Cancer stem-like cell behavior in anaplastic thyroid cancer: A challenging dilemma

Abstract

Aims

Anaplastic thyroid carcinoma (ATC) is an undifferentiated tumor of the thyroid which is characterized with poor prognosis, leading to its aggressive behavior and resistance to conventional therapies. Cancer stem cells (CSCs) are tumor cells that have self-renewal and clonal tumor initiation. Like other cancers, many studies have shown that ATC also has tumor cells with properties like stem cells. To evaluate the concept of cancer stem-like cell theory of ATC, we conducted this study to emphasize both on the concept of cancer stemness origin of these cells and target them for further therapeutic purposes. In the current study, we showed that two ATC cell lines, SW1736 and C643, have subpopulations (SP) that are similar to CSCs.

Materials and methods

Using MACS technique, cells positive for CD133 were isolated and subsequently validated with flow cytometry. For further analysis, expression of some stemness markers was evaluated.

Key findings

ABCG2, CD133, and Sox2 were significantly up-regulated, while Nestin was down-regulated in CD133^pos subpopulation compared to CD133^neg cells. In contrast to previous reports that over-expression of Nestin was considered as a marker for thyroid CSCs, we noticed that expression of Nestin was declined in stem cell-like tumor cells, derived from ATC cell lines.

Significance

This study reconfirmed the concept of cancer stem-like cell identity of SW1736 and C643 cells. Indeed, the characterization of CSCs should not be merely based on surface markers. Cell origin and genetic background should be additionally considered on CSCs subpopulation of ATCs for therapeutics.

Introduction

Cancer is one of the most life-threatening diseases with no conclusive therapy so far. Although there is no agreement on the factors responsible for some cancers' features such as metastasis, relapse and drug resistance, it seems that tumor heterogeneity plays the major role. Indeed, tumor is a complex environment containing tumor cells and various infiltrating cells such as endothelial, hematopoietic and stromal cells. These exterior cells affect tumor environment (TME) by changing metabolic status of tumor cells (like creating hypoxic condition) [23].

Generally, two models have been used for explaining tumor heterogeneity, stochastic and hierarchical models. In one hand, stochastic model relies on presumption that cancer is the result of mutations in cell cycle's genes that lead to cell hyperproliferation. In this model, cells that over express oncogenes or down regulate tumor-suppressor genes are capable of producing cancer. On the other hand, hierarchical theory emphasizes that tumor cells are originated from a small population of cells (CSCs) that have similar characteristics to adult stem cells (Fig. 1). According to this model, carcinogenesis occurs when an adult stem cell bypasses regulation or even when a terminally differentiated cell (with cancerous properties) is dedifferentiated (Fig. 1A, B) [35]. However, these two models are not mutually exclusive and can be harmonized by considering cellular plasticity, genetic diversity and non-genetic influences [27] [32], [35].

CSCs are tumor cells that have self-renewal, clonal tumor initiation and clonal long-term repopulation capacity (Fig. 1B) [35], [37]. Also, CSCs are able to transit between stem cell and non-stem cell states, an ability that refers as plasticity. CSCs are able to be quiescent for a long time and metastasize to different organs. These cells are resistant to usual treatments [34]. CSCs anatomically and functionally do not resemble the original organs' adult stem cells. Actually, it has been shown that CSCs lose multipotency (generation of an entire tissue) and asymmetric cell division in comparison with adult stem cells [27]. Hence, many of researchers prefer to use “tumor-initiating cells (T-ICs)” term instead of CSCs. However, phenotypes and gene-expression patterns of the cell of origin may differ considerably from that “CSCs” [35].

Most of signaling pathways that are important for maintaining stemness in stem cells, are similarly vital for CSCs such as Oct4/Sox2, Notch, and Wnt. It has been shown that disrupting CSCs' signaling pathways, leading to differentiation of them, can inhibit cancer drug resistance and relapse (known as differentiation therapy) [5], [20], [36].

First, CSCs were isolated from leukemia followed by their isolation from many solid tumors [1], [28]. Generally, techniques that are used for CSCs' isolation are based on the enrichment of CSCs via Hoechst dye or surface markers [18]. CD44 and CD133 are two common cell surface markers that have been used for the isolation of CSCs from many tumors [45], [51]. In addition, some studies have been successful in isolating CSCs from cancer cell lines [10], [18], [49]. Isolating CSCs from cell cultures can facilitate studies since they are more accessible than clinical samples.

Based on differentiation state, there are two types of thyroid cancers; differentiated thyroid cancer (papillary thyroid carcinoma and follicular thyroid carcinoma) and undifferentiated thyroid cancer (ATC). Differentiated thyroid cancers show good survival rate and are the most frequent type of thyroid cancers [7]. In contrast, ATC is an undifferentiated, aggressive and most deadly of all thyroid cancers. This type of cancer has a very low rate of cure and most patients die one year after diagnosis [24]. It is believed that chemo- and radio-resistant nature of ATC is due to the presence of stem cell-like tumor cells within the ATC. Therefore, many attempts have been done to isolate and characterize CSCs in this type of tumor [7], [26], [31]. Here, we observed that two ATC cell lines, SW1736 and C643, have SP that resembles CSCs. CD133^pos cells, derived from these two cell lines, have gene expression pattern similar to CSCs and express stemness factors (Fig. 1C).

To evaluate the concept of cancer stem-like cell theory of ATC, we conducted this study to emphasize both on the concept of cancer stemness origin of these cells and target them for further therapeutic purposes.