Current Enzyme Inhibition - Volume 3, Issue 4, 2007

Several xenobiotics cause oxidative DNA damage by reactive oxygen species (ROS) induction. The antioxidant defence system, including antioxidant enzymes, radical scavengers and chain breakers, limits cell injury induced by ROS. In particular the antioxidants such as reduced glutathione (GSH) and ascorbic acid (AA) and the antioxidant enzymes superoxide dismutase (SOD), catalase, glutathione peroxidase (GPx), glutatione reductase (GR) are implicated in antioxidant cellular response. Oxidative stress occurs when there is an imbalance between production of ROS and cellular antioxidant capacity or when there is a decrease in this capacity. Failure of antioxidant systems may lead to mutagenic oxidative DNA damage as well as deregulation of cell cycle control, resulting in carcinogenesis. In the last years oxidative DNA damage as consequence of anti-oxidant system reduced activity or inhibition has been widely studied by comet test, a rapid and sensitive technique that allows to evaluate DNA damage and its repair on single whole cell. In particular Comet assay modified with Fpg (formamidopyrimidine glycosylase) or endo III enzymes, that recognize and cut oxidized DNA bases, allows to evaluate oxidative DNA damage measuring oxidized purines and pyrimidines respectively. Comet assay also permits to study oxidative DNA repair evaluating the persistence of oxidative DNA damage in relation to repair time, performing the test on cells immediately after induction of DNA damage and after DNA recovery time. Recent studies indicate that oxidant-generating enzymes such as inducible nitric oxide synthase (iNOS) and the inducible cyclooxygenase- 2 (COX-2) are associated with growth and progression of tumour malignancy acting as mediators of inflammation, angiogenesis inductors and cancer proliferation promoters. It allowed to develop new anticancer treatments based on COX-2 inhibitors also in combination with radiation or antineoplastic drugs that may reduce side effects of cancer therapy.

The activation and transduction of several signalling pathways are dependent on tyrosine phosphorylation. The non-transmembrane protein tyrosine phosphatase SHP-2 (Src homology 2 domain containing tyrosine phosphatase 2) has been shown to be involved in several signalling pathways initiated by different growth factors, cytokines, hormones and extracellular matrix receptors. SHP-2 directly interacts with several growth factors, cell surface adhesion molecules and different adaptor molecules such as the Grb 2 associated binder 1 (Gab-1), Grb2 and the insulin receptor substrate 1 (IRS- 1). It has been shown to be required for activation of the mitogen activated protein kinase (MAP-Kinase) pathway upon fibroblast growth factor (FGF), epidermal growth factor (EGF) and insulin stimulation. Moreover, SHP-2 has been found to influence the phosphoinositide 3-Kinase (PI3-Kinase)/kt pathway upon stimulation with EGF, insulin like growth factor (IGF) and platelet derived growth factor (PDGF), thus affecting cell survival. SHP-2 also plays a negative role in certain signalling pathways, such as the janus activated kinase (JAK)-signal transducers and activators of transcription (STAT) pathway. Recently, SHP-2 has become clinically relevant as germ-line missense mutations in the gene encoding SHP-2 (Ptpn11) have been found to cause the developmental disorders Noonan syndrome and the Leopard syndrome. Other mutations in this gene lead to myeloid and lymphoid malignancies. Moreover, SHP-2 has also been implicated to play a role in diabetes and in the development of gastric adenocarcinoma following H. Pylori infection. This article deals with the role of SHP-2 in different signalling pathways and the involvement of SHP-2 in human disorders.

Protein Tyrosine kinases (TKs) play important roles in regulating the most fundamental cell processes, such as the cell cycle, proliferation, differentiation, motility, and cell death or survival. In many tumor cells, key TKs may no longer be adequately controlled, and excessive phosphorylation sustains signal transduction pathways in an activated state. Imatinib mesylate is an oral multitargeted tyrosine kinase inhibitor with antitumor activity. It recently received approval from the US Food and Drug Administration for the treatment of patients with BCR/ABL translocation defining chronic myeloid leukaemia, and subsequently for the treatment of patients with KIT (CD117)-positive non-resectable and/or metastatic malignant gastrointestinal stromal tumors. It has also shown promising clinical activity against other advanced solid tumors. The review provides an updated summary of emerging clinical experience with this promising new anticancer agent.

Endothelial dysfunction due to reduced bioavailability of nitric oxide (NO) is an early step in the course of atherosclerotic cardiovascular disease. NO is synthesized from L-arginine via the action of NO synthase, which is known to be blocked by endogenous L-arginine analogues such as asymmetric dimethylarginine (ADMA). ADMA is a naturally occurring amino acid found in plasma and various types of tissues. Plasma level of ADMA is reported to be associated with cardiovascular risk factors such as chronic kidney disease (CKD), being a strong predictor for cardiovascular disease and the progression of renal dysfunction in these patients. In this review, we discuss the molecular mechanisms for the elevation of ADMA levels in CKD. We also review here the pathological role for ADMA in cardiovascular complications in patients with CKD.

Phospholipases A2 (PLA2) comprise a set of extracellular and intracellular enzymes that catalyze the hydrolysis of the sn-2 fatty acyl bond of phospholipids to yield fatty acids and lysophospholipids. The PLA2 reaction is the primary pathway through which arachidonic acid (AA) is released from phospholipids. PLA2s have an important role in cellular death that occurs via necrosis or apoptosis. Reactive oxygen species are known to contribute to tissue damage during injury and inflammation. However, the species can also be sensed by the cells and trigger intracellular signaling cascades during cell death. This review examines recent evidence on the involvement of reactive oxygen species in lipid signaling during apoptosis and necrosis. Attention is focused on activation of PLA2s and lipoxygenases, which play important roles in mediating AA release and metabolism. The participation of several types of PLA2s in AA mobilization from phospholipids is discussed. The involvement of alternative routes for AA mobilization under oxidative stress is also considered.

Mitogen-activated protein kinases (MAPKs) are a family of serine-threonine kinases that respond to various extracellular signals and are involved in many cellular processes. The MAPK family consists of four major groups extracellular signal regulated kinase 1/2 (ERK1/2), c-Jun N-terminal kinase (JNK)/stress-activated protein kinase (SAPK), p38 and ERK5. Additional MAPKs (ERK3, ERK4, ERK7, ERK8) have been identified on the basis of their homology with the ERK1/2 sequence but their functions and activation have not yet been fully described. MAPKs are activated by a “three kinase cascade” and after activation they phosphorylate specific cytoplasmic and nuclear substrates. MAPK activity is specifically regulated by phosphatases and by interaction with scaffold and/or anchor proteins. MAPK inhibitors are useful tools for studying MAPK requirements in physiological and pathological processes and it is thought that they may constitute a promising new therapeutic strategy for the treatment of tumors, inflammatory and neurodegenerative diseases. MAPK inhibitors are specific to each member of the MAPK family and can act at different levels of the MAPK cascades. These inhibiting molecules may be ATP-competitive or ATP-noncompetitive depending on their binding sites. Other classes of MAPK inhibitors are represented by peptide inhibitors whose sequences derive from scaffold protein sequences, and by low molecular weight compounds that interact with specific MAPK docking domains. MAPKs play an important role in the nervous system. In vitro studies using cell lines and primary neuronal cultures have demonstrated that MAPKs play a crucial role in neuronal survival and differentiation, apoptotic and non-apoptotic neuronal death, neuronal plasticity, learning and memory. In this review, we summarize the studies in which MAPK involvement in neuronal differentiation and neuritogenesis of different cellular models has been demonstrated by MAPK inhibitors.

One of the most promising strategies for drug design and development is the identification of new molecules able to selectively inhibit those enzymes involved in pathological processes, without affecting other enzymes associated with physiological functions. Nevertheless, some enzymes can show a double-edge aspect of their own inhibition, which can lead to positive as well as negative consequences according to the pathological state. Phospholipase D (PLD), an ubiquitous enzyme nowadays considered as a critical regulator of several aspects of cell biology and signal transduction pathways, is a clear example of those double-dealer enzymes. While a great deal has been learned about PLD structure, biological functions and activation/regulation mechanisms, little yet is known about the derivable effects of its potential negative regulation, also due to the lack of specific inhibitors. Multiple evidences on PLD involvement in many pathological states development and progression, including inflammation, carcinogenesis and metastases, have been supplied, so that a deregulation of its activity could contribute to attenuate or slow down the inflammatory and tumour formation/ progression processes. On the other hand, in agreement with other previous observations, we have recently demonstrated the direct contribution of PLD activation in promoting intracellular mycobacterial killing. In this case, PLD inhibition resulted in a significant reduction of antimicrobial innate immune response and, hence, in a possible harmful effect. In the light of the above reported considerations, besides the recent advances in characterising new compounds able to selectively inhibit PLD activity and/or signalling, this review aimed at elucidating the potential dual beneficial/harmful consequences of PLD activity modulation.