Implications of reactive oxygen species on cancer formation and its treatment

Elevated levels of reactive oxygen species (ROS) are a hallmark of cancer. Although increased ROS concentrations play important roles in cancer formation and progression, levels above a cytotoxic threshold cause cancer cell death. Cancer cells adapt to high concentrations of ROS via antioxidant production and reprogrammed cellular metabolism (eg, the Warburg effect). Because some widely used anticancer therapies such as radiation therapy and chemotherapy rely on ROS accumulation as a mechanism to induce cancer cell death, a cancer cell's ability to control ROS levels is a driver of treatment resistance and a critical consideration for successful cancer treatment. The necessity for cancer cells to adapt to elevated levels of ROS to survive may represent an Achilles heel for some malignancies, as therapies designed to interfere with this adaptation would be expected to kill cancer cells. In this review, we provide an overview of the implications of ROS on cancer formation and anticancer treatment strategies, with a focus on treatment-resistant disease.

Bioactivation of Napabucasin Triggers Reactive Oxygen Species-Mediated Cancer Cell Death

PURPOSE

Napabucasin (2-acetylfuro-1,4-naphthoquinone or BBI-608) is a small molecule currently being clinically evaluated in various cancer types. It has mostly been recognized for its ability to inhibit STAT3 signaling. However, based on its chemical structure, we hypothesized that napabucasin is a substrate for intracellular oxidoreductases and therefore may exert its anticancer effect through redox cycling, resulting in reactive oxygen species (ROS) production and cell death.

EXPERIMENTAL DESIGN

Binding of napabucasin to NAD(P)H:quinone oxidoreductase-1 (NQO1), and other oxidoreductases, was measured. Pancreatic cancer cell lines were treated with napabucasin, and cell survival, ROS generation, DNA damage, transcriptomic changes, and alterations in STAT3 activation were assayed in vitro and in vivo. Genetic knockout or pharmacologic inhibition with dicoumarol was used to evaluate the dependency on NQO1.

RESULTS

Napabucasin was found to bind with high affinity to NQO1 and to a lesser degree to cytochrome P450 oxidoreductase (POR). Treatment resulted in marked induction of ROS and DNA damage with an NQO1- and ROS-dependent decrease in STAT3 phosphorylation. Differential cytotoxic effects were observed, where NQO1-expressing cells generating cytotoxic levels of ROS at low napabucasin concentrations were more sensitive. Cells with low or no baseline NQO1 expression also produced ROS in response to napabucasin, albeit to a lesser extent, through the one-electron reductase POR.

CONCLUSIONS

Napabucasin is bioactivated by NQO1, and to a lesser degree by POR, resulting in futile redox cycling and ROS generation. The increased ROS levels result in DNA damage and multiple intracellular changes, one of which is a reduction in STAT3 phosphorylation.

Abstract LB-142: Identification of STK33 as a cancer stemness kinase and regulator of Nanog function

In recent years evidence has accumulated in support of the cancer stem cell (CSC) model in cancer chemotherapy resistance, highlighting the urgency and necessity of identifying CSC targets for developing novel cancer therapeutics. A number of studies have suggested that Nanog is a crucial factor that can confer stemness properties to a portion of the heterogeneous cancer cell population. Although latent in normal somatic cells, aberrant expression of Nanog has been reported in many types of human cancers. Importantly, the expression levels of Nanog are often positively correlated with treatment resistance and poor survival of cancer patients. Various studies have shown that upregulation of Nanog expression enhances the tumorigenicity both in vivo and in vitro whereas repression or ablation of Nanog inhibits tumor initiation. Thus, expression of the stemness factor Nanog is linked to tumor progression, therapeutic resistance, relapse and metastasis.

However, Nanog is considered a non-druggable target. Here we provide data to support a role for STK33 (Serine Threonine Kinase 33) as a novel regulator of Nanog and as a potential therapeutic target. Ectopic STK33 expression promotes stemness phenotypes of cancer cells and increases expression levels of CSC drivers including Nanog, KLF4, and SOX2. On the other hand, knockdown of STK33 inhibits expression of Nanog and results in a reduction of the stemness phenotype. STK33 directly interacts with Nanog and appears to promote its stabilization through phosphorylation, resulting in increased Nanog transcriptional activity. Moreover, BBI-503 (Amcasertib), a first-in-class cancer stemness inhibitor, potently inhibits STK33, which led to inhibition of phosphorylation of Nanog. Collectively, our data demonstrate STK33 is a critical element in the signaling network that governs the stemness of cancer cells, and as a promising therapeutic target for cancer.

Citation Format: Susan L. Tran, Yudai Furuta, Chen Zhu, Ao Yang, Xiangao Sun, Harry A. Rogoff, Chiang J. Li. Identification of STK33 as a cancer stemness kinase and regulator of Nanog function [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr LB-142. doi:10.1158/1538-7445.AM2017-LB-142

Abstract 4777: Cancer stemness and resistance: Napabucasin (BBI-608) sensitizes stemness-high cancer cells to Paclitaxel by inhibiting the STAT3-MUC1 pathway

Stemness-high cancer cells, or cancer stem cells (CSC) represent a subpopulation of cancer cells with enhanced tumorigenic capacity, metastasis-forming potential, and resistance to conventional chemotherapy and radiation. One such key CSC pathway is regulated by STAT3, a transcription factor that is downstream of several cytokines and growth factor receptors which controls the expression of a broad range of target genes and plays essential roles in CSC biology in many tumor types. Cancer stemness inhibitor Napabucasin (BBI-608), a small molecule that inhibits gene transcription driven by STAT3, can inhibit stemness gene expression, block spherogenesis, and kill CSCs. In vivo, BBI-608 effectively blocks cancer relapse and metastasis in xenograft models while sparing normal hematopoietic stem cells, suggesting that targeting stemness-high cancer cells is a feasible approach for developing next-generation cancer therapeutics to combat cancer recurrence.

SOX2 (sex-determining region Y-box protein 2), a transcription factor that is essential for self-renewal and pluripotency, is amplified in various cancer types and has been shown to be required for CSC self-renewal and maintenance. Mucin 1 (MUC1), a glycoprotein normally expressed at the apical surface of epithelial cells, is overexpressed in most human epithelial cancers. MUC1 has been implicated in regulating tumor proliferation, metabolism, invasion, angiogenesis, chemoresistance, and inflammation.

Here, we generated a stemness-high culture system based on the reporter activity of a SOX2-regulatory region construct in MKN28 gastric cancer cells (MKN28 SOX2-reporter GFP+ cells). BBI-608 treatment inhibited the STAT3-MUC1 pathway in stemness-high cells. High MUC1 status in stemness-high cells was associated with Paclitaxel resistance. Down-regulation of MUC1 sensitized stemness-high cells to Paclitaxel. Moreover, BBI-608 synergized with Paclitaxel in inhibiting spherogenesis of stemness-high cells. This study provides a new mechanism for the association of cancer stemness with drug resistance. Our findings support this combination therapy, which pairs a conventional chemotherapy (Paclitaxel) with a cancer stemness inhibitor (BBI-608), as a promising strategy to combat cancer.

Citation Format: Harry A. Rogoff, Juying Li, Chiang Li. Cancer stemness and resistance: Napabucasin (BBI-608) sensitizes stemness-high cancer cells to Paclitaxel by inhibiting the STAT3-MUC1 pathway [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 4777. doi:10.1158/1538-7445.AM2017-4777

Abstract 2532: Alginate-based 3D system for the enrichment and culture of cancer stem cells

Cancer stem cells (CSCs)—cancer cells which have the ability to undergo “self-renewal” are considered to be responsible for the recurrence and metastasis of tumors. The development of a culture system which can mimic aspects of the tumor microenvironment within the cancer stem cell niche is important to elucidate the mechanisms that underlie the growth and survival of CSCs and to identify compounds that can target and eliminate this highly malignant cell population.

Here we demonstrate the use of an alginate-based 3D cell culture as an in vitro system for enriching and maintaining the stemness properties of cancer cell lines. In this work, we used alginate gels as a three-dimensional (3D) matrix to culture cancer cell lines. Alginate is an ideal system for CSC because of its inertness and ability to provide a hypoxic environment essential for maintaining the stem-like properties of these cells. We found that cells cultured in the alginate 3D matrix have increased CSC-related gene expression and alterations in metabolic-related gene expression that are consistent with changes reported to occur in CSC that help them to maintain their drug resistant and invasive properties.

In order to be useful as a drug screening platform to identify CSC targeting agents, we verified that the high stemness characteristics of the cells cultured in the alginate 3D matrix can be maintained over multiple passages and many months of continuous culture. The CSC cultures in the alginate 3D matrix were found to be resistant to both conventional chemotherapy and targeted therapeutics, while remaining sensitive to CSC targeting agents BB608 and BB503. These results support the potential of the alginate 3D culture system in the enrichment and expansion of CSCs, and provide a reliable in vitro system for the development and evaluation of CSC-targeting agents.

Citation Format: Karen A. Simon, Sylaja Murikipudi, Harry A. Rogoff, Chiang J. Li. Alginate-based 3D system for the enrichment and culture of cancer stem cells. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 2532.

Abstract 2222: Dual inhibition of cancer stemness and immune checkpoint genes by targeting Stat3

Highly malignant stemness-high cancer cells, also termed cancer stem cells, have been found to hijack stemness genes important for embryonic stem cells to acquire a state of high-stemness. Cancer stemness has been associated with enhanced tumorigenicity and resistance to chemotherapies. Moreover, stemness-high cancer cells have been isolated from a variety of human cancer types. Therefore, targeting cancer stemness holds significant promise for advancing cancer treatment.

Recent clinical success with antibodies targeting PD1 and PDL1 has validated that cancer cells can hijack immune checkpoint genes to subvert endogenous anticancer surveillance by the immune system. It is, therefore, highly desirable to develop therapeutics that simultaneously target both cancer cell stemness and immune checkpoints.

The transcription factor Signal Transducer and Activator of Transcription 3 (Stat3) is activated by a multitude of upstream oncogenic pathways and cytokine receptors. Aberrant and constitutive activation of Stat3 has been found in a wide variety of human cancers, and has also been implicated in cancer cell proliferation, survival, and immune evasion.

By using aiRNA (asymmetric RNA duplexes) we have achieved potent Stat3 silencing with precision in stemness-high cancer cells. We observed that Stat3 silencing leads to simultaneous down regulation of cancer cell stemness and immune checkpoint gene expression. Treatment of stemness-high cancer cells with BB608, a small molecule inhibitor of Stat3, led to simultaneous inhibition of stemness-high cancer cell survival and self-renewal, as well as downregulation of immune checkpoint genes, including IDO1 in vitro and in vivo. Furthermore, while expression of stemness genes and immune checkpoint genes in tumor tissues are known to predict poor patient survival, patients with higher expression levels of stemness genes and immune checkpoint genes showed prolonged overall survival after treatment with BB608 in clinical trials. Targeting Stat3 is, therefore, a promising strategy to achieve dual inhibition of cancer cell stemness and immune evasion.

Citation Format: Yuan Gao, Sarah Keates, Eric Hsu, Janet Huang, Emily Brooks, Matt Hitron, Youzhi Li, Harry A. Rogoff, Chiang Li. Dual inhibition of cancer stemness and immune checkpoint genes by targeting Stat3. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 2222.

Abstract LB-253: Inhibition of stemness by BBI608 is sufficient to suppress cancer relapse and metastasis

Cancer cells are extremely heterogeneous, even in each individual patient, in terms of their malignant potential, drug-senstivity, and their potential to metastasize and cause relapse. Subpopulations of cancer cells with extremely high tumorigenic potential have been isolated from cancer patients with a variety of tumor types and found to have high stemness properties termed cancer stem cells. These stemness-high cancer cells are extremely tumorigenic and are resistant to conventional therapeutics due to activation of pro-survival and anti-apoptotic pathways, overexpression of drug efflux pumps, and increased DNA repair capacity. Moreover, chemotherapy and radiation have been found to induce stemness genes in cancer cells, converting stemness-low cancer cells to stemness-high cancer cells. Such highly tumorigenic and drug-resistant stemness-high cancer stem cells are, therefore, likely to be “left-over” following chemotherapy or radiotherapy and ultimately responsible for relapse. We hypothesized that cancer stemness inhibition is sufficient to suppress metastasis and relapse. Stemness, initially defined by the expression of stem cells genes, is a property shared by embryonic stem cells and adult stem cells. It has been demonstrated that the gene expression profiles of cancer stem cells more closely resemble embryonic stem cells than adult stem cells, suggesting the feasibility to identify molecular targets that are required for cancer stemness, but not (or less so) by normal adult stem cells. Through gene-silencing approaches, we have identified Stat3 as critically important for maintaining cancer stemness, yet largely dispensable for adult stem cells. Here we show that BBI608, a small molecule identified by its ability to inhibit gene-transcription driven by Stat3 and cancer cell stemness properties, displays anticancer properties that are highly different from chemotherapeutics agents. Stemness-high cancer cells enriched by multiple techniques are resistance to chemotherapeutics, yet highly sensitive to the stemness inhibitor BBI608. Blockade of spherogenesis and reduction of stemness gene expression by BBI608 were observed in stemness-high cancer cells isolated from a variety of cancer types. While treatment of xenografted tumor models with chemotherapeutics enriched stemness-high cancer cells, BBI608 induced significant depletion of stemness-high populations in vivo. Moreover, the inhibition of stemness by BBI608 is sufficient to suppress cancer relapse and metastasis in xenografted human cancers in mice. These data demonstrate targeting cancer stemness as an effective way to suppress cancer relapse and metastasis.

Citation Format: Youzhi Li, Harry A. Rogoff, Sarah Keates, Yuan Gao, Sylaja Murikipudi, Keith Mikule, David Leggett, Wei Li, Arthur Pardee, Chiang J. Li. Inhibition of stemness by BBI608 is sufficient to suppress cancer relapse and metastasis. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr LB-253. doi:10.1158/1538-7445.AM2015-LB-253

Asymmetric shorter-duplex siRNA structures trigger efficient gene silencing with reduced nonspecific effects

Small interfering RNAs (siRNAs) are short, double-stranded RNAs that mediate efficient gene silencing in a sequence-specific manner by utilizing the endogenous RNA interference (RNAi) pathway. The current standard synthetic siRNA structure harbors a 19-base-pair duplex region with 3' overhangs of 2 nucleotides (the so-called 19+2 form). However, the synthetic 19+2 siRNA structure exhibits several sequence-independent, nonspecific effects, which has posed challenges to the development of RNAi therapeutics and specific silencing of genes in research. In this study, we report on the identification of truncated siRNA backbone structures with duplex regions shorter than 19 bp (referred to as asymmetric shorter-duplex siRNAs or asiRNAs) that can efficiently trigger gene silencing in human cell lines. Importantly, this asiRNA structure significantly reduces nonspecific effects triggered by conventional 19+2 siRNA scaffold, such as sense-strand-mediated off-target gene silencing and saturation of the cellular RNAi machinery. Our results suggest that this asiRNA structure is an important alternative to conventional siRNAs for both functional genomics studies and therapeutic applications.

E2F1 induces MRN foci formation and a cell cycle checkpoint response in human fibroblasts

Deregulation of the Rb/E2F pathway in human fibroblasts results in an E2F1-mediated apoptosis dependent on Atm, Nbs1, Chk2 and p53. Here, we show that E2F1 expression results in MRN foci formation, which is independent of the Nbs1 interacting region and the DNA-binding domain of E2F1. E2F1-induced MRN foci are similar to irradiation-induced foci (IRIF) that result from double-strand DNA breaks because they correlate with 53BP1 and gammaH2AX foci, do not form in NBS cells, do form in AT cells and do not correlate with cell cycle entry. In fact, we find that in human fibroblasts deregulated E2F1 causes a G1 arrest, blocking serum-induced cell cycle progression, in part through an Nbs1/53BP1/p53/p21(WAF1/CIP1) checkpoint pathway. This checkpoint protects against apoptosis because depletion of 53BP1 or p21(WAF1/CIP1) increases both the rate and extent of apoptosis. Nbs1 and p53 contribute to both checkpoint and apoptosis pathways. These results suggest that E2F1-induced foci generate a cell cycle checkpoint that, with sustained E2F1 activity, eventually yields to apoptosis. Uncontrolled proliferation due to Rb/E2F deregulation as well as inactivation of both checkpoint and apoptosis programs would then be required for transformation of normal cells to tumor cells.

Human cytomegalovirus IE1-72 activates ataxia telangiectasia mutated kinase and a p53/p21-mediated growth arrest response

Human cytomegalovirus (HCMV) encodes several proteins that can modulate components of the cell cycle machinery. The UL123 gene product, IE1-72, binds the Rb-related, p107 protein and relieves its repression of E2F-responsive promoters; however, it is unable to induce quiescent cells to enter S phase in wild-type (p53(+/+)) cells. IE1-72 also induces p53 accumulation through an unknown mechanism. We present here evidence suggesting that IE1-72 may activate the p53 pathway by increasing the levels of p19(Arf) and by inducing the phosphorylation of p53 at Ser15. Phosphorylation of this residue by IE1-72 expression alone or HCMV infection is found to be dependent on the ataxia-telangiectasia mutated kinase. IE2-86 expression leads to p53 phosphorylation and may contribute to this phenotype in HCMV-infected cells. We also found that IE1-72 promotes p53 nuclear accumulation by abrogating p53 nuclear shuttling. These events result in the stimulation of p53 activity, leading to a p53- and p21-dependent inhibition of cell cycle progression from G(1) to S phase in cells transiently expressing IE1-72. Thus, like many of the small DNA tumor viruses, the first protein expressed upon HCMV infection activates a p53 response by the host cell.

Dual Induction of Apoptosis and Senescence in Cancer Cells by Chk2 Activation: Checkpoint Activation as a Strategy against Cancer

The human checkpoint kinase 2 (Chk2) plays a central role in regulation of the cellular response to DNA damage, resulting in cell cycle arrest, DNA repair, or apoptosis depending on severity of DNA damage and the cellular context. Chk2 inhibitors are being developed as sensitizers for chemotherapeutic agents. In contrast, here we report that direct activation of Chk2 alone (without chemotherapeutic agents) led to potent inhibition of cancer cell proliferation. In the absence of de novo DNA damage, checkpoint activation was achieved by increased Chk2 expression, as evidenced by its phosphorylation at Thr68, resulting in senescence and apoptosis of cancer cells (DLD1 and HeLa). The Chk2-induced apoptosis was p53 independent and was mediated by caspase activation triggered by loss of mitochondrial potential. The Chk2-induced senescence was also p53 independent and was associated with induction of p21. These results suggest that direct activation of checkpoint kinases may be a novel approach for cancer therapy.

Life, death and E2F: linking proliferation control and DNA damage signaling via E2F1

Proper regulation of cellular proliferation is critical for normal development and cancer prevention. Most, if not all, cancer cells contain mutations in the Rb/E2F pathway, which controls cellular proliferation. Inactivation of the retinoblastoma (Rb) family of proteins can occur through Rb loss, mutation, or inactivation by cellular or viral oncoproteins leading to unrestrained proliferation and, often times, results in apoptosis. The loss of growth control occurs primarily by derepression and activation of the E2F transcription factors. E2F1 in particular, serves as the primary link between loss of Rb function and activation of p53-dependent apoptosis. E2F1 function is crucial for responding to loss of proper Rb-mediated growth control to activate p53 and the apoptotic program. Recently, we described the requirement for the DNA damage response proteins Atm, Nbs1, and Chk2 in the E2F1 apoptosis pathway. These findings suggest that there may be a more intimate relationship between the apoptosis pathways resulting from loss of proper Rb-mediated growth control and apoptosis resulting from the accumulation of DNA damage.

Apoptosis associated with deregulated E2F activity is dependent on E2F1 and Atm/Nbs1/Chk2

The retinoblastoma protein (Rb)/E2F pathway links cellular proliferation control to apoptosis and is critical for normal development and cancer prevention. Here we define a transcription-mediated pathway in which deregulation of E2F1 by ectopic E2F expression or Rb inactivation by E7 of human papillomavirus type 16 signals apoptosis by inducing the expression of Chk2, a component of the DNA damage response. E2F1- and E7-mediated apoptosis are compromised in cells from patients with the related disorders ataxia telangiectasia and Nijmegen breakage syndrome lacking functional Atm and Nbs1 gene products, respectively. Both Atm and Nbs1 contribute to Chk2 activation and p53 phosphorylation following deregulation of normal Rb growth control. E2F2, a related E2F family member that does not induce apoptosis, also activates Atm, resulting in phosphorylation of p53. However, we found that the key commitment step in apoptosis induction is the ability of E2F1, and not E2F2, to upregulate Chk2 expression. Our results suggest that E2F1 plays a central role in signaling disturbances in the Rb growth control pathway and, by upregulation of Chk2, may sensitize cells to undergo apoptosis.

E2F1 induces phosphorylation of p53 that is coincident with p53 accumulation and apoptosis

It has been proposed that the E2F1 transcription factor serves as a link between the Rb/E2F proliferation pathway and the p53 apoptosis pathway by inducing the expression of p19ARF, a protein that regulates p53 stability. We find that although p19ARF contributes to p53 accumulation in response to E2F expression, p19ARF is not required for E2F1-mediated apoptosis. E2F1 can signal p53 phosphorylation in the absence of p19ARF, similar to the observed modifications to p53 in response to DNA damage. These modifications are not observed in the absence of p19ARF following expression of E2F2, an E2F family member that does not induce apoptosis in mouse embryo fibroblasts but can induce p19ARF and p53 protein expression. p53 modification is found to be crucial for E2F1-mediated apoptosis, and this apoptosis is compromised when E2F1 is coexpressed with a p53 mutant lacking many N- and C-terminal phosphorylation sites. Additionally, E2F1-mediated apoptosis is abolished in the presence of caffeine, an inhibitor of phosphatidylinositol 3-kinase-related kinases that phosphorylate p53. These findings suggest that p53 phosphorylation is a key step in E2F1-mediated apoptosis and that this modification can occur in the absence of p19ARF.