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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
To
To
To
To
learn the important dietary lipids;
learn the role of free fatty acids;
understand the main metabolic lipids’ pathways ;
learn the functions of lipoproteins;
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
Short 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
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
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
18:3 α-Linolenic acid
20:5 Eicosapentaenoic acid
22:5 Docosapentaenoic acid
22:6 Docosahexaenoic acid
9-trans,12-cis 18:2;
9-cis,12-trans 18:2
Saturated
FAs
Medium chain
Long Chain
Mono
unsatureated
Cis FA
n-6 FAs
Polyunsaturated
Cis FA
Polyunsaturated
Cis FA
Trans FAs
n-3 FAs
TAG
of
adipose
tissue in human %
5%
24%
8%
46%
7%
7%
1%
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 reesterified 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 mgkg-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 mgkg-1d-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). Lecithincholesterol 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 ratelimiting 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.
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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 wholebody substrate metabolism and energy homeostasis.
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