Current Enzyme Inhibition - Volume 3, Issue 1, 2007

MET (hepatocyte growth factor receptor) and RON (recepteur d'origine Nantaise) are members of the MET proto-oncogene family of receptor tyrosine kinases (RTKs). Signalling from MET or RON activates multiple signalling pathways and ultimately promotes tumorigenesis and the formation of metastases. Mutations in MET have been detected in abundance in squamous cell carcinoma of the head and neck (SCCHN) metastases relative to the primary tumour, suggesting that this is a critical oncogene regulating dissemination. The biological significance of RON in SCCHN is still relatively unexplored. As survival has plateaued for patients with SCCHN, novel therapies with effects on the primary tumour and metastatic disease are urgently required. Small molecule inhibition of MET has been achieved in the pre-clinical setting and future clinical development is an exciting prospect. In this review, we summarise the biology of MET and RON RTKs and their contribution to an invasive tumour phenotype. We highlight their potential as therapeutic targets and address putative roles for MET and RON in resistance to conventional therapy, with particular reference to SCCHN.

Lysophosphatidic acid (LPA) and sphingosine 1-phosphate (S1P) are bioactive lysophospholipid mediators with a wide variety of biological actions. G-protein-coupled receptors for LPA and S1P have been identified, and physiological and pathological significances of the lysophospholipids and their receptors are under intensive investigation. Furthermore, specific agonists and antagonists for the receptors have been developed for clinical applications. However, mechanisms underlying their production have not yet been fully elucidated. Recently, autotaxin, an exo-phosphodiesterase implicated in tumor cell migration, has been discovered as lysophospholipase D that produces LPA and S1P from lysophosphatidylcholine and sphingosylphosphorylcholine, respectively. In this article, I reviewed the structure, expression, substrate specificity, enzymatic function, specific inhibitors and pathophysiological significances of lysophospholipase D/autotaxin.

In most organisms around 2% of the genes code for peptidases being this number only surmounted by the genes that code for transcriptional factors. This ubiquitous presence is almost unequaled and has for long fascinated biochemists. Although peptidases have been classified in four mechanistic classes (aspartic-, cysteine-, serine- and metallo-peptidases), the more recent MEROPS database recognizes 42 evolutionary distinct peptidase structures corresponding to 42 different families. As peptidases are involved in several physiological processes they are an obvious target for the development of therapeutic agents to treat infectious disease. The Trypanosomatidae family includes etiologic agents for human and veterinary diseases, such as Trypanosoma cruzi, Leishmania spp. and the African trypanosomes that are responsible for the Chagas disease, for a wide spectrum of clinical manifestations known as leishmaniasis, and for the “sleeping sickness”, respectively. These microorganisms present a complex life cycle that includes dimorphic developmental stages in distinct hosts and by extension show nutritional adaptation. This review covers the recent advances in the biochemical characterization of trypanosomatid proteolytic enzymes and that of specific inhibitors to block their hydrolytic activity, in accordance to the peptidases potential role as target to the treatment of the aforementioned illnesses.

Aldose reductase (E.C. 1.1.1.21), an intracellular enzyme of polyol pathway, catalyzes NADPHdependent reduction of glucose to sorbitol. Under normoglycemia, most of the cellular glucose is phosphorylated into glucose-6-phosphate by hexokinase. A minor part of non-phosphorylated glucose enters the polyol pathway, the alternate route of glucose metabolism. However, under hyperglycemia, because of saturation of hexokinase with ambient glucose, aldose reductase is activated, leading to excessive production of sorbitol. Intracellular accumulation of sorbitol is thought to result in irreversible damage. In the diabetic eye, the increased sorbitol accumulation in retina has been implicated in the pathogenesis of retinopathy, characterized by pericyte loss, basal membrane thickening, the major ocular complications of diabetes. Nearly all diabetic subjects have the same degree of retinopathy after 20 years of diabetes. 50% of patients with insulin dependent diabetes mellitus have proliferative retinopathy after 15 years. In addition, macular edema frequently produces central vision loss and blindness most commonly in non-insulin dependent diabetes mellitus. Therefore, aldose reductase enzyme inhibition is becoming one of the therapeutic strategies that have been proposed to prevent or ameliorate long-term diabetic complications.

We found that fat-soluble vitamins (i.e., vitamins A, D, E and K) have novel functions such as anticancer activity, and we therefore investigated the inhibitory activity of DNA polymerases (pols) from various species by vitamins and their related compounds. In vitamin A-related compounds, fucoxanthin, astaxanthin (provitamin A), all-trans retinal (vitamin A aldehyde) and all-trans retinoic acid (vitamin A acid) inhibited the activities of mammalian replicative pols α, δ and e with IC50 values of 18-190, 14-17 and 8-30 mM, respectively, and all-trans retinol (vitamin A) did not influence any pol activities. In vitamin D-related compounds, vitamins D2 and D3 were found to be selective inhibitors of calf pol a with IC50 values of 123 and 96 μM, respectively, and provitamins D2, D3 and the active form of vitamin D3 such as 1α, 25- dihydroxyvitamin D3 could not influence any pol activities. Tocotrienols, vitamin E compounds with an unsaturated side chain with three double bonds, selectively inhibited the activity of human pol λ with IC50 values of 18-81 μM. In vitamin K compounds, vitamin K3 selectively inhibited mitochondrial pol γ activity with an IC50 value of 6 μM, although vitamins K1 and K2 did not inhibit the activities of any pols. On the other hand, no compounds tested influenced the activities of plant pols from cauliflower, prokaryotic pols, or DNA metabolic enzymes tested. These compounds suppressed the cell growth of a human gastric cancer cell line, NUGC-3, and halted at the G1 phase in the cell cycle. We discussed the molecular mechanism and relationship between pol inhibitory activity and anti-cancer activity by fat-soluble vitamins.

The GHMP kinases are a structurally related family of small molecule kinases named after four of its members - galactokinase, homoserine kinase, mevalonate kinase and phosphomevalonate kinase. The group also includes the enzymes N-acetylgalactosamine kinase, arabinose kinase, mevalonate 5-diphosphate decarboxylase, archeal shikimate kinase and 4-(cytidine 5'-diphospho)-2-c-methyl-D-erythritol kinase. In addition the group includes two members not known to be catalytically active, the Caenorhabditis elegans sex-fate determining protein XOL-1 and the Saccharomyces cerevisiae transcriptional activator Gal3p. Two catalytic mechanisms have been proposed for GHMP kinases. The structure of mevalonate kinase suggests that an aspartate residue acts as an active site base, removing a proton from the substrate to facilitate attack on the γ- phosphate of MgATP. In contrast, in homoserine kinase there is no potential catalytic base and it is proposed that catalysis is driven by transition state stabilisation. Potential chemotherapeutic interventions against GHMP kinases fall into three main categories: inhibition of galactokinase to assist suffers of galactosemia, inhibition of mevalonate kinase or mevalonate 5-diphosphate decarboxylase to reduce flux through the cholesterol biosynthesis pathway and inhibition of bacterial GHMP kinases for novel anti-microbial therapies. These are in the early stages of development, but the accumulation of structural and mechanistic data will assist future progress.