Cancer stem cells in esophageal cancer and response to therapy

Abstract

Cancer stem cells (CSCs) in esophageal cancer have a key role in tumor-initiation, progression, metastases, immune evasion, and therapy resistance. CSCs can be characterized by unique biomarkers, however, there exists an immense complexity of changing characteristics driven by tumor evolution and exposure to therapy. Attempts to target CSCs are ongoing but more in depth research is necessary.

Esophageal cancer (EC) is the eleventh most common cause of cancer worldwide (459,299 cases) and the sixth most common cause of cancer mortality (439,000 deaths) ¹. Esophageal cancer has two common histologic subtypes: esophageal squamous cell carcinoma (ESCC) and esophageal adenocarcinoma (EAC). The incidence of EAC has been rising in the western world, and ESCC is common especially in Asia ¹. For resectable EC, preoperative chemoradiation or chemotherapy followed by surgery is currently standard treatment ². If we could identify patients destined to have a pathological complete response (pCR) for therapy, hypothetically we could avoid esophagectomy and preserve the organ. For metastatic EC, despite the development of chemotherapy, targeted therapy, and immunotherapy, the prognosis remains dismal. Therefore, in depth research to characterize molecular profile could help to establish robust predictive/prognostic biomarkers and novel therapeutic targets.

Cancer stem cells (CSCs) harbor unique properties to include self-renewal, tumor maintenance (proliferation), migration, immune evasion, and therapy resistance (including ability to manage ROS/NOS). CSCs are defined in the 2006 American Association for Cancer Research Workshop on Cancer Stem Cells as a cell within a tumor that possesses the capacity to self-renew and to give rise to the heterogeneous lineages of cancer cells that comprise the tumor ³. Two hypotheses regarding the origin of CSCs have been proposed ⁴. One is that normal stem cells transform into CSCs as a result of accumulated genetic alterations and epigenetic modifications. Another is that dedifferentiated cancer cells acquire capability (plasticity) of CSCs ⁴.

CSCs can be identified by specific surface and intracellular markers. In EC, several surface markers to identify CSCs have been discovered. Firstly, CD44+/CD24− EC cells were found to have a higher proliferation and tumor sphere forming capacity, and a small cohort study showed that no patients with CD44 positive EAC achieved pCR after preoperative chemoradiation ⁵. Secondly, our group found that low ALDH-1 expression in EAC tumor was significantly associated with pCR, longer overall survival (OS) and progression free survival (PFS) ⁶. Thirdly, CD133 was identified as a CSC marker in several solid tumors ⁷. One meta-analysis showed that CD133 expression in ESCC was significantly associated with lymph node metastases, higher clinical stage and higher histopathological grade ⁷. Fourthly, ABCG2 (ATP-Binding Cassette Transporter G2), which functions as transporter of cytotoxic substances, is associated with tumor initiation in ESCC ⁸. Moreover, ABCG2 expression in ESCC was significantly associated with higher TNM stages ⁸. Other surface proteins, such as Integrin7, LGR5, CD90, CD271, and ICAM1, have been reported as potential CSC markers ⁹. Clearly, more pathways are available to CSCs depending on the context (therapy as insult or tumor evolution).

Several pathways are activated and drive stemness in EC, such as the Wnt/beta-catenin pathway, Hedgehog pathway, Notch pathway, JAK-STAT3 pathway, and Hippo pathway. Firstly, the Wnt/beta-catenin pathway was found to contribute to CSC renewal ¹⁰. In the intestinal crypt, the Wnt/beta-catenin pathway has a role for maintaining homeostasis, but suppressor of this signal, adenomatous polyposis coli (APC), is deleted in 10% EAC ¹¹. Secondly, the Hedgehog pathway is essential for regulating the proliferation not only in normal embryonic cells, but also in cancer cells ¹². Transcription factors of downstream of Hedgehog signal, glioma-associated oncogene family (Gli-1), is associated with resistance for chemoradiation in EAC ¹². Thirdly, the Notch pathway is essential for determining cell fate thorough the direct interaction between the Notch receptors and ligands on adjacent cells. The Notch pathway was associated with stemness, resistance to therapy and differentiation ¹³. Fourthly, the JAK-STAT3 pathway works together with Nanog homeobox and octamer-binding transcription factor 4 (OCT4), which are required for modulating pluripotency ¹⁴. Finally, the Hippo pathway appears essential for cancer stemness, such as cancer initiation, progression, and metastases ¹⁵. Our group demonstrated that YAP overexpression in EAC cells conferred CSC properties and resistance to therapy by direct upregulation of SOX9 ¹⁶.

Therapy targeting CSCs could provide many benefits that do not occur with traditional cytotoxic agents that tend to kill susceptible later differentiated progenitors. Many therapies that target specific CSC pathways are making their way into the clinics. A Hedgehog inhibitor, Vismodegib, was assessed in combination with FOLFOX in a randomized phase 2 trial but did not prolong PFS (11.5 months vs 9.3 months; p= 0.34) (NCT00982592). The BRIGHTER study assessed napabucasin, a STAT3 inhibitor, in combination with paclitaxel in second line setting. Although the detail results are to be presented at ASCO 2018, napabucasin combined with paclitaxel vs. paclitaxel did not prolong OS (NCT02178956) ¹⁷. In these two trials, patients were not enriched by tumor/blood biomarkers which may be essential in future trials. Our group has initiated an NCI supported phase I/II trial evaluating Taladegib, which inhibit Hedgehog signaling by binding to smoothened (SMO), in combination with weekly paclitaxel, carboplatin, and radiation in localized EAC that are selected based on having ≥5% labeling index of nuclear Gli-1 (NCT02530437) ¹². CD44 is also a potential therapeutic target in EC. A variant form of CD44 (CD44v) interacts with cysteine-glutamate transporter and maintain high levels of intracellular reduced glutathione (GSH), leading to protect cells from oxidative stress ¹⁸. Sulfasalazine, inhibitor of cystine-glutamate transporter, has been evaluated in phase 1 trial, which showed that sulfasalazine could reduce CD44v-positive cancer cells in the posttreatment biopsy tissues ¹⁹. The Wnt/b-Catenin pathway inhibitors and the Notch inhibitors are in clinical trials for various solid tumors, but currently not for EC. Inhibiting the Hippo pathway suppresses CSCs characteristics therefore strategies to target the Hippo pathway are emerging. Recently, our group showed that a novel YAP inhibitor, CA3, is effective against CSCs with high YAP1 expression in EAC ²⁰. Several substances, such as verteporfin, VGLL4-mimicking peptide, and statins, were shown as the Hippo pathway inhibitor. Thus, more clinical trials against the Hippo pathway are expected.

The complexities lie in the fact that as the tumors multiply and/or experience threats (treatments such targeted agents, cytotoxic agents, or radiation), novel species of CSCs with higher capabilities are formed. Thus this level of heterogeneity remains a major impediment for cure. Emerging research also suggests that the CSC pathways are engaged in immune evasion as a survival tactic. Therefore, in the future, it would be important to consider combination therapy that targets CSCs and the immune cells for a greater advantage. Because of improving biotechnology, there is hope that the new portfolio of therapeutics against EC will be much better that what we have today.

Biographies

Kazuto Harada

Kazuto Harada is a Postdoctoral Fellow of Gastrointestinal Oncology at UT.MD Anderson Cancer Center in Houston. He received his Doctor of Medicine and Philosophy at Graduate School of Medical Science Kumamoto University, Japan. His research interests are translational research, cancer cell biology, and oncology in gastrointestinal cancer.

Melissa P Pizzi

Melissa P Pizzi is a Research Assistant II in MD Anderson Cancer Center. She received her master’s degree at A.C Camargo Cancer Center in São Paulo, Brazil, and her Biomedicine Bachelor’s degree at Universidade Federal do Triângulo Mineiro (UFTM), Brazil.

Namita Shanbhag, MS

Namita D. Shanbhag is a Research Investigator in the Department of Gastrointestinal Oncology at UT.MD Anderson Cancer Center in Houston. She received her Masters degree in Biotechnology at Johns Hopkins University. She has conducted translational research projects and managed clinical trials.

Hideo Baba

Hideo Baba is a Professor and Chairman, department of Gastroenterological Surgery, Graduate School of Medical Science Kumamoto University, Japan. His research interests are cancer stem cell biology and oncology in gastrointestinal cancer. At present, he is the director of the Japan Surgical Society and Japan Society of Clinical Oncology.

Shumei Song

Shumei Song is an associate professor of Gastrointestinal Oncology at UT.MD Anderson Cancer Center in Houston, TX, where she has taken charge of the translational gastroesophageal cancer research program and utilized patients’ derived samples, tissues and other resources aiming to identify novel drivers and new therapeutic strategies for advanced gastroesophageal cancer patients.

Jaffer A. Ajani

Dr. Ajani is professor of medicine in the Department of GI Medical Oncology. His clinical practice, clinical research and translational/basic research is focused on gastric and esophageal cancers. He has conducted many clinical and translational projects. He is the recipient of several NCI and DOD grants. He chairs two panels (gastric cancer and esophageal cancer) for NCCN.