Eradicating cancer cells: struggle with a chameleon

Immune Curbing of Cancer Stem Cells by CTLs Directed to NANOG

Cancer stem cells (CSCs) have been identified as the source of tumor growth and disease recurrence. Eradication of CSCs is thus essential to achieve durable responses, but CSCs are resistant to current anti-tumor therapies. Novel therapeutic approaches that specifically target CSCs will, therefore, be crucial to improve patient outcome. Immunotherapies, which boost the body's own immune system to eliminate cancerous cells, could be an alternative approach to target CSCs. Vaccines of dendritic cells (DCs) loaded with tumor antigens can evoke highly specific anti-tumor T cell responses. Importantly, DC vaccination also promotes immunological memory formation, paving the way for long-term cancer control. Here, we propose a DC vaccination that specifically targets CSCs. DCs loaded with NANOG peptides, a protein required for maintaining stem cell properties, could evoke a potent anti-tumor immune response against CSCs. We hypothesize that the resulting immunological memory will also control newly formed CSCs, thereby preventing disease recurrence.

Effects of baicalein on pancreatic cancer stem cells via modulation of sonic Hedgehog pathway

Recent studies have suggested that sonic Hedgehog (Shh) signaling pathway is aberrantly activated in cancer stem cells (CSCs). A seven-herb Chinese medicinal formula composed of Amorphophallus rivieri Durieu, Oldenlandia diffusa (Wild) Roxb, Scutellaria barbata D. Don, Gynostemma pentaphyllum (Thunb.) Mak and Amomum cardamomum L, i.e. Qingyihuaji (QYHJ) formula, has been shown to inhibit proliferation of pancreatic CSCs by inhibiting Shh signaling pathway and thereby prolong the overall survival of pancreatic cancer patients. Mass spectrometry analysis revealed that baicalein is one of the major compounds of QYHJ formula. The objective of this study was to investigate the role of Shh pathway in pancreatic cancer and to examine the molecular mechanisms of baicalein involved in pancreatic cancer treatment. We examined the effects of baicalein on pancreatic CSCs both in vivo and in vitro. The results indicated that baicalein attenuated the pluripotency of pancreatic CSCs. Then, we investigated the underlying mechanism and found that nuclear transcription factors, such as Sox-2 and Oct-4 as well as members in Shh signaling pathway, e.g. SHH, SMO, and Gli-2, were downregulated after baicalein treatment. Furthermore, silencing Gli-2 expression by small interfering RNA decreased Sox-2 expression and blocked the inhibitory effects of baicalein, suggesting that the effects of baicalein may be mediated through inhibition of Shh pathway. Our results suggested that baicalein, an active compound in QYHJ formula, could suppress the self-renewal of pancreatic CSCs through inhibition of Shh signaling pathway.

Retinoic Acid Reduces Stem Cell-Like Features in Pancreatic Cancer Cells

OBJECTIVES

Retinoic acid (RA) has important functions during embryonic development being involved in cell growth and differentiation. Although approved for the treatment of acute promyelocytic leukemia, it is still under investigation for different solid tumors including pancreatic cancer. The objective of this study was to analyze how RA affects pancreatic cancer stem cells and how its combination with chemotherapy could impact cell growth.

METHODS

Using different pancreatic cancer cell lines, we evaluated the effect of RA alone or in combination with chemotherapy regulating cancer stem cells properties and pathways.

RESULTS

Retinoic acid treatment reduces the expression of pancreatic stem cell markers CD24, CD44, CD133, and aldehyde dehydrogenase 1 but not c-Met. Although gemcitabine treatment increases the expression of some of these markers especially CD44 when it is combined with RA, a notable reduction in all of them is observed. Retinoic acid induces a G0/G1 arrest and combined with gemcitabine increases the apoptotic effect produced by chemotherapy probably as a consequence of a regulation of specific stem cell transcription factors.

CONCLUSIONS

Retinoic acid regulates self-renewal capacity of cells in pancreatic tumors and should be further investigated in combination with chemotherapy as therapeutic strategy in pancreatic cancer.

Restoring immunosurveillance by dendritic cell vaccines and manipulation of the tumor microenvironment

Cancer cells evolve from normal cells throughout life and are usually recognized by our immune system and destroyed, a process called immunosurveillance. Unfortunately, in some instances cancer cells paralyze our immune system, resulting in outgrowth and spreading of the tumor. Understanding the complexity of immunomodulation by tumors is important for the development of therapeutical strategies. Nowadays, various approaches have been developed to enhance anti-tumor immune responses and abrogate the immune dampening effect of the tumor and its surrounding environment, including dendritic cell-based vaccines, therapies to counteract myeloid derived suppressor cell function within the tumor and antagonists of inhibitory signaling pathways to overcome 'immune checkpoints'. The challenge is now to find the right combination of immune based therapies to fully restore immune function and provide a more efficacious and enduring anti-tumor response.

The stem cell markers Oct4A, Nanog and c-Myc are expressed in ascites cells and tumor tissue of ovarian cancer patients

PURPOSE

The aim of this study was to examine the expression of established stem cell markers in ascites and tumor tissue obtained from ovarian cancer patients.

METHODS

Mononuclear cells present in ascites were collected by density gradient centrifugation. Intracellular flowcytometry was used to assess the putative presence of stem cell markers. RT-PCR was used to detect full length Oct4A, a splice variant Oct4B, implicated in glioma and breast cancer, Oct4 pseudogenes and c-Myc. Genes were cloned and sequenced to determine putative mutations. Confocal laser scanning microscopy was performed to localize the markers in ascites cells as well as in tumor tissue. Material from carcinomas other than epithelial ovarian carcinoma served as control.

RESULTS

A small quantity of cells in ascites and in tumor tissue of ovarian cancer patients was detected that expresses c-Myc, Oct4A and Nanog. Besides Oct4A, present in the nucleus, also the cytoplasmic resident Oct4B splice variant was detected. Remarkably, c-Myc was found partially in the cytoplasm. Since no mutations in c-Myc were found that could explain the cytoplasmic localization, we hypothesize that this is due an IL-6 induced c-Myc shuttle factor.

CONCLUSIONS

The expression of stem cell genes was detected in a small proportion of tumor cells present in ascites as well as in tumor tissue. IL-6 plays an important role in the induction of c-Myc.

SOX2 promotes dedifferentiation and imparts stem cell-like features to pancreatic cancer cells

SOX2 (Sex-determining region Y (SRY)-Box2) has important functions during embryonic development and is involved in cancer stem cell (CSC) maintenance, in which it impairs cell growth and tumorigenicity. However, the function of SOX2 in pancreatic cancer cells is unclear. The objective of this study was to analyze SOX2 expression in human pancreatic tumors and determine the role of SOX2 in pancreatic cancer cells regulating CSC properties. In this report, we show that SOX2 is not expressed in normal pancreatic acinar or ductal cells. However, ectopic expression of SOX2 is observed in 19.3% of human pancreatic tumors. SOX2 knockdown in pancreatic cancer cells results in cell growth inhibition via cell cycle arrest associated with p21^Cip1 and p27^Kip1 induction, whereas SOX2 overexpression promotes S-phase entry and cell proliferation associated with cyclin D3 induction. SOX2 expression is associated with increased levels of the pancreatic CSC markers ALDH1, ESA and CD44. Importantly, we show that SOX2 is enriched in the ESA⁺/CD44⁺ CSC population from two different patient samples. Moreover, we show that SOX2 directly binds to the Snail, Slug and Twist promoters, leading to a loss of E-Cadherin and ZO-1 expression. Taken together, our findings show that SOX2 is aberrantly expressed in pancreatic cancer and contributes to cell proliferation and stemness/dedifferentiation through the regulation of a set of genes controlling G1/S transition and epithelial-to-mesenchymal transition (EMT) phenotype, suggesting that targeting SOX2-positive cancer cells could be a promising therapeutic strategy.

High-throughput chemical screens identify disulfiram as an inhibitor of human glioblastoma stem cells

Glioblastoma Multiforme (GBM) continues to have a poor patient prognosis despite optimal standard of care. Glioma stem cells (GSCs) have been implicated as the presumed cause of tumor recurrence and resistance to therapy. With this in mind, we screened a diverse chemical library of 2,000 compounds to identify therapeutic agents that inhibit GSC proliferation and therefore have the potential to extend patient survival. High-throughput screens (HTS) identified 78 compounds that repeatedly inhibited cellular proliferation, of which 47 are clinically approved for other indications and 31 are experimental drugs. Several compounds (such as digitoxin, deguelin, patulin and phenethyl caffeate) exhibited high cytotoxicity, with half maximal inhibitory concentrations (IC₅₀) in the low nanomolar range. In particular, the FDA approved drug for the treatment of alcoholism, disulfiram (DSF), was significantly potent across multiple patient samples (IC₅₀ of 31.1 nM). The activity of DSF was potentiated by copper (Cu), which markedly increased GSC death. DSF–Cu inhibited the chymotrypsin-like proteasomal activity in cultured GSCs, consistent with inactivation of the ubiquitin-proteasome pathway and the subsequent induction of tumor cell death. Given that DSF is a relatively non-toxic drug that can penetrate the blood-brain barrier, we suggest that DSF should be tested (as either a monotherapy or as an adjuvant) in pre-clinical models of human GBM. Data also support targeting of the ubiquitin-proteasome pathway as a therapeutic approach in the treatment of GBM.

Two hits are better than one: targeting both phosphatidylinositol 3-kinase and mammalian target of rapamycin as a therapeutic strategy for acute leukemia treatment

Phosphatidylinositol 3-kinase (PI3K) and mammalian target of rapamycin (mTOR) are two key components of the PI3K/Akt/mTOR signaling pathway. This signal transduction cascade regulates a wide range of physiological cell processes, that include differentiation, proliferation, apoptosis, autophagy, metabolism, motility, and exocytosis. However, constitutively active PI3K/Akt/mTOR signaling characterizes many types of tumors where it negatively influences response to therapeutic treatments. Hence, targeting PI3K/Akt/mTOR signaling with small molecule inhibitors may improve cancer patient outcome. The PI3K/Akt/mTOR signaling cascade is overactive in acute leukemias, where it correlates with enhanced drug-resistance and poor prognosis. The catalytic sites of PI3K and mTOR share a high degree of sequence homology. This feature has allowed the synthesis of ATP-competitive compounds targeting the catalytic site of both kinases. In preclinical models, dual PI3K/mTOR inhibitors displayed a much stronger cytotoxicity against acute leukemia cells than either PI3K inhibitors or allosteric mTOR inhibitors, such as rapamycin. At variance with rapamycin, dual PI3K/mTOR inhibitors targeted both mTOR complex 1 and mTOR complex 2, and inhibited the rapamycin-resistant phosphorylation of eukaryotic initiation factor 4E-binding protein 1, resulting in a marked inhibition of oncogenic protein translation. Therefore, they strongly reduced cell proliferation and induced an important apoptotic response. Here, we reviewed the evidence documenting that dual PI3K/mTOR inhibitors may represent a promising option for future targeted therapies of acute leukemia patients.

Regression of metastatic melanoma in a patient by antibody targeting of cancer stem cells

Current therapeutic regimens attempt to eliminate all malignant cells of a melanoma lesion. Pre-clinical data, however, indicate that melanoma is maintained by a minor subset of cancer cells, which are characterized by CD20 expression. We attempted to eliminate those cells in a progressing, chemotherapy-refractory metastatic melanoma patient by lesional injections of the anti-CD20 therapeutic antibody rituximab and concomitant dacarbazine treatment, which was ineffective as monotherapy. Although the frequency of CD20⁺ melanoma cells within the tumor lesions was initially about 2% and the bulk of tumor cells did not express CD20, rituximab treatment produced lasting remission accompanied by a decline of the melanoma serum marker S-100 to physiological levels. Detailed in-depth-analyses revealed a switch of serum cytokines from a T helper-2 to a pro-inflammatory T helper-1 cell profile. Apart from B cell elimination and decline in gammaglobulin levels, no grade 3/4 toxicity related to treatment was observed. Data provide the first clinical evidence that targeting the minor subset of CD20⁺ “melanoma sustaining cells” produces regression of chemotherapy-refractory melanoma and highlight the potency of selective cancer cell targeting in the treatment of melanoma.

Ovarian carcinoma tumor-initiating cells have a mesenchymal phenotype

Solid tumors appear to contain a subpopulation of cells (tumor-initiating cells, TICs) that not only drives and sustains tumor growth, but is possibly responsible for recurrence. We isolated, after enzymatic digestion of primary ovarian carcinoma samples, a subpopulation of cells propagating as non-adherent spheres in medium suitable for tumor stem cells. These cells were able to self-renew in vitro, as suggested by PKH-26 staining studies, were tumorigenic and acquired an epithelial morphology when grown in FBS-supplemented medium, losing their tumorigenic potential. Interestingly, the tumorigenic potential of PKH-26 (high) - and PKH-26 (neg) -sorted cells was similar. These TIC-enriched cultures showed higher levels of genes involved in stemness than differentiated cells derived from them and were more resistant to the cytotoxic effects of some drugs but equally sensitive to others. The higher level of ABCG2 efflux pump could explain increased resistance to taxol and VP16, and higher levels of genes involved in nucleotide excision repair partially explain the resistance to cisplatin. These cells express mesenchymal markers, and epithelial transition could be induced when cultured in differentiating conditions, with a loss of invasive potential. These data suggest that ovarian cancer is a stem cell disease and should help elucidate the role of these cells in the aggressive phenotype of this tumor and find new therapeutic strategies to reduce resistance to current chemotherapeutic drugs.

Pancreatic stellate cells form a niche for cancer stem cells and promote their self-renewal and invasiveness

Chronic pancreatitis and pancreatic ductal adenocarcinoma (PDAC) are characterized by extensive fibrosis. Importantly, in PDAC, this results in poor vascularization and impaired drug delivery to the cancer cells. Therefore, the combined targeting of pancreatic tumor stroma and chemotherapy should enhance response rates, but the negative outcome of a recent phase III clinical trial for the combination of chemotherapy and hedgehog pathway inhibition suggests that other means also need to be considered. Emerging data indicate that elimination of cancer stem cells as the root of the cancer is of pivotal importance for efficient treatment of pancreatic cancer. Recently, we demonstrated in a highly relevant preclinical mouse model for primary pancreatic cancers that the combination of cancer stem cell-targeting strategies in combination with a stroma-targeting agent, such as a hedgehog pathway inhibitor and chemotherapy, results in significantly enhanced long-term and progression-free survival. In the present study, we demonstrate mechanistically that Nodal-expressing pancreatic stellate cells are an important component of the tumor stroma for creating a paracrine niche for pancreatic cancer stem cells. Secretion of the embryonic morphogens Nodal/Activin by pancreatic stellate cells promoted in vitro sphere formation and invasiveness of pancreatic cancer stem cells in an Alk4-dependent manner. These data imply that the pancreatic cancer stem cell phenotype is promoted by paracrine Nodal/Activin signaling at the tumor-stroma interface. Therefore, targeting the tumor microenvironment is not only able to improve drug delivery but, even more importantly, destroys the cancer stem cell niche and, therefore, should be an integral part of cancer stem cell-based treatment strategies.

Genes at work in random bouts: stochastically discontinuous gene activity makes cell cycle duration and cell fate decisions variable, thus providing for stem cells plasticity

Cell interdivision periods (IDP) in homogenous cell populations vary stochastically. Another aspect of probabilistic cell behavior is randomness in cell differentiation. These features are suggested to result from competing stochastic events of assembly/disassembly of the transcription pre-initiation complex (PIC) at gene promoters. The time needed to assemble a proper PIC from different proteins, which must be numerous enough to make their combination gene specific, may be comparable to the IDP. Nascent mRNA visualization at defined genes and inferences from protein level fluctuations in single cells suggest that some genes do operate in this way. The onset of mRNA production by such genes may miss the time windows provided by the cell cycle, resulting in cells differentiating into those in which the respective mRNAs are either present or absent. This creates a way to generate cell phenotype diversity in multicellular organisms.