PAX8PPARgamma stimulates cell viability and modulates expression of thyroid-specific genes in a human thyroid cell line

Pax-8: Molecular biology, pathophysiology, and potential pathogenesis

Transcription factors, as the convergence points of multiple signaling pathways in eukaryotic cells, are closely involved in disease development. Pax-8, an important transcription factor belonging to the Pax family, exerts a crucial influence on the regulation of gene expression required for both physiological conditions and pathological processes. Pax-8 contributes to the pathogenesis of many human diseases, ranging from cardiovascular disease to many cancers, and therefore, it can be imagined that Pax-8 holds great therapeutic potential. In this review, we summarize the structure, distribution, function, and regulatory mechanisms of Pax-8 to provide a new research direction for Pax-8.

PKCα mediated by the PI3K/Akt-FOXA1 cascade facilitates cypermethrin-induced hyperthyroidism

Cypermethrin (CYP), a broad-spectrum pyrethroid insecticide is extensively used. CYP is also considered as a potential endocrine disruptor with the thyroid-disturbing property. Protein kinase C alpha (PKCα) is a pleiotropic signal transduction molecule that functions crucially in thyroid hormone (TH) homeostasis and thyroid functions. To explore underlying roles of PKCα in CYP-mediated disturbance of TH homeostasis, Sprague-Dawley rats and rat thyroid cells were used in this study. Results showed that β-CYP stimulated TH biosynthesis, as shown by the increase in plasma levels of TT4, FT4, TT3, FT3, and TSH. After β-CYP treatment, expressions of PKCα, three miRNAs (miR-17-5p, miR-330-3p, and miR-331-3p), thyroid transcription factor TTF-1, and thyroid-specific proteins (TSHr, TPO, and Tg) were significantly increased, while expressions of PI3K p110α, p-Akt, FOXA1, and thyroid transcription factors (TTF-2 and Pax8) were decreased. Further studies found that β-CYP induced PKCα translation by the miR-330-3p-targeted PI3K/Akt-FOXA1 cascade and then PKCα positively regulated TTF-1 to promote TPO and Tg expressions, which in turn facilitated TH biosynthesis. Likewise, PKCα positively modulated TSHr expressions to strengthen the TSH/TSHr signal in the HPT axis, thereby synergistically contributing to TH biosynthesis. Moreover, β-CYP also disturbed TH biotransformation and biotransport by inducing DIO1 and inhibiting DIO3 in thyroids and TTR expressions in livers. Taken together, β-CYP has the thyroid-disturbing effect and could promote TH biosynthesis, and PKCα plays vital roles in β-CYP-caused hyperthyroidism.

Thyrotropin regulation of differentiated gene transcription in adult human thyrocytes in primary culture

Thyroid transcription factors (TTFs) - NKX2-1, FOXE1, PAX8 and HHEX - regulate multiple genes involved in thyroid development in mice but little is known about TTF regulation of thyroid-specific genes - thyroglobulin (TG), thyroid peroxidase (TPO), deiodinase type 2 (DIO2), sodium/iodide symporter (NIS) and TSH receptor (TSHR) - in adult, human thyrocytes. Thyrotropin (thyroid-stimulating hormone, TSH) regulation of thyroid-specific gene expression in primary cultures of human thyrocytes is biphasic yielding an inverted U-shaped dose-response curve (IUDRC) with upregulation at low doses and decreases at high doses. Herein we show that NKX2-1, FOXE1 and PAX8 are required for TSH-induced upregulation of the mRNA levels of TG, TPO, DIO2, NIS, and TSHR whereas HHEX has little effect on the levels of these thyroid-specific gene mRNAs. We show that TSH-induced upregulation is mediated by changes in their transcription and not by changes in the degradation of their mRNAs. In contrast to the IUDRC of thyroid-specific genes, TSH effects on the levels of the mRNAs for NKX2-1, FOXE1 and PAX8 exhibit monophasic decreases at high doses of TSH whereas TSH regulation of HHEX mRNA levels exhibits an IUDRC that overlaps the IUDRC of thyroid-specific genes. In contrast to findings during mouse development, TTFs do not have major effects on the levels of other TTF mRNAs in adult, human thyrocytes. Thus, we found similarities and important differences in the regulation of thyroid-specific genes in mouse development and TSH regulation of these genes in adult, human thyrocytes.

The management of thyroid nodules and cancer in the molecular era

The incidence of thyroid cancer is increasing worldwide. Current standards in the diagnosis and management of thyroid cancer are limited by the uncertainty of fine-needle aspiration samples that are indeterminate in nature. Molecular markers have the potential to improve the accuracy of thyroid fine-needle aspiration and to aid the physician in giving a more accurate diagnosis and prognosis. This paper summarizes the various molecular markers currently available.

Pax-8-PPAR-γ fusion protein in thyroid carcinoma

Thyroid carcinoma is the most common endocrine malignancy, and its incidence is continuing to increase. Most thyroid carcinomas contain one of several known driver mutations, such as the Val600Glu substitution in B-Raf, Ras mutations, RET gene fusions, or PAX8-PPARG gene fusions. The PAX8-PPARG gene fusion results in the production of a Pax-8-PPAR-γ fusion protein (PPFP), which is found in approximately one-third of follicular thyroid carcinomas, as well as some follicular-variant papillary thyroid carcinomas. In vitro and in vivo evidence indicates that PPFP is an oncoprotein. Although specific mechanisms of action remain to be defined, PPFP is considered to act as a dominant-negative inhibitor of wild-type PPAR-γ and/or as a unique transcriptional activator of subsets of PPAR-γ-responsive and Pax-8-responsive genes. Detection of the fusion transcript in thyroid nodule biopsy specimens can aid clinical decision-making when cytological findings are indeterminate. The PPAR-γ agonist pioglitazone is highly therapeutic in a transgenic mouse model of PPFP-positive thyroid carcinoma, suggesting that PPAR-γ agonists might be beneficial in patients with PPFP-positive thyroid carcinomas.

Genetics and epigenetics of sporadic thyroid cancer

Thyroid carcinoma is the most common endocrine malignancy, and although the disease generally has an excellent prognosis, therapeutic options are limited for patients not cured by surgery and radioiodine. Thyroid carcinomas commonly contain one of a small number of recurrent genetic mutations. The identification and study of these mutations has led to a deeper understanding of the pathophysiology of this disease and is providing new approaches to diagnosis and therapy. Papillary thyroid carcinomas usually contain an activating mutation in the RAS cascade, most commonly in BRAF and less commonly in RAS itself or through gene fusions that activate RET. A chromosomal translocation that results in production of a PAX8-PPARG fusion protein is found in follicular carcinomas. Anaplastic carcinomas may contain some of the above changes as well as additional mutations. Therapies that are targeted to these mutations are being used in patient care and clinical trials.

The thyroid cancer PAX8-PPARG fusion protein activates Wnt/TCF-responsive cells that have a transformed phenotype

A chromosomal translocation results in the production of a paired box 8-peroxisome proliferator-activated receptor gamma (PAX8-PPARG) fusion protein (PPFP) in ∼35% of follicular thyroid carcinomas. To examine the role of PPFP in thyroid oncogenesis, the fusion protein was stably expressed in the non-transformed rat thyroid cell line PCCL3. PPFP conferred on PCCL3 cells the ability to invade through Matrigel and to form colonies in anchorage-independent conditions. PPFP also increased the fraction of cells with Wnt/TCF-responsive green fluorescent protein reporter gene expression. This Wnt/TCF-activated population was enriched for colony-forming and invading cells. These actions of PPFP required a functional PPARG DNA binding domain (DBD) within PPFP and were further stimulated by PPARG agonists. These data indicate that PPFP, through its PPARG DBD, induces Wnt/TCF pathway activation in a subpopulation of cells, and these cells have properties of cellular transformation including increased invasiveness and anchorage-independent growth.

Pax8 has a critical role in epithelial cell survival and proliferation

The transcription factor Pax8, a member of the Paired-box gene family, is a critical regulator required for proper development and differentiation of thyroid follicular cells. Despite being Pax8 well characterized with respect to its role in regulating genes responsible for thyroid differentiation, its involvement in cell survival and proliferation has been hypothesized but remains unclear. Here, we show that Pax8 overexpression significantly increases proliferation and colony-forming efficiency of Fischer rat thyroid line 5 epithelial cells, although it is not sufficient to overcome their hormone dependence. More interestingly, we show that Pax8-specific silencing induces apoptosis through a p53-dependent pathway that involves caspase-3 activation and cleavage of poly(ADP)ribose polymerase. Our data indicate that tumor protein 53 induced nuclear protein 1 (tp53inp1), a positive regulator of p53-dependent cell cycle arrest and apoptosis, is a transcriptional target of Pax8 and is upregulated by Pax8 knockdown. Remarkably, tp53inp1 silencing significantly abolishes Pax8-induced apoptosis thus suggesting that tp53inp1 may be the mediator of the observed effects. In conclusion, our data highlight that Pax8 is required for the survival of differentiated epithelial cells and its expression levels are able to modulate the proliferation rate of such cells.

Proteomic analysis of differentially expressed proteins in normal human thyroid cells transfected with PPFP

The fusion gene encoding the thyroid-specific transcription factor PAX8 and peroxisome proliferator-activated receptor γ (PPARγ (PPARG)) (designated as the PPFP gene) is oncogenic and implicated in the development of follicular thyroid carcinoma (FTC). The effects of PPFP transfection on the biological characteristics of Nthy-ori 3-1 cells were studied by MTT assay, colony formation, soft-agar colony formation, and scratch wound-healing assays as well as by flow cytometry. Furthermore, the differentially expressed proteins were analyzed on 2-DE maps and identified by MALDI-TOF-MS. Validation of five identified proteins (prohibitin, galectin-1, cytokeratin 8 (CK8), CK19, and HSP27) was determined by western blot analysis. PPFP not only significantly increased the viability, proliferation, and mobility of the Nthy-ori 3-1 cells but also markedly inhibited cellular apoptosis. Twenty-eight differentially expressed proteins were identified, among which 19 proteins were upregulated and nine proteins were downregulated in Nthy-ori 3-1(PPFP) (Nthy-ori 3-1 cells transfected with PPFP). The western blot results, which were consistent with the proteome analysis results, showed that prohibitin was downregulated, whereas galectin-1, CK8, CK19, and HSP27 were upregulated in Nthy-ori 3-1(PPFP). Our results suggest that PPFP plays an important role in malignant thyroid transformation. Proteomic analysis of the differentially expressed proteins in PPFP-transfected cells provides important information for further study of the carcinogenic mechanism of PPFP in FTCs.

Expression of the PAX8/PPARγ Fusion Protein Is Associated with Decreased Neovascularization In Vivo: Impact on Tumorigenesis and Disease Prognosis

The PAX8/PPARγ fusion protein (PPFP) occurs in 36% of human follicular thyroid carcinoma (FTC) and is associated with favorable prognosis. To elucidate the function of PPFP in FTC, we analyzed the consequences of PPFP expression in immortalized thyrocytes in vitro and in vivo via xenograft tumorigenesis. While PPFP-expressing cells exhibited oncogenic hallmarks, including increased growth and decreased apoptosis, in vitro, xenograft tumors were initiated but not sustained in vivo. PPFP xenograft tumors exhibited reduced CD31 staining and VEGF expression, suggesting that PPFP modulates neovascularization. Microarray analysis demonstrated increased expression of tissue inhibitor of metalloproteinase (TIMP-3), an inhibitor of angiogenesis, in PPFP cells and tumors, a finding confirmed by quantitative PCR and immunohistochemistry. Immunohistochemical staining of archival human thyroid tumors demonstrates a significant decrease in CD31 staining in all adenomas and carcinomas containing the PAX8/PPARγ rearrangement. Decreased angiogenesis in PPFP-containing tumors is directly correlated with our observations in the xenograft model and provides evidence for the first time that PPFP may impact FTC tumorigenesis by modulating angiogenesis in vivo.

The role of the PAX8/PPARgamma fusion oncogene in the pathogenesis of follicular thyroid cancer

When identified at early stages, most well-differentiated thyroid cancers are readily treated and yield excellent outcomes. Follicular thyroid cancer (FTC) however, when diagnosed at a late stage, may be very resistant to treatment, and exhibits 10-year survival rates less than 40%. Despite substantial progress in recent years, we still have limited understanding of the molecular and biological interrelationships between the various subtypes of benign and malignant follicular thyroid neoplasms. In contrast to the wealth of information available regarding papillary thyroid carcinoma (PTC), the triggering mechanisms of FTC development and the major underlying genetic alterations leading to follicular thyroid carcinogenesis remain obscure. Recent studies have focused on a chromosomal translocation, t(2;3) (q13;p25), fusing PAX8, a transcription factor that is essential for normal thyroid gland development, with the peroxisome proliferator-activated receptor gamma (PPARgamma), a member of the steroid/thyroid nuclear receptor family. This chromatin rearrangement results in the expression of a PAX8/PPARgamma fusion protein, designated PPFP, whose incidence is relatively common in FTC and may represent an initiating event in the genesis of FTC. Here we review progress on the studies of PPFP that assess its involvement in FTC tumorigenesis.

Thyroid hormone receptors regulate adipogenesis and carcinogenesis via crosstalk signaling with peroxisome proliferator-activated receptors

Peroxisome proliferator-activated receptors (PPARs) and thyroid hormone receptors (TRs) are members of the nuclear receptor superfamily. They are ligand-dependent transcription factors that interact with their cognate hormone response elements in the promoters to regulate respective target gene expression to modulate cellular functions. While the transcription activity of each is regulated by their respective ligands, recent studies indicate that via multiple mechanisms PPARs and TRs crosstalk to affect diverse biological functions. Here, we review recent advances in the understanding of the molecular mechanisms and biological impact of crosstalk between these two important nuclear receptors, focusing on their roles in adipogenesis and carcinogenesis.

The Role of the PAX8/PPARgamma Fusion Oncogene in Thyroid Cancer

Thyroid cancer is uncommon and exhibits relatively low mortality rates. However, a subset of patients experience inexorable growth, metastatic spread, and mortality. Unfortunately, for these patients, there have been few significant advances in treatment during the last 50 years. While substantial advances have been made in recent years about the molecular genetic events underlying papillary thyroid cancer, the more aggressive follicular thyroid cancer remains poorly understood. The recent discovery of the PAX8/PPARγ translocation in follicular thyroid carcinoma has promoted progress in the role of PPARγ as a tumor suppressor and potential therapeutic target. The PAX8/PPARγ fusion gene appears to be an oncogene. It is most often expressed in follicular carcinomas and exerts a dominant-negative effect on wild-type PPARγ, and stimulates transcription of PAX8-responsive promoters. PPARγ agonists have shown promising results in vitro, although very few studies have been conducted to assess the clinical impact of these agents.

Pax genes in embryogenesis and oncogenesis

The paired box genes are a family of nine developmental control genes, which in human beings (PAX) and mice (Pax) encode nuclear transcription factors. The temporal and spatial expressions of these highly conserved genes are tightly regulated during foetal development including organogenesis. PAY/Paxgenes are switched off during the terminal differentiation of most structures. Specific mutations within a number of PAX/Pax genes lead to developmental abnormalities in both human beings and mice. Mutation in PAX3 causes Waardenburg syndrome, and craniofacial-deafness-hand syndrome. The Splotch phenotype in mouse exhibits defects in neural crest derivatives such as, pigment cells, sympathetic ganglia and cardiac neural crest-derived structures. The PAX family also plays key roles in several human malignancies. In particular, PAX3 is involved in rhabdomyosarcoma and tumours of neural crest origin, including melanoma and neuroblastoma. This review critically evaluates the roles of PAX/Pax in oncogenesis. It especially highlights recent advances in knowledge of how their genetic alterations directly interfere in the transcriptional networks that regulate cell differentiation, proliferation, migration and survival and may contribute to oncogenesis.

Genome-wide studies in thyroid neoplasia

There is much interest in the application of genome biology to the field of thyroid neoplasia, despite the relatively low mortality rate associated with thyroid cancer in general. The principal reason for this interest is that the field of thyroid neoplasia stands to benefit from the application of genomic information to address a variety of pathologic and clinical issues. In addition to practical patient care issues, there is an excellent opportunity of expand the basic understanding of thyroid carcinogenesis. In this article, the most relevant genomic work on thyroid tumors performed to date is reviewed along with some general comments about the potential impact of genomic biology on thyroid pathology and the management of patients with thyroid nodules and cancer.