The tumor microenvironment is created by the tumor and dominated by tumor-induced interactions. Although various immune effector cells are recruited to the tumor site, their anti-tumor functions are downregulated, largely in response to tumor-derived signals. Infiltrates of inflammatory cells present in human tumors are chronic in nature and are enriched in regulatory T cells (T-reg) as well as myeloid suppressor cells (MSC). Immune cells in the tumor microenvironment not only fail to exercise anti-tumor effector functions, but they are co-opted to promote tumor growth. Sustained activation of the NF-kappa B pathway in the tumor milieu represents one mechanism that appears to favor tumor survival and drive abortive activation of immune cells. The result is tumor escape from the host immune system. Tumor escape is accomplished through the activation of one or several molecular mechanisms that lead to inhibition of immune cell functions or to apoptosis of anti-tumor effector cells. The ability to block tumor escape depends on a better understanding of cellular and molecular pathways operating in the tumor microenvironment. Novel therapeutic strategies that emerge are designed to change the protumor microenvironment to one favoring acute responses and potent anti-tumor activity.
Recognition of non-self molecular patterns by pattern recognition receptors is a cornerstone of innate immunity. Toll-like receptors (TLRs) exert a key role in recognizing pathogen-associated molecular patterns (PAMPs) but have also been implicated in the recognition of damage-associated molecular patterns (DAMPs). As such, TLRs regulate a wide range of biological responses including inflammatory and immune responses during carcinogenesis. The high expression of TLRs by antigen-presenting cells, including dendritic cells, and their ability to induce antitumor mediators such as type I interferon has led to efforts to utilize TLR agonists in tumor therapy in order to convert the often tolerant immune response toward antitumor responses. However, TLRs are also increasingly recognized as regulators of tumor-promoting inflammation and promoters of tumor survival signals. Here, we will review in detail the dichotomous role of TLRs in tumor biology, focusing on relevant TLR-dependent pro-and antitumor pathways, and discuss clinical applications of TLR-targeted therapies for tumor prevention and treatment.
Most cancer cells exhibit increased glycolysis and use this metabolic pathway for generation of ATP as a main source of their energy supply. This phenomenon is known as the Warburg effect and is considered as one of the most fundamental metabolic alterations during malignant transformation. In recent years, there are significant progresses in our understanding of the underlying mechanisms and the potential therapeutic implications. Biochemical and molecular studies suggest several possible mechanisms by which this metabolic alteration may evolve during cancer development. These mechanisms include mitochondrial defects and malfunction, adaptation to hypoxic tumor microenvironment, oncogenic signaling, and abnormal expression of metabolic enzymes. Importantly, the increased dependence of cancer cells on glycolytic pathway for ATP generation provides a biochemical basis for the design of therapeutic strategies to preferentially kill cancer cells by pharmacological inhibition of glycolysis. Several small molecules have emerged that exhibit promising anticancer activity in vitro and in vivo, as single agent or in combination with other therapeutic modalities. The glycolytic inhibitors are particularly effective against cancer cells with mitochondrial defects or under hypoxic conditions, which are frequently associated with cellular resistance to conventional anticancer drugs and radiation therapy. Because increased aerobic glycolysis is commonly seen in a wide spectrum of human cancers and hypoxia is present in most tumor microenvironment, development of novel glycolytic inhibitors as a new class of anticancer agents is likely to have broad therapeutic applications.
Isocitrate dehydrogenase 1 and 2 (IDH1/2) are key enzymes in cellular metabolism, epigenetic regulation, redox states, and DNA repair. IDH1/2 mutations are causal in the development and/or progression of various types of cancer due to supraphysiological production of D-2-hydroxyglutarate. In various tumor types, IDH1/2-mutated cancers predict for improved responses to treatment with irradiation or chemotherapy. The present review discusses the molecular basis of the sensitivity of IDH1/2-mutated cancers with respect to the function of mutated IDH1/2 in cellular processes and their interactions with novel IDH1/2-mutant inhibitors. Finally, lessons learned from IDH1/2 mutations for future clinical applications in IDH1/2 wild-type cancers are discussed
Various factors and cellular components in the tumor microenvironment are key drivers associated with drug resistance in many cancers. Here, we analyzed the factors and molecular mechanisms involved in chemoresistance in patients with esophageal squamous cell carcinoma (ESCC). We found that interleukin 6 (IL6) derived mainly from cancer-associated fibroblasts played the most important role in chemoresistance by upregulating C-X-C motif chemokine receptor 7 (CXCR7) expression through signal transducer and activator of transcription 3/nuclear factor-kappa B pathway. CXCR7 knockdown resulted in the inhibition of IL6-induced proliferation and chemoresistance. In addition, CXCR7 silencing significantly decreased gene expression associated with stemness, chemoresistance and epithelial-mesenchymal transition and suppressed the proliferation ability of ESCC cells in three-dimensional culture systems and angiogenesis assay. In clinical samples, ESCC patients with high expression of CXCR7 and IL6 presented a significantly worse overall survival and progression-free survival upon receiving cisplatin after operation. These results suggest that the IL6-CXCR7 axis may provide a promising target for the treatment of ESCC.
In this review, we discuss the role of the tumour-associated carbonic anhydrase isoform IX (CAIX) in the context of pH regulation. We summarise recent experimental findings on the effect of CAIX on cell growth and survival, and present a diffusion-reaction model to help in the assessment of CAIX function under physiological conditions. CAIX emerges as an important facilitator of acid diffusion and acid transport, helping to overcome large cell-to-capillary distances that are characteristic of solid tumours. The source of substrate for CAIX catalysis is likely to be CO2, generated by adequately oxygenated mitochondria or from the titration of metabolic acids with HCO3- taken up from the extracellular milieu. The relative importance of these pathways will depend on oxygen and metabolite availability, the spatiotemporal patterns of the cell's exposure to hypoxia and on the regulation of metabolism by genes. This is now an important avenue for further investigation. The importance of CAIX in regulating tumour pH highlights the protein as a potential target for cancer therapy. Oncogene (2010) 29, 6509-6521; doi: 10.1038/onc.2010.455; published online 4 October 2010
Exosomes are carriers of pro-tumorigenic factors that participate in glioblastoma (GBM) progression, and many fusion genes are strong driver mutations in neoplasia and are involved in tumorigenesis. However, the ability of fusion genes to be transduced by exosomes is unknown. We characterized exosomes from GBM cells harbouring and not harbouring PTPRZ1-MET fusion (ZM fusion). We also determined the effect of the exosomes from ZM fusion cells (ZM exosomes) on pro-oncogenic secretions and showed that ZM exosomes are internalized by the recipient cells. In addition, we studied the effect of ZM exosome-mediated intercellular communication in the GBM microenvironment. MET proto-oncogene expression was higher in ZM exosomes. Moreover, phosphorylated MET was detected only in ZM exosomes and not in exosomes released by non-ZM fusion GBM cells. ZM exosomes transferred to non-ZM fusion GBM cells and normal human astrocytes altered gene expression and induced epithelial-mesenchymal transition. The uptake of ZM exosomes also induced an exosome-dependent phenotype defined by GBM cell migration and invasion, neurosphere growth and angiogenesis. In addition, ZM exosomes conferred temozolomide resistance to the GBM cells, and exosomederived ZM fusion network proteins targeted multiple pro-oncogenic effectors in recipient cells within the GBM microenvironment. Our findings show that exosomes mediate the aggressive character of GBM and demonstrate the role of ZM fusion in the exacerbation of this effect. These findings have possible implications for the foundation of gene fusion-based therapy for managing GBM.
Emerging evidence has shown that aberrantly expressed microRNAs (miRNAs) are highly associated with tumour development and progression. However, little is known about the potential role of miRNAs in gastric cancer (GC) metastasis. In this study, miR-409-3p was found to be downregulated frequently in human GCs, and its expression was significantly associated with tumor-node-metastasis (TNM) stage and lymph node metastasis. Enforced expression of miR-409 in GC cells significantly reduced their migration and invasion in vitro and their capacity to develop distal pulmonary metastases and peritoneal dissemination in vivo. Moreover, we found that miR-409 exerted its function predominantly through the mature miR-409-3p, but not miR-409-5p. Microarray and bioinformatics analysis identified the pro-metastatic gene radixin (RDX) as a potential miR-409-3p target. Further studies confirmed that miR-409-3p suppressed the expression of RDX by directly binding to its 3'-untranslated region. Silencing of RDX by small interfering RNAs phenocopied the effects of miR-409 overexpression, whereas restoration of RDX in miR-409-overexpressed GC cells reversed the suppressive effects of miR-409. Taken together, these results demonstrate that miR-409 suppresses GC cell invasion and metastasis by directly targeting RDX and that patients with downregulated miR-409-3p are prone to lymph node metastasis.
Previously, it has been shown that pancreatic ductal adenocarcinoma (PDA) tumors exhibit high levels of hypoxia, characterized by low oxygen pressure (pO(2)) and decreased O-2 intracellular perfusion. Chronic hypoxia is strongly associated with resistance to cytotoxic chemotherapy and chemoradiation in an understudied phenomenon known as hypoxia-induced chemoresistance. The hypoxia-inducible, pro-oncogenic, serine-threonine kinase PIM1 (Proviral Integration site for Moloney murine leukemia virus 1) has emerged as a key regulator of hypoxia-induced chemoresistance in PDA and other cancers. Although its role in therapeutic resistance has been described previously, the molecular mechanism behind PIM1 overexpression in PDA is unknown. Here, we demonstrate that cis-acting AU-rich elements (ARE) present within a 38-base pair region of the PIM1 mRNA 3'-untranslated region mediate a regulatory interaction with the mRNA stability factor HuR (Hu antigen R) in the context of tumor hypoxia. Predominantly expressed in the nucleus in PDA cells, HuR translocates to the cytoplasm in response to hypoxic stress and stabilizes the PIM1 mRNA transcript, resulting in PIM1 protein overexpression. A reverse-phase protein array revealed that HuR-mediated regulation of PIM1 protects cells from hypoxic stress through phosphorylation and inactivation of the apoptotic effector BAD and activation of MEK1/2. Importantly, pharmacological inhibition of HuR by MS-444 inhibits HuR homodimerization and its cytoplasmic translocation, abrogates hypoxia-induced PIM1 overexpression and markedly enhances PDA cell sensitivity to oxaliplatin and 5-fluorouracil under physiologic low oxygen conditions. Taken together, these results support the notion that HuR has prosurvival properties in PDA cells by enabling them with growth advantages in stressful tumor microenvironment niches. Accordingly, these studies provide evidence that therapeutic disruption of HuR's regulation of PIM1 may be a key strategy in breaking an elusive chemotherapeutic resistance mechanism acquired by PDA cells that reside in hypoxic PDA microenvironments.
The epithelial-mesenchymal transition (EMT) is a crucial morphological event that occurs during the progression of epithelial tumors. EMT can be induced by transforming growth factor beta (TGF-beta) in certain kinds of cancer cells through the induction of Snail, a key regulator of EMT. We have previously found that TGF-beta remarkably induces Snail expression in cooperation with Ras signals; however, the underlying mechanism of this synergism has not yet been determined. Here, we demonstrate that signal transducer and activator of transcription 3 (STAT3) acts as a mediator that synergizes TGF-beta and Ras signals. The overexpression of STAT3 enhanced Snail induction, whereas siRNA-mediated knockdown of STAT3 inhibited it. The STAT3-gamma F mutant, which has Tyr 705 substituted with Phe, did not enhance Snail induction. Several STAT3 mutants lacking transcriptional activity also failed to enhance it; however, the putative STAT3-binding elements in the Snail promoter regions were not required for STAT3-mediated Snail induction. Protein inhibitor of activated STAT3 (PIAS3) inhibited the enhanced Snail promoter activity induced by TGF-beta and Ras. The interaction between PIAS3 and STAT3 was reduced by TGF-beta in cells harboring oncogenic Ras, whereas TGF-beta promoted the binding of PIAS3 to Smad3, a crucial mediator of TGF-beta signaling. Therefore, these findings suggest that STAT3 enhances Snail induction when it is dissociated from PIAS3 by TGF-beta in cooperation with Ras signals.
High thymidylate synthase (TS) level in cancer tissue is considered to result in resistance to pemetrexed therapy for advanced stages of nonsquamous non-small cell lung cancers. To further investigate the mechanism of pemetrexed resistance and potential prognostic outcomes in lung cancer, we established pemetrexed-resistant lung adenocarcinoma cell sublines from CL1 harboring a mutated TP53 gene (R248W) and A549 harboring wild-type TP53. We found the TS expression is upregulated in both pemetrexed-resistant sublines and the reduced TS level achieved through shRNA inhibition resulted in higher pemetrexed sensitivity. We also demonstrated that the acquisitions of pemetrexed resistance enhances epithelial mesenchymal transition (EMT) in vivo with a mice animal model and in vitro with CL1 and A549 sublines, which was associated with upregulation of ZEB1 which, in turn, downregulates E-cadherin and upregulates fibronectin. When ERK1/2 phosphorylation was reduced by an inhibitor (U0126) or siRNA inhibition, both pemetrexed-resistant sublines reduced their migration and invasion abilities. Therefore, the ERK-mediated pathways induce apoptosis with pemetrexed treatment, and may in turn mediate EMT when cancer cells are resistant to pemetrexed. We further demonstrated that the growth of pemetrexed-resistant tumors could be inhibited by vinblastine in vivo and vincristine in vitro. Our data indicate that pemetrexed resistance could be relieved by non-cross-resistant chemotherapeutic drugs such as vinca alkaloids and might be independent to TP53 status. Furthermore, the phosphorylation of ERK was reduced by vincristine. This finding provides a new insight for overcoming pemetrexed resistance and metastasis by application of vinca alkaloids.
The hyperactivated Wnt/beta-catenin signaling acts as a switch to induce epithelial to mesenchymal transition and promote colorectal cancer. However, due to its essential role in gut homeostasis, therapeutic targeting of this pathway has proven challenging. Additionally, IL-6/Stat-3 signaling, activated by microbial translocation through the dysregulated mucosal barrier in colon adenomas, facilitates the adenoma to adenocarcinomas transition. However, inter-dependence between these signaling pathways and key mucosal barrier components in regulating colon tumorigenesis and cancer progression remains unclear. In current study, we have discovered, using a comprehensive investigative regimen, a novel and tissue-specific role of claudin-3, a tight junction integral protein, in inhibiting colon cancer progression by serving as the common rheostat of Stat-3 and Wnt-signaling activation. Loss of claudin-3 also predicted poor patient survival. These findings however contrasted an upregulated claudin-3 expression in other cancer types and implicated role of the epigenetic regulation. Claudin-3 -/- mice revealed dedifferentiated and leaky colonic epithelium, and developed invasive adenocarcinoma when subjected to colon cancer. Wnt-signaling hyperactivation, albeit in GSK-3 beta independent manner, differentiated colon cancer in claudin-3 -/- mice versus WT-mice. Claudin-3 loss also upregulated the gp130/IL6/Stat3 signaling in colonic epithelium potentially assisted by infiltrating immune components. Genetic and pharmacological studies confirmed that claudin-3 loss induces Wnt/beta-catenin activation, which is further exacerbated by Stat-3-activation and help promote colon cancer. Overall, these novel findings identify claudin-3 as a therapeutic target for inhibiting overactivation of Wnt-signaling to prevent CRC malignancy.