Topoisomerase I inhibition in colorectal cancer: biomarkers and therapeutic targets

The topoisomerase I (Top 1) poison irinotecan is an important component of the modern treatment of colorectal cancer. By stabilising Top 1-DNA complexes, irinotecan generates Top 1-linked DNA single-strand breaks that can evolve into double-strand breaks and ultimately cause cell death. However, cancer cells may overcome cell killing by releasing the stalled topoisomerase from DNA termini, thereby reducing the efficacy of Top 1 poisons in clinics. Thus, understanding the DNA repair mechanisms involved in the repair of Top 1-mediated DNA damage provides a useful tool to identify potential biomarkers that predict response to this class of chemotherapy. Furthermore, targeting these pathways could enhance the therapeutic benefits of Top 1 poisons. In this review, we describe the cellular mechanisms and consequences of targeting Top 1 activity in cells. We summarise preclinical data and discuss the potential clinical utility of small-molecule inhibitors of the key proteins.     

Topoisomerase inhibition by phenolic metabolites: a potential mechanism for benzene's clastogenic effects

Exposure to benzene, a human and animal carcinogen, resultsin the formation of structural chromosomal aberrations in thebone marrow and blood cells of animals and humans. The mechanismsunderlying these clastogenic effects are unknown. Inhibitionof enzymes involved in DNA replication and repair, such as topoisomeraseenzymes, by the metabolites of benzene represents a potentialmechanism for the formation of chromosomal aberrations. To testthis hypothesis, the inhibitory effects of various phenolicand qulnone metabolites of benzene on the activity of humantopoisomerases I and II were studied in vitro. No inhibitionof topolsomerase I was seen with any of the tested metabolites.Inhibitory effects on topoisomerase II were not observed forhydroqulnone, phenol, 2,2'-biphenol, 4,4'-biphenol and catecholat concentrations as high as 500 µM. 1,4-Benzoquinoneand 1,2,4-benzenetrlol inhibited topoisomerase II at relativelyhigh 500 and 250 µM concentrations, respectively. Howeverfollowing bioactlvation using a peroxldase/H₂O₂ system, Inhibitoryeffects were seen at concentrations as low as 50 µM forboth phenol and 2,2'-biphenol and 10 µM for 4,4'-biphenol.The addition of reduced glutathione (GSH) to the 4,4'-biphenoland horseradish peroxidase reaction system protected topoisomerase II from inhibition suggesting that diphenoquinone oranother oxidation product formed from 4,4'-biphenol might bethe reactive species. These in vitro results indicate that inhibitionof topolsomerase II may contribute to the clastogenic and carcinogeniceffects of benzene. In addition, metabolites formed from thesephenolic compounds appear to represent several new types oftopolsomerase II-Inhibiting compounds.     

Topoisomerase I-mediated cytotoxicity of N-methyl-N'-nitro-N-nitrosoguanidine: trapping of topoisomerase I by the O6-methylguanine

Alkylating agents such as N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) are known to covalently link alkyl groups at the position 6 of guanines (O6MG) in DNA. O6-alkylguanine-DNA alkyltransferase (AGT) specifically removes the methyl group of the O6MG. Using purified human topoisomerase I (Top1), we found an 8-10-fold enhancement of Top1 cleavage complexes when O6MG is incorporated in oligonucleotides at the +1 position relative to a unique Top1 cleavage site. Top1 poisoning by O6MG is attributable to a decrease of the Top1-mediated DNA religation as well as an increase in the enzyme cleavage step. Increased cleavage is probably linked to a change in the hydrogen bonding pattern, such as in the case of the 8-oxoguanine, whereas inhibition of religation could be attributed to altered base pairing, such as abasic sites or base mismatches, because incorporation of a 6-thioguanine did not affect Top1 activity. Top1-DNA covalent complexes are also induced in MNNG-treated CHO cells constitutively lacking the AGT enzyme. Conversely, no increase could be detected in CHO cells transfected with the wild-type human AGT. Moreover, we show that yeasts overexpressing the human Top1 are more sensitive to MNNG, whereas knock-out Top1 strain cells display some resistance to the drug. Altogether, these results suggest a role for Top1 poisoning by alkylated bases in the antiproliferative activity of alkylating agents as well as in the DNA lesions resulting from endogenous and carcinogenic DNA modifications.     

Unraveling the role of hypoxia-inducible factor in renal cell carcinoma: a biological and therapeutic perspective

The genetic control of hypoxia-inducible factor (HIF) has long been associated with the pathogenesis of clear cell renal cell carcinoma. Only recently have the complex genetics and biology of relevant HIF subtypes been unraveled, leading to potential novel strategies for treating this disease.     

缺氧诱导因子-1在肿瘤临床方面的研究进展

缺氧诱导因子-1(HIF-1)是介导缺氧适应性反应的转录因子,其活性的调节受多种因素的影响,其表达与肿瘤的生长、浸润、转移和预后等密切相关,并对肿瘤的治疗具有重要意义.现就其在肿瘤临床方面的研究进展进行综述.     

Regulation of cancer cell metabolism by hypoxia-inducible factor 1

The induction of hypoxia-inducible factor 1 (HIF-1) activity, either as a result of intratumoral hypoxia or loss-of-function mutations in the VHL gene, leads to a dramatic reprogramming of cancer cell metabolism involving increased glucose transport into the cell, increased conversion of glucose to pyruvate, and a concomitant decrease in mitochondrial metabolism and mitochondrial mass. Blocking these adaptive metabolic responses to hypoxia leads to cell death due to toxic levels of reactive oxygen species. Targeting HIF-1 or metabolic enzymes encoded by HIF-1 target genes may represent a novel therapeutic approach to cancer.     

Growth factor receptors signaling in glioblastoma cells: therapeutic implications

In this study, we investigated the protein expression of platelet-derived growth factor receptor (PDGFR), insulin like growth factor-1 receptor (IGF-1R), phosphatidylinositol 3-kinase (PI3-K) and extracellular signal-regulated kinase (ERK1/2) in five primary glioblastoma (GB), with a view to their possible use as therapeutic targets. Our results demonstrated that appreciable levels of these proteins could be detected in the analysed GB cell lines, except for a low level of PDGFR and ERK1/2 expression in one GB cell line. The small molecule inhibitors towards IGF-1R, PDGFR, PI3-K and ERK1/2 respectively, have only modest or no anti-tumour activity on GB cells and therefore their combination with other therapy modalities was analysed. The interaction between small inhibitors and radiation was mostly additive or sub-additive; synergistic interaction was found in five of forty analysed combinations. Our results showed that GB cells are in general resistant to treatment and illustrate the difficulties in predicting the treatment response in malignant gliomas.     

缺氧诱导因子-1在肿瘤发生与治疗中的作用

缺氧诱导因子-1(HIF-1)是机体的一种重要转录因子,直接调控肿瘤细胞对低氧环境的适应,影响肿瘤的血管生成及能量代谢.HIF-1的表达和肿瘤生长与转移有密切的关系,与肿瘤细胞凋亡也有一定的联系.干扰HIF-1的合成及稳定性可以有效抑制肿瘤的生长与转移,并且成倍增加肿瘤细胞对化疗的敏感性.HIF-1可望在将来成为治疗肿瘤的有效靶点之一.     

Regulation of colon carcinoma cell invasion by hypoxia-inducible factor 1

Hypoxia-inducible factor 1 (HIF-1) transactivates genes the products of which mediate tumor angiogenesis and glycolytic metabolism. Overexpression of the HIF-1 alpha subunit, resulting from intratumoral hypoxia and genetic alterations, has been demonstrated in common human cancers and is correlated with tumor angiogenesis and patient mortality. Here we demonstrate that hypoxia or HIF-1 alpha overexpression stimulates Matrigel invasion by HCT116 human colon carcinoma cells, whereas this process is inhibited by a small interfering RNA directed against HIF-1 alpha. We show that HIF-1 regulates the expression of genes encoding cathepsin D; matrix metalloproteinase 2; urokinase plasminogen activator receptor (uPAR); fibronectin 1; keratins 14, 18, and 19; vimentin; transforming growth factor alpha; and autocrine motility factor, which are proteins that play established roles in the pathophysiology of invasion. Neutralizing antibodies against uPAR block tumor cell invasion induced by hypoxia or HIF-1 alpha overexpression. These results provide a molecular basis for promotion of the invasive cancer phenotype by hypoxia and/or HIF-1 alpha overexpression.     

SETDB1 in cancer: overexpression and its therapeutic implications

SET Domain Bifurcated Histone Lysine Methyltransferase 1 (SETDB1, ESET, KMT1E) is a H3K9 methyltransferase involved in gene silencing. In recent years, SETDB1 has been implicated as an oncogene in various cancers, highlighting a critical need to better understand the mechanisms underlying SETDB1 amplification, overexpression, and activation. In the following review, we first examine the history of SETDB1, starting from its discovery in 1999 and ending with recent findings. We follow with an outline of the structure and subcellular location of SETDB1, as well as potential mechanisms for regulation of its nuclear transport. Subsequently, we introduce SETDB1’s various functions, including its roles in promyelocytic leukemia nuclear body (PML-NB) formation, the methylation and activation of Akt, the silencing of the androgen receptor (AR) gene, retroelement silencing, the inhibition of tumor suppressor p53, and its role in promoting intestinal differentiation and survival. The Cancer Cell Line Encyclopedia (CCLE) screened SETDB1 dependency in 796 cancer cell lines, identifying SETDB1 as a common essential gene in 531 of them, demonstrating that SETDB1 expression is critical for the survival of the majority of cancers. Therefore, we provide a detailed review of the oncogenic effects of SETDB1 overexpression in breast cancer, non-small cell lung cancer, prostate cancer, colorectal cancer, acute myeloid leukemia, glioma, melanoma, pancreatic ductal adenocarcinoma, liver cancer, nasopharyngeal carcinoma, gastric carcinoma, and endometrial cancer. Accordingly, we review several methods that have been used to target SETDB1, such as using Mithramycin A, Mithralog EC-8042, 3’-deazaneplanocin A (DZNep), and paclitaxel. Finally, we conclude by highlighting remaining gaps in knowledge and challenges surrounding SETDB1. Ultimately, our review captures the wide scope of findings on SETDB1’s history, function, its implications in cancer, and provides suggestions for future research in the field.