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Metabolism of Macro- and Micronutrients Topic 1 Module 1.3

Metabolism of Lipids: New Insight Regina Komsa-Penkova,

Department of Biochemistry,

Medical Faculty,

Medial University –Pleven, Bulgaria

Lubos Sobotka,

Department of Metabolic Care and Gerontology,

Medical Faculty, Charles University,

Hradec Kralove, Czech Republic

.

Learning objectives

To learn the important dietary lipids;

To learn the role of free fatty acids;

To understand the main metabolic lipids’ pathways ;

To learn the functions of lipoproteins;

To learn the main steps of lipid metabolism.

Contents:

1. Introduction: dietary lipids

2. Main classes of fatty acids (FA)

2.1. Saturated FA

2.2. Cis MUFAs

2.3. PUFAs

2.3.1. - 6 PUFAs

2.3.2. -3 PUFAs

2.4. Trans FA

2.5. Conjugated FA

3. Metabolism of lipoproteins

3.1. Chylomicron production

3.2. VLDL and LDL

3.3. HDL

4. Lipid metabolism

4.1. Lipolysis

4.2. FFA in circulation

4.3. Fatty acid oxidation

4.4. Reesterification of FA into TAGs

4.5. Lipid droplets

4.6. Adipokines

5. References

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1. Introduction: Dietary Lipid

Lipids are important source of energy but possess many other metabolic functions as the

structural components of cellular and organelle membranes and essential precursors for

hormones, local mediators and regulatory molecules.

The accumulation of energy as fat was very important for survival of our ancestors in the

past. However, at present this accumulation leads to the development of obesity.

The increase in obesity incidence led to extensive research in the area of lipid metabolism,

food intake, appetite control as well as its contribution to the metabolic changes in

dyslipidemias, cardiovascular disease, endocrine disorders and cancer. Recent investigations

on metabolism highlighted the role of lipids as inflammatory and allergic components by

variety of pro and anti-inflammatory eicosanoids, specific cell signalling molecules for PPAR,

GP 120, Nf-kB, toll like receptors (TLR), influencing cell’s receptiveness, involved in growth

and development processes etc. New investigated molecules of lipid mediators like resolvins,

protectins, sirtulins, and maresins provided new insights into the inflammation process.

SREBP (sterol regulatory element binding protein), grelin, leptin, fatty acid transporters have

changed the understanding of lipid regulatory mechanisms.

According to general recommendations lipids should provide around 20-35 % of energy

intake in healthy individual. Moreover, lipids are necessary for absorption and transport of

lipid-soluble vitamins. Ingested lipids are either oxidised or used as building material in the

body (cell membranes, neural tissue, etc.) Excess of fat is accumulated in fat stores as the

body energy reserve. The energy yield of lipid is approximately 9.4 Kcal/g (39.3 kJ/g),

compared to 4.2 Kcal/g (17.6 kJ/g) for carbohydrates. In fact, the energy storage capacity

for fat is almost unlimited in human body. Usually the total amount of energy stored in

adipose tissue as triacylglycerol (TAG) (80,000-140,000 kcal) is 40-70 times higher than

that stored under the form of glycogen (1,700-2,000 kcal)(1, 2) and in obese subjects is

even higher.

Fig. 1 Energy reserves of human body:

TAG- 80000-140000 kcal, usable protein 25-30000 kcal, liver glycogen 400-600 kcal, blood

glucose 40 kcal and daily needs 2500-2800 kcal. Blood glucose at 5mM is sufficient for a

few minutes.

However, the excessive accumulation of TAG in human adipose tissue leads to obesity

development. Moreover, the accumulation of lipids in other tissues like skeletal muscle and

the liver, can be associated with insulin resistance and organ dysfunction. For example

excessive TAG deposition in the liver is associated with nonalcoholic steatohepatitis, in

skeletal muscles with insulin resistance and in the heart with cardiomyopathy (3, 4).

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2. Main Classes of Fatty Acids (FA)

Dietary lipids consist primarily of TAG - (97%), phospholipids (PL) and sterols. They are also

transporters for fat soluble vitamins. Quantitatively the major component of TAG and PL are

fatty acids (FA).

FA with the chain length of 14 to 22 carbons, with minor quantity of shorter and longer chain

FA (see Table 1). FAs are classified according to their main characteristics: chain length as

short-chain (2-4 carbons), medium chain (6-12 carbons) and long chain FA (14 to 22

carbons), according to the presence or absence of double bonds (saturated / unsaturated)

and their number (mono-, polyunsaturated). The location of the first double bond from the

methyl end is also important for FA nomenclature: (-3, -6 and -9 or n-3, n-6, or n-9

respectively). The double bonds can occur in either the cis or trans configuration.

The major dietary FAs are saturated, monounsaturated and polyunsaturated.

Table 1

Classification of FA and the distribution in TAG of adipose tissue in human

Class FA Subclasses Individual FA TAG of adipose

tissue in human %

Saturated

FAs

Short chain

Medium chain

Long Chain

2:0 Acetic acid

3:0 Valeric acid

4:0 Butyric acid

6:0 Caproid acid

8:0 Caprylic acid

10:0 Caproic acid

12:0 Lauric acid

14:0 Myristic acid

16:0 Palmitic acid

18:0 Stearic acid

5 %

24%

8%

Mono unsatureated Cis FA

18:1n-9 Oleic acid

14:1n-7 Myristoleic acid 16:1n-7 Palmitoleic acid 18:1n-7 Vaccenic acid 20:1n-9 Eicosenoic acid 22:1n-9 Erucic acid

46%

7%

Poly-unsaturated Cis FA

n-6 FAs

18:2 Linoleic acid

18:3 γ-Linolenic acid 20:3 Dihomo-γ-linolenic acid 20:4 Arachidonic acid 22:4 Adrenic acid 22:5 Docosapentaenoic acid

7%

Poly-unsaturated Cis FA

n-3 FAs

18:3 α-Linolenic acid 20:5 Eicosapentaenoic acid 22:5 Docosapentaenoic acid 22:6 Docosahexaenoic acid

1%

Trans FAs

9-trans,12-cis 18:2; 9-cis,12-trans 18:2

In general, animal lipids have high content of saturated and monounsaturated FAs (MUFA),

and are mainly solid (lard, tallow). Plant lipids are mainly oils and have a high content of

unsaturated FAs. Exceptions to this rule are coconut oil and palm kernel oil, which are high

in saturated lipid or waxes.

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2.1 Saturated FA

Recently saturated FAs have been considered to be associated with increased atherogenic

risk and adverse health outcome (5). Numerous studies have been conducted to investigate

the effect of saturated FAs on serum cholesterol concentration. Meta-analysis of metabolic

studies showed that the higher the intake of saturated FAs, the higher the serum total

cholesterol (6).

It has been shown that especially lauric, myristic, and palmitic, acids (intermediate chain

lengths 12:0–16:0) may increase the synthesis and accumulation of triglyceride and

cholesterol in liver (7), each resulting in the suppression of hepatic LDL receptor mRNA

levels (8). Replacing saturated FA with MUFA and PUFA, can reduce plasma LDL cholesterol

(LDL-C) (9, 10) via an increase in LDL receptor (LDLR)-mediated uptake of LDL-C from

circulation (11). LDLR-mediated uptake, however, is impaired by obesity. The greater rate of

hepatic cholesterol synthesis in obese individuals suppresses the expression of hepatic LDL

receptors (LDLR), thereby reducing hepatic LDL uptake. Not all the saturated FAs exhibit this

effect. Shorter chain saturated FAs (6:0–10:0) have little effect on plasma cholesterol

concentrations. Stearic acid, is neutral with respect to HDL cholesterol, and can lower LDL

cholesterol and the ratio of total to HDL cholesterol. Stable isotope tracer methods have

shown that approximately 9 to 14 % of dietary stearic acid is converted to oleic acid (12).

2.2 Cis Monounsaturated FAs

Cis monounsaturated FAs (MUFAs) contain one double bond with the hydrogen atoms

positioned on the same side of the double bond. Plant lipids are rich in cis MUFA like olive

oil, sunflower oil and canola oils. The double bond is localised usually in -9 position and

partially in position -7 (n-7). The most frequent MUFA is oleic acid (-9), which accounts

for about 92 % of dietary MUFAs. Palmitoleic acid (-7) is presented in minor amount in the

diet. There is convincing evidence that replacing carbohydrates with MUFA increases HDL

cholesterol concentration in plasma and improves insulin sensitivity; replacing of SFA

(C12:0–C16:0) with MUFA reduces LDL cholesterol concentration and total/HDL cholesterol

ratio (13).

2.3 Polyunsaturated FAs

Polyunsaturated FAs (PUFAs) are essential FA as mammalian cells do not have the enzymatic

system which inserts a cis double bond at the -6 or -3 positions of a FA chain. There are

two important classes of PUFAs -3 and -6 (double bond located at 3 and 6 carbon atoms

from the methyl end respectively). Linoleic acid (LA; 18:2n-6) is the quantitatively most

important PUFA, comprising 84–89% of the total PUFA energy, whereas -linolenic acid

(ALA; 18:3n-3) contributes 9–11% of the total PUFA energy. The recommended adequate

intake of LA is 17 g/d for young men and 12 g/d for young women, whereas of ALA is 1.6

and 1.1 g/d for men and women, respectively.

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Fig. 2 3D structure of Linoleic acid (LA;18:2 n-6), -Linolenic acid (ALA;18:3 n-3)

and DHA (22: 6n- 3)

Usually in western diet PUFAs contribute <7% of total energy intake and 19–22% of energy

intake from lipid in the diets of adults. PUFAs serve as the precursors to eicosanoids,

components of membrane phospholipids, and are also important in cell signalling pathways.

2.3.1 -6 PUFA

The most abundant -6 polyunsaturated FAs in our diet are:

Linoleic acid - LA

Arachidonic acid - AA

Dihomolinoleic acid - DHLA

Arachidonic acid and other PUFAs are involved in regulation of gene expression resulting in

decreased expression of proteins that regulate the enzymes involved in FA synthesis (14).

This may partly explain the ability of PUFAs to influence the hepatic synthesis of FAs. A lack

of dietary -6 polyunsaturated FAs is characterized by rough and scaly skin, dermatitis, and

an elevated eicosatrienoic acid to arachidonic acid ratio.

AA is the substrate for the production of a wide variety of eicosanoids (20-carbon AA

metabolites). Some are proinflammatory, vasoconstrictive, and/or proaggregatory, such as

prostaglandin E2, thromboxane A2, and leukotriene B4. However, others are

antiinflammatory/antiaggregatory, such as prostacyclin, lipoxin A4, and epoxyeicosatrienoic

acids. 11, 12-Epoxyeicosatrienoic acids are FA epoxides produced from AA by a cytochrome

P450 epoxygenase. Dihomo-γ-linolenic acid, formed from linoleic acid, is also an eicosanoid

precursor.

2.3.2 -3 PUFA

Polyunsaturated FAs in diet are:

-linolenic acid - ALA

Eicosapentaenoic acid - EPA

Docosahexaenoic acid - DHA

They play an important role as structural membrane lipids, particularly in nerve tissue and

the retina, and also serve as precursors to regulatory eicosanoids. ALA (18:3n-3) is an

essential FA, lack of ALA results in adverse clinical symptoms, including neurological

abnormalities, scaly dermatitis and poor growth. Vegetable oils are the major source of ALA,

fish oils contains EPA (20:5n-3) and DHA (22:6n-3) (15). ALA is the precursor of

eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), which are synthesized in

human organism. These very long chain PUFAs (EPA, DHA) are direct precursors of

eicosanoids with lower inflammatory activity than majority of pro-inflammatory eicosanoids

which are metabolites of AA.

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It was shown that increased consumption of -3 FAs from fish or fish-oil supplements (20

and 22 carbons), but not of ALA (18 carbons), reduces the rates of all-cause mortality,

cardiac(16) and sudden death (17). This is probably due to their anti-inflammatory effects.

A considerable number of observational and interventional studies, systematic reviews and

meta-analyses of the relationship of dietary -3 FAs and CVD events have been published

(18, 19). The inverse relationship between -3 FA intake and CVD events was found and

confirmed for EPA and DHA.

Moreover, EPA and DHA from fish oil have regulatory role in the expression of genes

involved in lipid and energy metabolism. Two transcriptional factors, particularly, sterol

regulatory element binding protein-1c (SREBP-1c) and peroxisome proliferator activated

receptor (PPAR ), were investigated as the regulators of gene expression by PUFA. -3

PUFA suppress the induction of lipogenic enzymes by inhibiting the expression and

processing of SREBP-1c (by antagonizing LXR-dependent activation of SREBP-1c) (20).

PPAR plays a key role in metabolic adaptation to fasting by inducing the genes for

mitochondrial and peroxisomal FA oxidation as well as those for ketogenesis in mitochondria.

2.4 Trans FAs

Trans FAs contain at least one double bond in the trans configuration, which results in a

chain straight shape more similar to saturated FAs. A major trans FA is elaidic acid (9-trans

18:1). Trans FAs are mainly produced by industrial hydrogenation of plant oils to produce

margarines. Partial hydrogenation of polyunsaturated oils causes saturation and hardening

of oils to margarines, however, isomerisation and migration of double bonds, can result in a

production of mixed 9-trans,12-cis 18:2; 9-cis,12-trans 18:2 FA (20).

There is a positive linear trend between trans FA intake in diet and plasma LDL cholesterol

concentration and increased risk of CHD. An inverse association between total trans FAs and

AA and DHA and concentrations in cholesteryl esters in plasma was described (22) as well as

between plasma cholesteryl esters elaidic acid (18:1trans), and birth weight of premature

infants (23) were also reported.

2.5 Conjugated Linoleic Acids (CLAs)

CLAs are geometric and positional isomers of linoleic acid containing conjugated trans and

cis double bonds. Nine different isomers of CLA as minor constituents of food were reported,

but only two of the isomers, cis-9, trans-11 and trans-10, cis-12, possess biological activity

including anticancer, anti-atherosclerosis and prevention of obesity development (24). CLAs

are naturally present in dairy products and ruminant meats as a consequence of

biohydrogenation in the rumen.

3. Metabolism of Lipoproteins

3.1 Chylomicron Production

The main pathway for dietary fats to enter the bloodstream is through chylomicron (CM)

formation in the intestine. Once released by pancreatic lipase into the intestine, FA and

monoacylglycerol (MAG) molecules are absorbed into enterocytes, where they are re-

esterified into TAGs and incorporated into TAG-rich particles covered by phospholipids. After

acquiring of Apo B-48 (the main apoprotein of chylomicron) by the action of microsomal transfer protein they are called chylomicrons. The production of apo B-48 (1.3 mgkg-1

d-1)

varies on the basis of dietary fat intake. The average residence time of apo B-48 in plasma

is 4.8 h (25).

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CMs enter the lymphatic system and the circulation through thoracic duct. In peripheral

tissues (manly adipose and muscle tissue) chylomicrones gain apoprotein C and then they

are hydrolysed by lipoprotein lipase (LPL), which degrades their core of TAGs; this lead to

the release of Fas and surface phospholipids. Released Fas are taken up by various tissues.

However some FA are not rapt at the place of hydrolysis but rather leak out into the plasma

as FFA pool were they are bound on albumin. Then these FFA are taken up by the liver or

other tissues. LPL is activated by an apoprotein CII, transferred to CM from other

lipoproteins (mainly HDL). After CMs lose much of their TAG, they acquire cholesterol ester

from other lipoproteins via the action of cholesterol ester transfer protein (CETP). During this

process CM also acquire apo E from HDL and they turn into chylomicron remnants which are

taken up by the liver. TAG originating from CM-remnant as well these synthesized de novo

in liver are repacked into very low-density lipoproteins (VLDLs), thereby recycling the

dietary Fas. However, the rate of incorporation de novo synthesized Fas into VLDL-TAG is

much slower in contrast to the incorporation of Fas from the plasma FFA pool.

Impaired postprandial plasma TAG clearance by adipose tissue was reported in obese

subjects after ingestion of a single mixed meal(26). This is partly explained by a lower

functional LPL activity per unit fat mass in combination with the absence of postprandial up

regulation of adipose tissue LPL in obesity.

3.2 VLDL and LDL

In the endogenous pathway the hepatic TAGs synthesized from glucose, Fas and other lipids,

are packaged into VLDL – very low density lipoproteins which are rich in triacylglycerol and

contain mainly protein Apo B-100. Apo B-100 is the major apoprotein of VLDL and the sole

protein of LDL. About 20.4 mgkg-1d-1 of apo B is packed into VLDL along with cholesterol

and phospholipids (23, 27). VLDLs transport TAGs from the liver to peripheral tissues, such

as muscle and adipose tissue, where in capillaries on the endothelial cell surface undergo

intravascular lipolysis by LPL. In circulation VLDL acquire another apoproteins (apo C-I and –

III and apo E) and phospholipids from HDL particles. ApoC-I and ApoC-III are small proteins

that modulate lipolysis (activation of LPL) and interaction of TAG rich particles with

receptors. After losing core triacylglyceroles approximately half of the VLDL remnants

(intermediate density lipoproteins – IDL) are cleared from the circulation by LDL receptor

(LDL-R) mediated endocytosis in the liver, and the residue undergoes further lipolysis to

produce LDL (28). VLDL apo B-100 has a residence time of 3.6h. Most of LDL is removed

from the circulation after binding to the hepatic LDL-R via apoB-100 (29). An estimated 70%

of circulating LDL is cleared by LDL-R in the liver.

During postprandial lipolysis TAG lipoprotein derived Fas are stored in the adipose tissue,

but also contribute significantly to the plasma FA pool (30). In patients with elevated TAGs

(hyperlipidemia) VLDL are increased in number and size and contain more TAGs.

These larger TAG-rich VLDL have delayed lipolysis in insulin-resistant patients, which may be

due to a lower affinity for LPL, lower LPL activity or lower affinity to tissue and hepatic

receptors that promote the degradation and clearance of VLDL. VLDLs with increased plasma

residence time transfer some of their core TAG to HDL and LDL via CETP, in exchange for

cholesterol ester (CE).

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Fig. 3 Lipid metabolism and transport. Liver produces VLDL from Cholesterol

and TAGs synthesized de novo and from CM-remnants (exogenous). Lecithin-

cholesterol acyltransferase (LCAT) esterifies free cholesterol I, forming the

core of newly synthesized HDL molecules. LPL hydrolyzes TG in VLDL LPL

(releasing glycerol and FFA)which result in LDL molecules taken up by

extrahepatic tissues and/or liver. HDL take back cholesterol to the liver in a

process known as reverse cholesterol transport. (ACAT, acyl-CoA:cholesterol

acyltransferase; HL, hepatic lipase).

HDL take back cholesterol to the liver in a process known as reverse

cholesterol transport. (ACAT, acyl-CoA:cholesterol acyltransferase; HL,

hepatic lipase). Lecithin-cholesterol acyltransferase (LCAT) esterifies free

cholesterol I, forming the core of newly synthesized HDL molecules.

Increased numbers of apo B or LDL particles not cleared by hepatic LDL receptors have

increased plasma residence time and may enter the arterial intima. Persistence of LDL in

circulation leads to production of atherogenic small dense LDLs. In individuals with

dyslipidaemia, LDLs and other atherogenic lipoproteins enter the arterial wall where they

undergo chemical modification, including oxidation (31). These modified lipoproteins initiate

the inflammatory process that culminates in atherosclerosis lesion development and

coronary heart disease (CHD) (32).

Patients with marked elevations of LDL cholesterol and tendinous xanthomas generally have

familial hypercholesterolemia resulting from a delayed catabolism of LDL apo B-100 (33)

associated with various defects in the LDL receptor (see Module 22.1).

3.3 HDL

The first step in cholesterol reverse transport is the production of Apo A-I and A-II by the

liver and Apo A-I by the intestine and their combination with the phospholipids and

cholesterol with the subsequent formation of discoid aggregates which are HDL precursors

(34). Cholesterol is taken from the cells of peripheral tissues by HDL either by passive

diffusion or through the action of ATP-dependent transmembrane transporter: ATP-binding

cassette transporter-1 (ABCA-1) (35). In HDL particle cholesterol then undergoes

esterification by the LCAT enzyme, forming cholesterol esters. This esterification prevents

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re-diffusion of cholesterol from HDL back to the membrane and amplifies cholesterol efflux,

forming “mature spherical” HDL.

HDL particles are captured by liver via scavenger receptor B, class 1 (SRB1) and apoE

receptors (adrenal glands, ovaries). In individuals with dyslipidaemia, when LDLs and other

atherogenic lipoproteins chemically modified, in the arterial wall initiate the inflammatory

process and atherosclerosis lesion development, the inflammation can be reversed by HDLs

via promotion of cholesterol efflux and/or inhibition of LDL oxidation and reduction of

adhesion molecule expression (28).

4. Lipid Metabolism

Lipid stores of white adipose tissue represent the major energy reserves in humans. After

food intake, most of the FAs released by LPL during postprandial lipolysis are taken up by

adipose tissue (AT) and esterified into TAG, which are subsequently stored in cytosolic lipid

droplets (LDs) of adipocytes. On energy demand, TAGs are mobilized from their stores by

hydrolytic cleavage and the resulting FFAs are delivered via the circulation to peripheral

tissues for β-oxidation and ATP production.

Lipid droplet-associated TAGs are also found in most nonadipose tissues, including liver,

cardiac muscle, and skeletal muscle (36). However, whereas adipocytes are able to release

FAs and provide them as systemic energy substrate, non-adipose cells do not secrete FFAs

but utilize them locally for energy production or lipid synthesis. Excessive ectopic lipid

deposition in non-adipose tissues leads to lipotoxicity.

4.1 Lipolysis

Consistent with its essential importance in energy homeostasis, lipolysis occurs in essentially

all tissues and cell types, however, it is most abundant in white and brown adipose tissues

(37).

Triacylglycerol of adipose tissue can be rapidly mobilized by the hydrolytic action of the

three main lipases of the adipocyte (38). The complete hydrolysis of TAG depends on the

activity of three enzymes, adipose triglyceride lipase (ATGL), hormone-sensitive lipase (HSL)

and monoacylglycerol lipase (MGL). Until recently HSL was considered to be the key rate-

limiting enzyme responsible for regulating TAG mobilization. In addition to its activity

towards triacylglycerols, HSL hydrolyses diacylglycerols (10 times more effectively than

TAG), monoacylglycerols, retinyl esters and cholesterol esters.

Recently identified enzyme - ATGL has been discussed as playing an important role in the

control of fat cell lipolysis. The role of ATGL in lipolysis became evident from the

observations of a severe “lipid” phenotype in ATGL-deficient mice (39). Absence of ATGL

causes a reduction of FA release from white adipose tissue by more than 75%, leading to

increased total fat mass, increased ectopic fat mass, and increased body weight.

Fig. 4 The hydrolytic cleavage of TAG by consequent action of the enzymes

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The full hydrolysis of TAG is dependent on the activity of three lipases ATGL, HSL and MGL,

each of which possesses a distinct specificity and regulatory mechanism.

Considerable progress has been made in understanding the mechanisms of activation of the

various lipases, mostly HSL. The best understood hormonal effects on AT lipolysis concern

the opposing regulation by insulin and catecholamines/glucagon, natriuretic peptides and

numerous autocrine/paracrine factors originating from adipocytes.

Glucagon is secreted during low glucose fasting state, and epinephrine is associated with

increased metabolic demands. In these conditions the energy need is covered by oxidation

of FA. Glucagon, norepinephrine, and epinephrine bind to G protein-coupled receptors that

activate production of cyclic AMP, leading to activation of protein kinase A, which as a

consequence activates (phosphorylates) HSL, thus stimulating lipolysis. Insulin stimulates

the opposite (inhibitory) effect when blood glucose is high. Insulin activates protein

phosphatase 2A, which dephosphorylates HSL, thereby inhibiting its activity. Insulin also

activates the enzyme phosphodiesterase, which hydrolysis cAMP and stops the effects of

protein kinase A.

Although more is known about HSL, it has been shown that HSL and ATGL can be activated

simultaneously, that enables HSL to access the surface of lipid droplets and stimulates ATGL.

Expression of ATGL is under the influence of dietary status. In fasting animals the level of

ATGL increases and then declines following re-feeding. It has recently been reported that

PKA-mediated phosphorylation of perilipin A is important for ATGL-dependent lipolysis The

classical pathway of lipolysis activation in adipocytes is cAMP-dependent. Several agents

contribute to the control of lipolysis in adipocytes by modulating the activity of HSL and

ATGL. In addition, CGI-58 has also been shown to stimulate ATGL activity.

4.2 FFA in Circulation

Subcutaneous and abdominal adipose tissues are the largest fat depots and contributes the

major proportion of circulating nonesterified fatty acids (NEFS a synonym of FFA),

considerably less FFA comes from intraabdominal adipose tissue) (40). Circulating FFAs in

blood are bound to albumin.

During prolonged fasting or during longer aerobic physical activity lipids are utilised as major

energy source. In the fasting state, plasma FFA arise (double at night fast) almost entirely

from hydrolysis of TAG within the adipocyte. Prolonged fasting concentrations of FFA have

been related to adipose tissue mass (41) and also to the eventual presence of type 2

diabetes.

After a meal that contains fat, LPL in the capillaries of adipose tissue hydrolyses circulating

TAG mainly in the chylomicrons. FFA released are taken up into the adipocytes for storage.

However, a part of FFA always escapes and joins the plasma FFA pool (in a process called “spillover”) reaching 40–50% of the total plasma FFA pool in the postprandial period (42). The postprandial concentrations of FFA tend to remain somewhat higher in obese compared

with lean people.

4.3 Fatty Acid Oxidation

FFAs are taken for oxidation in many tissues; quantitatively major site is skeletal muscle (up

to 80%). Fatty acids are transported into the cell by tissue specific fatty acid transport protein

(FATP), fatty acid translocase (FAT/CD 36) and plasma membrane fatty acid binding protein

(FABPpm) (43). Muscle uptake of FAs is dependent on plasma FFA level.

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Once in the cell, FAs are activated to acyl-CoA by fatty acyl-CoA synthase (FACS). Carnitine

palmitoyltransferase 1 (CPT1), carnitine translocase (CAT) and mitochondrial membrane

CPT2 catalyse the consequent processes of conversion of long chain acyl-CoA into

acylcarnitine, its transport across the inner mitochondrial membrane and reconversion to

acyl-CoA. Medium chain fatty acids transport into mitochondria is carnitine-independent.

In mitochondria acyl-CoA undergoes β-oxidation to acetyl-CoA with concomitant production

of reduced NADH and FADH2. Acetyl-CoA enters the citric acid cycle to be oxidized

completely to carbon dioxide. Energy production from FA in β-oxidation pathway is aerobic

mitochondrial process, exceeding 100 ATP per FA molecule (around 130 for palmitic acid).

Fig. 5 The availability of FA in myocyte mitochondria depends on the

rate of lipolysis by lipases and rate of reesterification in adipose tissue

and the rate of LPL lipolysis of triacylglycerol rich particles in

postprandial conditions. FABPpm and FAT/CD36 proteins have been

identified in the plasma membrane of muscle cells, which facilitate

the transport of FA into the myocyte. Another pool of FA for oxidation

are intramuscular triacylglycerol molecules Hydrolysis of these TAGs

involves intramuscular HSL and adipose triglyceride lipase (ATGL)

During fasting period acetyl CoA molecules condense to form ketone bodies in liver

mitochondria. During starvation or prolonged low carbohydrate intake, ketone bodies are an

important energy substrate for many tissues including the brain and skeletal muscle.

Increased dietary intake of medium-chain FAs also results in the higher production of ketone

bodies. This is explained by the carnitine-independent influx of medium-chain FAs into the

mitochondria, thus by-passing this regulatory step of FA entry into β-oxidation.

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4.4 Reesterification of Fatty Acids into TAGs

FAs that do not enter into oxidative pathways can be re-esterified into TAGs or other lipids

after activation into acyl-CoA derivatives. The non-oxidised fatty acids are esterified with

glycerol-3 phosphate in G-3-P pathway GPAT is believed to be the rate-limiting factor in

glycerophospholipid synthesis. This process starts with the acylation of glycerol-3-phosphate

with a fatty acyl-CoA, production of lysophosphatidic acid (LPA), followed by further

acylation by LPA acyltransferase (LPAAT) and dephosphorylation to yield diacylglycerol

(DAG). DAG is then esterified with the third acyl-CoA molecule to produce TAG. DAG is also

the substrate for the synthesis of phospholipids as phosphatidic choline (PC) and

phosphatidic ethanolamine (PE).

Fig. 6 The 2 metabolic pathways involved

in the synthesis of triacylglycerol (TAG).

The monoacylglycerol (MAG) pathway, also known as the remodeling pathway, begins with

the acylation of MAG with fatty acyl-CoA catalyzed by monoacylglycerol acyltransferase

(MGAT) to form diacylglycerol (DAG) (44). Further acylation of DAG by diacylglycerol

acyltransferase (DGAT) leads to the synthesis of TAG.

This pathway plays a predominant role in the enterocytes after feeding, where large

amounts of 2-MAG and fatty acids are released from the digestion of dietary lipids. The MAG

pathway is also active in adipose tissue, likely playing a role in a storing excess of energy in

TAG.

These two pathways share the final reaction, catalyzed by diacylglycerol acyltransferase

(DGAT), for converting DAG to TAG. In fact there are two DGAT enzymes, which are

structurally and functionally distinct. DGAT1 is expressed in skeletal muscle, skin, mammary

gland and intestine, with lower levels of expression in liver and adipose tissue. DGAT2 is the

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main form in hepatocytes and adipocytes (lipid droplets). Both enzymes are important

modulators of energy metabolism, although DGAT2 appears to be especially important in controlling the homeostasis of triacylglycerols in vivo (45).

Triacylglycerol products of the DGAT reaction may be channelled into the cores of cytosolic

lipid droplets (46) or triacylglycerol-rich lipoproteins for secretion in cells such as

enterocytes and hepatocytes.

4.5 Lipid Droplets

This lipid droplets (LD) could be compared in their micellar structure to the plasma

lipoproteins (47). LDs of adipocytes are enriched in triacylglycerols (energy store), while

defending cells against lipotoxicity. They also contain structural components, including

cholesterol and retinol, for membrane synthesis and repair. The phospholipid component of

the monolayer contains significant amounts of phosphatidylcholine with a fatty acid

composition distinct from that of the endoplasmic reticulum and plasma membrane. Many

cell types, even ganglia in the brain, can contain small lipid droplets (of the order of 50 nm

in diameter), but in adipocytes these can range to up to 200 μm in diameter. The lipid

droplets (LD) like plasma lipoproteins on the surface contain a specific group of

constitutively associated protein members of the PAT family: perilipin, adipophilin, TIP47

(now renamed to perilipins 1–3). Perilipins probably regulate formation, growth and lipolysis

of LDs. The enzymes of lipid metabolism are also abundantly located at the LD surface.

Fig. 7 Hypothetical model of triacylglycerol synthesis and

the lipid droplets formation in the ER (43). The reaction of

triacylglycerol synthesis catalysed by DGAT at the cytosolic surface of the ER.

In the liver, TAGs can either be stored temporarily or incorporated into TAG-rich VLDL and

released into the plasma. In myotubes palmitic acid is accumulated as DAG and TAG,

whereas oleic acid mainly as free FA. Oleic acid, the major MUFA is oxidized, as all other

FAs, by β-oxidation.

Copyright © by ESPEN LLL Programme 2014

TAG synthesis is important in many of physiological processes, intestinal dietary fat

absorption, intracellular storage of extra energy, lactation, attenuation of lipotoxicity, lipid

transportation, and signal transduction. The importance of TAG synthesis is exemplified by

severe insulin resistance in patients with lipodystrophy, a genetic condition characterized by

defective TAG synthesis and storage in adipose tissues (48). Whereas excessive TAG

accumulation in adipose leads to obesity, ectopic storage of TAG in nonadipose tissues such

as liver and skeletal muscle is associated with insulin resistance (49).

4.6 Adipokines

Lipid synthesis and storage and consequent lipolysis are the main classical functions of

adipocytes. However, adipocytes also express and secrete various factors (adipokines) that

exert autocrine, paracrine, and endocrine effects in the body (50). There are currently over

50 different adipokines recognized as being secreted from adipose tissue including growth

factors, cytokines, chemokines, acute phase proteins, complement-like factors, and adhesion

molecules like leptin, adiponectin, resistin, visfatin, apelin, vaspin and IL-1β, IL-4, IL-6,

CRP et. The adipokines are involved in the modulation of several physiological responses

that includes control of appetite and energy balance. Specific metabolic processes regulated

by adipose tissue include lipid metabolism, glucose homeostasis, inflammatory process,

angiogenesis, regulation of coagulation) and blood pressure (51). Recent evidence has

demonstrated that many factors secreted from adipocytes are pro-inflammatory mediators.

Table 2

Main adipokines and their functions

Adipokines Function

leptin Repression of food intake

Stimulation of fatty acid oxidation in liver, pancreas and skeletal muscle

Modulation of hepatic gluconeogenesis

Modulation of pancreatic β-cell function

Suppression of resistin and retinol binding protein 4 expression

Stimulation of adiponectin expression)

adiponectin Stimulation of fatty acid oxidation in liver and skeletal muscle

Suppression of hepatic gluconeogenesis

Stimulation of glucose uptake in skeletal muscle

Stimulation of insulin secretion

Modulation of food intake and energy expenditure

resistin Stimulation of TNF-α and IL-6 expression

visfatin Stimulation of TNF-α and IL-6 expression

apelin apelin signaling participates in cell relaxation (smooth muscle cell) or

contraction (cardiomyocyte), migration and proliferation

vaspin Suppression of leptin, resistin, and TNF-α expression

Of particular relevance is the ability of white adipose tissue (WAT) to increase or reduce

leptin secretion under conditions of positive and negative energy balance, respectively. From

this point of view, leptin works as a signaling molecule that sends information to the central

nervous system (CNS) regarding the content of fat stored in the WAT. Through this

mechanism, the CNS can sense energy availability in the organism and make continuous

adjustments in food intake and energy expenditure. The WAT is viewed as a multifunctional

organ that has the ability to regulate metabolic rate of organs and tissues, as well as whole-

body substrate metabolism and energy homeostasis.

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