Current Enzyme Inhibition - Volume 4, Issue 1, 2008

Protein tyrosine phosphatases (PTPs) and their inhibitors have been more and more studied during the past decades. Dephosphorylation is implicated in many biological events including the progression of the cell cycle. Around two hundred PTPs are known in humans, divided into three main groups. They all have a common amino acid sequence in their active site, referred to as the C(X)5R motif, namely a cysteine followed by five varying residues and an arginine. The CDC25 and CDC14 families are examples of PTPs described for their potential in cancer treatment, since they are key regulators of the cell cycle progression. CDC25 inhibitors have already proved their antiproliferative properties whereas the effect of the inhibition of CDC14 remains to be studied.The current review describes how the homology of the active site among the PTPs leads to similarities in their mechanism of action, regulation and inhibition. These similarities make it possible for medicinal chemists to design inhibitors based on the knowledge acquired on PTP1B inhibitors.

Septic shock is a major cause of death following trauma and a persistent problem in surgical patients. It is a challenge to the critical care medicine specialist and carries an unacceptable high mortality rate, despite adequate antibiotic and vasopressor therapies. The prevalent hypothesis regarding its mechanism is that the syndrome is caused by an excessively defensive and inflammatory response. During the acute phase, some signaling mechanisms are activated, particularly changes in body temperature and hormonal release, which function to restore the host homeostasis that has been disturbed by the infection. Since the neuroendocrine and immune systems are functionally related, the exposure to antigens induces a synchronized response, which allows the organism to successfully endure immunologic changes. An important characteristic of this communication includes the appearance of proteins released into the circulation by activated immune cells. These proteins, called cytokines, may enter the circulation and reach neuroendocrine organs, where they either act, themselves or through the release of intermediates such as nitric oxide (NO). NO is a gaseous and free radical molecule, synthesized by nitric oxide synthases (NOS) during conversion of L-arginine in L-citrulline. These enzymes involve three different isoforms, neuronal, endothelial and inducible. The first and most common function attributed to NO is its role in controlling vascular tonus. Besides vasodilatation and antimicrobial defense, it has been demonstrated that NO has other functions, including body temperature control. The synthesis of NO may also be induced in the brain as a consequence of infection and may alter the function of the hypothalamic-pituitary axis. In this review we propose to discuss the pathophysiological effects of NOS inhibition in body temperature regulation during experimental sepsis and septic shock.

Aberrant arachidonic acid metabolism has been recognized as a dominant mechanism underlying the development and progression of a range of human cancers. Metabolism of arachidonic acid through the 5-lipoxygenase (5-Lox) pathway generates an array of highly reactive eicosanoids categorized into two distinct groups; the leukotrienes (LTs) and the 5-hydroxyeicosatetraenoids (5-HETEs). Evidence documenting prominent roles of these two groups of eicosanoids in the development and progression of cancer are accumulating rapidly both by in vitro cell culture experiments using cancer cells originating from many tissues, and by in vivo tumor model studies. Moreover, significantly higher levels of expression of 5-Lox has been observed in patient tumor specimens in grade-specific manner, and production of elevated levels of 5-Lox metabolites were observed in cancer cells compared to normal counterparts. Thus, 5-Lox has emerged as a new potential target for the prevention and treatment of cancer. Metabolites of 5-Lox are potent signaling molecules and elicit diverse biological activities that are involved in neoplastic transformation and progression, such as cell proliferation, invasion and motility, and apoptosis-resistance. Blocking 5-Lox activity by pharmacological agents will deprive cancer cells of the critical metabolites for their survival and growth, and thus may be effective in the prevention as well as treatment of cancer. Indeed some studies have already shown promise for using 5-Lox inhibitors for the prevention and treatment of cancers of the prostate, lung, pancreas, brain, esophagus, oral cavity, etc.

Because of its role in the T cell response, pharmacological inhibition of calcineurin is clinically useful to suppress the immune system following organ transplantation. Introduction of calcineurin inhibitors (CIs) in the early 1980s resulted in a profound improvement in graft survival. For this reason, calcineurin inhibitors have become a cornerstone for post-transplant management. CIs are also used to treat an increasingly wide array of immune-related disorders. However, inhibition of calcineurin is known to affect tissues other than the immune system and long-term use of CIs often produces therapeutically-limiting side-effects; the most common of which is nephrotoxicity. An additional complex issue arises with the use of CI during pregnancy as the role of calcineurin in development is still incompletely understood. Survey of the literature reveals a growing number of studies demonstrating direct links between calcineurin action and development, particularly of the immune system and kidney structures. Data thus far point to important roles for calcineurin in regulation of transcription, cell hypertrophy, extracellular matrix accumulation, and apoptosis effectively linking observed therapeutic side-effects with direct actions of calcineurin in immune and renal cells. Thus, it seems prudent to review what is known about calcineurin inhibition in animal models with an emphasis on developmental affects and to then compare with the albeit limited data available from human subjects. Just as laboratory research is demonstrating mechanistic explanations for some features of calcineurin inhibitor nephrotoxicity, so to may laboratory results yield important information about the contribution of calcineurin to development.

Endothelial cells (ECs) regulate blood flow and pressure, platelet aggregation, and angiogenesis. Chemical (e.g. acetylcholine, bradykinin) and mechanical stimuli (linear shear stress and pulse pressure) enhance endothelial production of nitric oxide (NO). In the mammalian three isoforms of NO synthase [endothelial (eNOS), inducible (iNOS) and neuronal (nNOS)] are expressed in different cell types showing a specific subcellular compartmentalization with colocalized effectors. The NOS-guanylyl-cyclase-protein kinase G (PKG) system plays a key role in mediating specific signaling involved in both short- and long-term control of several functions. In particular, mechanical-induced NO release may act as an amplifier of the local metabolic vasodilator(s) during exercise. It is likely that mechanical-induced increase in eNOS and extra-cellular superoxide dismutase expression and activity contribute to the augmentation of endothelial function during exercise training. NO-induced cGMP activates PKG, which phosphorylates other kinases to regulate their activity, leading to a reduction of cytosolic Ca2+ and, thus, to vascular relaxation. c-Src and other kinases are also activated and play a pivotal role as intracellular signaling molecules during exercise training, which may also lead to genomic and nongenomic changes. Endothelial dysfunction is a component of atherogenic activity, diabetes and other vascular risk factors which lead to coronary heart disease and heart failure. In patients, training may improve NO production that explains beneficial effects of exercise, such as lowered lipoprotein level, increased vasodilatation and reduced vasoconstriction. The improved outcomes may be due, in part, to the positive effect of exercise on NOSs function, kinase activation and, consequently, on endothelial dependent vasodilatation.

To indicate the importance of fungal enzyme inhibitors (FEI) it is only required to mention penicillin. However, many other natural products from fungi have been described. Combinatorial chemistry (CC) is not providing the medicines that were predicted and it is time to return to natural products (NP). The inhibitions of enzymes by FEI run the gamut of medicinal applications and toxicity. The field is, or should be, “big science”, although many areas are underfunded partly from an overemphasis of CC, and from ignorance of fungi as a whole. Furthermore, the use of PCR with fungi is growing exponentially, without regard to adequate controls because of FEI. This leads to false negative results (FNR) which is the worst possible outcome in diagnostic mycology. The field of mycotoxin research is huge and related to NP discovery. Much of the toxicity relates to FEI. In terms of therapeutics, the fungal compounds can be used against cancer, diabetes, poisonings, Alzheimer's disease, etc. The forms of inhibitions include acetylcholinesterase, nuclear factor- kappa B, protein kinase, tyrosine kinase, aromatase and sulphatase, matrix metalloproteinases, cyclooxygenase, DNA polymerase/topoisomerases, glycosidases and more. Endophytic fungi may be sources of undetected compounds. Fungi can also transform precursors into potential inhibitors. The time is right for a review in the field of FEI.