Regulation of plasma fatty acid metabolism

Regulation of plasma fatty acid metabolism

Clinica Chimica Acta 286 (1999) 163–180 Regulation of plasma fatty acid metabolism Jumana Saleh, Allan D. Sniderman, Katherine Cianflone* Mike Rosenb...

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Clinica Chimica Acta 286 (1999) 163–180

Regulation of plasma fatty acid metabolism Jumana Saleh, Allan D. Sniderman, Katherine Cianflone* Mike Rosenbloom Laboratory for Cardiovascular Research, McGill University Health Centre, Montreal, PQ , Canada H3 A 1 A1 Received 21 December 1998; accepted 25 March 1999

Abstract Although adipose tissue serves a crucial function in energy storage, excess adipose tissue – that is, obesity – is often associated with diabetes and cardiovascular disease. A common thread in the weave of complications is increased plasma concentrations of fatty acids. In the present review, we have focused on two specific points that relate to obesity: (i) What are the metabolic consequences of increased free fatty acid concentrations? and (ii) What are the physiological factors that are involved in the regulation of fatty acid uptake or release from adipose tissue? We have tried to emphasize new factors that act as hormones on adipose tissue and in so doing regulate the net concentration of circulating free fatty acids.  1999 Elsevier Science B.V. All rights reserved.

1. Introduction The main function of adipose tissue is to store and release energy in the form of fatty acids. The body’s supply of fatty acids originate from the diet or through endogenous fatty acid synthesis, primarily in the liver. Fatty acids are chains of hydrocarbon groups attached to a carboxyl group. They vary in chain length and saturation and can be nonessential or essential (unable to be synthesized within the body). Oxidation of fatty acids is our major metabolic source of energy to *Corresponding author. Tel.: 1 1-514-842-1231; fax: 1 1-514-982-0686. E-mail address: [email protected] (K. Cianflone) 0009-8981 / 99 / $ – see front matter  1999 Elsevier Science B.V. All rights reserved. PII: S0009-8981( 99 )00099-6

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support our biological survival. Dietary fatty acids are esterified to form triglyceride (TG), and assembled into large lipoprotein particles (chylomicrons), secreted from intestinal cells into the lymph and from there enter the general circulation. Triglyceride rich VLDL are assembled in a similar manner in the liver and secreted from there into the general circulation. Both of these triglyceride rich particles (chylomicrons and VLDL) are substrates for the lipolytic action of lipoprotein lipase as they pass through the capillary space (Fig. 1). Energy in excess of the body’s needs is assimilated by fat cells in the form of fatty acids and stored as triglycerides in lipid droplets. Upon demand, intracellular triglyceride stores are hydrolysed by the action of hormone sensitive lipase to release free fatty acids that are transported to different tissues and oxidized to generate energy (Fig. 1), [1]. The balance between lipogenesis (TG storage) and lipolysis (TG breakdown) in adipocytes is of critical importance and regulation of these processes is under complex hormonal and neuronal influence. Dysregulation of the lipogenesis-lipolysis balance in adipose tissue leads to significant metabolic complications. A positive imbalance between storage and utilization leads to obesity in which

Fig. 1. Adipose tissue extracellular free fatty acid (FFA) concentrations are influenced by (i) intracellular hormone-sensitive lipase hydrolysis of storage triglyceride (TG); (ii) lipoprotein lipase lipolysis of triglyceride rich lipoproteins, (chylomicrons and VLDL); and (iii) fatty acid uptake and esterification in adipose tissue to form storage TG.

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there are increased numbers (hyperplasia) as well as increased size (hypertrophy) of adipocytes. Obesity is a major health problem associated with serious complications and, in humans, is generally divided into (i) subcutaneous obesity (female type or gynoid obesity) which is characterized by an accumulation of fat under the skin, especially in lower body parts; and (ii) omental (male type or android obesity) which is characterized by the accumulation of fat within the omentum [2]. Many studies have investigated the regional variations in adipocyte morphology and the relation to metabolic profiles and disease. The first scientific evidence that showed a link between abdominal body fat distribution and disease was provided by Vague et al. [3]. Since then, many studies have shown that abdominal obesity, particularly excess visceral (or omental) fat, is associated with a series of metabolic complications including abnormal glucose metabolism [4,5], hyperinsulinemia [4], familial combined hyperlipidemia [6], increased VLDL and small dense LDL production [7], all of which are associated with cardiovascular disease and diabetes [8]. A common underlying feature of these subjects is an increase in circulating free fatty acid levels [9–11]. This has been demonstrated in many studies exemplified by those of Despres and colleagues [8,12], as summarized in a number of reviews [13].

2. Consequences of increased fatty acid The concentration of plasma fatty acids may be regulated by the utilization or release of fatty acids by a number of tissues such as adipose tissue, muscle and liver. In the present discussion, we will focus on the regulation of fatty acid storage and release by the adipose tissue as one factor which can influence net plasma fatty acid concentrations. Nonetheless, regardless of the cause of increased plasma fatty acids, the consequence is significant perturbations at the level of many tissues. At the immediate site of generation of fatty acids, that is on the endothelial cell surface where lipoprotein lipase is located, high levels of free fatty acids can inhibit lipoprotein lipase activity through product inhibition [14], interfere with interaction of lipoprotein lipase with its cofactor apoCII [15], or dislodge lipoprotein lipase from its endothelial cell anchorage in adipose tissue and muscle [16]. Consequently postprandial TG clearance may be delayed. The increased circulating fatty acids (FFA) also affect liver, muscle, pancreas, arterial wall and adipose tissue. Many in vitro studies have shown that increased hepatic availability of circulating FFA and partially hydrolyzed chylomicrons will result in enhanced production of apoB lipoproteins [17,18]. The net result is an increase in VLDL and LDL production and increased plasma LDL, and therefore an increased risk for atherosclerosis. In addition, fatty acids stimulate hepatic glucose production [13] and modulate gene expression in positive or

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negative ways of enzymes such as fatty acid synthase, acetyl CoA carboxylase, malic enzyme, pyruvate kinase and PEPCK [19–21]. The increased fatty acids interfere with insulin extraction by the liver [5] and stimulate secretion of other factors by the liver such as plasminogen activator inhibitor (PAl-I) [22]. In muscle, increased plasma FFA also interfere with normal glucose utilization in several ways. An increased flux of fatty acids decreases muscle glycogen synthase [13] and increases muscle triglyceride stores [23], thereby reducing glucose oxidation. Whether these effects are accompanied by increased or decreased fatty acid oxidation is controversial [23,24]. In some cases, no change in fatty acid oxidation has been noted in obese vs. normal subjects [25]. The reduction in glucose utilization and the reduced insulin stimulation of glucose transport and uptake into this tissue are associated with increases in both plasma glucose and insulin (insulin resistance) [5,12]. Can this be a direct result of increased fatty acid levels? In fact, these same physiological consequences which are termed ‘‘insulin resistance’’ when plasma glucose remains within the normal range can be generated experimentally in vivo by administering infusions of fatty acids which results in both increased insulin levels and decreased muscle glucose uptake [26]. Fatty acids can also affect the pancreas by promoting insulin secretion [13] and, with chronic exposure, lead to beta cell apoptosis [27]. Within the arterial wall, fatty acids have been shown to promote the growth of smooth muscle cells [28,29] and the activation of T lymphocytes [30], both processes involved in the development of atherosclerosis. Finally, in adipose tissue, fatty acids serve as more than just a substrate for triglyceride synthesis. For example, fatty acids directly increase glycerol-3phosphate acyltransferase activity which enhances triglyceride storage [31]. At the same time, fatty acids can also interfere with insulin stimulated glucose transport [32,33], even though there is no effect on insulin stimulated glycogen synthesis [32]. Fatty acids have a profound influence on adipocyte membrane composition [33], gene regulation [19,20], and adipocyte differentiation [31]. These effects are mediated by specific fatty acids themselves (polyunsaturated fatty acids, arachidonic acid), or via molecules which are derived from fatty acids (as described below). These effects may be transduced in part through direct interaction with PPARs (peroxisome proliferator activated receptors), which are members of the steroid / thyroid nuclear receptor superfamily of ligand activated transcription factors [31]. The gamma isotype (PPARg) is mainly expressed in adipose tissue and appears to play a critical role in adipocyte differentiation and lipid storage [31]. Although the identification of native PPAR ligands is still underway, both fatty acids and eicosanoids (such as PGJ 2 ) have been identified as potential PPAR activators [34,35]. Fatty acids are precursors for other cellular effectors such as lysophosphatidic acid, prostaglandins and arachidonic acid which, in turn, can affect both cell

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differentiation and lipolysis [36]. Phosphoinositolglycan-peptide (PIG-P), isolated from insulin sensitive tissue, has been demonstrated to exert partial insulin like effects on glucose and lipid metabolism. Treatment of rat adipocytes with PIG-P as well as with other agents that have partial insulin-mimetic activity, such as phosphatidyl inositol-specific phospholipase C and sulfonylurea glimepiride, trigger protein tyrosine phosphorylation which correlates with stimulation of lipogenesis and TG storage in adipose tissue [37]. Fatty acids also have potent regulatory effects on key adipocyte enzymes and secreted hormones. Fatty acids stimulate the secretion of angiotensin II [38] and leptin [26,39] by adipocytes. Of note, dietary lipoproteins (chylomicrons) increase the production of acylation stimulating protein (ASP) from adipocytes both in vitro and in vivo [40–42]. Thus the consequences of increased circulating fatty acids are many and result in metabolic changes which promote increased obesity, insulin resistance and hyperlipidemia.

3. Causes of increased plasma fatty acids Many studies have been performed in order to understand the basic mechanisms regulating fatty acid movement into and out of adipose tissue, the major TG storage site (Fig. 1). The size of the extracellular pool of fatty acids is a balance between (i) the rate of intracellular lipolysis by hormone sensitive lipase and release of fatty acids from the cells; (ii) lipoprotein lipase hydrolysis (independent of cellular fatty acid uptake); and (iii) adipose tissue FFA uptake and esterification of fatty acids. Each of these three aspects is discussed below. Firstly, many studies have demonstrated increased hormone-stimulated lipolysis in omental adipocytes as compared to subcutaneous adipocytes (for review see [43–46]). However, the basal (unstimulated) lipolytic rate is, in fact, lower in omental than subcutaneous adipocytes. Furthermore, in vitro studies have shown that the maximum lipolytic rate of glycerol release following adrenergic stimulation was higher in subcutaneous than in omental fat cells and this was parallelled by increases in hormone sensitive lipase activity and mRNA expression [47]. However, unlike other fat depots, omental fat tissue has direct access to the liver through the portal vein and the metabolic complications that accompany excess visceral fat may reflect direct fatty acid input to the liver. Secondly, the general view that lipoprotein lipase is the rate limiting step that determines the rate of hydrolysis of plasma triglyceride and consequently uptake of free fatty acid by fat cells was re-examined by Olivecrona and colleagues [48]. They showed that the amount of lipoprotein lipase appears to be in excess of normal requirements and thus not rate limiting [48]. In addition, the correlation between plasma triglyceride clearance and lipoprotein lipase activity was poor [48,49]. Although lipoprotein lipase is essential for efficient clearance

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of plasma triglycerides and normal adipose tissue composition, other factors may also be important determinants of TG clearance such as tissue uptake and esterification of free fatty acids.

4. Basal fatty acid esterification: influence of size, sex and site Fatty acid cellular uptake and intracellular fatty acid esterification to triglyceride are two tightly coupled processes. Fatty acid uptake itself (independent of esterification) is regulated by differentiation [50], is variable in different adipose tissue sites [51], and is influenced by the presence or absence of ap2, the adipocyte fatty acid binding protein [52]. Studies have shown that fatty acid incorporation into adipose tissue (FIAT) increases linearly with increasing concentrations of a physiological spectrum of fatty acids in vitro [53]. FIAT and glucose (GLIAT) incorporation activities per cell were positively correlated with fat cell diameter but not fat cell number [54]. The basal capacity to synthesize triglyceride in a mature adipocyte is not only influenced by cell size but also by the sex of the donor as well as the site of sampling. For example, in fat tissue from morbidly obese patients, triglyceride synthesis in subcutaneous adipose tissue from women had higher triglyceride synthetic capacity compared to omental tissue and was greater than the triglyceride synthesis in both adipose tissue depots in males [55]. It is true that subcutaneous adipocytes are larger than omental adipocytes [56], and since larger cells appear to have greater triglyceride synthetic capacity than smaller cells [55] these differences may simply be a function of cell size. However, even in preadipocytes which have not yet differentiated to contain a triglyceride droplet, in vitro studies showed that basal triglyceride synthesis was greater in subcutaneous than omental preadipocytes [57]. Studies also suggest a major role of sex steroids in determining the anatomical distribution of fat. Their function may be exerted through both androgen and estrogen receptors which have been demonstrated recently on adipose tissue [58]. The level of androgen receptors in omental tissue is twice as high as in subcutaneous preadipocytes [58]. Testosterone inhibits lipoprotein lipase activity but can stimulate hormone sensitive lipolysis markedly [46,58], and this may enhance fatty acid release in male omental tissue. Female sex steroids (estrogens) appear to have similar effects to testosterone [45,46]. On the other hand, progesterone inhibits lipolysis by decreasing cAMP levels probably mediated through phosphodiesterase IV [59] and counteracts the stimulatory effect of estrogen. The anti-lipolytic effect of insulin also appears to be greater in women than in men [60].

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5. Insulin and catecholamines as regulators Both insulin and catecholamines have long been recognized to have profound influences on lipolysis / lipogenesis and these effects have been reported widely [43,44,61–65]. Insulin activates phosphodiesterase, reducing the intracellular levels of cAMP, thus reducing the activation of hormone sensitive lipase. Insulin increases glucose transport through its effects on translocation of Glut 4 [66] and has direct, although less marked, effects on the triglyceride synthetic enzymes [67] as well as lipoprotein lipase [68]. Since insulin is an effective lipogenic factor, could it play a role in enhancing obesity? Although obesity is often associated with insulin resistance, nonetheless, it is evident that fat storage is still effective in obese and insulin resistant subjects. Arner et al. studied the anti-lipolytic effect of insulin in obese subjects. There was no change in insulin responsiveness or sensitivity compared to the normal controls. When the obese subjects were divided into hyperinsulinemic and normal fasting serum insulin levels, a similar anti-lipolytic effect of insulin was observed in both groups [69]. However, other studies have suggested that there is a decrease in insulin responsiveness in fat cells from obese subjects [70]. The second well known lipolytic regulators are norepinephrine / epinephrine. Their effects on hormone sensitive lipase mediated release of fatty acids have been extensively studied. The effects appear to be mediated through interaction with a number of adrenergic receptors: b1, b2, al and a2 receptors. Recently, however, a new member of this family, the b3 receptor has been described. Although this receptor appears to be specific to fat tissue [44], it appears to play a minor role in stimulation of overall lipolysis [63]. The variable expression of these multiple receptors on adipose tissue from different depots will influence the overall response to a lipolytic stimulus.

6. Lipolysis and lipogenesis: new regulators There are a number of recently described endocrine and autocrine factors that are emerging as additional potential regulators of lipid turnover in adipocytes. Several of them exert pronounced effects in vitro on lipolysis or lipogenesis and their effects may vary significantly in different adipose depots. For the purposes of this discussion, in the references indicated, lipolysis is measured experimentally as the mass of fatty acid or glycerol released from adipose tissue or by increases in hormone sensitive lipase activity or mRNA. Lipogenesis is measured as radio-labelled glucose or fatty acid incorporation into intracellular triglyceride, increases in fatty acid synthesis or glucose transport, increases in activity or mRNA of any of the enzymes or transporters inherent to these

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Table 1 Endocrine and autocrine hormones which influence lipolysis Endocrine

Effects

Ref.

Adenosine Testosterone Estrogen Progesterone Growth hormone Neuropeptide Y Prostaglandins

⇓ ⇑ ⇓ ⇓ ⇑ ⇑ ⇓ ⇑ ⇓ ⇓

activates PDE (decreases cAMP), inhibits HSL activates b1, b2, b3, al receptors, activates HSL (OM d . SC e) activates a2 receptors, inhibits HSL (OM . SC) inhibits lipolysis (SC . OM) interacts with androgen receptor, increases lipolysis interacts with estrogen receptors, increases lipolysis (OM . SC) decreases lipolysis and cAMP by increasing PDE (type IV) interacts with growth hormone receptor (OM . SC) decreases lipolysis (SC 5 OM) inhibits lipolysis (SC . OM)

[62,63,103] [43] [58] [45,48] [50] [48,71] [73,104] [43,105]

Autocrine TNFa f NO g Leptin Agouti

⇑ ⇑ ⇑ ⇓

increases lipolysis increases lipolysis increases lipolysis decreases lipolysis

[74] [75,76] [77–79] [80]

Insulin NE/EP c

a

b

[66–68]

a

PDE: phosphodiesterase. HSL: hormone sensitive lipase. c NE / EP: Norepinephrine / epinephrine. d OM: omental. e SC: subcutaneous. f TNFa: tumor necrosis factor a. g NO: nitric oxide. b

processes (including lipoprotein lipase), or by an increase in cell fat content. A summary of endocrine and autocrone factors and their effects on lipolysis and lipogenesis is described in Tables 1,2 respectively. As described above, factors known to influence lipolysis include insulin, catecholamines and sex hormones such as estrogen, testosterone and progesterone. Growth hormones [46,71] also increase lipolysis and growth hormone receptors are present at a higher density in omental tissue vs. subcutaneous depots [45]. In addition to insulin, prostaglandins and adenosine also inhibit lipolysis and have greater effects in subcutaneous compared to omental adipose tissue [43]. Finally, neuropeptide Y, in addition to its neuronal effects, has been shown to have direct peripheral effects and also inhibits lipolysis equally in both subcutaneous and omental adipose tissue [72,73]. Within the adipocyte itself, several secreted autocrine regulators of lipolysis have been described. These include TNFa [74], NO [75,76] and leptin [77–79], all of which stimulate lipolysis. Agouti, by contrast, inhibits lipolysis [80]. Factors that influence lipogenesis have been studied less intensely. In addition to its anti-lipolytic activities, insulin is known to be a major lipogenic hormone (as described above). However, new factors are regularly being proposed as

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Table 2 Endocrine and autocrine hormones which influence lipogenesis Endocrine

Effects

Insulin



Testosterone Estrogen Growth hormone

⇓ ⇓ ⇓

Cortisol DHEAe GLP-1 f

⇑ ⇓ ⇑ ⇑

Autocrine ASP g



Angiotensin II



TNFa h NO i Leptin Agouti

⇓ ⇓ ⇓ ⇑

Ref. a

b

increases LPL (SC ), increases glucose transport (GLUT 4 c), increases fatty acid esterification interacts with androgen receptor, inhibits lipoprotein lipase interacts with estrogen receptor, inhibits lipoprotein lipase interacts with growth hormone receptor (OM d . SC), inhibits insulin stimulated lipogenesis, decreases fat content increases lipoprotein lipase decreases fatty acid synthase increases lipoprotein lipase increases lipoprotein lipase, fatty acid incorporation, enhances insulin effect on glucose uptake & lipid synthesis (decreases cAMP) increases fatty acid incorporation and glucose transport in adipocytes and muscle cells, additive to insulin increases triglyceride content, fatty acid synthase, glycerol-3-phosphate acyltransferase decreases lipogenesis decreases insulin mediated glucose uptake decreases lipid synthesis (acetyl CoA carboxylase) increases mRNA fatty acid synthase, additive to insulin

[66–68] [48,58] [45] [71] [45,48,71] [81] [82,83]

[93] [87,88] [85] [86] [84,85] [80,89]

a

LPL: lipoprotein lipase. SC: subcutaneous. c GLUT 4: glucose transporter 4. d OM: omental. e DHEA: dehydroepiandrosterone. f GLP: glucagon-like peptide. g ASP: acylation stimulating protein. h TNFa: tumor necrosis factor a. i NO: nitric oxide. b

lipogenic modifiers. Thus, as described above, testosterone and estrogen interact with specific receptors and decrease lipoprotein lipase activity [45,46,58]. Growth hormones act through specific receptors and inhibit insulin stimulated lipogenesis and decrease fat content [71]. Cortisol inhibits de novo fatty acid synthesis, but increases lipoprotein lipase activity [45,46,71]. DHEA and GLP-I also both stimulate lipoprotein lipase [81,82]. GLP-1 also stimulates FIAT and enhances insulin-mediated effects on glucose uptake and lipid synthesis [82,83]. An array of adipocyte-derived secreted autocrine factors also influence lipogenesis. Both TNFa and leptin decrease lipid synthesis, through, for example, inhibition of acetyl CoA carboxylase [84,85]. NO also appears to inhibit insulin mediated glucose uptake [86].

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Recently, it was found that angiotensin II was synthesized and secreted by adipocytes, and increases the triglyceride content of adipocytes and the activities of two lipogenic enzymes: fatty acid synthase and glycerol-3-phosphate dehydrogenase [87]. These appear to be mediated through the type-2 angiotensin II receptor and the effect can be blocked by specific receptor blockade [88]. Moreover, angiotensin II increased the transcription rate of fatty acid synthase and leptin (obese) genes in 3T3 and human adipocytes [87,88]. Increased fat stores in obese yellow mice triggered studies to investigate the role of the agouti protein in lipogenesis. It was shown that agouti and agoutirelated proteins increased adipocyte fatty acid synthase gene expression in a Ca 21 dependent manner. Agouti and insulin exert similar but additive effects on adipocyte lipogenesis, indicating different signalling pathways [80,89]. A major autocrine lipogenic factor is ASP, the factor on which we have focused our studies. ASP is produced via the interaction of the complement proteins C3, factor B and adipsin. All three of these proteins are synthesized and secreted by adipocytes in a differentiation dependent manner [90,91], resulting in increased ASP production [92]. ASP is a potent stimulant of both triglyceride synthesis and glucose transport in adipocytes (for review see [93]). The autoregulatory effect of ASP on its principal physiological target, adipose tissue, may serve as a critical factor in determining regional adipose distribution and related metabolic disorders. ASP levels were shown to be increased in gynoid obesity and levels drop after a prolonged fast [94]. Although ASP levels are increased in obesity, the adipocytes from patients with morbid obesity remain as responsive to ASP as adipocytes from normal weight subjects [95]. This contrasts with the resistance to insulin action in some studies of obese subjects [96,97]. In addition to their greater capacity to store triglyceride, in vitro studies have shown that larger adipocytes can produce more ASP than smaller ones [90], thus higher circulating levels of ASP may enhance further TG storage. These studies and other evidence point to ASP as a major player in the pathophysiology of obesity, hyperlipidemia and related lipid disorders. Recently it was shown that a functional ASP knockout mouse had delayed triglyceride clearance [98] and intraperitoneal administration of ASP in genetically obese mice (ob /ob and db /db) increased triglyceride clearance markedly [99].

7. Positive feedback regulation There are two interesting properties which characterize many of these lipolytic / lipogenic factors. The first is that in many instances the hormones have opposite effects on lipolysis vs. lipogenesis (Tables 1,2). For example, insulin and agouti protein both decrease lipolysis but increase lipogenesis. Similarly, testosterone, estrogen, growth hormone, TNFa, NO and leptin all increase

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lipolysis, but decrease lipogenesis. The second feature is that a number of these factors, both endocrine and autocrine (such as insulin, leptin, angiotensin II and ASP), are stimulated by fatty acids or dietary lipids and they, in turn, then regulate the processes of lipolysis and lipogenesis which control the tissue disposition of these fatty acids and lipids. Certain fatty acids influence leptin secretion [26,39], and the secreted leptin not only has neuronal effects, but also appears to enhance lipolysis peripherally in adipocytes. Fatty acids increase angiotensin II [38] secretion from adipose tissue and insulin secretion from the pancreas [13]. Dietary chylomicrons increase ASP secretion from adipose tissue [40–42] and all of these hormones then stimulate lipogenesis in adipocytes enhancing storage of triglycerides. Thus a number of factors, especially autocrine factors, demonstrate positive / negative feedback regulation in adipocytes by the substrates which they influence.

8. Pharmacologic targets A consequence of the recent advances in adipose tissue biology is the development of a number of pharmacologic agents which target the pathways of lipolysis and / or lipogenesis directly or through the hormone effectors described above for the treatment of obesity and or diabetes. For example, AOD4O1 is an anti-obesity drug which is a synthetic analogue of growth hormones. It increases lipolysis and decreases lipogenesis in adipose tissue (both esterification of fatty acids and de novo fatty acid synthesis). This occurs in both subcutaneous and omental adipose tissue, however the effects were more pronounced in subcutaneous adipose tissue [100]. Vanadate and peroxovanadate, which act as insulinmimetic drugs, inhibit tyrosine phosphatases resulting in increased lipoprotein lipase secretion and decreased hormone-sensitive lipolysis [72,101]. Compound UK-14304 acts as an alpha-2 adrenergic agonist to decrease lipolysis in adipose tissue [64]. Thiazolidinediones (TD) are a new family of insulin sensitizing agents and appear to have multiple effects (for review see [34,35]). They improve the secretory response of beta cells to insulin secretagogues, improve insulin sensitivity in skeletal muscles, adipose tissue and hepatocytes and activate glycolysis in hepatocytes, thus reducing fasting hyperglycemia and insulinemia. In adipose tissue, TD increase expression of GLUT 1 and GLUT 4, increase lipoprotein lipase activity [102], and block the lipolytic effect of TNFa but not catecholamines [74]. TD markedly influence lipid metabolism by decreasing plasma triglyceride. Beyond triglyceride, there are limited data suggesting possible effects on fatty acids, LDL cholesterol and HDL cholesterol. Thiazolidinediones act directly through binding to the nuclear receptor PPARg, activating the receptor and influencing the lipogenic process as well as adipocyte differentiation. Thus many new pharmacologic opportunities are available which

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can target hormone receptor interactions (endocrine and autocrine) through the use of agonist / antagonist compounds, the intracellular pathways (signal transduction or nuclear receptors) or the enzymes directly involved in lipolysis / lipogenesis.

9. Summary In this review we have summarized briefly the multiple metabolic consequences of increased circulating fatty acids, and focused on the cellular processes which can influence this fatty acid trafficking in adipose tissue. One of the most interesting recent developments in adipose tissue biology has been the recognition that there are multiple autocrine and endocrine factors produced by adipose tissue. Preliminary data suggests that several of these newly described hormones may demonstrate potential lipolytic or lipogenic functions with positive / negative feedback regulation. Future experimental data will certainly define these pathways, and explore their interactions with each other as well as their potential as pharmacologic targets.

10. Abbreviations Ap2: ASP: DHEA: EP: FFA: FIAT: GLIAT: GLP-1: GLUT 1: GLUT 4: HDL: HSL: LDL: LPL: NE: NO: OM: PAl-I: PDE: PGJ 2 :

adipocyte fatty acid binding protein acylation stimulating protein dehydroepiandrosterone epinephrine free fatty acids fatty acid incorporation into adipose tissue glucose incorporation into adipose tissue glucagon like peptide glucose transporter 1 glucose transporter 4 high density lipoprotein hormone sensitive lipase low density lipoprotein lipoprotein lipase norepinephrine nitric oxide omental plasminogen activator inhibitor-1 phosphodiesterase prostaglandin J 2

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PIG-P: PPAR: SC: TD: TG: TNFa: VLDL:

175

phosphoinositolglycan-peptide peroxisome proliferator activated receptors subcutaneous thiazolidinedione triglyceride tumor necrosis factor a very low density lipoprotein

Acknowledgements This work was supported by grants from the Medical Research Council of Canada (KC), Heart and Stroke Foundation of Quebec (KC), and Servier Pharmaceuticals (ADS). KC is a research scholar of the Heart and Stroke Foundation of Canada.

References [1] Bjorntorp P, Bradoff BN. In: Obesity, Philadelphia Penn: Lippincott J B Company, 1992, pp. 3–12. [2] Vague J. The degree of masculine differentiation of obesities: a factor determining predisposition to diabetes, atherosclerosis, gout and uric calculous disease. Obes Res 1996;4:204–12. ´ ´ ´ Presse Med ´ [3] Vague J. La differenciation sexuelle, facteur determinant des formes de l’obesite. 1947;30:339. [4] Wing RR, Bunker CH, Kuller LH, Mathews KA. Insulin, body mass index and cardiovascular risk factors in premenopausal women. Arteriosclerosis 1989;9:479–84. [5] Peiris AN, Struve MF, Kissebah AH. Relationship of body fat distribution to the metabolic clearance of insulin in premenopausal women. Intl J Obesity 1987;11:581–9. [6] Karjalainen L, Pihlajamaki J, Karhapaa P, Laakso M. Impaired insulin-stimulated glucose oxidation and free fatty acid suppression in patients with familial combined hyperlipidemia: a precursor defect for dyslipidemia? Arterioscler Thromb Vasc Biol 1998;18:1548–53. [7] Despres JP. The insulin resistance-dyslipidemia syndrome: the most prevalent cause of coronary artery disease? Can Med Assoc J 1993;148:1339–40. [8] Despres JP, Moorjani S, Lupien PJ, Tremblay A, Nadeau A, Bouchard C. Regional distribution of body fat, plasma lipoproteins, and cardiovascular disease. Arteriosclerosis 1990;10:497–511. [9] Bjorntorp P. Portal adipose tissue as a generator of risk factors for cardiovascular disease and diabetes. Arteriosclerosis 1990;10:493–6. [10] Bjorntorp P. How should obesity be defined? J Intern Med 1990;227:147–9. [11] Jensen MD, Haymond MW, Rizza RA, Cryer PE, Miles PE. Influence of body fat distribution on free fatty acid metabolism in obesity. J Clin Invest 1989;83:1168–73. [12] Despres JP. The insulin resistance-dyslipidemic syndrome of visceral obesity: effect on patients’ risk. Obes Res 1998;6:85–175.

176

J. Saleh et al. / Clinica Chimica Acta 286 (1999) 163 – 180

[13] Boden G. Role of fatty acids in the pathogenesis of insulin resistance and NIDDM. Diabetes 1997;46:3–10. [14] Bengtsson G, Olivecrona T. Lipoprotein lipase. Mechanism of product inhibition. Eur J Biochem 1980;106:557–62. [15] Posner I, DeSanctis J. Kinetics of product inhibition and mechanisms of lipoprotein lipase activation by apolipoprotein C-II. Biochemistry 1987;26:3711–7. [16] Saxena U, Witte LD, Goldberg IJ. Release of endothelial lipoprotein lipase by plasma lipoproteins and free fatty acids. J Biol Chem 1989;264:4349–55. [17] Sniderman AD, Cianflone K. Substrate delivery as a determinant of hepatic apoB secretion. Arterioscler Thromb 1993;13:629–36. [18] Lewis GF. Fatty acid regulation of very low density lipoprotein production. Curr Opin Lipidology 1997;8:146–53. [19] Sessler AM, Ntambi JM. Polyunsaturated fatty acid regulation of gene expression. J Nutr 1998;128:923–6. [20] Niot I, Poirier H, Besnard P. Regulation of gene expression by fatty acids: special reference to fatty acid-binding protein (FABP). Biochimie 1997;79:129–33. [21] Cheema SK, Clandinin MT. Diet fat alters expression of genes for enzymes of lipogenesis in lean and obese mice. Biochim Biophys Acta 1996;1299:284–8. [22] Bastard JP, Bruckert E, Robert JJ. Are free fatty acids related to plasma plasminogen activator inhibitor I in android obesity? Int J Obes Relat Metab Disord 1995;19:836–8. [23] McGarry JD. Glucose-fatty acid interactions in health and disease. Am J Clin Nutri 1998;67:5500–4. [24] Colberg SR, Simoneau JA, Thaete FL, Keeley DE. Skeletal muscle utilization of free fatty acids in women with visceral obesity. J Clin Invest 1995;95:1846–53. [25] Van Baak MA, Schiffelers SLH, Saris WHM. Increased NEFA availability leads to a similar increase in energy expenditure and fat oxidation in lean and obese men. Int J Obes 1998;22:S157. [26] Vettor R, Lombardi AM, Fabris R. FFA. infusion induces insulin resistance and increases ob gene expression and leptin plasma levels in rats. Int J Obes 1998;22:S168. [27] Shimabukuro M, Zhou YT, Levi M, Unger RH. Fatty acid-induced beta cell apoptosis: a link between obesity and diabetes. Proc Nat Acad Sci USA 1998;95:2498–502. [28] Lu G, Morinelli TA, Meier KE, Rosenzweig SA, Egan BM. Oleic acid-induced mitogenic signaling in vascular smooth muscle cells. A role for protein kinase C. Circ Res 1996;79:611–8. [29] Morisaki N, Lindsey JA, Milo GE, Cornwell DG. Fatty acid metabolism and cell proliferation. III. Effect of prostaglandin biosynthesis either from exogenous fatty acid or endogenous fatty acid release with hydralazine. Lipids 1983;18:349–52. [30] Khan WA, Blobe GC, Hannun YA. Arachidonic acid and free fatty acids as second messengers and the role of protein kinase C. Cell Signal 1995;7:171–84. [31] Lee YM, Birgel M, Hube F, Hauner H. Free fatty acids are potent stimulators of differentiation in human preadipocytes. Int J Obes 1998;22:514. [32] Van Epps-Fung M, Williford J, Wells A, Hardy RW. Fatty acid-induced insulin resistance in adipocytes. Endocrinology 1997;138:4338–45. [33] Fickova M, Hubert P, Cremel G, Leray C. Dietary (n-3) and (n-6) polyunsaturated fatty acids rapidly modify fatty acid composition and insulin effects in rat adipocytes. J Nutrition 1998;128:512–9. [34] Komers R, Vrana A. Thiazolidinediones – tools for the research of metabolic syndrome X. Physiol Res 1998;47:215–25. [35] Schoonjans K, Martin G, Staels B, Auwerx J. Peroxisome proliferator-activated receptors, orphans with ligands and functions. Curr Opin Lipidology 1997;8:159–66.

J. Saleh et al. / Clinica Chimica Acta 286 (1999) 163 – 180

177

[36] Negrel R. Paracrine / autocrine signals and adipogenesis. Int J Obes 1998;22:520. [37] Muller G, Wied S, Piossek C, Bauer A, Bauer J, Frick W. Convergence and divergence of the signaling pathways for insulin and phosphoinositolglycans. Mol Med 1998;4:299–323. [38] Safonova I, Aubert J, Negrel R, Ailhaud G. Regulation by fatty acids of angiotensinogen gene expression in preadipose cells. Biochem J 1997;322:235–9. [39] Bergene E, Ukropec J, Liska B et al. n-3 Fatty acids decrease plasma leptin levels in dietary induced hypertriglyceridemia and insulin resistance. Int J Obes 1998;22:5221. [40] Maslowska M, Scantlebury T, Germinario R, Cianflone K. Acute in vitro production of ASP in differentiated adipocytes. J Lipid Res 1997;38:21–31. [41] Scantlebury T, Maslowska M, Cianflone K. Chylomicron specific enhancement of Acylation Stimulating Protein (ASP) and precursor protein C3 production in differentiated human adipocytes. J Biol Chem 1998;273:22903–9. [42] Saleh J, Summers LKM, Cianflone K, Fielding BA, Sniderman AD, Frayn KN. Coordinated release of acylation stimulating protein (ASP) and triacyglycerol clearance by human adipose tissue in vivo in the postprandial period. J Lipid Res 1998;39:884–91. [43] Amer P. Obesity and the adipocyte: regional adipocity in man. J Endo 1997;155:191–2. [44] Arner P. Differences in lipolysis between human subcutaneous and omental adipose tissues. Ann Med 1995;27:435–8. [45] Bjorntorp P. The regulation of adipose tissue distribution in humans. Int J Obes 1996;20:291–302. [46] Bjorntorp P. Hormonal control of regional fat distribution. Hum Reprod 1997;12:21–5. [47] Reynisdottir S, Dauzats M, Thorne A, Langin D. Comparison of hormone-sensitive lipase activity in visceral and subcutaneous human adipose tissue. J Clin Edocrinol Metab 1997;82:4162–6. [48] Olivecrona G, Olivecrona T. Triglyceride lipases and atherosclerosis. Curr Opin Lipidology 1995;6:291–305. [49] Brun LD, Gagne C, Julien P. Familial lipoprotein lipase activity deficiency: study of total body fatness and subcutaneous fat tissue distribution. Metabolism 1989;38:1005–9. [50] Abumrad NA, el-Maghrabi MR, Amri EZ, Lopez E, Grimaldi PA. Cloning of a rat adipocyte membrane protein implicated in binding or transport of long-chain fatty acids that is induced during preadipocyte differentiation. Homology with human CD36. J Biol Chem 1993;268:17665–8. [51] Caserta F, Civelek V, Hamilton JA, Kirkland JL, Corkey BE. Differences in free fatty acid uptake in differentiated preadipocytes from different anatomic regions. Int J Obes 1998;22:557. [52] Scheja L, Makowski L, Shimshek DR. Impaired lipolysis and altered insulin secretion in mice deficient for the adipocyte fatty acid-binding protein, aP2. Int J Obes 1998;22:532. [53] Walldius G. What happens after lipoprotein lipase? A low fatty acid incorporation into adipose tissue (FIAT) in hypertriglyceridaemia-experimental and clinical studies. Int J Obes 1981;5:707–16. [54] Walldius G. Serum triglycerides and fatty acid incorporation into human adipose tissue (FIAT). Their relations with adipose tissue characteristics and glucose tolerance. Acta Medica Scand 1976;200:409–21. [55] Edens NK, Fried SK, Kral JG, Hirsch J, Leibel RL. In vitro lipid synthesis in human adipose tissue from three abdominal sites. Am J Physiol 1993;265:E374–9. [56] Ostman J, Amer P, Engfeldt P, Kager L. Regional differences in the control of lipolysis in human adipose tissue. Metabolism 1979;28:1198–205. [57] Maslowska M, Sniderman AD, MacLean LD, Cianflone K. Regional differences in triacylglycerol synthesis in adipose tissue. J Lipid Res 1993;34:219–28.

178

J. Saleh et al. / Clinica Chimica Acta 286 (1999) 163 – 180

[58] Giudicelli Y. Expression and possible role of sex steroid receptors in rat and human preadipocytes during adipogenesis. Int J Obes 1998;22:529. [59] Sutter-Dub MT, Cordoba P. Acute effects of progesterone on glucose metabolism in rat adipocytes: are they modulated by endogenous adenosine? Metab Clin Exp 1997;46:595– 604. [60] Goto T, Hinata T, Suda T. Gender difference in antilipolytic action of insulin in relation to body fat distributions. Int J Obes 1998;22:517. [61] Schiffelers SLH, Saris WHM, Van Baak MA. b 2 -adrenoceptor mediated lipolysis and fat oxidation are reduced in obese men. Int J Obes 1998;22:575. [62] Guo Z, Johnson CM, Jensen MD. Regional lipolytic responses to isoproterenol in women. Am J Physiol 1997;273:E108–12. [63] Van Harmelen V, Lonnqvist F, Thorne A. Noradrenaline-induced lipolysis in isolated mesenteric, omental and subcutaneous adipocytes from obese subjects. Int J Obes Relat Metab Disord 1997;21:972–9. [64] Kolehmainen M, Ohisalo JJ, Paakkonen M, Poikolainen E, Alhava E, Uusitupa MIJ. Weakened inhibition of adipose tissue lipolysis is related to disturbed glucose metabolism in severe obesity. Int J Obes 1998;22:5108. [65] Lonnqvist F, Thorne A, Large V, Amer P. Sex differences in visceral fat lipolysis and metabolic complications of obesity. Arterioscler Thromb Vasc Biol 1997;17:1472–80. [66] Jones JP, Dohm GL. Regulation of glucose transporter GLUT-4 and hexokinase II gene transcription by insulin and epinephrine. Am J Physiol (Endocrinol Metab) 1997;273:E682– 7. [67] Stevenson RW, Kreutter DK, Andrews KM, Genereux PE, Gibbs EM. Possibility of distinct insulin-signaling pathways beyond phosphatidylinositol 3-kinase-mediating glucose transport and lipogenesis. Diabetes 1998;47:179–85. [68] Fried SK, Russell CD, Papaspyrou-Rao S, Ricci MR, Bunkin DA. Regional differences in the hormonal regulation of lipoprotein lipase and leptin expression in human omental and subcutaneous adipose tissue. Int J Obes 1998;22:520. [69] Arner P, Bolinder J, Engfeldt P, Ostman J. The antilipolytic effect of insulin in human adipose tissue in obesity, diabetes mellitus, hyperinsulinemia and starvation. Metabolism 1981;30:753–60. [70] Sinha MK, Taylor LG, Pories WJ. Long-term effect of insulin on glucose transport and insulin binding in cultured adipocytes from normal and obese humans with and without non-insulin-dependent diabetes. J Clin Invest 1987;80:1073–81. [71] Skarda J. Effect of bovine growth hormone on growth, organ weights, tissue composition and adipose tissue metabolism in young castrated male goats. Livestock Prod Sci 1998;55:215– 25. [72] Castan I, Wijkander J, Manganiello V, Belfrage P, Degerman E. Mechanisms of inhibition of lipolysis by insulin, vanadate and peroxovanadate in rat adipocytes. Int J Obes 1998;22:S109. [73] Aguado M, Fruhbeck G, Margareto J, Martinez JA. Neuropeptide Y decreases basal lipolytic activity on isolated adipocytes from wistar rats. Int J Obes 1998;22:S109. [74] Souza SC, Yamamoto MT, Franciosa MD, Lien P, Greenberg AS. BRL 49653 blocks the lipolytic actions of tumor necrosis factor-alpha: a potential new insulin-sensitizing mechanism for thiazol idinediones. Diabetes 1998;47:691–5. [75] Gaudiot N, Jaubert AM, Lacasa D, Sabourault D, Giudicelli Y, Ribiere C. Modulation of white adipose tissue lipolysis by nitric oxide (NO): role of the NO redox state. Int J Obes 1998;22:S101. [76] Lacasa D, Giudicelli Y, Ribiere C. Modulation of white adipose tissue lipolysis by nitric oxide. J Biol Chem 1998;273:13475–81.

J. Saleh et al. / Clinica Chimica Acta 286 (1999) 163 – 180

179

[77] Fruhbeck G, Aguado M, Martinez JA. In vitro lipolytic effect of leptin on mouse adipocytes: evidence for a possible autocrine / paracrine role of leptin. Biochem Biophys Res Commun 1997;240:590–4. [78] Fruhbeck G, Aguado M, Gomez-Ambrosi J, Martinez JA. Lipolytic effect of in vivo leptin administration on adipocytes of lean and ob /ob mice, but not db /db mice. Biochem Biophys Rese Commun 1998;250:99–102. [79] Fruhbeck G, Aguado M, Gomez-Ambrosi J, Martinez JA. In vivo leptin administration increases basal adipocyte lipolysis of ob /ob mice, but not db /db mice. Int J Obes 1998;22:S176. [80] Wilkison WO, Moustaid-Moussa N, Mynatt RL. Role of adipocyte and pancreas intracellular calcium in agouti-induced obesity. Int J Obes 1998;22:557. [81] LeBlanc J, Picard F, Huang Q, Lalonde J, Richard D, Deshaies Y. DHEA and its metabolite etiocholanedione acutely increase adipose tissue lipoprotein lipase in mice. Int J Obes 1998;22:5189. [82] Yip RGC, Boylan MO, Wolfe MM. Glucose-dependent insulinotropic polypeptide receptors are present and functional on fat cells. Int J Obes 1998;22(3):530. [83] Montrose-Rafizadeh C, Yang H, Wang Y, Roth J, Montrose MH, Adams LG. Novel signal transduction and peptide specificity of glucagon-like peptide receptor in 3T3-L1 adipocytes. J Cell Physiol 1997;172:275–83. [84] Bai Y, Zhang S, Kim KS, Lee JK, Kim KH. Obese gene expression alters the ability of 30A5 preadipocytes to respond to lipogenic hormones. J Biol Chem 1996;271:13939–42. [85] Prins JB, Niesler CU, Winterford CM. Tumor necrosis factor-alpha induces apoptosis of human adipose cells. Diabetes 1997;46:1939–44. ´ [86] Marette A, Perreault M, Bedard S, Roy D, Kapur S. Nitric oxide: a new player in skeletal muscle and adipose tissue metabolism. Int J Obes 1998;22:S101. [87] Jones BH, Standridge MK, Moustaid N. Angiotensin II increases lipogenesis in 3T3-LI and human adipose cells. Endocrinology 1997;138:1512–9. [88] Moustaid-Moussa N, Standridge M, Jones BH. Differential effects of angiotensin II receptor blockade in lean and obese mice. Int J Obes 1998;22:574. [89] Claycombe KJ, Moustaid-Moussa N, Xue B, Zemel MB, Mynatt R, Wilkison WO. Mechanisms of agouti-induced obesity: effects on adipocyte metabolism. Int J Obes 1998;22:S102. [90] Cianflone K, Roncari DAK, Maslowska M, Baldo A, Forden J, Sniderman AD. The adipsin-acylation stimulating protein system in human adipocytes: regulation of triacylglycerol synthesis. Biochemistry 1994;33:9489–95. [91] Choy LN, Rosen BS, Spiegelman BM. Adipsin and an endogenous pathway of complement from adipose cells. J Biol Chem 1992;267:12736–41. [92] Cianflone K, Maslowska M. Differentiation induced production of ASP in human adipocytes. Eur J Clin Invest 1995;25:817–25. [93] Cianflone K. The acylation stimulating protein pathway: clinical implications. Clin Biochem 1997;30:301–12. [94] Cianflone K, Sniderman AD, Kalant D, Marliss EB, Gougeon R. Response of plasma ASP to a prolonged fast. Intl J Obesity 1995;19:604–9. [95] Walsh MJ, Sniderman AD, Cianflone K, Vu H, Rodriguez MA, Forse RA. The effect of ASP on the adipocyte of the morbidly obese. J Surg Res 1989;46:470–3. [96] Kissebah AH, Vydelingum N, Murray R. Relation of body fat distribution to metabolic complications of obesity. J Clin Endocrinol Metab 1982;54:254–60. [97] Ciaraldi TP, Kolterman OB, Olefsky JM. Mechanism of the postreceptor defect in insulin action in human obesity. Decrease in glucose transport system activity. J Clin Invest 1981;68:875–80.

180

J. Saleh et al. / Clinica Chimica Acta 286 (1999) 163 – 180

[98] Murray I, Sniderman A, Phelis S, Wetsel R, Colten H, Cianflone K. Postprandial triglyceride metabolism in a functional acylation stimulating (ASP) knockout mouse. Int J Obesity 1998;22:533. [99] Saleh J, Sniderman A, Cianflone K. Postprandial triacylglycerol clearance in ob /ob and db /db mice: physiological effects of ASP in mice models of obesity and hypertriglyceridemia. Int J Obesity 1998;22:S183. [100] Heffernan M, Ogru E, Igniatovic V, Gianello R, Jiang WJ, Ng F. Sensitivity of human subcutaneous adipose tissues to the lipolytic action of an anti-obesity compound A0D9401. Int J Obesity 1998;22:S110. [101] Motoyashiki T, Miyake M, Morita M, Mizutani K, Ueki H. Enhancement of the vanadatestimulated release of lipoprotein lipase activity by astilbin from the leaves of Engelhardtia chrysolepis. Biol Pharm Bull 1998;21:517–9. [102] Ottoson M, Oden B, Eden S. The effects of BRL49653, an insulin-sensitizing agent, on lipoprotein lipase activity in human adipose tissue. Int J Obes 1998;22:570. [103] Iwao N, Oshida Y, Sato Y. Regional difference in ipolysis caused by a beta-adrenergic agonist as determined by the microdialysis technique. Acta Physiol Scand 1997;161:481–7. [104] Labelle M, Boulanger Y, Fournier A, St.Pierre S, Savard R. Tissue-specific regulation of fat cell lipolysis by NPY in 6-OHDA-treated rats. Peptides 1997;18:801–8. [105] Girouard H, Savard R. The lack of bimodality in the effects of endogenous and exogenous prostaglandins on fat cell lipolysis in rats. Prostaglandis and Other Lipid Mediators 1998;56:43–52.