The role of cancer stem cells in neoplasia of the lung: past, present and future

Stem cell characteristics of dormant cells and cisplatin‑induced effects on the stemness of epithelial ovarian cancer cells

Tumor dormancy is a common biological property of malignancies and a leading factor in treatment failure, metastasis and tumor recurrence. The present study generated mouse xenograft models by injection of PKH26‑labeled SKOV3 ovarian cancer cells, which were divided into two groups: The control group (SKOV3‑P tumors,) and the treatment group that generated resistant tumors following prolonged administration of cisplatin (SKOV3‑R tumors). Administration of cisplatin resulted in inhibition of the tumor growth and SKOV3‑R tumors coexisted with their host at a stable size. According to fluorochrome PKH26 retention, there were multiple cell clones (PKH26hi, PKH26low and PKH26neg cells) in the single cell line generated from xenograft tumors. PKH26hi subsets in SKOV3‑P and SKOV3‑R tumors were dormant cells, as the majority were arrested in G0/G1 phase and expressed high levels of the stem cell markers Oct‑4, Nestin, CD117 and CD44. PKH26hi subsets also demonstrated greater clonogenic capability in vitro and tumorigenicity in vivo, as compared with PKH26low and PKH26neg cells. Notably, chemotherapy was demonstrated to lead to the enrichment and enhanced stem‑like characteristics of dormant/slow‑cycling PKH26hi cells. The results of the present study have demonstrated for the first time, to the best of our knowledge, that dormant tumor cells exhibit stem‑like characteristics, and that cisplatin enhances these characteristics in epithelial ovarian cancer cells.

Colony-stimulating factor 1 potentiates lung cancer bone metastasis

Colony-stimulating factor 1 (CSF1) is essential for osteoclastogenesis that mediates osteolysis in metastatic tumors. Patients with lung cancer have increased CSF1 in serum and high levels are associated with poor survival. Adenocarcinomas metastasize rapidly and many patients suffer from bone metastasis. Lung cancer stem-like cells sustain tumor growth and potentiate metastasis. The purpose of this study was to determine the role of CSF1 in lung cancer bone metastasis and whether inhibition of CSF1 ameliorates the disease. Human lung adenocarcinoma A549 cells were examined in vitro for CSF1/CSF1R. A549-luc cells were injected intracardiac in NOD/SCID mice and metastasis was assessed. To determine the effect of CSF1 knockdown (KD) in A549 cells on bone metastasis, cells were stably transfected with a retroviral vector containing short-hairpin CSF1 (KD) or empty vector (CT). Results showed that A549 cells express CSF1/CSF1R; CSF1 increased their proliferation and invasion, whereas soluble CSF1R inhibited invasion. Mice injected with A549-luc cells showed osteolytic bone lesions 3.5 weeks after injection and lesions increased over 5 weeks. Tumors recapitulated adenocarcinoma morphology and showed osteoclasts along the tumor/bone interface, trabecular, and cortical bone loss. Analyses of KD cells showed decreased CSF1 protein levels, reduced colony formation in soft agar assay, and decreased fraction of stem-like cells. In CSF1KD mice, the incidence of tumor metastasis was similar to controls, although fewer CSF1KD mice had metastasis in both hind limbs. KD tumors showed reduced CSF1 expression, Ki-67+ cells, and osteoclasts. Importantly, there was a low incidence of large tumors >0.1 mm(2) in CSF1KD mice compared with control mice (10% vs 62.5%). This study established a lung osteolytic bone metastasis model that resembles human disease and suggests that CSF1 is a key determinant of cancer stem cell survival and tumor growth. Results may lead to novel strategies to inhibit CSF1 in lung cancer and improve management of bone metastasis.

Lung cancer stem cells: progress and prospects

Epithelial stem cells are critical for tissue generation during development and for repair following injury. In both gestational and postnatal stages, the highly branched and compartmentalized organization of the lung is maintained by multiple, resident stem/progenitor cell populations that are responsible for the homeostatic maintenance and injury repair of pulmonary epithelium. Though lung epithelial injury in the absence of oncogenic mutation is more commonly expressed as chronic lung disease, lung cancer is the most common form of death worldwide and poses a highly significant risk to human health. Cancer is defined by the cell of origin, responsible for initiating the disease. The Cancer Stem Cell Hypothesis proposes that cancer stem cells, identified by stem-like properties of self-renewal and generation of differentiated progeny, are responsible for propagating growth and spread of the disease. In lung cancer, it is hypothesized that cancer stem cells derive from several possible cell sources. The stem cell-like resistance to injury and proliferative potentials of bronchioalveolar stem cells (BASCs) and alveolar epithelial type II cells (AEC2), as well as cells that express the cancer stem cell marker glycoprotein prominin-1 (CD133) or markers for side populations make them potential reservoirs of lung cancer stem cells. The abnormal activation of pathways that normally regulate embryonic lung development, as well as adult tissue maintenance and injury repair, including the Wnt, Hedgehog (Hh) and Notch pathways, has also been identified in lung tumor cells. It is postulated that therapies for lung cancer that specifically target stem cell signaling pathways utilized by lung cancer stem cells could be beneficial in combating this disease.

Targeting the mechanisms of resistance to chemotherapy and radiotherapy with the cancer stem cell hypothesis

Despite advances in treatment, cancer remains the 2nd most common cause of death in the United States. Poor cure rates may result from the ability of cancer to recur and spread after initial therapies have seemingly eliminated detectable signs of disease. A growing body of evidence supports a role for cancer stem cells (CSCs) in tumor regrowth and spread after initial treatment. Thus, targeting CSCs in combination with traditional induction therapies may improve treatment outcomes and survival rates. Unfortunately, CSCs tend to be resistant to chemo- and radiation therapy, and a better understanding of the mechanisms underlying CSC resistance to treatment is necessary. This paper provides an update on evidence that supports a fundamental role for CSCs in cancer progression, summarizes potential mechanisms of CSC resistance to treatment, and discusses classes of drugs currently in preclinical or clinical testing that show promise at targeting CSCs.

Cancer stem cells in lung tumorigenesis

Although stem cells were discovered more than 50 years ago, we have only recently begun to understand their potential importance in cancer biology. Recent advances in our ability to describe, isolate, and study lung stem cell populations has led to a growing recognition of the central importance cells with stem cell-like properties may have in lung tumorigenesis. This article reviews the major studies supporting the existence and importance of cancer stem cells in lung tumorigenesis. Continued research in the field of lung cancer stem cell biology is vital, as ongoing efforts promise to yield new prognostic and therapeutic targets.

Lung cancer cell lines: Useless artifacts or invaluable tools for medical science?

Multiple cell lines (estimated at 300-400) have been established from human small cell (SCLC) and non-small cell lung cancers (NSCLC). These cell lines have been widely dispersed to and used by the scientific community worldwide, with over 8000 citations resulting from their study. However, there remains considerable skepticism on the part of the scientific community as to the validity of research resulting from their use. These questions center around the genomic instability of cultured cells, lack of differentiation of cultured cells and absence of stromal-vascular-inflammatory cell compartments. In this report we discuss the advantages and disadvantages of the use of cell lines, address the issues of instability and lack of differentiation. Perhaps the most important finding is that every important, recurrent genetic and epigenetic change including gene mutations, deletions, amplifications, translocations and methylation-induced gene silencing found in tumors has been identified in cell lines and vice versa. These "driver mutations" represented in cell lines offer opportunities for biological characterization and application to translational research. Another potential shortcoming of cell lines is the difficulty of studying multistage pathogenesis in vitro. To overcome this problem, we have developed cultures from central and peripheral airways that serve as models for the multistage pathogenesis of tumors arising in these two very different compartments. Finally the issue of cell line contamination must be addressed and safeguarded against. A full understanding of the advantages and shortcomings of cell lines is required for the investigator to derive the maximum benefit from their use.

Cancer stem cell: implications in cancer biology and therapy with special reference to lung cancer

The cancer stem cell (CSC) theory is currently central to the field of cancer research, because it is not only a matter of academic interest but also crucial in cancer therapy. CSCs share a variety of biological properties with normal somatic stem cells in terms of self-renewal, the propagation of differentiated progeny, the expression of specific cell markers and stem cell genes, and the utilization of common signaling pathways and the stem cell niche. However, CSCs differ from normal stem cells in their tumorigenic activity. Thus, CSCs are also termed cancer initiating cells. In this paper, we briefly review hitherto described study results and refer to some excellent review articles to understand the basic properties of CSCs. In addition, we focus upon CSCs of lung cancers, since lung cancer is still increasing in incidence worldwide and remains the leading cause of cancer deaths. Understanding the properties of, and exploring cell markers and signaling pathways specific to, CSCs of lung cancers, will lead to progress in therapy, intervention, and improvement of the prognosis of patients with lung cancer. In the near future, the evaluation of CSCs may be a routine part of practical diagnostic pathology.