1. Introduction
Sarcomas are rare mesenchymal malignant tumors with unique biological and clinical features. Sarcomas are unique malignancies for several reasons. First, sarcomas originate from diverse mesenchymal tissue lineages such as adipose, muscle, fibrous, cartilage, nervous, and vascular tissues, or bone. Since these tissues are distributed throughout the human body, sarcomas can occur in almost all organs. Second, sarcomas are heterogeneous diseases that are pathologically grouped into more than 70 described subtypes [
1]. The histological appearances do not necessarily represent their normal counterparts, and indeed, the original normal cells are not identified for most sarcomas. Third, sarcomas have high complexity at the molecular level, classifying them into two groups: genetically simple sarcomas, such as those bearing specific genetic alterations, and sarcomas with multiple, complex karyotypic abnormalities with no specific pattern [
2,
3].
Recent advances in genomic technology using next-generation sequencing have enabled the classification of sarcomas, which did not fit into known specific diagnostic categories [
4]. Such classification may lead to innovative therapies. Finally, despite their diversity, heterogeneity, and complexity, sarcomas are rare, accounting for less than 1% of all malignancies. The reasons for this rarity are not well understood. Possible reasons include the need for unique genetic mutations for carcinogenesis, the small number of original cells, and the resistance of the original cells to carcinogenesis. Interestingly, sarcomas are prevalent in children and adolescents, where they account for approximately 20% of cancer-related deaths [
5]. Although several molecular predispositions have been suggested [
6,
7,
8,
9,
10,
11,
12,
13], the mechanisms of early onset of sarcomas in children are not understood.
Given their diversity, heterogeneity, complexity, and rarity, the clinical management of sarcomas is quite challenging. However, there are many successful examples of innovative drugs for sarcomas. For example, a tyrosine kinase inhibitor, imatinib mesylate, was originally developed for the treatment of chronic myelocytic leukemia, which has a unique chromosome translocation; later, imatinib mesylate was repurposed to gastrointestinal stromal tumor (GIST), which is characterized by mutations or overexpression of c-kit and PDGFR [
14,
15,
16,
17,
18]. Imatinib has also shown activity in metastatic dermatofibrosarcoma protuberans (DFSP) [
19] and fibrosarcomatous DFSP [
20]. Following imatinib, other tyrosine kinase inhibitors, such as sunitinib [
21] and regorafenib [
22], have been approved for GISTs.
Other drugs include trabectedin, pazopanib, eribulin, olaratumab, and denosumab. Trabectedin, which binds to the minor groove of DNA to cause DNA damage, demonstrated evidence of cytotoxic activity against soft tissue sarcomas [
23,
24,
25,
26,
27]. Pazopanib, an inhibitor for VEGFR, PDGFR, FGFR, c-kit, and many other tyrosine kinases [
28,
29], has suppressive effects on angiogenesis and has been approved for the treatment of non-GIST soft tissue sarcomas [
30,
31,
32]. Eribulin is a microtubule inhibitor, which binds to the vinca domain of tubulin and inhibits the polymerization of tubulin and the assembly of microtubules, inducing cell cycle arrest at the G2/M phase [
33,
34] and exhibiting anti-tumor effects [
35,
36]. Eribulin was approved for metastatic breast cancer, and later, it significantly prolonged overall survival in patients with leiomyosarcoma or liposarcoma in a randomized, phase 3 trial with an active control [
37]. Olaratumab, a monoclonal antibody targeting platelet-derived growth factor receptor (PDGFR)-alpha, extended the overall survival of metastatic GIST [
38] as well as non-GIST sarcomas [
39]. New drugs, such as denosumab, a receptor activator of nuclear factor kB ligand (RANKL) [
40], have yielded favorable results against osteosarcomas in pre-clinical studies [
41,
42,
43] and giant cell rich tumors [
44]. Although the results of these clinical and pre-clinical trials seemed to be encouraging, they were often discrepant and needed to be interpreted with care. For example, Piuperno-Neumann et al. reported a discrepancy between OS206 trial data and preclinical data, and finally did not recommend zoledronate for osteosarcoma patients [
45].
This recent progress in the development of novel anti-cancer drugs suggests improvement in the clinical outcomes of patients with sarcomas in the near future [
46,
47,
48]. However, considering the complexity of sarcomas, a larger number of effective anti-cancer drugs should be developed. As the success rate of drug development remains generally low in oncology, and the number of patients who can be recruited into clinical trials is limited for sarcomas, a pre-clinical study to evaluate the eligibility of potential anti-cancer drugs is important, especially in rare malignancies, such as sarcomas.
Cell lines have been used as indispensable tools for both basic research and pre-clinical studies. Since animal cell culture became a common laboratory technique in the mid-1900s, it has served as a model for human cancer research. Since cell lines are maintained in artificial tissue culture conditions, there are critical arguments about the utility of cell lines; however, the advantages of other cancer models, such as xenografts [
49] and organoids [
50], for cancer research have been emphasized to complement the inherent drawbacks of cell lines. Indeed, the generation of cell lines may involve extensive selection and adaptation to in vitro culture conditions, and in some cell lines, only rare clones may expand with considerable genetic changes [
51]. Therefore, the results of experiments using cell lines should be interpreted with caution [
52]. However, the advantages of cell lines over other cancer models are obvious. Once the cell lines are stably established, they constantly expand in the tissue culture conditions and they are useful to examine the functional effects and mechanisms of genes or drugs with significant reproducibility. In this context, because cell lines are deposited in public cell banks or shared in the research community, we can integrate research results obtained in different laboratories. Previous reports suggested that the unique characteristics of cell lines can contribute to the development of cancer therapy. For example, cell lines allow the screening of a compound library [
53] and the prediction of reactions to treatments [
54,
55]. In addition to drug development, cell lines are required for biomarker studies. In a biomarker study, the functional evaluation of biomarker candidates is mandatory to convince collaborators to perform multi-institutional validation studies. Cell lines are mandatory to investigate the biological properties of biomarker candidates. Overall, without using cell lines, most of the anti-cancer drugs and biomarkers used in hospitals and the scientific discoveries written in text books could not have been achieved for cancers.
To date, different sarcoma cell lines have been developed. These cell lines represent a useful experimental model to examine the hypothesis about the etiology of diseases, to evaluate the molecular mechanisms of cancer progression, and to examine the effect of potent anti-cancer drugs at the cellular and subcellular levels. At the same time, besides the obvious utilities of cell lines in cancer research, researchers may be empirically aware that sarcoma cell lines are not readily available, probably due to the rarity of the disease, and a lack of proper cell lines hinders basic studies and development of effective therapies for sarcomas. In this review, we provide an overview of the current status of reported sarcoma cell lines, and finally discuss what types of sarcoma cell lines need to be established, what system needs to be created to promote sarcoma research using cell lines, and what biological studies need to be performed to improve the present status of sarcoma cell lines.
4. Discussion
A lack of sarcoma cell lines is empirically noticed in the research community; it is important to know their availability from a practical view point. In this review, we investigated the current status of sarcoma cell lines to reveal what cell lines have to be established to promote sarcoma research. The cell line database, Cellosaurus, used in this study includes more than one hundred thousand cell lines and is frequently updated. Thus, Cellosaurus is an adequate cell line database for investigation.
We grouped the cell lines according to the histology of their original tumor. We found that 45 histological subtypes were covered by the currently reported cell lines, while 133 were not. Considering the diversity and complexity of sarcomas, we need more cell lines that represent the different histological subtypes. In addition, we found that multiple cell lines were established for 36 histological subtypes, with a single cell line reported for nine subtypes. During the course of the cell line establishment, clonal selection and expansion may occur, and only limited cell populations may survive under tissue culture conditions. To understand the sustainability of the original characteristics of the established cell lines, the capability of tumor tissue formation and the histology of the formed tumors can be evaluated by xenograft experiments. In addition, patient-to-patient variations are clinically considerable even if they have tumors with the same histology. Therefore, no single cell line can represent the characteristics of whole tumor tissues; we need to use multiple cell lines. In this sense, we also need more cell lines for sarcomas that already have corresponding cell lines.
Cell lines are most frequently established from Ewing’s sarcoma and osteosarcoma samples. However, the absolute number of patients with Ewing’s sarcoma and osteosarcoma is small according to medical statistics; undifferentiated pleomorphic sarcoma, liposarcoma, and leiomyosarcoma are more common sarcomas [
58]. Thus, the number of patients may not be a critical factor to determine the established cell lines. Cell lines with a higher malignant potential may be easier to establish, and the clinical stage of donor patients, pathological grading, and prognosis may be correlated with the success rate of the cell line establishment. However, during our investigation, there was no report discussing the efficacy of the cell line establishment in terms of histology. This issue is quite important because we can refine the experimental protocols and improve the efficacy of experiments by clarifying the biological and clinical factors that determine the success rate of establishment.
The histological diagnosis of original tumors of cell lines may need to be updated in cases where the name of cell lines did not match the official classification. Among the 844 sarcoma cell lines investigated, 42 were not named according to the 2013 World Health Organization
Classification of Tumours of Soft Tissue and Bone [
1,
59]. The diagnosis of sarcomas has been achieved based on morphological observations, and sarcomas are reclassified by the genetic characterization and subsequent phenotypic correlations. Thus, the diagnosis of cell lines with the official name should be refined by pathological examinations according to the most recent diagnosis criteria. This is a dilemma for a study using clinical materials, because the criteria of histological subtypes may have been updated after the cell lines were reported. To take full advantage of patient-derived sarcoma cell lines, we should investigate the pathology archives and update the diagnosis. However, this will be a challenging task.
Unfortunately, cell lines are not always deposited in cell banks. We found that only 139 of 819 sarcoma cell lines named according to the WHO classification were deposited in public cell banks. Probably, the rest of the cell lines can be provided upon request by researchers. The current cell bank systems may rely on researchers and institutes to undertake the cell line establishment. Establishing novel cell lines costs a considerable amount of resources, such as time and money; furthermore, because cell lines are properties of the institutes to which researchers are affiliated, it may be difficult to deposit all cell lines in public cell banks and share them with other researches. As the establishment of cell lines itself is not necessarily a novel discovery, nor would the publication be in high-impact journals, researchers may not be motivated to establish and share cell lines. A system to motivate cell line establishers and their institutes may be required to improve the availability by depositing cell lines.
This systematic review has several limitations. First, although the genetic background and biological characteristics of some but not all cell lines were reported in publications, this review did not summarize those data. In our research, 692 cell lines were reported in previous papers, and 108 of them were deposited in cell banks (
Figure 2). Although the experiments were performed individually using different methods, it is worth integrating the relevant genetic and biological data of reported cell lines to evaluate their possible applications. Second, the clinical features of donor patients, such as metastasis and resistance against therapy, were not investigated in this review. Bernardo et al. [
60] performed a systematic review for patient-derived xenografts in bladder cancers and discussed the clinical factors that may influence the take-rate of xenografts. Lu et al. [
61] investigated previous studies on xenograft establishment, and correlated the higher engraftment rates with tumor stage. A similar approach could be used for cell lines of sarcomas. Thirdly, the pathological diagnosis should be updated using the most recent pathological criteria of sarcomas. It is possible that some of the reported cell lines may actually represent other subtypes. However, because we cannot access the original pathological archives and it takes too much effort to validate the results of pathological diagnosis, we cannot know the correct histology according to the most recent WHO classification. This is a general problem of sarcoma research, as observed when we conducted histology-based research using previously published data. Finally, the applications of cell lines are diverse, and probably depend on the cell lines and the experiments. In addition to the number of established cell lines, it would be worth investigating the literature to determine how the established cell lines were used by the researchers who received them.