Fetal cell carcinogenesis: a new hypothesis for better understanding of thyroid carcinoma

Dynamic Cancer Cell Heterogeneity: Diagnostic and Therapeutic Implications

Though heterogeneity of cancers is recognized and has been much discussed in recent years, the concept often remains overlooked in different routine examinations. Indeed, in clinical or biological articles, reviews, and textbooks, cancers and cancer cells are generally presented as evolving distinct entities rather than as an independent heterogeneous cooperative cell population with its self-oriented biology. There are, therefore, conceptual gaps which can mislead the interpretations/diagnostic and therapeutic approaches. In this short review, we wish to summarize and discuss various aspects of this dynamic evolving heterogeneity and its biological, pathological, clinical, diagnostic, and therapeutic implications, using thyroid carcinoma as an illustrative example.

Thyroid Cancer Stem-Like Cells: From Microenvironmental Niches to Therapeutic Strategies

Thyroid cancer (TC) is the most common endocrine malignancy. Recent progress in thyroid cancer biology revealed a certain degree of intratumoral heterogeneity, highlighting the coexistence of cellular subpopulations with distinct proliferative capacities and differentiation abilities. Among those subpopulations, cancer stem-like cells (CSCs) are hypothesized to drive TC heterogeneity, contributing to its metastatic potential and therapy resistance. CSCs principally exist in tumor areas with specific microenvironmental conditions, the so-called stem cell niches. In particular, in thyroid cancer, CSCs' survival is enhanced in the hypoxic niche, the immune niche, and some areas with specific extracellular matrix composition. In this review, we summarize the current knowledge about thyroid CSCs, the tumoral niches that allow their survival, and the implications for TC therapy.

The Possible Role of Cancer Stem Cells in the Resistance to Kinase Inhibitors of Advanced Thyroid Cancer

Target therapy with various kinase inhibitors (KIs) has been extended to patients with advanced thyroid cancer, but only a subset of these compounds has displayed efficacy in clinical use. However, after an initial response to KIs, dramatic disease progression occurs in most cases. With the discovery of cancer stem cells (CSCs), it is possible to postulate that thyroid cancer resistance to KI therapies, both intrinsic and acquired, may be sustained by this cell subtype. Indeed, CSCs have been considered as the main drivers of metastatic activity and therapeutic resistance, because of their ability to generate heterogeneous secondary cell populations and survive treatment by remaining in a quiescent state. Hence, despite the impressive progress in understanding of the molecular basis of thyroid tumorigenesis, drug resistance is still the major challenge in advanced thyroid cancer management. In this view, definition of the role of CSCs in thyroid cancer resistance may be crucial to identifying new therapeutic targets and preventing resistance to anti-cancer treatments and tumor relapse. The aim of this review is to elucidate the possible role of CSCs in the development of resistance of advanced thyroid cancer to current anti-cancer therapies and their potential implications in the management of these patients.

Overdiagnosis of Juvenile Thyroid Cancer

Overdiagnosis is the detection of a disease that does not do any harm to the patient throughout the lifetime. Thyroid cancer in children is a rare disease; however, since 2011, many children in Fukushima, Japan, have been diagnosed with it, and the number has shown a steady increase to over 200 cases at present. Some experts have stated that this phenomenon is due to overdiagnosis caused by thyroid ultrasound (US)-based thyroid screening detecting self-limiting thyroid cancer, which will not lead to clinical symptoms in the future. Harm caused by overdiagnosis of thyroid cancer is more serious in the young, since it is difficult to perform active surveillance and children diagnosed with cancer are likely to suffer from stigma. Thus, overdiagnosis of thyroid cancer in the young is not only a health problem but also a problem of human rights. Conflicts of interest among people related to screening programs and some experts with incomplete knowledge on overdiagnosis help to spread misleading opinions together with fear of radiation exposure among residents, which has led to their erroneous understanding of the nature of US-based thyroid screening. Scientific and honest discussions among experts to enhance education of residents and consideration of medical ethics are crucial to prevent the expansion of overdiagnosis.

Cancer Stem Cells in Thyroid Tumors: From the Origin to Metastasis

Thyroid tumors are extremely heterogeneous varying from almost benign tumors with good prognosis as papillary or follicular tumors, to the undifferentiated ones with severe prognosis. Recently, several models of thyroid carcinogenesis have been described, mostly hypothesizing a major role of the thyroid cancer stem cell (TCSC) population in both cancer initiation and metastasis formation. However, the cellular origin of TCSC is still incompletely understood. Here, we review the principal epigenetic mechanisms relevant to TCSC origin and maintenance in both well-differentiated and anaplastic thyroid tumors. Specifically, we describe the alterations in DNA methylation, histone modifiers, and microRNAs (miRNAs) involved in TCSC survival, focusing on the potential of targeting aberrant epigenetic modifications for developing novel therapeutic approaches. Moreover, we discuss the bidirectional relationship between TCSCs and immune cells. The cells of innate and adaptive response can promote the TCSC-driven tumorigenesis, and conversely, TCSCs may favor the expansion of immune cells with protumorigenic functions. Finally, we evaluate the role of the tumor microenvironment and the complex cross-talk of chemokines, hormones, and cytokines in regulating thyroid tumor initiation, progression, and therapy refractoriness. The re-education of the stromal cells can be an effective strategy to fight thyroid cancer. Dissecting the genetic and epigenetic landscape of TCSCs and their interactions with tumor microenvironment cells is urgently needed to select more appropriate treatment and improve the outcome of patients affected by advanced differentiated and undifferentiated thyroid cancers.

Functions of stem cells of thyroid glands in health and disease

Thyroid gland has been implicated in the regulation of many functions using endocrine, paracrine and autocrine signals. Functional thyroid follicular cells derived from stem cells attracted a great interest from researchers as a strategy for thyroid's regenerative therapy. Thyroid has a very low rate of turnover; however, studies showed that the regenerative ability is enhanced following diseases or thyroidectomy, which promotes the role of stem cell. The objective of this review is to summarize the morphological characterization and the expression of stem cell genes/markers in the thyroid. Also, to highlight the mechanisms of tumor formation in thyroid via its stem cells. The most important thyroid stem cell's markers are: stem cell antigen 1 (SCA-1), octamer-binding transcription 4 (OCT-4), p63, CD34+ CD45-, paired box gene 8 (PAX-8), thyroid transcription factor 1 (TTF-1), thyroid transcription factor 2 (TTF-2), hematopoietically expressed homeobox protein HHEX, the transcription factor GATA-4, hepatocyte nuclear factor 4-α (HNF-4-α) and homeobox transcription factor Nanog (hNanog). This review highlights the functional characterization describing the mechanisms of stem cell's differentiation into functional thyroid follicle and proposing mechanisms involving in cancer formation through one of these cell types: fetal cell, thyroblasts, prothyrocytes, certain genetic mutation in the mature thyroid cells or presence of a special type of cells (cancer stem cell) which are responsible for different types of cancer formation. Understanding the mechanisms of thyroid's stem cell in cancer formation and the expression of the biomarkers in normal and abnormal thyroid status are promising physiological tools in promoting thyroid regeneration and in provision management for thyroid cancer.

Gene master regulators of papillary and anaplastic thyroid cancers

We hypothesize that distinct cell phenotypes are governed by different sets of gene master regulators (GMRs) whose strongly protected (by the homeostatic mechanisms) abundance modulates most cell processes by coordinating the expression of numerous genes from the corresponding functional pathways. Gene Commanding Height (GCH), a composite measure of gene expression control and coordination, is introduced to establish the gene hierarchy in each phenotype. If the hypothesis is true, than one can selectively destroy cancer nodules from a heterogeneous tissue by altering the expression of genes whose GCHs are high in cancer but low in normal cell phenotype. Here, we test the hypothesis and show its utility for the thyroid cancer (TC) gene therapy. First, we prove that malignant and cancer free surrounding areas of a surgically removed papillary TC (PTC) tumor are governed by different GMRs. Second, we show that stable transfection of a gene induces larger transcriptomic alterations in the cells where it has higher GCH than in other cells. For this, we profiled the transcriptomes of the papillary BCPAP and anaplastic 8505C TC cell lines before and after stable transfection with NEMP1, DDX19B, PANK2 or UBALD1. The four genes were selected to have similar expression levels but significantly different GCH scores in the two cell lines before transfection. Indeed, each of the four genes triggered larger alterations in the cells where they had larger GCH. Our results prove the feasibility of a personalized gene therapy approach that selectively targets the cancer cells from a tissue.

Role of Krüppel-Like Factor 4 in the Maintenance of Chemoresistance of Anaplastic Thyroid Cancer

BACKGROUND

Anaplastic thyroid cancer (ATC) has a very poor prognosis due to its aggressive nature and resistance to conventional treatment. Radiotherapy and chemotherapy are not fully effective because of the undifferentiated phenotype and enhanced drug resistance of ATC. The objective of this study was to evaluate the involvement of Krüppel-like factor 4 (KLF4), a stemness-associated transcription factor, in the undifferentiated phenotype and drug resistance of ATC.

METHODS

ATC cells were compared to papillary thyroid cancer cells in drug resistance and gene expression. The effects of KLF4 knockdown in ATC cells on in vitro and in vivo drug resistance were measured. The effects of KLF4 overexpression and knockdown on ABC transporter activity were determined.

RESULTS

ATC cells, such as HTH83, 8505C, and SW1736, exhibited higher resistance to the anticancer drug paclitaxel and higher expression of KLF4 than TPC-1 papillary thyroid cancer cells. Knockdown of KLF4 expression in ATC cells increased the expression of the thyroid-specific differentiation genes, such as thyrotropin receptor, thyroid peroxidase, thyroglobulin, and sodium-iodide symporter. Knockdown of KLF4 expression in ATC cells decreased the resistance to doxorubicin and paclitaxel, and reduced ABC transporter expression. Luciferase reporter assay results showed that KLF4 overexpression increased ABCG2 promoter activity, which was abolished by KLF4 knockdown. A tumorigenicity assay showed that the combination of paclitaxel treatment and KLF4 knockdown significantly decreased tumor mass originated from HTH83 cells in mice.

CONCLUSIONS

ATC cells show high expression of KLF4, and KLF4 expression is necessary for maintaining the undifferentiated phenotype and drug resistance in vitro and in vivo. The present study identifies KLF4 as a potential therapeutic target for eliminating ATC cells.

Pathological processes and therapeutic advances in radioiodide refractory thyroid cancer

While in most patients with non-medullary thyroid cancer (TC), disease remission is achieved by thyroidectomy and ablation of tumor remnants by radioactive iodide (RAI), a substantial subgroup of patients with metastatic disease present tumor lesions that have acquired RAI resistance as a result of dedifferentiation. Although oncogenic mutations in BRAF, TERT promoter and TP53 are associated with an increased propensity for induction of dedifferentiation, the role of genetic and epigenetic aberrations and their effects on important intracellular signaling pathways is not yet fully elucidated. Also immune, metabolic, stemness and microRNA pathways have emerged as important determinants of TC dedifferentiation and RAI resistance. These signaling pathways have major clinical implications since their targeting could inhibit TC progression and could enable redifferentiation to restore RAI sensitivity. In this review, we discuss the current insights into the pathological processes conferring dedifferentiation and RAI resistance in TC and elaborate on novel advances in diagnostics and therapy to improve the clinical outcome of RAI-refractory TC patients.

Natural history of thyroid cancer [Review]

Thyroid cancers have long been considered to arise in middle age and, after their repeated proliferation, resulting in further damage to the genome, they progress to more aggressive and lethal cancers. However, in 2014, some studies were reported that might lead to a marked change in our understanding of the natural history of thyroid cancer. A high prevalence of papillary carcinoma in the young suggested that the first initiation of thyroid cancer is likely to occur in the infantile period. Such a conclusion was also supported by a very slow growth rate of papillary microcarcinomas (PMCs) in an observation trial. The proliferation rate of PMCs was negatively correlated with the age, and surgery to remove PMCs did not contribute to reduce mortality from thyroid cancer. These findings strongly suggested the existence of self-limiting cancers, which are truly malignant but do not progress to lethal cancers, for the first time in human history. The early detection of self-limiting cancers results in overdiagnosis. Ultrasonographic screening of the thyroid in the young should be avoided. Lethal thyroid cancers, whose origin is still unknown, appear suddenly after middle age. In the elderly, thyroid cancers are a mixture of self-limiting and lethal cancers; thus, when thyroid cancer is detected, careful follow-up with examination of its growth rate is required.

Cancer stem cells as a potential therapeutic target in thyroid carcinoma

A number of studies have indicated that tumor growth and proliferation is dependent on a small subset of cells, defined as cancer stem cells (CSCs). CSCs have the capability to self-renew, and are involved with cancer propagation, relapse and metastatic dissemination. CSCs have been isolated from numerous tissues, including normal and cancerous thyroid tissue. A regulatory network of signaling pathways and microRNAs (miRNAs) control the properties of CSCs. Differentiated thyroid carcinoma is the most common type of endocrine cancer, with an increasing incidence. Anaplastic thyroid carcinoma is the most rare type of endocrine cancer; however, it also exhibits the highest mortality rate among thyroid malignancies, with an extremely short survival time. Thyroid CSCs are invasive and highly resistant to conventional therapies, including radiotherapy and chemotherapy, which results in disease relapse even when the primary lesion has been eradicated. Therefore, targeting thyroid CSCs may represent an effective treatment strategy against aggressive neoplasms, including recurrent and radioresistant tumors. The present review summarizes the current literature regarding thyroid CSCs and discusses therapeutic strategies that target these cells, with a focus on the function of self-renewal pathways and miRNAs. Elucidation of the mechanisms that regulate CSC growth and survival may improve novel therapeutic approaches for treatment-resistant thyroid cancers.

Normal vs cancer thyroid stem cells: the road to transformation

Recent investigations in thyroid carcinogenesis have led to the isolation and characterisation of a subpopulation of stem-like cells, responsible for tumour initiation, progression and metastasis. Nevertheless, the cellular origin of thyroid cancer stem cells (SCs) remains unknown and it is still necessary to define the process and the target population that sustain malignant transformation of tissue-resident SCs or the reprogramming of a more differentiated cell. Here, we will critically discuss new insights into thyroid SCs as a potential source of cancer formation in light of the available information on the oncogenic role of genetic modifications that occur during thyroid cancer development. Understanding the fine mechanisms that regulate tumour transformation may provide new ground for clinical intervention in terms of prevention, diagnosis and therapy.

An improved protocol for mRNA quantification after fluorescence-activated cell sorting with an increased signal to noise ratio in flow cytometry

We established a method to analyze cells collected by fluorescence-activated cell sorting (FACS) named mRNA quantification after FACS (FACS-mQ), in which cells are labeled with a fluorescent dye in a manner that minimizes RNA degradation, and then cells sorted by FACS are examined by analyzing their gene expression profile. In this study, we established a modified protocol to analyze molecules with a low expression level, such as N-cadherin and thyroid transcription factor, by improving the signal to noise ratio in flow cytometry. Use of a fluorophore-conjugated second antibody and the appropriate choice of a fluorescence dye showed a marked increase in the signal to noise ratio. Use of the Can Get Signal Immunostain in diluting antibodies shortened the reaction time. In real-time reverse transcription-PCR, a significant decrease in the copy number of intracellular mRNAs was not observed after in-tube immunostaining. These results indicated that the present protocol is useful for separating and analyzing cells by FACS-mQ, targeting a molecule with a low expression level.

Stem cell biology in thyroid cancer: Insights for novel therapies

Currently, thyroid cancer is one of the most common endocrine cancer in the United States. A recent involvement of sub-population of stem cells, cancer stem cells, has been proposed in different histological types of thyroid cancer. Because of their ability of self-renewal and differentiation into various specialized cells in the body, these putative cells drive tumor genesis, metastatic activity and are responsible to provide chemo- and radioresistant nature to the cancer cells in the thyroid gland. Our Review was conducted from previously published literature to provide latest apprises to investigate the role of embryonic, somatic and cancer stem cells, and discusses the hypothesis of epithelial-mesenchymal transition. Different methods for their identification and isolation through stemness markers using various in vivo and in vitro methods such as flow cytometry, thyrosphere formation assay, aldehyde dehydrogenase activity and ATP-binding cassette sub-family G member 2 efflux-pump mediated Hoechst 33342 dye exclusion have been discussed. The review also outlines various setbacks that still remain to target these tumor initiating cells. Future perspectives of therapeutic strategies and their potential to treat advanced stages of thyroid cancer are also disclosed in this review.

The Beneficial Effects of Valproic Acid in Thyroid Cancer Are Mediated through Promoting Redifferentiation and Reducing Stemness Level: An In Vitro Study

Valproic acid (VPA) has been identified as a histone deacetylase inhibitor, inducing differentiation in transformed cells. However, no study has shown the effect of VPA in the redifferentiation induction and stemness of anaplastic thyroid. The main objective of this study was to evaluate the efficacy of VPA as a differentiation therapy agent in human thyroid cancer based on its effect on stemness and differentiation process. Indications for differentiation of 8305C and B-CPAP cell lines following VPA treatment were obtained by analyzing cell proliferation rate, morphological changes, adherent-dependent colony formation, and Hoechst 33342 staining. The expressions of stemness, differentiation, and aggressiveness specific marker genes were measured by quantitative RT-PCR. VPA treatment effectively showed growth inhibition in both cell lines. The high nuclear-cytoplasmic (N : C) ratio of 8305C cells markedly decreased and treated cells became more epithelial-like. Treated cells showed stronger Hoechst 33342 fluorescence compared with control cells. The hTERT and OCT-4 reduction was paralleled with adherent-dependent colony formation decrement in both cell lines. VPA effectively induced NIS and TTF-1 in anaplastic cells, it whereas showed no clear pattern in papillary cell line. VPA treatment also resulted in the reduction of MMP-2 and MMP-9. These finding suggest that VPA could redifferentiate the anaplastic thyroid cancer cells.

Thyroid tumor-initiating cells: increasing evidence and opportunities for anticancer therapy (review)

Accumulating evidence supports the notion that thyroid cancer is initiated by tumor-initiating cells (TICs) (commonly known as cancer stem cells), which are thought to play a crucial role in malignant progression, therapeutic resistance and recurrence. Thyroid TICs have been isolated and identified using specific biomarkers (such as CD133), the side population, sphere formation and aldehyde dehydrogenase activity assays. Although their characteristics remain largely unknown, TICs provide an attractive cellular mechanism to explain therapeutic refractoriness. Efforts are currently being directed toward the identification of therapeutic strategies that could target these cells. The present review discusses the cellular origins of TICs and the main approaches used to isolate and identify thyroid TICs, with a focus on the remaining challenges and opportunities for anticancer therapy.

Estrogens and stem cells in thyroid cancer

Recent discoveries highlight the emerging role of estrogens in the initiation and progression of different malignancies through their interaction with stem cell (SC) compartment. Estrogens play a relevant role especially for those tumors bearing a gender disparity in incidence and aggressiveness, as occurs for most thyroid diseases. Although several experimental lines suggest that estrogens promote thyroid cell proliferation and invasion, their precise contribution in SC compartment still remains unclear. This review underlines the interplay between hormones and thyroid function, which could help to complete the puzzle of gender discrepancy in thyroid malignancies. Defining the association between estrogen receptors' status and signaling pathways by which estrogens exert their effects on thyroid cells is a potential tool that provides important insights in pathogenetic mechanisms of thyroid tumors.

Fetal cell carcinogenesis of the thyroid: a modified theory based on recent evidence

Thyroid cancer cells were believed to be generated by multi-step carcinogenesis, in which cancer cells are derived from thyrocytes, via multiple incidences of damage to their genome, especially in oncogenes or anti-oncogenes that accelerate proliferation or foster malignant phenotypes, such as the ability to invade the surrounding tissue or metastasize to distant organs, until a new hypothesis, fetal cell carcinogenesis, was presented. In fetal cell carcinogenesis, thyroid tumor cells are assumed to be derived from three types of fetal thyroid cell which only exist in fetuses or young children, namely, thyroid stem cells (TSCs), thyroblasts and prothyrocytes, by proliferation without differentiation. Genomic alternations, such as RET/PTC and PAX8-PPARγ1 rearrangements and a mutation in the BRAF gene, play an oncogenic role by preventing thyroid fetal cells from differentiating. Fetal cell carcinogenesis effectively explains recent molecular and clinical evidence regarding thyroid cancer, including thyroid cancer initiating cells (TCICs), and it underscores the importance of identifying a stem cells and clarifying the molecular mechanism of organ development in cancer research. It introduces three important concepts, the reverse approach, stem cell crisis and mature and immature cancers. Further, it implies that analysis of a small population of cells in a cancer tissue will be a key technique in establishing future laboratory tests. In the contrary, mass analysis such as gene expression profiling, whole genomic scan, and proteomics analysis may have definite limitations since they can only provide information based on many cells.

Multiple pluripotent stem cell markers in human anaplastic thyroid cancer: the putative upstream role of SOX2

BACKGROUND

Anaplastic thyroid carcinoma (ATC) is a rare and aggressive endocrine tumor with highly undifferentiated morphology. It has been suggested that cancer stem cells (CSCs) might play a central role in ATC. The objectives of this study were (i) to characterize CSCs from ex vivo ATC specimens by investigating the expression of several pluripotent stem cell markers, and (ii) to evaluate in vitro drug resistance modifications after specific CSC transcription factor switch-off.

METHODS

In ex vivo experiments, eight formalin-fixed, paraffin-embedded ATC specimens were analyzed by reverse-transcription and real-time quantitative PCR and immunohistochemistry. In in vitro experiments using ATC SW1736 cells, the expression levels of OCT-4, NANOG, and ABCG2 and the sensitivity to either cisplatin or doxorubicin were evaluated after silencing.

RESULTS

OCT-4, KLF4, and SOX2 transcription factors and C-KIT and THY-1 stem surface antigens showed variable up-regulation in all ATC cases. The SW1736 cell line was characterized by a high percentage of stem population (10.4±2.1% of cells were aldehyde dehydrogenase positive) and high expression of several CSC markers (SOX2, OCT4, NANOG, C-MYC, and SSEA4). SOX2 silencing down-regulated OCT-4, NANOG, and ABCG2. SOX2 silencing sensitized SW1736 cells, causing a significant cell death increase (1.8-fold) in comparison to control cells with 10 μM cisplatin (93.9±3.4% vs. 52.6±9.4%, p<0.01) and 2.7 fold with 0.5 μM doxorubicin (45.8±9.9% vs. 17.1±3.4% p<0.01). ABCG2 silencing caused increased cell death with both cisplatin (74.9±1.4%) and doxorubicin treatment (74.1±0.1%) vs. no-target-treated cells (respectively, 45.8±1.0% and 48.6±1.0%, p<0.001).

CONCLUSIONS

The characterization of CSCs in ATC through the analysis of multiple pluripotent stem cell markers might be useful in identifying cells with a stem-like phenotype capable of resisting conventional chemotherapy. In addition, our data demonstrate that SOX2 switch-off through ABCG2 transporter down-regulation has a major role in overcoming CSC chemotherapy resistance.

Epithelial-mesenchymal transition triggers cancer stem cell generation in human thyroid cancer cells

Increasing evidence has shown that cancer stem cells or tumor initiating cells are the 'root cause' of malignant cancers. However, the exact origin of cancer stem cells still remains obscure in thyroid research. EMT has been implicated in the initiation and conversion of early-stage tumors into invasive malignancies and is associated with the stemness of cancer cells. Based on these facts, a new hypothesis was suggested that EMT induces cancer stem cell generation and tumor progression in human thyroid cancer cells in vitro. In the present study, FTC133 cells identified as EMT-negative cells were used for EMT induction by HIF‑1α transfection. Overexpression of HIF-1α induced FTC133 cells to undergo EMT, downregulated the epithelial markers E-cadherin, upregulated the mesenchymal marker vimentin, and associated with highly invasive and metastatic properties. Most importantly, the induction of EMT promoted the stem-like side population cell proportion in the FTC133 cells. These results indicate that EMT induction promotes CSC traits and cell proportions in the thyroid cancer cells, which implies that EMT could induce cancer stem cell generation and tumor progression in thyroid cancers. Further understanding of the role of EMT and cancer stem cells in cancer progression may reveal new targets for the prevention or therapy of thyroid cancers.

Preparation of thyroid follicular cells for mRNA quantification after fluorescence-activated cell sorting

We established a novel method to analyze cells collected by fluorescence-activated cell sorting (FACS) named mRNA quantification after FACS (FACS-mQ) in which cells are labeled with a fluorescence dye in a manner that minimizes RNA degradation, and then cells sorted by FACS are examined by analyzing their gene expression profile. In order to analyze cells using FACS-mQ, it is essential to prepare single-cell suspensions without RNA degradation. We found that a new tissue preservation medium, ThelioKeep™, which contains epigallocatechin-3-gallate (EGCG), was suitable for preservation of thyroid tissues. The aim of this study was to establish a cell dispersion method of thyroid follicular cells using ThelioKeep™. We compared the efficiency of cell dispersion between the two methods, the conventional cold pre-incubation method and the ThelioKeep™ method; then we determined if cells obtained by the ThelioKeep™ method were suitable for FACS-mQ analysis. We found that a larger number of cells were recovered using ThelioKeep™ than using the conventional cold pre-incubation method. Furthermore, cell viability was higher with the ThelioKeep™ method than with the cold pre-incubation method. Thyroid cells collected by this method were analyzed by FACS-mQ. A clear shift in flow cytometry analysis was observed when cells were stained with an anti-thyroglobulin or anti-thyroid transcription factor-1 antibody. After sorting, the same copy number of ACTB mRNA was detected in thyroid cells as in an anaplastic carcinoma cell line, 8305C. These findings imply that preparation of thyroid cells using the present method is suitable for FACS-mQ analysis.

Stem cells and cancer stem-like cells in endocrine tissues

Cancer stem-like cells are a subpopulation of self-renewing cells that are more resistant to chemotherapy and radiation therapy than the other surrounding cancer cells. The cancer stem cell model predicts that only a subset of cancer cells possess the ability to self-renew and produce progenitor cells that can reconstitute and sustain tumor growth. Evidence supporting the existence of cancer stem-like cells in the thyroid, pituitary, and in other endocrine tissues is rapidly accumulating. These cells have been studied using specific biomarkers including: CD133, CD44, Nestin, Nanog, and aldehyde dehydrogenase enzyme. Putative cancer stem-like cells can be studied in vitro using serum-free media supplemented with basic fibroblast growth factor and epidermal growth factor grown in low attachment plates or in extracellular matrix leading to sphere formation in vitro. Cancer stem-like cells can also be separated by fluorescent cell sorting and used for in vitro or in vivo studies. Injection of enriched populations of cancer stem-like cells (also referred to as tumor initiating cells) into immunodeficient mice results in growth of xenografts which express cancer stem-like biomarkers. Human cancer stem-like cells have been identified in thyroid cancer cell lines, in primary thyroid cancers, in normal pituitary, and in pituitary tumors. Other recent studies suggest the existence of stem cells and cancer stem-like cells in endocrine tumors of the gastrointestinal tract, pancreas, lungs, adrenal, parathyroid, and skin. New discoveries in this field may lead to more effective therapies for highly aggressive and lethal endocrine cancers.

Differential proteomic and phenotypic behaviour of papillary and anaplastic thyroid cell lines

Thyroid carcinomas account for a minority of all malignant tumours but, after those of the gonads, they represent the most common forms of endocrine cancers. They include several types, among which the papillary thyroid cancer (PTC) and the anaplastic thyroid cancer (ATC) are the best known. The two hystotypes display significant biological and clinical differences: PTC is a well differentiated form of tumour with a high incidence and a good prognosis, while the ATC is less frequent but represents one of the most aggressive endocrine tumours with morphological features of an undifferentiated type. To date, as far as we know, no conclusive studies, useful to design arrays of molecular markers, have been published illustrating the phenotypic and proteomic differences between these two tumours. The aim of this work was to perform a comparative analysis of two thyroid cancer cell lines, derived respectively from papillary (BCPAP) and anaplastic (8505C) thyroid carcinomas. The comparative analysis included cell behaviour assays and proteomic analysis by 2D-PAGE and mass spectrometry. The results have highlighted a new proteomic signature for the anaplastic carcinoma-derived cells, consistent with their high proliferation rate, motility propensity and metabolic shift, in relation to the well-differentiated PTC cells.

Genes that characterize T3-predominant Graves' thyroid tissues

OBJECTIVE

3,5,3'-Triiodothyronine (T(3))-predominant Graves' disease is characterized by the increasing volume of thyroid goiter resulting in poor prognosis. Although type 1 and type 2 iodothyronine deiodinases (DIO1 and DIO2 respectively) are known to be overexpressed in the thyroid tissues of T(3)-predominant Graves' disease, the pathogenesis of this disease is still unclear. The aim of our study is to identify genes that characterize T(3)-predominant Graves' disease tissue in order to clarify the molecular mechanism of this disease.

DESIGN AND METHODS

mRNAs from two thyroid tissues of both typical T(3)-predominant and common-type Graves' disease were analyzed with DNA microarrays with probes for 28 869 genes. Genes identified to be differentially expressed between the two groups were further analyzed in the second and third screenings using 70 Graves' thyroid tissues by real-time quantitative RT-PCR.

RESULTS

Twenty-three candidate genes were selected as being differentially expressed in the first screening with microarrays. Among these, seven genes, leucine-rich repeat neuronal 1 (LRRN1), bone morphogenetic protein 8a (BMP8A), N-cadherin (CDH2), phosphodiesterase 1A (PDE1A), creatine kinase mitochondrial 2 (CKMT2), integrin beta-3 (ITGB3), and protein tyrosine phosphatase non-receptor type 4 (PTPN4), were confirmed to be differentially expressed in DIO1 or DIO2 over- and underexpressing Graves' tissues.

CONCLUSIONS

These genes are related to the characteristics of T(3)-predominant Graves' disease, such as high titer level of serum anti-TSH receptor antibody, high free T(3) to free thyroxine ratio, and a large goiter size. They might play a role in the pathogenesis of T(3)-predominant Graves' disease.

Prolonged hybridization with a cRNA probe improves the signal to noise ratio for in-tube in situ hybridization for quantification of mRNA after fluorescence-activated cell sorting

We developed an in-tube in situ hybridization method for mRNA quantification after fluorescence-activated cell sorting (FACS-mQ). A specific RNA in a particular cell type is stained with a cRNA probe and a fluorescent dye, which allows the stained cells to be selected by FACS without excessive RNA degradation. Our previous protocol required 4 h for hybridization with a cRNA probe, which might not produce enough fluorescence signal for sorting genes with low expressions. We determined the effect of prolonged hybridization for in-tube in situ hybridization on quantitative measurement of intracellular RNAs. During the hybridization step, the quantity of ACTB mRNA decreased gradually until 4 h, but remained constant from 4 to 16 h below 63.6° C. For flow cytometry, cells hybridization with cRNA probes for TG mRNA at 60° C for 16 h showed both increased signal and decreased background fluorescence compared to those hybridized for 4 h. These results indicate that when performing in-tube in situ hybridization, hybridization temperature can be raised to 63.6° C and the hybridization step can be extended up to 16 h without excessive intracellular RNA degradation.

Blood presence of circulating oncofetal fibronectin mRNA, by RT-PCR, does not represent a useful specific marker for the management and follow-up of thyroid cancer patients

BACKGROUND

Recent studies strongly suggest the use of oncofetal fibronectin (onfFN) mRNA in diagnostic follow-up and staging due to its very high specificity for thyroid cancers. Since the use of this marker has not been well established yet, particularly in the monitoring of minimal residual disease, we have tried to verify the diagnostic power of onfFN and its usefulness as a prognostic molecular marker. For this reason, we evaluated (by RT-PCR) the presence of onfFN mRNAs, not only in blood samples and thyroid tissues (both normal and neoplastic), but also in different biological fluids (such as K3-EDTA blood samples, saliva and urine) belonging to healthy individuals.

METHODS

Molecular investigations, such as RT-PCR protocol, and sequencing of onfFN cDNAs evaluation of the above-mentioned samples were performed.

RESULTS

The onfFN transcript was largely expressed in all benign and malignant thyroid tissues [differentiated thyroid carcinomas (DTCs)] tested as well as in a large number of biological fluids; in particular, 100% urine samples were positive for onfFN transcript as compared to the thyroglobulin (Tg) mRNA (75%), while saliva was always positive for onfFN and never for Tg. These findings indicate that onfFN cannot be considered a marker specific for thyroid cancer presence. Finally, Tg results were positive in a large part of the samples, but not always in concomitance with onfFN.

CONCLUSIONS

We underline how the complexity of onfFN transcripts could affect the RT-PCR procedure. In addition, the presence of onfFN transcripts in several normal and cancer tissues, along with non-thyroid biological fluids or cells, does not allow the use of this marker for cancer monitoring.

mRNA quantification after fluorescence activated cell sorting using locked nucleic acid probes

Recently, we have established an in-tube in situ hybridization method named mRNA quantification after fluorescence activated cell sorting (FACS-mQ), in which a specific RNA in a particular cell type is stained with a florescent dye, allowing the stained cells to be selected by FACS without suffering excessive RNA degradation. Using this method, the biological characteristics of the sorted cells can be determined by analyzing their gene expression profile. In this study, we used locked nucleic acid (LNA) oligonucleotides, which are known to enhance both the sensitivity and specificity of RNA detection, as hybridization probes in FACS-mQ. When we used a LNA probe targeting the human 28S sequence, we were able to efficiently separate human cells from rat cells. Using LNA probes, the hybridization step was shortened to 1 h. After the hybridization step, 84.6% RNA was preserved; thus, we were able to successfully measure gene expression levels in each type of cell after FACS. Providing the LNA probe efficiently hybridizes with the target sequence, FACS-mQ with an LNA probe is a powerful tool for separating particular cells and determining their biological characteristics by analyzing their gene expression profile.

Clinical review: The emerging cell biology of thyroid stem cells

CONTEXT

Stem cells are undifferentiated cells with the property of self-renewal and give rise to highly specialized cells under appropriate local conditions. The use of stem cells in regenerative medicine holds great promise for the treatment of many diseases, including those of the thyroid gland.

EVIDENCE ACQUISITION

This review focuses on the progress that has been made in thyroid stem cell research including an overview of cellular and molecular events (most of which were drawn from the period 1990-2011) and discusses the remaining problems encountered in their differentiation.

EVIDENCE SYNTHESIS

Protocols for the in vitro differentiation of embryonic stem cells, based on normal developmental processes, have generated thyroid-like cells but without full thyrocyte function. However, agents have been identified, including activin A, insulin, and IGF-I, which are able to stimulate the generation of thyroid-like cells in vitro. In addition, thyroid stem/progenitor cells have been identified within the normal thyroid gland and within thyroid cancers.

CONCLUSIONS

Advances in thyroid stem cell biology are providing not only insight into thyroid development but may offer therapeutic potential in thyroid cancer and future thyroid cell replacement therapy.

Thyroid cancer stem cells

Thyroid cancer is the most frequently diagnosed endocrine cancer and causes more deaths than all other endocrine cancers combined. Research findings support the concept that a subpopulation of thyroid cancer cells displays properties characteristic of stem cells. These putative cancer-forming entities drive tumorigenesis as a result of their dual ability to undergo self-renewal and to differentiate into various types of cancer cells; they also mediate metastasis and are resistant to the effects of chemotherapy and radiation therapy. This Review discusses the cellular origin of thyroid cancer and the properties of the thyroid cancer stem cell niche. The article critically evaluates the methods used to identify molecular markers expressed by thyroid-cancer-initiating cells and outlines prospective therapeutic strategies to directly target these cells. Stem-cell technology offers an unprecedented opportunity to investigate these crucial cancer stem cell populations and to advance understanding of the molecular mechanisms that control disease processes. Such knowledge could potentially lead to the development of more effective and safer treatment regimens for late-stage thyroid cancer than are currently available.

Messenger RNA quantification after fluorescence-activated cell sorting using in situ hybridization

Recent studies using stem cells or cancer stem cells have revealed the importance of detecting minor populations of cells in blood or tissue and analyzing their biological characteristics. The only possible method for carrying out such procedures is fluorescence-activated cell sorting (FACS). However, FACS has the following two limitations. First, cells without an appropriate cell surface marker cannot be sorted. Second, some laborious procedures such as rapid sorting or treatment under sterilized conditions may require in order to analyze their biological characteristics. If a specific mRNA in a particular cell type can be stained with a florescent dye and then the cells can be sorted without causing RNA degradation, a more simple and universal method for sorting and analyzing cells with a specific gene expression pattern could be established since the biological characteristics of the sorted cells could then be determined by analyzing their gene expression profile. In this study, we established a basic protocol for messenger RNA quantification after FACS (FACS-mQ) using a cRNA probe. This method could be used for the detection and analysis of stem cells or cancer stem cells in various tissues.

Messenger RNA quantification after fluorescence activated cell sorting using intracellular antigens

Recent studies using stem cells or cancer stem cells have revealed the importance of detecting minor populations of cells in blood or tissue and analyzing their biological characteristics. The only possible method for carrying out such procedures is fluorescence activated cell sorting (FACS). However, FACS has the following limitations. First, cells without an appropriate cell surface marker cannot be sorted. Second, the cells have to be kept alive during the sorting process in order to analyze their biological characteristics. If an intracellular antigen that was specific to a particular cell type could be stained with a florescent dye and then the cells can be sorted without causing RNA degradation, a more simple and universal method for sorting and analyzing cells with a specific gene expression pattern could be established since the biological characteristics of the sorted cells could then be determined by analyzing their gene expression profile. In this study, we established a basic protocol for messenger RNA quantification after FACS (FACS-mQ) targeting intracellular antigens. This method can be used for the detection and analysis of stem cells or cancer stem cells in various tissues.

Thyroid stem cells and cancer

BACKGROUND

Thyroid gland development and function are essential for life, and recent findings indicate the presence of stem/progenitor cells within the thyroid gland as a potential source of tissue regeneration and cancer formation.

SUMMARY

This review summarizes the current knowledge on early differentiation of thyroid cells from embryonic stem cells and highlights exciting concepts and recent novel findings on adult thyroid stem/progenitor cells in the normal thyroid gland and in thyroid cancer. Other potential sources and markers of stem/progenitor cells in the thyroid include bone marrow, microchimerism, and embryological remnant-derived multifocal solid cell nests. Finally, we discuss new therapeutic strategies that target thyroid cancer stem cells.

CONCLUSIONS

Thyroid stem/progenitor cell populations are present in the normal and diseased thyroid gland. Advances in normal and cancer thyroid stem cell biology will be essential for future targeted therapies.

Expression of nestin mRNA is a differentiation marker in thyroid tumors

Nestin is a maker that identifies stem cells in some adult tissues, and its expression is believed to relate to malignancy in cancer cells. In this study, we measured the expression levels of nestin mRNA in various kinds of thyroid tumor by the real-time quantitative reverse transcription-polymerase chain reaction. Unexpectedly, nestin mRNA was detected in almost all differentiated thyroid tumors and normal thyroid tissues, whereas extremely decreased expression was observed in anaplastic carcinomas, which are the most malignant of the thyroid follicular cell-derived tumors. These results suggest that nestin mRNA is a differentiation marker, and its expression does not relate to malignant characteristics in thyroid tumors.

The tumor stem cell concept-implications for endocrine tumors?

The cancer stem cell hypothesis has recently evolved from an increasing body of evidence suggesting that in some cancers a small population of tumor cells with stem cell-like properties represents a critical component that dictates the malignant behavior of a given tumor. These observations challenge classical cancer biology and its theory, that tumor growth is mainly based on genomic alterations followed by modulation of cell cycle pathways, which finally result in uncontrolled clonal proliferation. Over the last few years, much progress in the field of tumor stem cells has been achieved in non-endocrine malignancies. In this review, we summarize the existing evidence regarding the tumor stem cell concept for tumor pathophysiology in general and highlight current models that have the potential to further impact research on endocrine tumors.

In vitro identification and characterization of CD133(pos) cancer stem-like cells in anaplastic thyroid carcinoma cell lines

Background

Recent publications suggest that neoplastic initiation and growth are dependent on a small subset of cells, termed cancer stem cells (CSCs). Anaplastic Thyroid Carcinoma (ATC) is a very aggressive solid tumor with poor prognosis, characterized by high dedifferentiation. The existence of CSCs might account for the heterogeneity of ATC lesions. CD133 has been identified as a stem cell marker for normal and cancerous tissues, although its biological function remains unknown.

Methodology/Principal Findings

ATC cell lines ARO, KAT-4, KAT-18 and FRO were analyzed for CD133 expression. Flow cytometry showed CD133^pos cells only in ARO and KAT-4 (64±9% and 57±12%, respectively). These data were confirmed by qRT-PCR and immunocytochemistry. ARO and KAT-4 were also positive for fetal marker oncofetal fibronectin and negative for thyrocyte-specific differentiating markers thyroglobulin, thyroperoxidase and sodium/iodide symporter. Sorted ARO/CD133^pos cells exhibited higher proliferation, self-renewal, colony-forming ability in comparison with ARO/CD133^neg. Furthermore, ARO/CD133^pos showed levels of thyroid transcription factor TTF-1 similar to the fetal thyroid cell line TAD-2, while the expression in ARO/CD133^neg was negligible. The expression of the stem cell marker OCT-4 detected by RT-PCR and flow cytometry was markedly higher in ARO/CD133^pos in comparison to ARO/CD133^neg cells. The stem cell markers c-KIT and THY-1 were negative. Sensitivity to chemotherapy agents was investigated, showing remarkable resistance to chemotherapy-induced apoptosis in ARO/CD133^pos when compared with ARO/CD133^neg cells.

Conclusions/Significance

We describe CD133^pos cells in ATC cell lines. ARO/CD133^pos cells exhibit stem cell-like features - such as high proliferation, self-renewal ability, expression of OCT-4 - and are characterized by higher resistance to chemotherapy. The simultaneous positivity for thyroid specific factor TTF-1 and onfFN suggest they might represent putative thyroid cancer stem-like cells. Our in vitro findings might provide new insights for novel therapeutic approaches.

TSH signalling and cancer

Thyroid cancers are the most frequent endocrine neoplasms and mutations in the thyrotropin receptor (TSHR) are unusually frequent. Here we present the state-of-the-art concerning the role of TSHR in thyroid cancer and discuss it in light of the cancer stem cell theory or the classical view. We briefly review the gene and protein structure updating the cancer related TSHR mutations database. Intriguingly, hyperfunctioning TSHR mutants characterise differentiated cancers in contrast to undifferentiated thyroid cancers which very often bear silenced TSHR. It remains unclear whether TSHR alterations in thyroid cancers play a role in the onset or they appear as a consequence of genetic instability during evolution, but the presence of functional TSHR is exploited in therapy. We outline the signalling network build up in the thyrocyte between TSHR/PKA and other proliferative pathways such as Wnt, PI3K and MAPK. This networks integrity surely plays a role in the onset/evolution of thyroid cancer and needs further research. Lastly, future investigation of epigenetic events occurring at the TSHR and other loci may give better clues for molecular based therapy of undifferentiated thyroid carcinomas. Targeted demethylating agents, histone deacetylase inhibitors combined with retinoids and specific RNAis may help treatment in the future.

Thyroid stem cells: lessons from normal development and thyroid cancer

Ongoing advances in stem cell research have opened new avenues for therapy for many human disorders. Until recently, however, thyroid stem cells have been relatively understudied. Here, we review what is known about thyroid stem cells and explore their utility as models of normal and malignant biological development. We also discuss the cellular origin of thyroid cancer stem cells and explore the clinical implications of cancer stem cells in the thyroid gland. Since thyroid cancer is the most common form of endocrine cancer and that thyroid hormone is needed for the growth and metabolism of each cell in the body, understanding the molecular and the cellular aspects of thyroid stem cell biology will ultimately provide insights into mechanisms underlying human disease.

New insights into thyroid stem cells

Stem cells exhibit an extraordinary ability for self-renewal. They also give rise to many specialized cells. The potential of stem cells in regenerative medicine, developmental biology, and drug discovery has been well documented. Although advances in stem cell science have raised broad ethical concerns, it is clear that stem cell technology has revolutionized our thinking in modern biology and medicine and provided the basis for understanding many of the mechanisms controlling basic biological processes and disease mechanisms. This review details the nascent field of thyroid stem cell research, exploring the current status of thyroid stem cell differentiation from the perspectives of both developmental biology and cell replacement therapy. It highlights successes to date in the generation of thyroid follicular cells from embryonic stem cells in the laboratory and the identification and characterization of adult stem cells from human thyroid glands and thyroid cancers. Finally, it outlines future challenges with a focus on potential stem cell therapy for thyroid patients.

Stem cells derived from goiters in adults form spheres in response to intense growth stimulation and require thyrotropin for differentiation into thyrocytes

OBJECTIVE

This study aimed to analyze under which conditions quiescent stem cells derived from human goiters can be propagated to outgrow and whether these cells have retained the capacity to differentiate into thyroid cells.

DESIGN

Stem cells were isolated by fluorescence-activated cell sorting as a side population by the Hoechst 33342 efflux technique. Growth pattern of stem cells and cocultures of stem cells with thyrocytes grown as monolayer and in Matrigel was investigated. Expression of stem cell markers, endodermal markers, and thyroid-specific markers was analyzed by RT-PCR. In stem cell-derived thyrocytes, embedded in collagen to form follicles, TSH-dependent (125)iodide uptake was measured.

RESULTS

Stem cells were isolated as a side population from a non-side population fraction that consisted of endodermal marker-positive cells and thyroid cells. Intense growth stimulation of stem cells in coculture with thyrocytes resulted in formation of nonadherent, three-dimensional spheres that consisted of highly proliferating stem cells with their characteristic expression profiles. In response to TSH and serum, sphere-derived progenitor cells differentiated into thyrocytes that expressed paired box gene 8, thyroglobulin, sodium iodide symporter, thyroid-stimulating hormone receptor, and thyroperoxidase mRNA and showed TSH-dependent (125)iodide uptake.

CONCLUSION

Quiescent stem cells derived from goiters can be propagated to form spheres that consist of highly proliferating stem cells that are able to differentiate TSH dependently into thyroid cells. Compared with thyrocytes, stem cells display a much higher proliferation rate on acute growth stimulation, which may suggest a putative role of the offspring of stem cells in the chronic growth factor-stimulated nodular transformation of the thyroid.

Apolipoprotein e expression in anaplastic thyroid carcinoma

OBJECTIVES

Apolipoprotein E (Apo E) has been known to play a role in cholesterol transport and metabolism. However, recently the relationship between Apo E and carcinoma progression has been investigated. In this study, we investigated Apo E expression in thyroid carcinoma at both the protein and molecular levels.

METHODS

We investigated Apo E expression at the protein and molecular level in 124 thyroid neoplasms.

RESULTS

In RT-PCR and in situ hybridization, the Apo E mRNA expression level was very low in papillary and follicular carcinomas as well as normal thyroid, but was dramatically elevated in anaplastic carcinoma. In an immunohistochemical study, 32 of 33 anaplastic carcinomas (97.0%) showed high levels of Apo E expression, but this phenomenon was seen only in 1 of 51 papillary carcinomas (2.0%). None of the follicular carcinomas or adenomas showed high levels of Apo E expression.

CONCLUSIONS

These findings suggest that Apo E is one of the typical biological characteristics of anaplastic thyroid carcinoma.

BRAF V600E mutation in anaplastic thyroid carcinomas and their accompanying differentiated carcinomas

Frequency of a BRAF^V600E mutation in anaplastic thyroid carcinoma, which is thought to be derived mainly from papillary carcinoma by multi-step carcinogenesis, is much lower than that in papillary carcinomas. To clarify this phenomenon, we analysed BRAF^V600E mutation in 20 cases of anaplastic carcinoma and 13 accompanying differentiated carcinomas. Among twenty cases of anaplastic carcinomas, nine and four accompanied papillary and follicular carcinomas, respectively. BRAF^V600E mutation was found in four (20%) cases. BRAF^V600E mutation was found in three of nine (33.3%), none of four and one of seven (14.3%) anaplastic carcinomas with papillary carcinoma, follicular carcinoma and without differentiated components, respectively. All three papillary carcinomas accompanied by anaplastic carcinoma with a BRAF^V600E mutation were also shown to have a BRAF^V600E mutation. In summary, BRAF^V600E mutation was occasionally observed in anaplastic carcinomas with papillary carcinoma, and the low frequency of BRAF^V600E mutation in anaplastic carcinoma was thought to be due to the low frequency of anaplastic carcinomas with papillary carcinoma. These findings raise a question about the classical model of anaplastic transformation and suggest some roles of thyroid cancer stem cells in the generation of anaplastic carcinoma.

Expression of endoderm stem cell markers: evidence for the presence of adult stem cells in human thyroid glands

OBJECTIVE

Adult stem cells have been detected in several human tissues. The object of this study was to investigate whether they also occur in the human thyroid gland.

DESIGN

The expression of the stem cell marker Oct- 4 and the early endodermal markers GATA-4 and HNF4alpha was analyzed in histologic slides and cultured cells derived from goiters, in the FRTL5 cell line, and the HTh74, HTC, C643, and XTC133 thyroid carcinoma cell lines.

MAIN OUTCOME

Stem cell markers were detectable in all primary cultures whereas in the differentiated FRTL5 cell line no expression was observed. Expression of stem cell marker mRNA was not affected by thyrotropin (TSH) stimulation and did not decrease when cells underwent several passages. Immunostaining of cultured cells and of histologic slides of goitrous tissues showed only single cells that were immunoreactive for Oct-4, GATA-4, and HNF4a. Expression of Oct-4 but not of endodermal marker GATA-4 was also detectable in some thyroid carcinoma cell lines. Fluorescence-activated cell sorter (FACS) analysis demonstrated cell populations that were positive for either Oct-4, GATA-4, or HNF4alpha but negative for thyroglobulin. When these putative, FACS-sorted stem cell populations were further analyzed by reverse transcriptase-polymerase chain reaction (RT-PCR), expression of all stem cell markers and of Pax8 but not of thyroglobulin mRNA was detectable.

CONCLUSIONS

These data provide evidence for the presence of adult stem and precursor cells of endodermal origin in the human thyroid gland.

Fetal cell carcinogenesis of the thyroid: theory and practice

A novel hypothesis of thyroid carcinogenesis, the "fetal cell carcinogenesis" hypothesis, in which cancer cells are derived from the remnants of three types of fetal thyroid cells, instead of normal thyroid follicular cells, is proposed. In this hypothesis, thyroid cancer cells are generated from fetal cells by proliferation without differentiation and oncogenes play an oncogenic role by preventing fetal cells from differentiating. This hypothesis explains well the clinical and biological features and recent molecular evidence of thyroid carcinoma. It underscores the importance of clarifying the molecular mechanism of thyroid development and the identification of fetal thyroid cells, especially thyroid stem cells (TSCs), since such data will lead to better understanding of thyroid carcinoma and other thyroid diseases.