Current Enzyme Inhibition - Volume 1, Issue 2, 2005

An increase in the expression and activity of both inducible nitric oxide synthase (iNOS) and cyclooxygenase-2 (COX-2) has been shown in several types of human tumors. A large body of evidence has demonstrated that these two enzymes are involved in tumor progression through several molecular mechanisms, such as promotion of tumor cell proliferation, inhibition of apoptosis and stimulation of angiogenesis. iNOS and COX-2 share a number of similarities in terms of pathophysiological phenomena and are often co-expressed in cancer tissues. The product of iNOS, nitric oxide (NO), has been demonstrated to modulate COX-2 expression and prostaglandin production in both inflammatory and tumor experimental models. Cyclic GMP and peroxynitrite, the coupling product of NO and superoxide, appear as the most important pathways by which NO may regulate COX-2 expression. We have recently shown that both NO- and COX-2-related angiogenesis is mediated by an increase in VEGF production in colorectal cancer. We also provided evidence that NO can stimulate COX activity and that its pro-angiogenic effect is mainly mediated by COX-2-related PGE2 production. The purpose of this review is to summarize experimental data on the molecular mechanisms underlying iNOS-COX-2 cross-talk and investigate the pathophysiological significance of this interaction in cancer. Given the availability of highly selective inhibitors of both iNOS and COX-2, dual inhibition of these enzymes appears as a promising therapeutic tool in the treatment of various types of human cancers by producing a possible synergistic anti-tumor effect.

Chronically elevated glucocorticoid concentrations, either systemically or locally, are associated with adverse metabolic effects, including osteoporosis, Cushing's syndrome, obesity, dyslipidemia, type 2 diabetes, and cardiovascular disease. Elevated glucocorticoid levels impair insulin and leptin sensitivity, ultimately resulting in the development of the metabolic syndrome. β-hydroxysteroid dehydrogenase type 1 (β-HSD1) catalyzes the oxoreduction of inactive 11-ketoglucocorticoids (cortisone in humans, 11- dehydrocorticosterone in rodents) to active β-hydroxyglucocorticoids (cortisol in humans, corticosterone in rodents) and regulates the local activation of glucocorticoid receptors (GR). Studies in transgenic mice demonstrated a causal role of β-HSD1 for the development of the metabolic syndrome. In humans, β-HSD1 expression and visceral obesity positively correlate, making this enzyme a promising target for therapeutic interventions. Administration of a selective synthetic non-steroidal β-HSD1 inhibitor lowered circulating glucose and increased insulin sensitivity in hyperglycemic mouse models. Moreover, agonists of PPARg and LXRα, used in diabetes treatment, decrease β-HSD1 expression, suggesting that some of the beneficial metabolic effects of these anti-diabetic drugs may be due to reduced local GR activation. In addition to its role in the activation of glucocorticoids, β-HSD1 catalyzes the detoxification of reactive carbonyl-compounds, including the major dietary oxysterol 7-ketocholesterol, the potent tobacco carcinogen nicotine-derived nitrosamine ketone (NNK) and the anti-cancer drug oracin. To achieve efficient and safe therapeutic treatment further studies have to address the structure-function relationship of β-HSD1 and the pathophysiological relevance of this enzyme in detoxification processes. Appropriate dosing and tissue-specific delivery or activity of highly selective β-HSD1 inhibitors may be required for successful clinical applications.

This review will focus on glutathione depletion performed in different biological systems to elucidate molecular implications of this tripeptide on physiological and pathological processes. Cellular and molecular changes produced in liver, brain and kidney by glutathione depleting drugs, such as D,Lbuthionine- S,R-sulfoximine, menadione and acetaminophen, will be described. Special attention will be given to the inhibitory mechanisms produced by those drugs on intestinal calcium absorption and alkaline phosphatase activity. The role of DL-buthionine-S,R-sulfoximine and menadione on mechanisms of apoptosis, reactive oxygen species production and mitochondrial function from mammalian breast cancer cells will be analyzed. Finally, reversion or prevention of some of the previous effects by administration of lipoic acid, glutathione monoester, KR31378 and 2(RS)-n-propylthiazolidine-4(R)-carboxylic acid will be discussed.

Arachidonic acid (AA) metabolites are lipid signalling mediators that play a central role in a broad array of physiological and pathophysiological processes, including cell proliferation and differentiation. AA metabolism diverges into two main pathways, the cyclooxygenase (COX) pathway, which leads to prostaglandin and thromboxane production, and the lipoxygenase (LOX) pathway, which leads to the leukotriene and hydroxyeicosatetraenoic acid production. These inflammatory molecules exert profound biological effects that enhance the development and progression of human cancers. A large number of synthetic drugs and natural molecules inhibit the enzymes involved in these pathways and thus prevent, delay or reverse inflammation. They may also prove useful in the chemoprevention of cancer. These studies have primarily used non-steroidal anti-inflammatory drugs, which block the COX pathway. Recent pre-clinical studies indicate that the LOX pathway is also a key target for cancer prevention strategy. This article reviews the role of COX and LOX inhibitors in cell proliferation and cancer.

Organophosphorus (OP) compounds are extensively used in widespread applications, causing considerable poisonings around the world. Current treatments against poisoning by organophosphates, nerve agents, or other acetylcholinesterase (AChE) inhibitors are based on the intravenous or intramuscular administration of a muscarinic antagonist (atropine), a cholinesterase reactivator (an oxime), and an anticonvulsant (a benzodiazepine like diazepam). Since these antidotes have serious adverse effects, the prophylaxis against OP intoxication is based principally on the administration of reversible cholinesterase inhibitors. However, evidence demonstrates that, in addition to the AChE inhibition, the toxicity of OP compounds is mediated by the generation of nitric oxide and other free radicals. These toxic molecules can be counteracted by antioxidants such as vitamins C and E, spin traps, melatonin, and low-molecular-weight thiols. The latter compounds can also increase the synthesis of glutathione, which can both ameliorate the OP-induced oxidative stress and enhance OP detoxification. Therefore, the low side effects of antioxidants make them suitable for pretreatment against a potential OP poisoning and might be used in combination with the standard treatments. The present review focuses on how OP compounds cause oxidative stress, apoptosis, and necrosis; how cells respond to these processes; and how antioxidants can improve OP poisoning.

Common (surface) epithelial cancer of the ovary is epitomized by metastasis to organs of the abdominal cavity. Proteolytic enzymes, predominately those of the matrix metalloproteinase and plasminogen activator-plasmin systems, have been implicated as mediators of tissue invasion. This review highlights the salient features of ovarian cancer progression, the proteases involved, and experimental approaches of inhibition. Enzyme inhibitors hold promise in the treatment of early-stage and recurrent (following cytoreductive surgery) disease.

Functional characterization of enzymes plays an essential role in one of the major areas of proteomics research: the modelling of sections of the cellular metabolism with a view to being able to model the whole cellular metabolism and the interaction of cells within tissues and organs. With these purposes in mind, the scientific community established a new branch within the life sciences, called systems biology. However, meaningful modelling, by necessity, requires comparable and reliable data from standardized enzyme characterizations. From a short, but detailed, investigation of the BRENDA database, it is shown here that the quality of experimental data of enzymes is insufficient for the needs of theoretical biology. Here we describe the dilemma of modern enzymology which generates functional data from enzymes under nonstandardized experimental conditions followed by suggestions how to remedy this situation by initiating broad discussions within the scientific community and introducing to a new initiative, called STRENDA.

Protein prenylation is a type of lipid modification involving attachment of either a farnesyl (15- carbon) or geranylgeranyl (20-carbon) isoprenoid group via thioether linkages to the cysteine residues at or near the C-terminus of various eukaryotic proteins. According to their substrate specificity the three known types of prenyltransferases can be categorized in two classes: the farnesyltransferase and geranylgeranyltransferase- I recognize CAAX motifs whereas geranylgeranyltransferase-II recognizes a non-CAAX motif. Protein substrates of prenyltransferases include G-proteins and small GTP-binding proteins, which are critical intermediates of cell signaling and cytoskeletal organization including the Ras protein. Activated Ras proteins trigger a cascade of phosphorylation events through sequential activation of the PI3 kinase/AKT pathway, which is critical for cell survival, and the Raf/Mek/Erk kinase pathway that is involved in cell proliferation. Because farnesylation of Ras is needed for its transforming and proliferative activity, protein-farnesyltransferase inhibitors (FTIs) provide a rational target of anticancer therapy. Like FTIs, geranylgeranyltransferase inhibitors can also be used as antitumor agents and are more effective as a part of combinational therapy with taxanes in blocking the proliferation of tumor cells. FTIs can also be used to treat several viral diseases like Hepatitis delta virus and Herpes simplex virus. FTIs can inhibit the growth of trypanosomal and malarial parasites. Inhibitors have been designed based on both the protein substrate and the isoprenoid diphosphate substrate.