogenic mechanism could be repaired using drugs to inhibit corepressors.

PAX8 activates metabolic genes via enhancer elements in Renal Cell Carcinoma

Transcription factor networks shape the gene expression programs responsible for normal cell identity and pathogenic state. Using Core Regulatory Circuitry analysis (CRC), we identify PAX8 as a candidate oncogene in Renal Cell Carcinoma (RCC) cells. Validation of large-scale functional genomic screens confirms that PAX8 silencing leads to decreased proliferation of RCC cell lines. Epigenomic analyses of PAX8-dependent cistrome demonstrate that PAX8 largely occupies active enhancer elements controlling genes involved in various metabolic pathways. We selected the ferroxidase Ceruloplasmin (CP) as an exemplary gene to dissect PAX8 molecular functions. PAX8 recruits histone acetylation activity at bound enhancers looping onto the CP promoter. Importantly, CP expression correlates with sensitivity to PAX8 silencing and identifies a subset of RCC cases with poor survival. Our data identifies PAX8 as a candidate oncogene in RCC and provides a potential biomarker to monitor its activity.

Transcription factors are critical regulators of cell identity. Here, the authors use computational and functional genomic approaches to show an oncogenic role of PAX8 in renal cancer. Mechanistic dissection of PAX8 functions reveal its role in activating genes associated with metabolic pathways.

Regulation of Foxe1 by Thyrotropin and Transforming Growth Factor Beta Depends on the Interplay Between Thyroid-Specific, CREB and SMAD Transcription Factors

Background: Thyroid follicular cells are characterized by the expression of a specific set of genes necessary for the synthesis and secretion of thyroid hormones, which are in turn regulated by the transcription factors Nkx2-1, Pax8, and Foxe1. Thyroid differentiation is finely tuned by the balance between positive regulatory signals, including thyrotropin (TSH), and by negative regulatory signals, such as transforming growth factor beta (TGF-β), which counteracts the action of TSH. A role for Foxe1 as a mediator of hormonal and growth-factor control of thyroid differentiation has been previously suggested. Therefore, the aim of this work was to study the mechanisms governing Foxe1 expression to define the ligands and signals that regulate one of the important factors in thyroid differentiation. Methods: Expression of Foxe1 was evaluated in rat PCCl3 thyroid follicular cells under different treatments. The mouse Foxe1 promoter was cloned, and site-directed mutagenesis was undertaken to study its transcriptional regulation and to identify response elements. Protein/DNA binding assays were performed to evaluate the binding of different transcription factors, and gene-silencing approaches were used to elucidate their functional roles. Results:In silico analysis of the Foxe1 promoter identified binding sites for Nkx2-1, Pax8, Foxe1, and Smad proteins, as well as cAMP-response element (CRE) sites. It was found that both CRE-binding protein and CRE modulator were necessary for the TSH-mediated induction of Foxe1 expression via the cAMP/PKA signaling pathway. Moreover, transcription of Foxe1 was regulated by Nkx2-1 and Pax8 and by itself, suggesting an autoregulatory mechanism of activation and an important role for thyroid transcription factors. Finally, TGF-β, through Smad proteins, inhibited the TSH-induced Foxe1 expression. Conclusions: This study shows that Foxe1 is the final target of TSH/cAMP and TGF-β regulation that mediates expression of thyroid differentiation genes, and provides evidence of an interplay between CRE-binding proteins, thyroid transcription factors, and Smad proteins in its regulation. Thus, Foxe1 plays an important role in the complex transcriptional network that regulates thyroid follicular cell differentiation.

Mapping of Human FOXP2 Enhancers Reveals Complex Regulation

Mutations of the FOXP2 gene cause a severe speech and language disorder, providing a molecular window into the neurobiology of language. Individuals with FOXP2 mutations have structural and functional alterations affecting brain circuits that overlap with sites of FOXP2 expression, including regions of the cortex, striatum, and cerebellum. FOXP2 displays complex patterns of expression in the brain, as well as in non-neuronal tissues, suggesting that sophisticated regulatory mechanisms control its spatio-temporal expression. However, to date, little is known about the regulation of FOXP2 or the genomic elements that control its expression. Using chromatin conformation capture (3C), we mapped the human FOXP2 locus to identify putative enhancer regions that engage in long-range interactions with the promoter of this gene. We demonstrate the ability of the identified enhancer regions to drive gene expression. We also show regulation of the FOXP2 promoter and enhancer regions by candidate regulators - FOXP family and TBR1 transcription factors. These data point to regulatory elements that may contribute to the temporal- or tissue-specific expression patterns of human FOXP2. Understanding the upstream regulatory pathways controlling FOXP2 expression will bring new insight into the molecular networks contributing to human language and related disorders.

Resistance to thyrotropin

Resistance to thyrotropin (RTSH) is broadly defined as reduced sensitivity of thyroid follicle cells to stimulation by biologically active TSH due to genetic defects. Affected individuals have elevated serum TSH in the absence of goiter, with the severity ranging from nongoitrous isolated hyperthyrotropinemia to severe congenital hypothyroidism with thyroid hypoplasia. Conceptually, defects leading to RTSH impair both aspects of TSH-mediated action, namely thyroid hormone synthesis and gland growth. These include inactivating mutations in the genes encoding the TSH receptor and the PAX8 transcription factor. A common third cause has been genetically mapped to a locus on chromosome 15, but the underlying pathophysiology has not yet been elucidated. This review provides a succinct overview of currently defined causes of nonsyndromic RTSH, their differential diagnoses (autoimmune; partial iodine organification defects; syndromic forms of RTSH) and implications for the clinical approach to patients with RTSH.

Thyroid transcription factors in development, differentiation and disease

Identification of the thyroid transcription factors (TTFs), NKX2-1, FOXE1, PAX8 and HHEX, has considerably advanced our understanding of thyroid development, congenital thyroid disorders and thyroid cancer. The TTFs are fundamental to proper formation of the thyroid gland and for maintaining the functional differentiated state of the adult thyroid; however, they are not individually required for precursor cell commitment to a thyroid fate. Although knowledge of the mechanisms involved in thyroid development has increased, the full complement of genes involved in thyroid gland specification and the signals that trigger expression of the genes that encode the TTFs remain unknown. The mechanisms involved in thyroid organogenesis and differentiation have provided clues to identifying the genes that are involved in human congenital thyroid disorders and thyroid cancer. Mutations in the genes that encode the TTFs, as well as polymorphisms and epigenetic modifications, have been associated with thyroid pathologies. Here, we summarize the roles of the TTFs in thyroid development and the mechanisms by which they regulate expression of the genes involved in thyroid differentiation. We also address the implications of mutations in TTFs in thyroid diseases and in diseases not related to the thyroid gland.

Wnt-independent role of β-catenin in thyroid cell proliferation and differentiation

The Wnt/β-catenin pathway has been associated with thyroid cell growth and tumorigenesis. However, little is known regarding its involvement in the response to the key regulators of thyroid cell proliferation and differentiation. Here we show that TSH and IGF-1 increase β-catenin nuclear accumulation and its transcriptional activity in differentiated thyroid cells. This effect takes place in a Wnt-independent manner because TSH and IGF-1, through the activation of protein kinase A and protein kinase B/Akt, phosphorylate β-catenin at S552 and S675, which results in β-catenin release from E-cadherin at the adherens junctions. Nuclear β-catenin regulates thyroid cell proliferation, because its silencing or the overexpression of a dominant-negative form of T-cell factor 4 resulted in reduced levels of cyclin D1 and DNA synthesis. Furthermore, the β-catenin silencing markedly reduced the expression of Pax8, the main transcription factor involved in epithelial thyroid cell differentiation. Finally, we observed that β-catenin physically interacts with the transcription factor Pax8, increasing its transcriptional activity on the sodium iodide symporter (NIS) gene, a critical gene required for thyroid cell physiology. Taken together, our findings show that β-catenin plays a not yet described role in thyroid function including a functional interaction with Pax8.

Mechanisms of thyroid development and dysgenesis: an analysis based on developmental stages and concurrent embryonic anatomy

Thyroid dysgenesis is the most common cause of congenital hypothyroidism that affects 1 in 3000 newborns. Although a number of pathogenetic mutations in thyroid developmental genes have been identified, the molecular mechanism of disease is unknown in most cases. This chapter summarizes the current knowledge of normal thyroid development and puts the different developmental stages in perspective, from the time of foregut endoderm patterning to the final shaping of pharyngeal anatomy, for understanding how specific malformations may arise. At the cellular level, we will also discuss fate determination of follicular and C-cell progenitors and their subsequent embryonic growth, migration, and differentiation as the different thyroid primordia evolve and merge to establish the final size and shape of the gland.