Current Pharmaceutical Design - Volume 13, Issue 21, 2007

Angiogenesis is a tightly regulated process that leads to the formation of new blood vessels in limited physiological conditions, and can also occur under pathological situations as retinopathies, arthritis, endometriosis and cancer. Enhanced angiogenesis is present in tumors that need new blood capillaries to grow, remove metabolic waste and transport the cells to locations distal to the primary tumor, facilitating metastasis formation. For these reasons, blockade of angiogenesis is an attractive approach for the treatment of both solid and haematological malignancies. Antiangiogenic therapy should be less toxic in comparison with conventional treatments such as chemotherapy, being angiogenesis a process relatively restricted to the growing tumor. The Src family of tyrosine kinases has been implicated in the intracellular signaling cascade that acts downstream of cell surface receptors to elicit different cellular functions, including growth, proliferation, adhesion and motility. Src kinases are frequently activated in human malignancies, causing tumor progression, metastasis formation and deregulating expression of proangiogenic molecules. This review reports several studies performed by different authors demonstrating the involvement of Src tyrosine kinases in angiogenesis by regulating different signalling pathways. Moreover, we report selective Src inhibitors for which a direct involvement with angiogenesis has been demonstrated, even if every Src inhibitor could potentially possesses also antiangiogenic properties. Biological data, structures and mechanisms of action of selected molecules, in terms of Src protein-inhibitor interactions, are also reported.

The formation of capillary-like structures during angiogenesis requires a series of well-orchestrated cellular events allowing endothelial cells and pericytes to migrate into the perivascular space. The proper activation of the migratory machinery in these cells is fine controlled by the presence of angiogenic challenges and by the interactions with extracellular matrix. The two members of the focal adhesion protein tyrosine kinases (FA-PTKs), FAK and PYK2, play a central role in modulating endothelial and vascular smooth muscle cells migration confirming the well consolidated observations in other migrating cell types. However accumulating data reveal that FAK and PYK2 are involved in several cell processes including cell proliferation and survival. FAK, once localized to focal adhesions, is thought to be one of the principal effectors in linking signals initiated by integrins and growth factor receptors to cytoskeleton, thus controlling migration. Although more obscure, and differently regulated, the function of PYK2 seems to be similar to that of FAK, but restricted to few cell types, including vasculature forming cells. FAK and PYK2 exert a primary role as adaptor proteins able to recruit, with high turnover, several proteins which in turn, through their docking domains and tyrosine kinase activity, determine both the turnover in focal adhesion assembly and the specificity of downstream signaling. The characterization of functional interactions of FAPTKs may provide new potential therapeutic targets in order to control vascular pathological processes including angiogenesis.

Affirming that excess fat can be an unfavourable medical condition is definitively not an observation of great originality. In fact, in 400 BC, Hippocrates astutely observed that “sudden death was more common in those who are naturally fat than in the lean” [1]. By the contrast, the concept of targeting the adipose tissue and internal organs with increased fat content during pharmaceutical or lifestyle intervention is of great novelty. If the reduction of excess fat is still a therapeutic goal in improving a poor cardio-metabolic profile, new insights suggest the potential of improving the effectiveness of therapeutic intervention through the adiposity, not only against. In fact, there is now a compelling need of emerging therapeutic strategies targeted to the adipose tissue. The reason for the growing scientific interest into the fat is the widely-accepted acknowledgement that adipose tissue is not a silent organ, but a very active source of multiple bioactive cytokines, called adipokines. The adipocyte, mini-organ within this neglected organ, sends outputs (adipokines) and accept inputs (nuclear receptors). The adipose tissue communicates with almost all other organs through endocrine, paracrine and also autocrine interactions. Hence, both systemic and local regulations of internal organs’ function and morphology have been recently attributed to the adipose tissue. Fat tissue is also a potential great responder, by the presence of multiple receptors that can be modulated, stimulated or inhibited by drugs with different mechanisms of action and therapeutic purposes. Of additional and supportive note is the fact that the adipose tissue can now be clinically measured and quantified by simple, accurate and reliable diagnostic tools. Both biological and clinical characteristics of the adipose tissue seem to warrant a successful development of new therapeutic strategies. The concept and importance of proximity of adipose tissue to the organs is also intriguing. Intuitively, the visceral adipose tissue has been evoked as the most desirable therapeutic target. In fact, great interest has been recently focused on the visceral adiposity, namely the fat depots that surround the internal organs. The evidences supporting the visceral adiposity as independent cardio-metabolic risk factor are rapidly emerging. A body of studies suggest that the increased fat, particularly the visceral fat, may play a significant role in the development of the metabolic syndrome, a cluster of diseases apparently independent, but actually linked by common pathogenic key factors. In this Current Pharmaceutical Design issue, leading experts in clinical and bio-molecular aspects of adiposity, and its relationship with metabolic syndrome, addressed a topic of growing and exciting interest, as well as the potential use of different adipose tissue depots and organs as therapeutic targets. A systematic and detailed overview of basic and clinical aspects of adiposity-related diseases and an extremely up-to-date of the potential within targeting adipose tissue and fatty organs, are provided in this Current Pharmaceutical Design issue. Leonetti et al. [2] introduced the concept of metabolic syndrome, with its diagnostic aspects still controversial and under continue evolution. They extensively discussed the importance of regional fat distribution rather total adiposity in cardiovascular risk stratification. Waist circumference and imaging diagnostics have been evaluated as traditional and new markers of visceral adiposity. Great attention is focused on reinforce the notion that reductions in visceral adipose tissue should be a primary aim of strategies designed to reduce health risks associated with metabolic syndrome. Human adipose tissues are not only located in different anatomic compartments, but differ for embryological origin, bio-molecular properties, and therefore different therapeutic use.......

The metabolic syndrome is a long-term process, explained by the interaction of genetic and environmental factors, that starts early in life and is involved in the pathophysiology of a large percentage of cases with type 2 diabetes and atherosclerosis. A number of clinical studies have demonstrated the importance of fat distribution and especially the contribution of visceral fat accumulation to the development of metabolic disorders. Visceral adipose tissue can be studied through different imaging techniques. The accumulation of visceral adipose tissue, as opposed to subcutaneous fat, increases the risk of developing metabolic disease and cardiovascular diseases (CVD). Visceral adipocytes secrete a variety of cytokines known as adipocytokines suggesting that adipose tissue is an endocrine organ that may affect the function of other organs. Weight loss, particularly a reduction in waist circumference, improves insulin sensitivity, lipid profile, and serum adipocytokines, reducing the risk of developing chronic disease and CVD. Waist circumference is a required component of metabolic syndrome under the International Diabetes Federation (IDF) criteria, rather than an optional component as used by other previous classifications. Studies have shown that using a lower waist circumference threshold within the context of metabolic syndrome increases the prevalence, but decreases the risk of mortality and type 2 diabetes. It is possible that waist circumference acts as a marker for other risk factors. These findings reinforce the notion that reductions in visceral adipose tissue should be a primary aim of strategies designed to reduce health risks associated with metabolic syndrome.

The metabolic syndrome represents a constellation of co-morbidities that include central adiposity, insulin resistance, dyslipidemia and hypertension, which results from an elevated prevalence of obesity. An increased abdominal adiposity is observed in upperbody obesity with preferential accumulation of fat in the visceral depot, which renders these individuals more prone to metabolic and cardiovascular problems. The pathophysiology of the metabolic syndrome seems to be closely associated to an elevated efflux of free fatty acids from the visceral fat compartment and a dysregulation of the expression of adipose tissue-derived factors (also termed “adipokines”). Weight reduction and increased physical activity represent the main approach to tackle the “diabesity” epidemic. Nonetheless, taking advantage of the different biochemical and molecular characteristics of visceral and subcutaneous adipose tissue may open up novel pharmacological strategies to combat the metabolic and cardiovascular derangements accompanying the metabolic syndrome.

In most countries the prevalence of obesity now exceeds 15%, the figure used by the World Health Organization to define the critical threshold for intervention in nutritional epidemics. Here we describe Homo obesus (man the obese) as a recent phenotypic expression of Homo sapiens. Specifically, we classified Homo obesus as a species deficient of metabotrophic factors (metabotrophins), including endogenous proteins, which play essential role in the maintenance of glucose, lipid, energy and vascular homeostasis, and also improve metabolism-related processes such as inflammation and wound healing. Here we propose that pharmaceuticals, nutraceuticals and xenohormetics targeting transcriptional, secretory and/or signaling pathways of metabotrophins, particularly adiponectin, nerve growth factor, brain-derived neurotrophic factor, interleukin-10, and sirtuins, might be new tools for therapy of Homo obesus. Brief comment is also given to (i) exogenous metabotrophic agents represented by various classes of drugs, and (ii) adiponutrigenomics of lifspan.

Increased visceral adiposity, is an emerging cardiovascular risk factor. There is now a compelling need to quantify visceral adipose tissue not only for diagnostic purposes, but also for therapeutic interventions with weight reduction drugs or pharmaceuticals targeted to adipose tissue, as well as anti-obesity medications, thiazolidinediones, fibrates, angiotensin receptor blockers, highly active antiretroviral therapy and hormone replacement therapy. Among visceral adipose tissues, growing evidences suggest that cardiac adiposity may play an important role in the development of an unfavorable cardiovascular risk profile. Recent papers suggest that epicardial fat, index of cardiac and visceral adiposity, could locally modulate the morphology and function of the heart. The close anatomical relationship between epicardial adipose tissue and the adjacent myocardium should readily allow local paracrine interactions between these tissues. Echocardiography has been recently proposed for the direct assessment of epicardial adipose tissue. Echocardiographic assessment of epicardial fat may be a helpful tool not only for diagnostic purposes, as marker of visceral adiposity and inflammation, but also for therapeutic interventions with drugs that can modulate the adipose tissue. In this article, epicardial adipose tissue's structure, function, method of assessment and reliability as a diagnostic tool and potential therapeutic target is reviewed.

Almost every systemic vessel is surrounded by a layer of perivascular adipose tissue (PVAT), which had been mainly considered as a mechanical support for vasculature. However, recent advances have revealed that PVAT is an active player in controlling vessel function. PVAT releases relaxation factor(s) with unknown chemical identity (named perivascular adipocyte-derived relaxation factor, PVRF) that attenuates vasoconstriction to various agonists including phenylephrine, serotonin, angiotensin II, and U 46619 (a thromboxane A2 mimic), through activation of K+ channels. PVAT also promotes vasoconstriction to perivascular nerve stimulation by producing vasoconstrictor or facilitator (named perivascular adipocyte-derived constricting factor, PVCF), which includes superoxide and was mediated through activation of tyrosine kinase and MAPK/ERK pathways. Therefore, PVAT has a dual regulatory role in modulating vessel function, attenuating vasoconstriction to agonists by PVRF and promoting constriction to perivascular nerve excitation by PVCF. In vivo, normal amount of PVAT (total body fat as well) is likely to be important in maintaining the homeostasis of vascular tone and blood pressure, since lipoatrophic mice developed hypertension. On the other end, excessive accumulation of body fat (obesity) impaired PVRF production/action, despite an increase in the amount of PVAT. In spontaneously hypertensive rats, an animal model of hypertension without obesity, the ability of PVAT to attenuate vasoconstriction to agonists was reduced, and treatment with atorvastatin improved PVAT function. PVAT, vasodilating and constricting factors of PVAT origin, and signalling pathways of these factors may represent new targets for developing new strategies to treat vascular disorders associated with abnormal adiposity.

Non-alcoholic fatty liver disease (NAFLD) is often associated with features of the metabolic syndrome, carrying an increased risk to develop non-alcoholic steatohepatitis (NASH), the inflammatory form of liver steatosis. Epidemiological data confirm that obesity, diabetes, hypertension and hyperlipidemia are frequently found in NAFLD and worsen its prognosis because of increased risk of fibrotic evolution, eventually leading to liver cirrhosis. Recent studies confirm the close relationship between the metabolic syndrome and liver steatosis, and further support the detrimental role of oxidative stress and lipid peroxidation, which are pathophysiological processes present in both conditions. Novel diagnostic tools and life style modifications together with targeted therapeutic actions are urgently needed for the management of liver dysfunction in course of metabolic syndrome.

The metabolic syndrome is an emerging global epidemic characterized by clustering of metabolic abnormalities leading to increased cardiovascular risk: glucose intolerance or type 2 diabetes, dyslipidemia, hypertension, and “central” obesity. Scientists are decoding and piecing together the molecular texture underlying the metabolic syndrome: insulin resistance and dyslipidemia stand out as central pathophysiological events. In this picture, the liver rises as the leading organ in the maintenance of metabolic fitness; it serves as the first relay station for processing dietary information, and encloses the whole biochemical machinery for both glucose and lipid storage and disposal. In addition, the liver is a target of the different endocrine molecules secreted by pancreatic β-cells and adipose tissue. Evidence collected in animal models supports the central role of the liver in the metabolic syndrome. While specific bereft of insulin sensitivity in skeletal muscle and adipose tissue fails to induce diabetes at certain extent, this is constantly the outcome in case of hepatic insulin resistance. Also, dyslipidemia is currently interpreted as the result of increased flux of free fatty acids to the liver with ensuing misbalance of lipoprotein synthesis and removal. In this review we bring together recent advances in the field of lipid sensing nuclear receptors, adipokines and other molecules responsible for metabolic fitness, and provide a putative coherent frame to conceive the pathophysiology of the metabolic syndrome.

HIV-associated lipodystrophy or lipoatrophy, unreported before the introduction of highly active antiretroviral therapy (HAART), was first described in 1998, and has a prevalence ranging from 18% to 83%. As in genetic lipodystrophy syndromes, fat redistribution may precede the development of metabolic complications (dyslipidemia, insulin resistance) in HIV-infected patients receiving HAART. The pathogenesis of HAART-associated lipodystrophy and metabolic syndrome is complex and a number of factors are involved, including direct effects of HAART on lipid metabolism, endothelial and adipocyte cell function, and mitochondria.Protease inhibitors are responsible for a decrease in cytoplasmic retinoic-acid protein-1, in low density lipoprotein-receptor-related protein and in peroxisome proliferator activated receptor type-gamma. Nucleoside reverse transcriptase inhibitors, and thymidine analogues, are responsible for mitochondrial dysfunction as demonstrated by a decrease in subcutaneous adipose tissue mitochondrial DNA content. Both phenomena are responsible for a decreased differentiation of adipocytes, increased levels of free fatty acids and lipoatrophy. The increased levels of proinflammatory cytokines, such as tumor necrosis factor (TNF)-alpha and interleukin-6 may further contribute in development of lipodystrophy. TNF-alfa activates 11-beta-hydroxysteroid dehydrogenase type-1, which converts inactive cortisone to active cortisol, resulting in increased lipid accumulation in adipocytes and insulin resistance. HAART drugs and inflammatory cytokines are associated with a decrease in adiponectin. The levels of adiponectin and adiponectin-to-leptin ratio correlate positively with insulin resistance in HIV-infected patients with lipodystrophy. HAART-associated metabolic syndrome is an increasingly recognized clinical entity. The atherogenic profile of this syndrome may increase the risk of cardiovascular disease even in young HIV-infected patients. A better understanding of the molecular mechanisms responsible for this syndrome will lead to the discovery of new drugs that will reduce the incidence of lipodystrophy and related metabolic complications in HIV-infected patients receiving HAART.