White Adipose Tissue

Lipoprotein particles (chylomicrons, VLDL)

Chylomicrons Vldl

(^Jbumip) Fatty acids

[Insulin +] Glucose-(GLUT4

Glycerol

[Insulin +] Glucose-(GLUT4

(^Jbumip) Fatty acids

Glycerol

Fat storage Fat mobilisation

Fig. 4.15 Overview of fatty acid and glucose metabolism in white adipose tissue.

The body's main store of chemical energy is in the form of triacylglycerol (TAG) in white adipose tissue. Fat storage is the process of deposition of TAG; fat mobilisation (or lipolysis) is the process of hydrolysis of the stored TAG to release non-esterified fatty acids into the plasma (bound to the carrier protein albumin), so that they can be taken up by other tissues. LPL, lipoprotein lipase; HSL, hormone-sensitive lipase; glycerol 3-P, glycerol 3-phosphate; VLDL, very-low-density lipoprotein. The major pathways and main sites of hormonal regulation are shown: a plus sign indicates stimulation, a minus sign inhibition. Dashed lines show multiple enzymatic steps.

Triacylglycerol in the plasma is present in the lipoprotein particles (covered fully in Chapter 9). The largest of these particles, which carry most of the triacylglycerol, are too big to escape from the capillaries into the interstitial fluid; therefore, the adipocytes cannot take them up directly. There is an interesting mechanism to overcome this difficulty. Adipocytes produce the enzyme lipoprotein lipase, which hydrolyses the triacylglycerol in lipoprotein particles to release fatty acids, which can then diffuse into the interstitial space and so reach the adipocytes. Since lipoprotein lipase must act in the capillaries, it is exported from the adipocytes to the endothelial cells lining the capillaries of adipose tissue. Here it is attached by chains of the complex amino-glycan heparan sulphate, a carbohydrate with highly negatively charged sulphate groups to which the enzyme molecules attach through a charge interaction. Lipoprotein lipase can thus come into contact with, and act upon, passing lipoprotein particles (Fig. 4.16). It acts on them to hydrolyse their triacylglycerol, thus releasing fatty acids. These fatty acids diffuse a short distance through the interstitial space towards the adipocytes, which take them up.4 The fatty acids are mainly taken up into the cells by a carrier-mediated process involving FAT/CD36 (Table 2.4), although this is an area of current research. The diffusion of the fatty acids from the site of lipoprotein lipase action into the cells is probably regulated by concentration gradients. The concentration gradient from capillary to cell will be produced, after a meal, by the activation of lipoprotein lipase, stimulation by insulin of the esterification pathway, and the suppression of the release of fatty acids from the triacylglycerol store within the cell.

Ppar Pathway White Adipose Tissue

Fig. 4.16 The action of lipoprotein lipase in white adipose tissue. Lipoprotein lipase is attached to the branching glycosamino-glycan chains that form the glycocalyx (a fuzzy surface lining the capillary, attached to the endothelial cells). It acts on lipoprotein particles in the capillaries which contain triacylglycerol (TAG), hydrolysing this TAG to release fatty acids which are taken up into adipocytes and re-esterified for storage as TAG. More than one molecule of the enzyme acts on a lipoprotein particle at once.

Fig. 4.16 The action of lipoprotein lipase in white adipose tissue. Lipoprotein lipase is attached to the branching glycosamino-glycan chains that form the glycocalyx (a fuzzy surface lining the capillary, attached to the endothelial cells). It acts on lipoprotein particles in the capillaries which contain triacylglycerol (TAG), hydrolysing this TAG to release fatty acids which are taken up into adipocytes and re-esterified for storage as TAG. More than one molecule of the enzyme acts on a lipoprotein particle at once.

Once inside the cells, the fatty acids are esterified to form triacylglycerol which joins the lipid droplet for storage. The pathway of esterification is the usual one in which the fatty acids are firstly activated by formation of CoA derivatives, then linked to glycerol 3-phosphate (the phosphatidic acid pathway - see Fig. 3.8). The glycerol 3-phosphate is formed through glycolysis; it is in equilibrium with dihydroxyacetone phosphate, an intermediate in glycolysis, their interconversion being catalysed by the enzyme glycerol-3-phosphate dehydrogenase.

The activity of lipoprotein lipase in adipose tissue is stimulated by insulin, secreted in response to an elevation in the blood glucose concentration. Since we rarely eat fat alone, this means that after a typical meal containing both fat and carbohydrate, the uptake of fat into adipose tissue will be stimulated. The activation of lipoprotein lipase by insulin is rather complex, because of the rather complicated 'life cycle' of this enzyme. The effect of insulin involves increased transcription, altered processing of the enzyme within adipocytes and probably increased export to the endothelial cells. It is therefore not a rapid process and takes a matter of 3 - 4 hours. This time-course will be highly relevant when we consider the coordination of metabolism in different tissues by insulin (Chapter 6). Within adipose tissue, the esterification of fatty acids is also stimulated by the production of glycerol 3-phosphate through glycolysis, increased by insulin (Fig. 4.15). Thus, insulin stimulates both the uptake and storage in adipose tissue of fat circulating as triacylglyc-erol in the plasma.

The other potential pathway of fat deposition in adipose tissue is that of de novo lipogenesis. The pathway is the same as that in the liver (Box 4.3). It is stimulated by insulin at multiple points. Thus, again, insulin acts to promote fat storage in adipose tissue.

4.5.3.2 Fat mobilisation

The mobilisation of fat results in the liberation of fatty acids from the stored triacylglycerol; these fatty acids are released into the plasma as non-esterified fatty acids bound to albumin, and so are made available to other tissues. Since the mobilisation of fat involves the hydrolysis of stored lipid, it is also called lipolysis. The breakdown of triacylglycerol is catalysed by a lipase, although this enzyme is necessarily situated within the adipocytes, in contrast to lipoprotein lipase which is exported to the capillaries. It is known as hormonesensitive lipase, because its responsiveness to hormones was recognised before that of lipoprotein lipase. (It is better, perhaps, to think of this enzyme as the intracellular lipase.) It acts at the surface of the triacylglycerol droplet, and catalyses the hydrolysis of the ester bonds of two fatty acids. Another enzyme, a monoacylglycerol lipase, present in high activity, is responsible for removal of the third fatty acid. Thus, three fatty acids and one glycerol molecule are produced from each molecule of stored triacylglycerol. The fatty acids for the most part leave the cells and enter the plasma non-esterified fatty acid pool. The glycerol also leaves the cell; it cannot be utilised for esterification of fatty acids since adipose tissue almost completely lacks the enzyme glycerol kinase which would be necessary for this.

The activity of hormone-sensitive lipase must clearly be regulated very precisely, such that it is inactive when insulin levels are high. Hormone-sensitive lipase is regulated by phosphorylation in a manner similar to glycogen phos-phorylase in the liver: this was discussed in Chapter 2 (Box 2.4). Phosphoryla-tion is brought about by elevation of the cellular level of cAMP, in response to a number of regulators. It is probable in humans that the most important of these are adrenaline (in the plasma) and noradrenaline (released by sympathetic nerves). Glucagon has a potent effect in isolated fat cells in the laboratory, but appears not to affect fat mobilisation in humans in vivo. Equally important is the inactivation of hormone-sensitive lipase by dephosphorylation in response to insulin. This is a very potent effect - it responds to relatively low concentrations of insulin - and very rapid, occurring within a matter of minutes of raising the insulin concentration. Thus, insulin not only promotes fat storage, but it restrains fat mobilisation.

Phosphorylation of hormone-sensitive lipase appears to do more than simply change its conformation. It appears that, in its dephosphorylated, inactive form, hormone-sensitive lipase is present in the cytosol of the cell. When it is phosphorylated, it moves (translocates) to the surface of the lipid droplet and begins hydrolysing the stored triacylglycerol. Another protein is involved in this process: perilipin. Perilipin is an abundant protein in white adipocytes, and seems to coat the lipid droplet. Perilipin is also a substrate for phosphorylation with a signal chain similar to that for hormone-sensitive lipase (Box 2.4). When it is phosphorylated, it moves, or curls up, away from the lipid droplet, allowing hormone-sensitive lipase access.

Insulin has a further effect in restraining fat mobilisation. The fatty acids released by the action of hormone-sensitive lipase are available for esterification by the phosphatidic acid pathway already described. Insulin, as we have seen, stimulates this pathway by increasing the provision of glycerol 3-phosphate. Thus, insulin both inhibits the activity of hormone-sensitive lipase and 'mops up' any fatty acids it may liberate by increasing their re-esterification. These two actions are illustrated in Fig. 4.17.

4.5.3.3 Adipocyte differentiation and longer-term regulation of fat storage

We have seen above how adipocytes will take up excess fatty acids in the short term, for instance in the period following a meal that contains both carbohydrate (to stimulate insulin) and fat. Normally the uptake of fatty acids after meals will be balanced by fat mobilisation in the postabsorptive state (e.g. during the night-time fast) and during exercise, so that in many people the size of the fat stores remains relatively constant over long periods. We all know, however, that there are situations in which there is a gradual excess of fat deposition over mobilisation, or, of course, vice versa. Adipose tissue has well-developed

Glucose

Glucose

Agent Based Model

Fig. 4.17 Suppression of fat mobilisation by insulin. Insulin restrains fat mobilisation by two mechanisms: suppression of the activity of hormone-sensitive lipase (HSL), and stimulation of the re-esterification of fatty acids within the adipocytes. (The same process of esterification will also be simultaneously incorporating fatty acids from circulating triacyl-glycerol, released by lipoprotein lipase (LPL), into stored triacylglycerol.)

L^Non-esterified ' fatty acids

Glycerol

Fig. 4.17 Suppression of fat mobilisation by insulin. Insulin restrains fat mobilisation by two mechanisms: suppression of the activity of hormone-sensitive lipase (HSL), and stimulation of the re-esterification of fatty acids within the adipocytes. (The same process of esterification will also be simultaneously incorporating fatty acids from circulating triacyl-glycerol, released by lipoprotein lipase (LPL), into stored triacylglycerol.)

regulatory mechanisms to cope with these situations. If there is a long-term situation of positive energy balance, SREBP-1c expression will be increased by insulin and the PPARy system will be activated by the excess availability of fatty acids (Sections 2.4.1.1, 2.4.2.2). Between them, these two systems will up-regulate expression of the key enzymes involved in fat storage (Table 4.3). Each fat cell will increase in size as it stores more fat. Activation of these systems has another important effect: it is the stimulus for the differentiation of fat cell precursors, or preadipocytes, into new adipocytes. SREBP-1c was discovered independently in adipocytes as a factor causing adipocyte differentiation, and it is also called adipocyte determination and differentiation factor-1 (ADD-1). As Table 4.3 shows, SREBP-1c is itself a stimulus to increased expression of PPARy, which is another adipocyte differentiation factor. Thus, a long-term positive energy balance will result in both an increase in adipocyte size (hypertrophy) and an increase in the number of fat cells (hyperplasia).

Table 4.3 Some genes whose expression in adipose tissue will be increased during long-term energy excess.

SREBP-1c PPARy

Acetyl-CoA carboxylase Lipoprotein lipase

Fatty acid synthase Fatty acid transport protein

Glycerol phosphate acyl transferase Acyl-CoA synthase

Lipoprotein lipase GLUT4

PPARy

Only genes with relevance to increasing lipid storage are listed.

Small fat cells seem to be more metabolically active than big, fat-full cells, and may be particularly avid in taking up excess fatty acids. This may be the key to understanding the action of the new antidiabetic drugs, the thiazolid-inediones (TZDs), which are PPARy activators (Section 2.4.2.2). If the TZDs stimulate the proliferation of new fat cells, which are very active in 'trapping' fatty acids, then circulating fatty acid concentrations may fall and removal of metabolic competition may allow glucose utilisation by other tissues to increase - but this will be at the expense of additional fat deposition. In fact, prominent features of TZD action are a reduction in circulating non-esterified fatty acid concentrations, an increase in body weight, and an improvement in the ability of other tissues to metabolise glucose. We will revisit this topic in a later chapter (Chapter 10).

4.5.3.4 Adipose tissue as an endocrine organ

Several decades ago it was recognised that adipose tissue could produce certain steroid hormones, including oestrogens (female sex hormones). This is because cells within adipose tissue (probably mainly cells other than the adipocytes) express the enzymes to interconvert steroid hormones. Oestrogens (such as oestradiol) can be produced from androgens (such as androstenedione) that are produced by the adrenal cortex (Section 5.5.1). This has important ramifications. In obesity, when there is an excess of adipose tissue, more oestrogens may be produced. That has some beneficial effects: obese postmenopausal women (whose ovaries no longer produce oestrogens) are somewhat protected from osteoporosis, compared with lean women, because of this. The hormone cortisol is also produced from the inactive precursor cortisone. That may have untoward effects in obese men, adding to a metabolic 'stress' state.

The real impetus to this field came, however, in 1994 with the recognition that adipose tissue secretes a peptide hormone, now called leptin (from the Greek leptos, thin). The story of leptin will be further developed in the next chapter (Section 5.6) when we consider adipose tissue with other hormone-secreting organs, and in Chapter 11 (see Section 11.2) when we consider energy balance.

Along with leptin, which is certainly a true hormone, we now recognise adipose tissue to produce a number of other proteins, many of which are relevant to energy metabolism. One, of course, is lipoprotein lipase (see Section 4.5.3.1 above). Others include apolipoprotein E and cholesteryl ester transfer protein (both relevant to lipid metabolism: see Chapter 9), a number of cytokines (peptides that signal between cells and may play a role in inflammatory responses), proteins involved with blood clotting, and a number of components of the complement pathway involved in immunological defences. The complement story is interesting. Some years ago adipocytes were found to express in large quantities a protein that was called (for want of a better name) adipsin. Later, adipsin was found to be identical to factor D, a component of the so-called alternative complement pathway. Now we know that adipocytes produce several factors involved in this pathway and that these may interact, somewhere outside the fat cell, to produce a fragment known as acylation stimulating protein (ASP). ASP is a potent stimulator of fatty acid esterification in adipocytes. Thus, adipocytes seem to act locally to regulate their own fat storage.

4.6 The kidneys 4.6.1 General description

The two kidneys sit fairly high up towards the back of the abdomen. Strictly speaking, they are not in the abdominal cavity; they are behind the peritoneum, the membrane which surrounds the other abdominal organs such as the liver and intestines. The kidneys weigh about 150 g each in the adult.

The adjective renal (from the Latin renes, the kidneys) is used to describe the properties and functions of the kidneys. The kidneys are supplied with blood through the renal arteries which branch off the aorta, and the blood is returned to the inferior vena cava through the renal veins. The major purpose of the kidneys is to produce urine. This is a vehicle for excretion of (1) those products of metabolism that the body needs to dispose of; and (2) regulated amounts of water, in order to maintain the correct osmolarity of the body fluids.

The details of renal physiology are outside the scope of this book, although a brief outline is necessary in order to understand the energy requirements of the kidneys. Blood flows through a series of complex structures known as the glomeruli, where 'tangles' of blood vessels are surrounded by a cup-shaped structure, the glomerular capsule or Bowman's capsule. The endothelium of these blood capillaries is highly fenestrated (see Section 1.3.1) to allow ready passage of molecules out of the blood, into the capsule: this process is known as glomerular filtration. It is aided by the fact that the blood in the glomerular capillaries is under higher pressure than usual in capillaries. Thus, some of the plasma water, together with its complement of the smaller molecules dissolved in plasma, is lost into the capsule. The capsule is the termination of a tube, the renal tubule; the complete assembly of glomerular capsule and tubule is called a nephron, of which there are about half a million in each human kidney. The fluid thus entering the renal tubule is the beginning of urine. However, before the urine is fully formed, much of the water and many of the solutes filtered at the glomerulus will be reabsorbed into the blood. In contrast to filtration, reabsorption is a very selective process and much of it involves active transport - energy-requiring transport of substances up a concentration gradient back into the plasma. The renal tubule forms a long loop, the loop of Henle, with descending and ascending limbs, following it in order from the glomerular capsule towards its end, where it joins a larger duct collecting urine from a number of tubules. These collecting ducts merge and eventually form the ureter, the tube carrying fully formed urine from the kidney to the bladder.

4.6.2 The scale of kidney function

About 1 litre of blood passes each minute through the glomeruli, or almost 800 litres each day. Of this, about 20% is filtered through into the nephrons, producing almost 200 litres of filtrate each day. About 99% of the volume of this filtrate is reabsorbed, so that only 1-2 litres leaves the body as urine.

Water-soluble substances in the plasma are filtered along with the plasma water. Consider glucose as an example. A typical concentration of glucose in the plasma is about 5 mmol/litre, or 0.9 g/litre. Thus, almost 1000 mmol (180 g) of glucose are lost into the glomerular filtrate each day. Since the body does not 'want' to excrete glucose, virtually all of this is reabsorbed from the renal tubule.

Reabsorption of glucose has many similarities to the absorption of glucose from the intestine, discussed in Chapter 3. The epithelial cells lining the tubules have microvilli, like the intestinal mucosal cells, increasing their absorptive surface area. At least in the first part of the tubule, glucose is carried into the cells by the sodium-glucose co-transporters SGLT-1, SGLT-2 and SGLT-3 and leaves - into the interstitial fluid and thus the venous plasma - by the facilitated transporter GLUT2 (as in the enterocyte, illustrated in Fig. 3.7). Thus, the energy for glucose reabsorption again comes from a gradient of concentration for sodium ions, which is maintained by the activity of the Na+-K+-ATPase, and ultimately from hydrolysis of ATP.

4.6.3 Energy metabolism in the kidney

Given the very large quantities of solutes other than glucose which also have to be reabsorbed, it should not surprise us to learn that the kidneys have a high demand for energy. In fact they consume about 10% of the total oxygen consumption of the body at rest, although they contribute less than 0.5% of body mass. However, this metabolic activity is not spread evenly throughout the kidney.

In cross-section, the kidney can be seen to be formed of three fairly distinct parts. There is an outer lighter coloured layer, the renal cortex, surrounding a darker centre, the renal medulla. In the concave part of the 'bean' shape is the renal pelvis, the area where the collecting ducts gather together and form the ureter. The glomeruli are situated in the cortex, and some nephrons are completely contained in the cortex. Others have their loops 'dipping down' into the medulla; but most of the energy-requiring reabsorption of solutes goes on in the cortex. The cortex has a high blood supply and it has a correspondingly aerobic pattern of metabolism; it oxidises glucose, fatty acids and ketone bodies to provide its metabolic energy. The medulla, on the other hand, is much less well supplied with blood and derives its metabolic energy from the anaerobic metabolism of glucose. This is illustrated in Fig. 4.18.

During starvation, the kidney becomes a relatively important site of glu-coneogenesis. This process seems to take a few days of starvation to adapt, but may then contribute up to half the body's need for glucose. The development of gluconeogenesis in the kidney is also related to mechanisms for excretion

Glucose NEFA Ketone bodies 0„

Glucose NEFA Ketone bodies 0„

Glucose

Ureter

Fig. 4.18 Schematic view of energy metabolism in different regions of the kidney.

The cortex (outer layer) is well supplied with blood, has a high energy demand and is largely aerobic; the medulla has a poor blood supply and is largely anaerobic. It may derive its glucose by reabsorption from the tubules.

Glucose

Ureter

Fig. 4.18 Schematic view of energy metabolism in different regions of the kidney.

The cortex (outer layer) is well supplied with blood, has a high energy demand and is largely aerobic; the medulla has a poor blood supply and is largely anaerobic. It may derive its glucose by reabsorption from the tubules.

of hydrogen ions to assist maintenance of acid-base balance (see later, Section 8.3.2.4).

Keep Your Weight In Check During The Holidays

Keep Your Weight In Check During The Holidays

A time for giving and receiving, getting closer with the ones we love and marking the end of another year and all the eating also. We eat because the food is yummy and plentiful but we don't usually count calories at this time of year. This book will help you do just this.

Get My Free Ebook


Post a comment