Current Medicinal Chemistry - Immunology, Endocrine & Metabolic Agents - Volume 5, Issue 2, 2005

For thousand of years plants provided drugs to therapists, but humans have eaten such compounds at various concentrations in common foods for even more years. It is conceivable that during evolution, phytochemicals shaped the human genoma, as bioactive substances synthesized by plants for their own purposes could produce either harmful or beneficial effects depending on human digestion, absorption and metabolism and on human ability to produce biological responses to their consumption. Now that the attention of medical research turns towards low-cost, effective and widespread approaches to contrast the pathogenic mechanisms of diseases for primary prevention, a more relevant role is warranted to foods and diet. Pharmacology classically pursues an approach of the kind “one drug-one biological target”. Foods do not work in this way: they contain an array of bioactive compounds acting on a wide range of biological function. This complexity is an true obstacle to understand how foods work in producing, modulating or preventing diseases. But the challenge is worthwhile, as by this study we can understand the role of foods in human evolutionary processes and we can produce powerful tools for human health. In order to enhance the functional quality of foods it is necessary to take into account a variety of factors that render foods different from drugs, each of whom is illustrated by one of the papers included in this hot topic. First, in general, the bioactive substances have a role in the organism producing them. In order to shape the functional profile of plants it is necessary to enhance their production of several compounds by means of genetic tools with a comprehensive knowledge of plant physiology. The review from Morandini and colleagues illustrates this concept. Second, it is different to administer the same compounds as drugs or as foods. In some cases, the composition of the diet itself, not the action of a single bioactive compound, can mimic effects produced by some drugs. The review from Bertoli and colleagues shows that the ketogenic diet determines effects similar to these of antiepileptic drugs, and that it is a valid alternative to control pharmaco-resistant seizures. Beyond the specific cases, it is thus important to take in mind that delivering bioactive compounds through foods invariably has an impact on the whole diet, and the latter factor could be equally important in producing effects on human health. Third, Tighe and colleagues describe an example of how food fortification can be successfully used to deliver components (folic acid) that are critical to prevent diseases as neural tube defects and even cardiovascular disease. However, the efficacy and safety of such approaches must be addressed in a way similar to pharmacological ones. In the case illustrated here, the importance to monitor potential adverse effects of increasing folic acid delivery is underscored, but it is definitely more difficult to perform surveillance for foods than for drugs. In her manuscript, Albertazzi gives a comprehensive review of the current process of providing clinical evidence for health effects of phytoestrogens. It is clearly pointed out that it is not sufficient to concentrate on biomarkers of disease, it is necessary to perform clinical trials and to measure clinical endpoints even when foods or food extracts are used, whose action arises from the mix of components and not from a single, pure, measurable one. Fifth, Battezzati and San Romerio show the recent developments in the understanding of chronic inflammation as a pathogenic mechanism at the root of many chronic degenerative diseases and the development of successful therapeutic and preventive strategies. Here is an example of the possibility to act on a target (inflammation) not only by means of pharmacological doses of a single drug, but by means of a number of different compounds that could act synergistically. Much work has yet to be done to achieve the target. It is exciting, however, that people from different backgrounds, plant and human genetists and physiologists, pharmacologists, physicians and epidemiologists, food scientists and nutritionists begin to converge to share knowledge and the common project to provide new foods and drugs for the increased duration of human life increasingly affected by chronic-degenerative diseases.

Directing metabolic fluxes in plants for the production of nutraceuticals or fine chemicals (e.g. drug precursors) is becoming increasingly attractive and feasible. We review first recent accomplishments of plant metabolic engineering. Both experimental evidence and theoretical predictions point out that (i) metabolic flux increases require manipulation of most of the enzymes in a biosynthetic pathway, (ii) modulating all enzymes in a pathway avoids extremes in metabolite concentration in the pathway and causes little disturbance in connected pathways. On these basis we conclude that the most general and effective way forward for increasing the production of plant metabolites is to manipulate factors which specifically and co-ordinately regulate the expression most of the genes coding for the enzymes of a biosynthetic pathway. We therefore discuss the methods to acquire knowledge on the regulatory circuitry of transcription of genes involved in metabolic pathways and the strategies to manipulate it. Providing precursors to the pathway and removing the product, which can be accomplished by creating metabolic shortcuts and engineering energy dependent pumps specific for product removal, are part of the strategy.

Despite the continued development and release of new antiepileptic drugs (AEDs), 20-30% of all patients with epilepsy do not respond to conventional therapy or have related side effects that preclude their continued use. Presently, the most important AEDs used in paediatric population are: phenytoin, carbamazepine, valproic acid, vigabatrin, lamotrigine and topiramate. Their mechanisms of action are only partially known. The side-effects of the common antiepileptic drugs are exhaustively described but the strategies to avoid them are still unsatisfactory. Among alternative therapies used for the “drugs resistant epilepsy”, the nutritional approach defined “ketogenic diet” is one of the most promising and several reports confirmed its efficacy especially in children defined “drug resistant”. However, after more than 80 years of its applications, the mechanism of action, the long-term side-effects and the indications of use of the ketogenic diet are not satisfactory clarified. The purpose of this work is to review recent advances in nutritional and pharmacological management of childhood epilepsy comparing the history, the mechanisms of action, the indications, and the side effects of the ketogenic diet and the principal antiepileptic drugs in order to assess the links between the two kind of approaches.

Elevated homocysteine is an independent risk factor for cardiovascular disease, with recent evidence supporting a causal relationship. The exact mechanism by which homocysteine causes vascular disease is unclear, but it may be both atherogenic and thrombogenic. Homocysteine concentration can be influenced by a variety of factors, including, age, sex, genetics, B-vitamin status, disease state and lifestyle. The B-vitamins, folate, vitamin B12 and vitamin B6, play important roles in homocysteine metabolism and of these, folate is recognized as the main nutritional determinant of homocysteine concentrations. The effectiveness of low dose folic acid supplementation in lowering homocysteine in the general population is well established. However, for certain population subgroups such as the elderly, those with cardiovascular disease and individuals with polymorphisms affecting key enzymes in homocysteine metabolism controversy still remains regarding the dose of folic acid required to produce optimal homocysteine lowering. The aim of this review was to assess the evidence for homocysteine as a risk factor for vascular disease and to provide an overview of the treatment of hyperhomocysteinemia with the relevant B-vitamins at both the pharmacological and physiological doses.

Phytoestrogens are a large family of plant derived molecules possessing various degrees oestrogen like activity. Food or food supplements containing phytoestrogen are often been advocated as an alternative to hormonal replacement therapy (HRT)in women with contra-indications to the use of conventional oestrogen replacement, or simply wanting a more "natural" alternatives. There have been several studies performed with phytoestrogen in various aspects of the postmenopausal women health. Results have been sometime conflicting and difficult to interpret. The lack of knowledge of what precisely is the active ingredient, its minimally effective doses, the lack of standardization of the preparations used as well as the large individual variability of metabolism of precursors introduced with the diet may all have played a role in confusing the issue about effectiveness of these compounds. Phytoestrogen fall in the gray area between food and drugs hence in spite of the vast public interest, there are no interests in company producing these supplements in investing in research from which they will not exclusively benefit from. It is often difficult for physician to know how to advise patients on this matter. In this paper I critically review the clinical data available to date in an attempt to answer some of the most commonly asked questions about dose and type of phytoestrogens supplementation most likely to be effective in different aspect of the climacteric woman health.

Chronic subclinical inflammation is frequently found in chronic degenerative diseases including cardiovascular and neurodegenerative diseases, diabetes and obesity. Much evidence suggests that inflammation is pathogenically involved in these diseases. The molecular mechanisms have been recently elucidated and the central role of the nuclear factor κB has been established in orchestrating the multiple cellular responses to pro-inflammatory stimuli. An interesting cross-talk between insulin and inflammatory signalling pathways could help to explain the relationship between inflammation and insulin resistance and to provide a therapeutic target as well. It emerged that a wide number of different signals converge on the nuclear factor, many of them being whose are of nutritional origin. This raises the possibility to modulate by diet, the inflammatory component of many chronic degenerative diseases.

One of the most important metabolic actions of insulin is to promote glucose transport in skeletal muscle and adipose tissue. Insulin-stimulated glucose transport in these target tissues is mediated by translocation of the insulinresponsive glucose transporter GLUT4 from an intracellular location to the plasma membrane where GLUT4 facilitates entry of glucose into the cell. Over the past decade, tremendous progress has been made in elucidating insulin signaling pathways regulating translocation of GLUT4. One essential signaling pathway in this process is a PI 3-kinase-dependent pathway that controls activation of downstream ser/thr kinases such as PDK-1, Akt, and PKC-ζ leading to an increase in the exocytosis rate for GLUT4. Although activation of PI 3-kinase is necessary for insulin-stimulated translocation of GLUT4, it is not sufficient. Recently, a PI 3-kinase-independent pathway involving activation of TC10 (a GTPase belonging to the rho family) has been identified as another necessary element. Insulin-stimulated phosphorylation of Cbl results in assembly of signaling complexes that activate TC10. This leads to rearrangements of actin structures that facilitate translocation of GLUT4. In this review, we will discuss details of both the PI 3-kinase-dependent and - independent pathways mediating translocation of GLUT4 in response to insulin.

Insulin is largely responsible for the mobilization of dietary glucose in muscle and fat tissues, through exocytosis of glucose transporters from intracellular stores to the plasma membrane. Here we review the current understanding of insulin-induced translocation of GLUT4 in cultured muscle cells expressing myc-tagged GLUT4, and compare and contrast this knowledge with that obtained using adipose cells in culture. We summarize 1) The insulin signalling components required for GLUT4 traffic; 2) The steps of GLUT4 traffic susceptible to regulation by insulin signals; 3) The molecular mechanisms of GLUT4-vesicle fusion with the cell surface; and 4) The role of the actin cytoskeleton in GLUT4 translocation. The findings summarized lead to a hypothetical model for GLUT4 traffic in muscle cells where, in the basal state, GLUT4 molecules recycle to and from the plasma membrane via endosomes independently of the actin cytoskeleton. Insulin activation of phosphatidylinositol (PI) 3-kinase and Akt accelerates GLUT4 transit through endosomes en route to a specialized compartment. The newly identified Akt substrate, AS160 appears to facilitate GLUT4 exit from the specialized compartment towards the plasma membrane. In parallel, insulin regulates the actin cytoskeleton via PI3- kinase-dependent activation of the small GTPase Rac. The resulting actin remodelling is thought to facilitate the spatial segregation of insulin-dependent signals and GLUT4-rich vesicles. Finally, fusion of GLUT4 vesicles with the plasma membrane also involves PI 3-kinase-signalling and requires the v-SNARE VAMP2.

Insulin increases the rate of glucose uptake into muscle and fat cells. Numerous studies provide evidence that this is accomplished through an increase in the cell-surface amount of the facilitative glucose transporter GLUT4. Diverse techniques have been used to document a time-dependent gain in surface GLUT4, often in conjunction with a decrease in its intracellular content. These include subcellular fractionation coupled to immunoblotting, affinity photolabelling coupled to immunoprecipitation, and surface detection by immunofluorescence and immunoelectron microscopy. However, there are also abundant reports of a discrepancy between the extent of stimulation of the rate of glucose uptake by insulin and the increase in the amount of cell-surface transporters. One interpretation of this discrepancy is that insulin regulates the intrinsic activity of GLUT4 in addition to recruiting transporters to the cell surface. Several interpretations of this discrepancy can be offered, based on the experimental techniques used to measure glucose uptake and cell-surface GLUT4. Here we discuss the methods used to quantify these observations. Subsequently, we inspect the studies that have measured the insulin-stimulated increase in cell-surface GLUT4 concurrently with glucose uptake, in order to determine if there is realistic evidence to support the hypothesis that insulin regulates GLUT4 intrinsic activity. We conclude that in spite of various possible explanations for the discrepancy between the stimulation of glucose uptake and that of cell surface GLUT4, there is solid evidence to suggest that insulin increases the intrinsic activity of GLUT4.

The glucose transporters (GLUTs) are currently a 13 member family of facilitative transmembrane proteins which transport glucose down its concentration gradient. The GLUTs have a tissue specific expression and regulation. Dysregulation of GLUTs have been implicated in the pathogenesis of a number of diseases including diabetes and cancer and are known to play an important role in the developing embryo. In addition, roles for GLUTs in cardiac function and embryonic development have been identified and will be discussed in this review. The ability to ablate or over-express GLUTs has advanced our understanding of the role these transporters play in the maintenance of normal glucose homeostasis and the pathogenesis of diabetes. The development of Cre-LoxP technology coupled with the existence of tissue specific promoters allows investigators to manipulate gene expression both globally and in a tissue specific manner. The major GLUTs which have been investigated using transgenic technology are GLUT1, GLUT4 and GLUT2. Overexpression of GLUT4 and GLUT1 results in increased glucose uptake and metabolism. However, only GLUT4 overexpression protects against the development of insulin resistance in transgenic mice. Genetic ablation of GLUT4 and GLUT2 results in impaired insulin tolerance and defects in both lipid and glucose metabolism. This review will present various transgenic models of GLUT modification and discuss what has been learned from these models about the role that GLUTs play in glucose homeostasis, insulin action and development.

The overall objective of this paper is to review the mechanisms by which various metabolic and cellular signals, as well as nuclear transcription factors, regulate the expression and function of the insulin responsive glucose transporter-4 (GLUT4) gene. Reviewing this information will help the reader to understand the molecular processes involved in both glucose homeostasis and the pathogenesis of abnormal metabolic states involving impaired insulin action, such as insulin resistance and diabetes mellitus type 2 (DM2). The same molecular mechanisms are also involved in tumorigenesis. Studies on GLUT4 regulation, its translocation machinery, and intrinsic activity have contributed valuable knowledge that may be useful for developing therapeutic strategies aimed at increasing GLUT4 protein levels, which could potentially improve glucose homeostasis in insulin resistance and diabetes.

The GLUT4 gene is subject to complex tissue-specific and metabolic regulation that has a profound impact on insulin-mediated glucose disposal. The regulation of this gene is of special clinical interest because insulin-mediated glucose homeostasis is highly sensitive to the levels of GLUT4 protein in muscle and adipose tissue. For this reason, the mechanisms of regulated expression of the GLUT4 gene have been intensively studied over the past decade. Understanding the transcriptional mechanisms that underlie the regulated expression of this highly differentiated gene have been slow to emerge, due to the paucity of suitable model systems available for detailed investigation. The development of transgenic mouse models to understand the mechanisms of transcriptional regulation has greatly enhanced out understanding of this gene. Information gained about the regulation of the GLUT4 gene has provided insight into mechanisms by which complex gene regulation occurs through a small number of cis-acting regulatory elements.