Ugur Eskiocak

Immortalized Epithelial Cells Derived From Human Colon Biopsies Express Stem Cell Markers and Differentiate In Vitro

BACKGROUND & AIMS Long-term propagation of human colonic epithelial cells (HCEC) of adult origin has been a challenge; currently used HCEC lines are of malignant origin and/or contain multiple cytogenetic changes. We sought to immortalize human colon biopsy-derived cells expressing stem cell markers and retaining multilineage epithelial differentiation capability. METHODS We isolated and cultured cells from biopsy samples of 2 patients undergoing routine screening colonoscopy. Cells were immortalized by expression of the nononcogenic proteins cyclin-dependent kinase 4 (Cdk4) and the catalytic component of human telomerase (hTERT) and maintained for more than 1 year in culture. RESULTS The actively proliferating HCECs expressed the mesenchymal markers vimentin and alpha-smooth muscle actin. Upon growth arrest, cells assumed a cuboidal shape, decreased their mesenchymal features, and expressed markers of colonic epithelial cells such as cytokeratin 18, zonula occludens-1, mucins-1 and -2, antigen A33, and dipeptidyl peptidase 4. Immortalized cells expressed stem cell markers that included LGR5, BMI1, CD29, and CD44. When placed in Matrigel in the absence of a mesenchymal feeder layer, individual cells divided and formed self-organizing, cyst-like structures; a subset of cells exhibited mucin-2 or polarized villin staining. CONCLUSIONS We established immortalized HCECs that are capable of self-renewal and multilineage differentiation. These cells should serve as valuable reagents for studying colon stem cell biology, differentiation, and pathogenesis.

Targeting of Nrf2 induces DNA damage signaling and protects colonic epithelial cells from ionizing radiation

Nuclear factor-erythroid 2–related factor 2 (Nrf2) is a key transcriptional regulator for antioxidant and anti-inflammation enzymes that binds to its endogenous inhibitor protein, Kelch-like ECH (erythroid cell-derived protein with CNC homology)-associated protein 1, in the cytoplasm under normal conditions. Various endogenous or environmental oxidative stresses, such as ionizing radiation (IR), can disrupt the Nrf2–Kelch-like ECH-associated protein 1 complex. This allows Nrf2 to translocate from the cytoplasm into the nucleus to induce transcription of heme oxygenase-1 and other cytoprotective enzymes through binding to antioxidant responsive elements. However, how Nrf2 protects cells from IR-induced damage remains unclear. Here, we report that Nrf2 activation by the synthetic triterpenoids, bardoxolone methyl (BARD) and 2-cyano-3,12-dioxooleana-1,9 (11)-dien-28-oic acid–ethyl amide, protects colonic epithelial cells against IR-induced damage, in part, by enhancing signaling of the DNA damage response. Pretreatment with BARD reduced the frequency of both G1 and S/G2 chromosome aberrations and enhanced the disappearance of repairosomes (C-terminal binding protein interacting protein, Rad51, and p53 binding protein-1 foci) after IR. BARD protected cells from IR toxicity in a Nrf2-dependent manner. The p53 binding protein-1 promoter contains three antioxidant responsive elements in which Nrf2 directly binds following BARD treatment. In addition, 2-cyano-3,12-dioxooleana-1,9 (11)-dien-28-oic acid–ethyl amide provided before exposure to a lethal dose of whole-body irradiation protected WT mice from DNA damage and acute gastrointestinal toxicity, which resulted in improved overall survival. These results demonstrate that Nrf2 activation by synthetic triterpenoids is a promising candidate target to protect the gastrointestinal tract against acute IR in vitro and in vivo.

Processive and Distributive Extension of Human Telomeres by Telomerase under Homeostatic and Nonequilibrium Conditions

Specific information about how telomerase acts in vivo is necessary for understanding telomere dynamics in human tumor cells. Our results imply that, under homeostatic telomere length-maintenance conditions, only one molecule of telomerase acts at each telomere during every cell division and processively adds ∼60 nt to each end. In contrast, multiple molecules of telomerase act at each telomere when telomeres are elongating (nonequilibrium conditions). Telomerase extension is less processive during the first few weeks following the reversal of long-term treatment with the telomerase inhibitor Imetelstat (GRN163L), a time when Cajal bodies fail to deliver telomerase RNA to telomeres. This result implies that processing of telomerase by Cajal bodies may affect its processivity. Overexpressed telomerase is also less processive than the endogenously expressed telomerase. These findings reveal two major distinct extension modes adopted by telomerase in vivo.

Human melanoma metastasis in NSG mice correlates with clinical outcome in patients.

The ability of melanomas to produce circulating tumor cells and to metastasize in immunocompromised mice correlates with their ability to form distant metastases in patients. Melanomas in Miniature No two patients are exactly alike, so studying cancer biology in humans is complicated. There is no way to standardize experimental conditions or to analyze tumor biology in isolation. To overcome these difficulties, researchers use model animals such as mice, but these are never a perfect model of human physiology. For example, the process of cancer metastasis in mice frequently does not reflect findings in human patients. Some metastatic human cancers do not grow well in mice or do not metastasize. Now, Quintana et al. have developed a new mouse model of melanoma that provides a more faithful reproduction of human tumor metastasis and offers insights into the biology of circulating tumor cells. Quintana and colleagues used highly immunosuppressed mice to implant tumors from patients with stage III melanomas—those that have spread locally (typically to lymph nodes), but not beyond. All of the patients’ tumors were successfully implanted in this mouse model, but not all metastasized. Pathological analysis and bioluminescence imaging demonstrated that the spread of melanoma in the mouse model correlated with eventual spread of the disease in human patients. The same tumors that would later metastasize and kill their human hosts also metastasized widely in the mice. Conversely, melanomas that did not spread after surgical removal of the parent tumors from patients also metastasized slowly or inefficiently in animals, even after repeated passages of tumor cells through several generations of mice. To explain this phenomenon, the authors searched for human cancer cells in the blood of tumor-bearing mice and found a correlation there as well—the mice whose tumors metastasized efficiently had easily detectable tumor cells in their bloodstream, whereas the mice with poorly metastatic tumors did not. In this study, tumors were removed from their normal microenvironment and transplanted into mice; thus, this protocol permits only a partial view of the metastatic process. The new mouse model can be used to study intrinsic factors in cancer cells that contribute to metastasis but not immune factors or other patient-specific characteristics that may contribute to survival. Nevertheless, this report nicely demonstrates that at least some of the key factors that regulate melanoma metastasis are intrinsic to the tumors. The new mouse model will allow the characterization of metastatic processes and possibly point to markers that predict metastasis or therapeutic targets for human melanoma interventions. Studies of human cancer metastasis have been limited by a lack of experimental assays in which cancer cells from patients metastasize in vivo in a way that correlates with clinical outcome. This makes it impossible to study intrinsic differences in the metastatic properties of cancers from different patients. We recently developed an assay in which human melanomas readily engraft in nonobese diabetic/severe combined immunodeficient interleukin-2 receptor-γ chain null (NSG) mice. We show that melanomas from 25 patients exhibited reproducible differences in the rate of spontaneous metastasis after transplantation into NSG mice and that these differences correlated with clinical outcome in the patients. Stage IIIB/C melanomas that formed distant metastases within 22 months in patients also formed tumors that metastasized widely in NSG mice, whereas stage IIIB/C melanomas that did not form distant metastases within 22 to 50 months in patients metastasized more slowly in NSG mice. These differences in the efficiency of metastasis correlated with the presence of circulating melanoma cells in the blood of NSG mice, suggesting that the rate of entry into the blood is one factor that limits the rate of metastasis. The study of NSG mice can therefore yield information about the metastasis of human melanomas in vivo, in this case revealing intrinsic differences among stage III melanomas in their ability to circulate/survive in the blood and to metastasize.

CDDO-Me Protects against Space Radiation-Induced Transformation of Human Colon Epithelial Cells

Abstract Radiation-induced carcinogenesis is a major concern both for astronauts on long-term space missions and for cancer patients being treated with therapeutic radiation. Exposure to radiation induces oxidative stress and chronic inflammation, which are critical initiators and promoters of carcinogenesis. Many studies have demonstrated that non-steroidal anti-inflammatory drugs and antioxidants can reduce the risk of radiation-induced cancer. In this study, we found that a synthetic triterpenoid, CDDO-Me (bardoxolone methyl), was able to protect human colon epithelial cells (HCECs) against radiation-induced transformation. HCECs that were immortalized by ectopic expression of hTERT and cdk4 and exhibit trisomy for chromosome 7 (a non-random chromosome change that occurs in 37% of premalignant colon adenomas) can be transformed experimentally with one combined exposure to 2 Gy of protons at 1 GeV/nucleon followed 24 h later by 50 cGy of 56Fe ions at 1 GeV/nucleon. Transformed cells showed an increase in proliferation rate and in both anchorage-dependent and independent colony formation ability. A spectrum of chromosome aberrations was observed in transformed cells, with 40% showing loss of 17p (e.g. loss of one copy of p53). Pretreatment of cells with pharmacological doses of CDDO-Me, which has been shown to induce antioxidative as well as anti-inflammatory responses, prevented the heavy-ion-induced increase in proliferation rate and anchorage-dependent and independent colony formation efficiencies. Taken together, these results demonstrate that experimentally immortalized human colon epithelial cells with a non-random chromosome 7 trisomy are valuable premalignant cellular reagents that can be used to study radiation-induced colorectal carcinogenesis. The utility of premalignant HCECs to test novel compounds such as CDDO-Me that can be used to protect against radiation-induced neoplastic transformation is also demonstrated.

Functional Parsing of Driver Mutations in the Colorectal Cancer Genome Reveals Numerous Suppressors of Anchorage-Independent Growth

Landmark cancer genome resequencing efforts are leading to the identification of mutated genes in many types of cancer. The extreme diversity of mutations being detected presents significant challenges to subdivide causal from coincidental mutations to elucidate how disrupted regulatory networks drive cancer processes. Given that a common early perturbation in solid tumor initiation is bypass of matrix-dependent proliferation restraints, we sought to functionally interrogate colorectal cancer candidate genes (CAN-genes) to identify driver tumor suppressors. We have employed an isogenic human colonic epithelial cell (HCEC) model to identify suppressors of anchorage-independent growth by conducting a soft agar-based short hairpin RNA (shRNA) screen within the cohort of CAN-genes. Remarkably, depletion of 65 of the 151 CAN-genes tested collaborated with ectopic expression of K-RAS(V12) and/or TP53 knockdown to promote anchorage-independent proliferation of HCECs. In contrast, only 5 of 362 random shRNAs (1.4%) enhanced soft agar growth. We have identified additional members of an extensive gene network specifying matrix-dependent proliferation, by constructing an interaction map of these confirmed progression suppressors with approximately 700 mutated genes that were excluded from CAN-genes, and experimentally verifying soft agar growth enhancement in response to depletion of a subset of these genes. Collectively, this study revealed a profound diversity of nodes within a fundamental tumor suppressor network that are susceptible to perturbation leading to enhanced cell-autonomous anchorage-independent proliferative fitness. Tumor suppressor network fragility as a paradigm within this and other regulatory systems perturbed in cancer could, in large part, account for the heterogeneity of somatic mutations detected in tumors.

Synergistic effects of ion transporter and MAP kinase pathway inhibitors in melanoma

New therapies are required for melanoma. Here, we report that multiple cardiac glycosides, including digitoxin and digoxin, are significantly more toxic to human melanoma cells than normal human cells. This reflects on-target inhibition of the ATP1A1 Na+/K+ pump, which is highly expressed by melanoma. MEK inhibitor and/or BRAF inhibitor additively or synergistically combined with digitoxin to induce cell death, inhibiting growth of patient-derived melanomas in NSG mice and synergistically extending survival. MEK inhibitor and digitoxin do not induce cell death in human melanocytes or haematopoietic cells in NSG mice. In melanoma, MEK inhibitor reduces ERK phosphorylation, while digitoxin disrupts ion gradients, altering plasma membrane and mitochondrial membrane potentials. MEK inhibitor and digitoxin together cause intracellular acidification, mitochondrial calcium dysregulation and ATP depletion in melanoma cells but not in normal cells. The disruption of ion homoeostasis in cancer cells can thus synergize with targeted agents to promote tumour regression in vivo.

RASSF1A inactivation unleashes a tumor suppressor/oncogene cascade with context-dependent consequences on cell cycle progression.

ABSTRACT The RASSF1A gene is one of the most frequently inactivated genes in over 30 different types of cancers (H. Donninger, M. D. Vos, and G. J. Clark, J. Cell Sci. 120:3163–3172, 2007, http://dx.doi.org/10.1242/jcs.010389). Despite the prevalence of RASSF1A silencing in human cancer, the mechanism by which RASSF1A functions as a tumor suppressor is not well understood. Characterization of the consequences of RASSF1A loss on epithelial cell proliferation revealed that RASSF1A expression suppresses both microRNA 21 (miR-21) expression and extracellular signal-regulated kinase 1/2 (ERK1/2) activation. The mechanism of the former is through restraint of SCFβTrCP-dependent destruction of the repressor element 1 silencing transcription factor (REST) tumor suppressor and consequent inhibition of miR-21 promoter activation. The mechanism of the latter is through physical sequestration of MST2, which results in accumulation of inactivating S259 phosphorylation of RAF1. Whether or not inactivation of these RASSF1A regulatory relationships can unleash enhanced proliferative capacity is dependent upon the coupling of SCFβTrCP and miR-21 to suppression of SKP2 protein translation and stability. Airway epithelial cultures retain this coupling and therefore respond to RASSF1A inactivation by p27-dependent cell cycle arrest. In contrast, colonic crypt-derived epithelial cells have uncoupled SCFβTrCP from SKP2 and respond to RASSF1A inactivation by enhanced proliferation rates. These observations help account for context-specific molecular etiology of oncogenic transformation and suggest intervention strategies for recently developed SKP2 inhibitors.

Selective targeting of mutant adenomatous polyposis coli (APC) in colorectal cancer

A small molecule specifically kills cancer cells with APC truncations and spares cells with wild-type APC. Truncating colorectal cancer The adenomatous polyposis gene (APC), which is a tumor suppressor, is commonly mutated in colon cancer and cannot be directly targeted by existing therapeutics. Zhang et al. identified a drug called TASIN-1, which specifically targets cells with APC truncations, the most common APC mutation seen in colorectal cancer. The authors showed that this drug interferes with cholesterol biosynthesis and that it selectively kills APC-truncated cancer cells in vitro and in mouse models without any detectable toxicity, paving the way for further development of TASIN-1. Mutations in the adenomatous polyposis coli (APC) gene are common in colorectal cancer (CRC), and more than 90% of those mutations generate stable truncated gene products. We describe a chemical screen using normal human colonic epithelial cells (HCECs) and a series of oncogenically progressed HCECs containing a truncated APC protein. With this screen, we identified a small molecule, TASIN-1 (truncated APC selective inhibitor–1), that specifically kills cells with APC truncations but spares normal and cancer cells with wild-type APC. TASIN-1 exerts its cytotoxic effects through inhibition of cholesterol biosynthesis. In vivo administration of TASIN-1 inhibits tumor growth of CRC cells with truncated APC but not APC wild-type CRC cells in xenograft models and in a genetically engineered CRC mouse model with minimal toxicity. TASIN-1 represents a potential therapeutic strategy for prevention and intervention in CRC with mutant APC.

Usp9x regulates Ets-1 ubiquitination and stability to control NRAS expression and tumorigenicity in melanoma

ETS transcription factors are commonly deregulated in cancer by chromosomal translocation, overexpression or post-translational modification to induce gene expression programs essential in tumorigenicity. Targeted destruction of these proteins may have therapeutic impact. Here we report that Ets-1 destruction is regulated by the deubiquitinating enzyme, Usp9x, and has major impact on the tumorigenic program of metastatic melanoma. Ets-1 deubiquitination blocks its proteasomal destruction and enhances tumorigenicity, which could be reversed by Usp9x knockdown or inhibition. Usp9x and Ets-1 levels are coincidently elevated in melanoma with highest levels detected in metastatic tumours versus normal skin or benign skin lesions. Notably, Ets-1 is induced by BRAF or MEK kinase inhibition, resulting in increased NRAS expression, which could be blocked by inactivation of Usp9x and therapeutic combination of Usp9x and MEK inhibitor fully suppressed melanoma growth. Thus, Usp9x modulates the Ets-1/NRAS regulatory network and may have biologic and therapeutic implications.