Targeting radioresistant osteosarcoma cells with parthenolide

Osteosarcoma is a devastating tumor of bone, primarily affecting adolescents. Osteosarcoma tumors are notoriously radioresistant. Radioresistant cancers, including osteosarcoma, typically exhibit a considerable potential for relapse and development of metastases following treatment. Relapse and metastatic potential can, in part, be due to a specific radioresistant subpopulation of cells with stem-like characteristics, cancer stem cells, which maintain the capacity to regenerate entire tumors. In the current study, we have investigated whether in vitro treatments with parthenolide, a naturally occurring small molecule that interferes with NF-κB signaling and has various other effects, will re-sensitize cancer stem cells and the entire cell population to radiotherapy in osteosarcoma. Our results indicate that parthenolide and ionizing radiation synergistically induce cell death in LM7 osteosarcoma cells. Importantly, the combination treatment results in a significant reduction in the viability of both the overall population of osteosarcoma cells and the cancer stem cell subpopulation. This effect is dependent on the ability of parthenolide to induce oxidative stress. Therefore, as a supplement to current multimodal therapy, parthenolide may sensitize osteosarcoma tumors to radiation and greatly reduce the prevalence of relapse and metastatic progression.     

Ablation of breast cancer stem cells with radiation

Tumor radioresistance leads to recurrence after radiation therapy. The radioresistant phenotype has been hypothesized to reside in the cancer stem cell (CSC) component of breast and other tumors and is considered to be an inherent property of CSC. In this study, we assessed the radiation resistance of breast CSCs using early passaged, patient-derived xenografts from two separate patients. We found a patient-derived tumor in which the CSC population was rapidly depleted 2 weeks after treatment with radiation, based on CD44(+) CD24(-) lin(-) phenotype and aldehyde dehydrogenase 1 immunofluorescence, suggesting sensitivity to radiotherapy. The reduction in CSCs according to phenotypic markers was accompanied by a decrease in functional CSC activity measured by tumor sphere frequency and the ability to form tumors in mice. In contrast, another patient tumor sample displayed enrichment of CSC after irradiation, signifying radioresistance, in agreement with others. CSC response to radiation did not correlate with the level of reactive oxygen species in CSC versus non-CSC. These findings demonstrate that not all breast tumor CSCs are radioresistant and suggest a mechanism for the observed variability in breast cancer local recurrence.     

Non-Invasive Bioluminescence Imaging to Monitor the Immunological Control of a Plasmablastic Lymphoma-Like B Cell Neoplasia after Hematopoietic Cell Transplantation

To promote cancer research and to develop innovative therapies, refined pre-clinical mouse tumor models that mimic the actual disease in humans are of dire need. A number of neoplasms along the B cell lineage are commonly initiated by a translocation recombining c-myc with the immunoglobulin heavy-chain gene locus. The translocation is modeled in the C.129S1-Igha^tm1(Myc)Janz/J mouse which has been previously engineered to express c-myc under the control of the endogenous IgH promoter. This transgenic mouse exhibits B cell hyperplasia and develops diverse B cell tumors. We have isolated tumor cells from the spleen of a C.129S1-Igha^tm1(Myc)Janz/J mouse that spontaneously developed a plasmablastic lymphoma-like disease. These cells were cultured, transduced to express eGFP and firefly luciferase, and gave rise to a highly aggressive, transplantable B cell lymphoma cell line, termed IM380. This model bears several advantages over other models as it is genetically induced and mimics the translocation that is detectable in a number of human B cell lymphomas. The growth of the tumor cells, their dissemination, and response to treatment within immunocompetent hosts can be imaged non-invasively in vivo due to their expression of firefly luciferase. IM380 cells are radioresistant in vivo and mice with established tumors can be allogeneically transplanted to analyze graft-versus-tumor effects of transplanted T cells. Allogeneic hematopoietic stem cell transplantation of tumor-bearing mice results in prolonged survival. These traits make the IM380 model very valuable for the study of B cell lymphoma pathophysiology and for the development of innovative cancer therapies.     

Neuroendocrine differentiation contributes to radioresistance development and metastatic potential increase in non-small cell lung cancer

Radiation treatment induces neuroendocrine differentiation (NED) in non-small cell lung cancer (NSCLC) A549 and H157 cells, so higher NE-like features in radioresistant A549 (A549R26-1) and H157 (H157R24-1) cells are observed than in parental cells. We detected higher NED marker expressions in A549R26-1 cell-derived tumors than in A549 cell-derived tumors. In mechanism studies, we found that NED induction in A549R26-1 and H157R24-1 cells was accompanied by increased intracellular cAMP and IL-6 levels. Treatment of radioresistant lung cancer cells with the inhibitor (SQ22536) of adenylate cyclase (AC) which is the enzyme responsible for the cAMP production, or the neutralizing antibody (Ab) of IL-6, resulted in decreased NE-like features in radioresistant lung cancer cells. In addition, we found MEK/Erk is the signaling pathway that triggers the cAMP- and IL-6-mediated NED induction in radioresistant lung cancer cells. Also, we found that MEK/Erk signaling pathway inhibition decreased NED in radioresistant cells. Radioresistant lung cancer cells exhibiting high NE-like features also showed higher radioresistance and higher metastatic potential than parental cells. When we inhibited cAMP-, or IL-6-mediated pathways, or the downstream MEK/Erk signaling pathway, radiosensitivity of radioresistant lung cancer cells was significantly increased and their metastatic potential was significantly reduced. In in vivo mouse studies, reducing NED by treating mice with the MEK/Erk inhibitor increased radiosensitivity. Immunohistochemical staining of tumor tissues lowered expressions of the NED/epithelial-mesenchymal transition (EMT)/metastatic markers when mice were treated with the MEK/Erk inhibitor.     

Metformin Radiosensitizes p53-Deficient Colorectal Cancer Cells through Induction of G2/M Arrest and Inhibition of DNA Repair Proteins

The present study addressed whether the combination of metformin and ionizing radiation (IR) would show enhanced antitumor effects in radioresistant p53-deficient colorectal cancer cells, focusing on repair pathways for IR-induced DNA damage. Metformin caused a higher reduction in clonogenic survival as well as greater radiosensitization and inhibition of tumor growth of p53^-/- than of p53^+/+ colorectal cancer cells and xenografts. Metformin combined with IR induced accumulation of tumor cells in the G2/M phase and delayed the repair of IR-induced DNA damage. In addition, this combination significantly decreased levels of p53-related homologous recombination (HR) repair compared with IR alone, especially in p53^-/- colorectal cancer cells and tumors. In conclusion, metformin enhanced radiosensitivity by inducing G2/M arrest and reducing the expression of DNA repair proteins even in radioresistant HCT116 p53^-/- colorectal cancer cells and tumors. Our study provides a scientific rationale for the clinical use of metformin as a radiosensitizer in patients with p53-deficient colorectal tumors, which are often resistant to radiotherapy.     

Single Unpurified Breast Tumor-Initiating Cells from Multiple Mouse Models Efficiently Elicit Tumors in Immune-Competent Hosts

The tumor-initiating cell (TIC) frequency of bulk tumor cell populations is one of the criteria used to distinguish malignancies that follow the cancer stem cell model from those that do not. However, tumor-initiating cell frequencies may be influenced by experimental conditions and the extent to which tumors have progressed, parameters that are not always addressed in studies of these cells. We employed limiting dilution cell transplantation of minimally manipulated tumor cells from mammary tumors of several transgenic mouse models to determine their tumor-initiating cell frequency. We determined whether the tumors that formed following tumor cell transplantation phenocopied the primary tumors from which they were isolated and whether they could be serially transplanted. Finally we investigated whether propagating primary tumor cells in different tissue culture conditions affected their resident tumor-initiating cell frequency. We found that tumor-initiating cells comprised between 15% and 50% of the bulk tumor cell population in multiple independent mammary tumors from three different transgenic mouse models of breast cancer. Culture of primary mammary tumor cells in chemically-defined, serum-free medium as non-adherent tumorspheres preserved TIC frequency to levels similar to that of the primary tumors from which they were established. By contrast, propagating the primary tumor cells in serum-containing medium as adherent populations resulted in a several thousand-fold reduction in their tumor-initiating cell fraction. Our findings suggest that experimental conditions, including the sensitivity of the transplantation assay, can dramatically affect estimates of tumor initiating cell frequency. Moreover, conditional on cell culture conditions, the tumor-initiating cell fraction of bulk mouse mammary tumor cell preparations can either be maintained at high or low frequency in vitro thus permitting comparative studies of tumorigenic and non-tumorigenic cancer cells.     

Evolution of Resistance to Targeted Anti-Cancer Therapies during Continuous and Pulsed Administration Strategies

The discovery of small molecules targeted to specific oncogenic pathways has revolutionized anti-cancer therapy. However, such therapy often fails due to the evolution of acquired resistance. One long-standing question in clinical cancer research is the identification of optimum therapeutic administration strategies so that the risk of resistance is minimized. In this paper, we investigate optimal drug dosing schedules to prevent, or at least delay, the emergence of resistance. We design and analyze a stochastic mathematical model describing the evolutionary dynamics of a tumor cell population during therapy. We consider drug resistance emerging due to a single (epi)genetic alteration and calculate the probability of resistance arising during specific dosing strategies. We then optimize treatment protocols such that the risk of resistance is minimal while considering drug toxicity and side effects as constraints. Our methodology can be used to identify optimum drug administration schedules to avoid resistance conferred by one (epi)genetic alteration for any cancer and treatment type.     

Role of neutrophil extracellular traps in regulation of lung cancer invasion and metastasis: Structural insights from a computational model

Lung cancer is one of the leading causes of cancer-related deaths worldwide and is characterized by hijacking immune system for active growth and aggressive metastasis. Neutrophils, which in their original form should establish immune activities to the tumor as a first line of defense, are undermined by tumor cells to promote tumor invasion in several ways. In this study, we investigate the mutual interactions between the tumor cells and the neutrophils that facilitate tumor invasion by developing a mathematical model that involves taxis-reaction-diffusion equations for the critical components in the interaction. These include the densities of tumor and neutrophils, and the concentrations of signaling molecules and structure such as neutrophil extracellular traps (NETs). We apply the mathematical model to a Boyden invasion assay used in the experiments to demonstrate that the tumor-associated neutrophils can enhance tumor cell invasion by secreting the neutrophil elastase. We show that the model can both reproduce the major experimental observation on NET-mediated cancer invasion and make several important predictions to guide future experiments with the goal of the development of new anti-tumor strategies. Moreover, using this model, we investigate the fundamental mechanism of NET-mediated invasion of cancer cells and the impact of internal and external heterogeneity on the migration patterning of tumour cells and their response to different treatment schedules.     

Tumor-Immune Interaction, Surgical Treatment, and Cancer Recurrence in a Mathematical Model of Melanoma

Malignant melanoma is a cancer of the skin arising in the melanocytes. We present a mathematical model of melanoma invasion into healthy tissue with an immune response. We use this model as a framework with which to investigate primary tumor invasion and treatment by surgical excision. We observe that the presence of immune cells can destroy tumors, hold them to minimal expansion, or, through the production of angiogenic factors, induce tumorigenic expansion. We also find that the tumor–immune system dynamic is critically important in determining the likelihood and extent of tumor regrowth following resection. We find that small metastatic lesions distal to the primary tumor mass can be held to a minimal size via the immune interaction with the larger primary tumor. Numerical experiments further suggest that metastatic disease is optimally suppressed by immune activation when the primary tumor is moderately, rather than minimally, metastatic. Furthermore, satellite lesions can become aggressively tumorigenic upon removal of the primary tumor and its associated immune tissue. This can lead to recurrence where total cancer mass increases more quickly than in primary tumor invasion, representing a clinically more dangerous disease state. These results are in line with clinical case studies involving resection of a primary melanoma followed by recurrence in local metastases.     

Cyclin D1 overexpression perturbs DNA replication and induces replication-associated DNA double-strand breaks in acquired radioresistant cells

Fractionated radiotherapy (RT) is widely used in cancer treatment, because it preserves normal tissues. However, repopulation of radioresistant tumors during fractionated RT limits the efficacy of RT. We recently demonstrated that a moderate level of long-term fractionated radiation confers acquired radioresistance to tumor cells, which is caused by DNA-PK/AKT/GSK3β-mediated cyclin D1 overexpression. The resulting cyclin D1 overexpression leads to forced progression of the cell cycle to S-phase, concomitant with induction of DNA double-strand breaks (DSBs). In this study, we investigated the molecular mechanisms underlying cyclin D1 overexpression-induced DSBs during DNA replication in acquired radioresistant cells. DNA fiber data demonstrated that replication forks progressed slowly in acquired radioresistant cells compared with corresponding parental cells in HepG2 and HeLa cell lines. Slowly progressing replication forks were also observed in HepG2 and HeLa cells that overexpressed a nondegradable cyclin D1 mutant. We also found that knockdown of Mus81endonuclease, which is responsible for resolving aberrant replication forks, suppressed DSB formation in acquired radioresistant cells. Consequently, Mus81 created DSBs to remove aberrant replication forks in response to replication perturbation triggered by cyclin D1 overexpression. After treating cells with a specific inhibitor for DNA-PK or ATM, apoptosis rates increased in acquired radioresistant cells but not in parental cells by inhibiting the DNA damage response to cyclin D1-mediated DSBs. This suggested that these inhibitors might eradicate acquired radioresistant cells and improve fractionated RT outcomes.     

A mathematical model of tumor growth and its response to single irradiation

Background

Mathematical modeling of biological processes is widely used to enhance quantitative understanding of bio-medical phenomena. This quantitative knowledge can be applied in both clinical and experimental settings. Recently, many investigators began studying mathematical models of tumor response to radiation therapy. We developed a simple mathematical model to simulate the growth of tumor volume and its response to a single fraction of high dose irradiation. The modelling study may provide clinicians important insights on radiation therapy strategies through identification of biological factors significantly influencing the treatment effectiveness.

Methods

We made several key assumptions of the model. Tumor volume is composed of proliferating (or dividing) cancer cells and non-dividing (or dead) cells. Tumor growth rate (or tumor volume doubling time) is proportional to the ratio of the volumes of tumor vasculature and the tumor. The vascular volume grows slower than the tumor by introducing the vascular growth retardation factor, θ. Upon irradiation, the proliferating cells gradually die over a fixed time period after irradiation. Dead cells are cleared away with cell clearance time. The model was applied to simulate pre-treatment growth and post-treatment radiation response of rat rhabdomyosarcoma tumors and metastatic brain tumors of five patients who were treated with Gamma Knife stereotactic radiosurgery (GKSRS).

Results

By selecting appropriate model parameters, we showed the temporal variation of the tumors for both the rat experiment and the clinical GKSRS cases could be easily replicated by the simple model. Additionally, the application of our model to the GKSRS cases showed that the α-value, which is an indicator of radiation sensitivity in the LQ model, and the value of θ could be predictors of the post-treatment volume change.

Conclusions

The proposed model was successful in representing both the animal experimental data and the clinically observed tumor volume changes. We showed that the model can be used to find the potential biological parameters, which may be able to predict the treatment outcome. However, there is a large statistical uncertainty of the result due to the small sample size. Therefore, a future clinical study with a larger number of patients is needed to confirm the finding.

     

A partial differential equation model and its reduction to an ordinary differential equation model for prostate tumor growth under intermittent hormone therapy

Hormonal therapy with androgen suppression is a common treatment for advanced prostate tumors. The emergence of androgen-independent cells, however, leads to a tumor relapse under a condition of long-term androgen deprivation. Clinical trials suggest that intermittent androgen suppression (IAS) with alternating on- and off-treatment periods can delay the relapse when compared with continuous androgen suppression (CAS). In this paper, we propose a mathematical model for prostate tumor growth under IAS therapy. The model elucidates initial hormone sensitivity, an eventual relapse of a tumor under CAS therapy, and a delay of a relapse under IAS therapy, which are due to the coexistence of androgen-dependent cells, androgen-independent cells resulting from reversible changes by adaptation, and androgen-independent cells resulting from irreversible changes by genetic mutations. The model is formulated as a free boundary problem of partial differential equations that describe the evolution of populations of the abovementioned three types of cells during on-treatment periods and off-treatment periods. Moreover, the model can be transformed into a piecewise linear ordinary differential equation model by introducing three new volume variables, and the study of the resulting model may help to devise optimal IAS schedules.     

Tumor Growth Rate Determines the Timing of Optimal Chronomodulated Treatment Schedules

In host and cancer tissues, drug metabolism and susceptibility to drugs vary in a circadian (24 h) manner. In particular, the efficacy of a cell cycle specific (CCS) cytotoxic agent is affected by the daily modulation of cell cycle activity in the target tissues. Anti-cancer chronotherapy, in which treatments are administered at a particular time each day, aims at exploiting these biological rhythms to reduce toxicity and improve efficacy of the treatment. The circadian status, which is the timing of physiological and behavioral activity relative to daily environmental cues, largely determines the best timing of treatments. However, the influence of variations in tumor kinetics has not been considered in determining appropriate treatment schedules. We used a simple model for cell populations under chronomodulated treatment to identify which biological parameters are important for the successful design of a chronotherapy strategy. We show that the duration of the phase of the cell cycle targeted by the treatment and the cell proliferation rate are crucial in determining the best times to administer CCS drugs. Thus, optimal treatment times depend not only on the circadian status of the patient but also on the cell cycle kinetics of the tumor. Then, we developed a theoretical analysis of treatment outcome (TATO) to relate the circadian status and cell cycle kinetic parameters to the treatment outcomes. We show that the best and the worst CCS drug administration schedules are those with 24 h intervals, implying that 24 h chronomodulated treatments can be ineffective or even harmful if administered at wrong circadian times. We show that for certain tumors, administration times at intervals different from 24 h may reduce these risks without compromising overall efficacy.     

The proangiogenic factor ephrin-A1 is up-regulated in radioresistant murine tumor by irradiation

While the pre-treatment status of cancer is generally correlated with outcome, little is known about microenvironmental change caused by anti-cancer treatment and how it may affect outcome. For example, treatment may lead to induction of gene expression that promotes resistance to therapy. In the present study, we attempted to find a gene that was both induced by irradiation and associated with radioresistance in tumors. Using single-color oligo-microarrays, we analyzed the gene expression profiles of two murine squamous cell carcinomas, NR-S1, which is highly radioresistant, and SCCVII, which is radiosensitive, after irradiation with 137-Cs gamma rays or carbon ions. Candidate genes were those differentially regulated between NR-S1 and SCCVII after any kind of irradiation. Four genes, Efna1 (Ephrin-A1), Sprr1a (small proline-rich protein 1A), Srgap3 (SLIT-ROBO Rho GTPase activating protein 3) and Xrra1 [RIKEN 2 days neonate thymus thymic cells (NOD) cDNA clone E430023D08 3'], were selected as candidate genes associated with radiotherapy-induced radioresistance. We focused on Efna1, which encodes a ligand for the Eph receptor tyrosine kinase known to be involved in the vascular endothelial growth factor (VEGF) pathway. We used immunohistochemical methods to detect expression of Ephrin-A1, VEGF, and the microvascular marker CD31 in radioresistant NR-S1 tumor cells. Ephrin-A1 was detected in the cytoplasm of NR-S1 tumor cells after irradiation, but not in SCCVII tumor cells. Irradiation of NR-S1 tumor cells also led to significant increases in microvascular density, and up-regulation of VEGF expression. Our results suggest that radiotherapy-induced changes in gene expression related with angiogenesis might also modulate microenvironment and influence responsiveness of tumors.     

Identification of differentially expressed genes contributing to radioresistance in lung cancer cells using microarray analysis

Radiotherapy has played a key role in the control of tumor growth in many cancer patients. It is usually difficult to determine what fraction of the tumor cell population is radioresistant after a course of radiotherapy. The response of tumor cells to radiation is believed to be accompanied by complex changes in the gene expression pattern. It may be possible to use these to sensitize radioresistant tumor cells and improve radiocurability. Based on the biological effects of ionizing radiation, in the present study, we developed one oligonucleotide microarray to analyze the expression of 143 genes in cells of two lung cancer cell lines with different radiosensitivities. Compared to NCI-H446 cells, expression of 18 genes significantly increased the basal levels in the radioresistant A549 cells, in which eight genes were up-regulated and 10 genes were down-regulated. In A549 cells irradiated with 5 Gy, 22 (19 up-regulated and three down-regulated) and 26 (eight up-regulated and 18 down-regulated) differentially expressed genes were found 6 and 24 h after irradiation, respectively. In NCI-H446 cells, the expression of 17 (nine up-regulated and eight down-regulated) and 18 (six up-regulated and 12 down-regulated) genes was altered 6 and 24 h after irradiation, respectively. RT-PCR was performed, and we found that MDM2, BCL2, PKCZ and PIM2 expression levels were increased in A549 cells and decreased in NCI-H446 cells after irradiation. Genes involved in DNA repair, such as XRCC5, ERCC5, ERCC1, RAD9A, ERCC4 and the gene encoding DNA-PK, were found to be increased to a higher level in A549 cells than in NCI-H446 cells. Antisense suppression of MDM2 resulted in increased radiosensitivity of A549 cells. Taken together, these results demonstrate the possibility that a group of genes involved in DNA repair, regulation of the cell cycle, cell proliferation and apoptosis is responsible for the different radioresistance of these two lung cancer cells. This list of genes may be useful in attempts to sensitize the radioresistant lung cancer cells.     

The therapeutic implications of plasticity of the cancer stem cell phenotype

The cancer stem cell hypothesis suggests that tumors contain a small population of cancer cells that have the ability to undergo symmetric self-renewing cell division. In tumors that follow this model, cancer stem cells produce various kinds of specified precursors that divide a limited number of times before terminally differentiating or undergoing apoptosis. As cells within the tumor mature, they become progressively more restricted in the cell types to which they can give rise. However, in some tumor types, the presence of certain extra- or intracellular signals can induce committed cancer progenitors to revert to a multipotential cancer stem cell state. In this paper, we design a novel mathematical model to investigate the dynamics of tumor progression in such situations, and study the implications of a reversible cancer stem cell phenotype for therapeutic interventions. We find that higher levels of dedifferentiation substantially reduce the effectiveness of therapy directed at cancer stem cells by leading to higher rates of resistance. We conclude that plasticity of the cancer stem cell phenotype is an important determinant of the prognosis of tumors. This model represents the first mathematical investigation of this tumor trait and contributes to a quantitative understanding of cancer.     

MCF-7 human breast carcinomas in nude mice as a model for evaluating aromatase inhibitors

While hormone-dependent, mammary tumors induced with carcinogens (DMBA or NMU) in intact rats have been used extensively for studying aromatase inhibitors, there is currently no suitable model to investigate their effects in human breast cancers in vivo. While hormone responsive tumors can be formed in the athymic mouse using human breast carcinoma MCF-7 cells, due to the low ovarian estrogen production, tumor growth is induced with estradiol supplementation. Thus, this model is unsuitable for studies of aromatase inhibitors. We have induced tumors without the need for estrogen supplementation by co-inoculating MCF-7 cells with Matrigel, a basement membrane preparation, into intact athymic mice. In one experiment, 45 days after inocubation, mice were assigned to the control group or 4-hydroxyandrostenedione (4-OHA) (1 mg/day s.c.) treatment for 52 days. Tumor volumes in the control mice increased 672%, whereas tumor volumes in the treated mice did not change significantly (178.9 ± 16.2 to 336.6 ± 120 mm³). In the second experiment, 55 days after inoculation, groups of mice were treated with the antiestrogen, tamoxifen (5 μg/day s.c.) or vehicle (controls). Tumor volumes in the control mice increased 325% in 58 days, whereas there was no significant change in tumor volume in the tamoxifen treated group (338.8 ± 55.3 to 330.6 ± 84.9 mm³). The results suggest that (1) the tumors resulting from MCF-7 cells co-inoculated with Matrigel are estrogen-dependent and (2) tamoxifen and 4-OHA were effective in suppressing growth of these tumors. The results suggest that this model should be useful for evaluating the effects of aromatase inhibitors and for comparing breast cancer treatments.     

Cancer stem cell, niche and EGFR decide tumor development and treatment response: A bio-computational simulation study

Recent research in cancer biology has suggested the hypothesis that tumors are initiated and driven by a small group of cancer stem cells (CSCs). Furthermore, cancer stem cell niches have been found to be essential in determining fates of CSCs, and several signaling pathways have been proven to play a crucial role in cellular behavior, which could be two important factors in cancer development. To better understand the progression, heterogeneity and treatment response of breast cancer, especially in the context of CSCs, we propose a mathematical model based on the cell compartment method. In this model, three compartments of cellular subpopulations are constructed: CSCs, progenitor cells (PCs), and terminal differentiated cells (TCs). Moreover, (1) the cancer stem cell niche is, considered by modeling its effect on division patterns (symmetric or asymmetric) of CSCs, and (2) the EGFR signaling pathway is integrated by modeling its role in cell proliferation, apoptosis. Our simulation results indicate that (1) a higher probability for symmetric division of CSC may result in a faster expansion of tumor population, and for a larger number of niches, the tumor grows at a slower rate, but the final tumor volume is larger; (2) higher EGFR expression correlates to tumors with larger volumes while a saturation function is observed, and (3) treatments that inhibit tyrosine kinase activity of EGFR may not only repress the tumor volume, but also decrease the CSCs percentages by shifting CSCs from symmetric divisions to asymmetric divisions. These findings suggest that therapies should be designed to effectively control or eliminate the symmetric division of CSCs and to reduce or destroy the CSC niches.