Introduction

Lipolysis is the metabolic process through which triacylglycerols (TAGs) break down via hydrolysis into their constituent molecules: glycerol and free fatty acids (FFAs). Fat storage in the body is through adipose TAGs and is utilized for heat, energy, and insulation. The body uses fat stores as its main source of energy during starvation, conserving protein. Overall, fats are quantitatively the most important fuel in the body, and the length of time that a person can survive without food depends mainly on the amount of fat stored in the adipose tissue. Thus, lipolysis is especially important in the fasting state of metabolism when blood glucose levels have decreased. However, it also occurs under non-stimulated (basal conditions).[1]

The glycerol produced by lipolysis is a source of carbon for gluconeogenesis in the liver. FFAs are transported in the blood bound to albumin and are either oxidized in tissues by a process called beta-oxidation or converted to ketone bodies. The byproducts of beta-oxidation, ATP, and NADH, promote gluconeogenesis. FFAs convert to ketone bodies in the liver, which serves as an energy source for the brain, thus decreasing further consumption of already depleted blood glucose. FFAs are utilized throughout the body for energy production or biosynthetic pathways except in white adipose tissue (WAT) where they are stored. In a metabolic "fasting" state, when the body is deprived of nutrients, WAT releases FFAs and glycerol to supply non-adipose tissues.[2] The major enzymes participating in lipolysis constitute adipose triglyceride lipase (ATGL), hormone-sensitive lipase (HSL), and monoglyceride lipase (MGL).

Fundamentals

Triacylglycerol Synthesis

TAGs, which provide the body with a significant source of energy, are obtained from the diet or are synthesized endogenously, mainly in the liver. They are transported in the blood as lipoproteins and are stored in adipose tissue. The major classes of blood lipoproteins involved include high-density lipoprotein (HDL),  intermediate-density lipoprotein (IDL), low-density lipoprotein (LDL), very-low-density lipoprotein (VLDL), and chylomicrons. Chylomicrons undergo synthesis in the small intestine and transport dietary TAGs from the small intestine to tissues such as muscle and adipose. The liver synthesizes VLDLs in the liver and transport TAGs from the liver to tissues in the same way. HDLs have multiple functions relative to lipid metabolism, including serving an integral role in the conversions of VLDL to LDL. HDL also serves as a reservoir for essential apoproteins such as Apo C-II. Apo C-II activates lipoprotein lipase, the enzyme responsible for digestion and breakdown of TAGs. Synthesis of TAGs stores in adipose tissue occurs in the fed state after a meal.

Triacylglycerol Hydrolysis

During times of energy deprivation, WAT is stimulated via homeostatic control to shift toward higher net rates of lipolysis. This change in nutritional state elucidates this compensatory process and is regulated through hormonal and biochemical signals. Lipolysis proceeds in an orderly and controlled manner, with different enzymes acting at each step. Catecholamines are the primary activators of lipolysis, while other hormones and dietary compounds also affect it. Each of these substances binds and work on their respective membrane-bound receptors to elicit a signaling cascade with the sole purpose of activating HSL. ATGL performs the first step of TAG hydrolysis (thus, it is rate-limiting), generating diacylglycerols and FFAs. HSL performs the second step and hydrolyzes DAGs, generating monoacylglycerols and FFAs. MGL is selective for MGs and produces glycerol and the third FFA.

Issues of Concern

Defective lipolysis in non-adipose tissues impairs their normal function, leading to excessive TAG accumulation and lipid storage disease.[2] Conversely, an overabundance of FFAs due to unregulated lipolysis results in lipotoxicity in non-adipose tissues. Failure to package FFAs into lipid droplets causes chronic elevation of circulating FFAs, which can lead to chronic inflammation, mitochondrial dysfunction, and cell death.[3]

Cellular Level

As previously described, hormones bind to cell surface receptors (i.e., norepinephrine binds beta-adrenergic receptors) to stimulate lipolysis in adipocytes. A number of lipid droplet–associated proteins are known to modulate rates of basal (non-stimulated) and stimulated lipolysis. These proteins include CGI-58 and perilipin. Perilipin is the major protein found in association with lipid droplets in adipocytes.[4] In WAT, there are two important mechanisms regulating lipolysis: the activation of ATGL by CGI-58 and the protein kinase A (PKA)-mediated phosphorylation of HSL and perilipin.

In the basal state, CGI-58 is bound to perilipin rendering it unable to bind to or activate ATGL. Both ATGL and HSL reside in the cytosol.

In the stimulated state, β-Adrenergic receptors signal adenylyl cyclase to generate cAMP. cAMP then binds PKA resulting in increased activity of the enzyme. PKA then phosphorylates HSL and perilipin, which causes HSL to translocate from the cytosol to the surface of the lipid droplet. Now phosphorylated perilipin releases CGI-58 so that it can bind to and activate ATGL. Similarly to HSL, ATGL also must translocate from the cytosol to the surface of the lipid droplet. It is important to note that MGL is localized on the surface of the lipid droplet, in the cytosol, and the ER independent of the metabolic state.[5]

Molecular Level

Lipids have diverse structures but are all similar in that they are insoluble in water. Fatty acids usually possess an even number of carbon atoms, are 16 to 20 carbons in length, and can be saturated or unsaturated (the latter applying to contain double bonds). They are described by the number of carbons they contain and the positions of the double bonds, if any. For example, arachidonic acid has 20 carbons and four double bonds and is written as 20:4, Δ5,8,11,14, or 20:4(ω-6).

All naturally occurring fatty acids possess double bonds in the cis configuration. Polyunsaturated fatty acid classification is often according to the position of the first double bond from the omega-end (the carbon farthest from the carboxyl group). Common examples of these are omega-3 and omega-6 fatty acids. Monoacylglycerols (monoglycerides), diacylglycerols (diglycerides), and triacylglycerols (triglycerides) contain one, two, and three fatty acids esterified to glycerol, respectively.

Function

Fatty acids are carried on the albumin in the blood. In tissues such as muscle and kidney, fatty acids undergo oxidation for energy. In the liver, fatty acids convert to ketone bodies that are oxidized by tissues such as muscle and kidney. During starvation (after fasting has lasted for about three or more days), the brain uses ketone bodies for energy. The ketone bodies, acetoacetate, and β-hydroxybutyrate serve as a source of fuel. The liver uses glycerol as a source of carbon for gluconeogenesis, which produces glucose for tissues, including the brain and red blood cells.

Mechanism

Triacylglycerol Synthesis

They are synthesized in two ways: (1) from the FFAs produced as a byproduct of lipoprotein lipase’s effect on chylomicrons and VLDL, and (2) from a glycerol moiety derived from glucose. In the liver and adipose tissue, glycerol-3-phosphate (G3P) provides the glycerol moiety. The liver can convert glycerol to G3P via an intermediate or directly because it has the enzyme glycerol kinase. Adipose cells lack this enzyme and must produce G3P solely via an intermediate. The storage of TAGs in adipose tissue is mediated by insulin, which stimulates adipose cells to secrete lipoprotein lipase and to take up glucose, which converts to glycerol (via a DHAP intermediate) for triacylglycerol synthesis. In this process, glucose converts to DHAP, which is reduced by NADH to form G3P. Ultimately, G3P reacts with two fatty acyl CoA molecules to form phosphatidic acid. The phosphate group is cleaved to form a diacylglycerol, which reacts with another fatty acyl CoA to form a triacylglycerol.

Triacylglycerol Hydrolysis

As stated previously, during times of energy deprivation, WAT is stimulated by hormonal and biochemical signals to increase lipolysis. Lipolysis proceeds in an orderly and controlled manner, with different enzymes acting at each step. The current model of lipolysis identifies three major enzymes involved: ATGL, HSL, and MGL. Catecholamines, particularly norepinephrine, are the primary activators of fasting-induced lipolysis, while other hormones also have an effect. These include cortisol, glucagon, growth hormone (GH), and adrenocorticotropic hormone (ACTH).

Dietary compounds, such as caffeine and calcium, also stimulate lipolysis. Each of these substances binds and act on their respective membrane-bound receptors and elicit a signaling cascade using a common second messenger, cyclic AMP. Cyclic AMP then binds to and activates protein kinase A (PKA). Once PKA is enzymatically active, it phosphorylates HSL, the most important of the three enzymes involved in initiating lipolysis because it is enzymatically activated in all stages of hydrolysis. ATGL performs the first step of TAG hydrolysis, generating diacylglycerols and FAs. Its activity is tightly regulated by two accessory proteins: CGI-58 and G0S2. CGI-58 coactivates the hydrolase activity of ATGL and G0S2 inactivates the hydrolase activity of ATGL. HSL performs the second step and hydrolyzes DAGs, generating monoacylglycerols and FAs. MGL is selective for MGs and generates glycerol and the third FA.

Fatty Acid Metabolism

Short and medium-chain fatty acids diffuse freely into the cytosol and mitochondria of cells. Long-chain fatty acids must undergo protein-mediated transport across the cell membrane into the cytosol via fatty acid translocase (FAT) or fatty acid-binding protein (FABP). Acyl-CoA synthase then converts the fatty acids to fatty acyl-CoA. The fatty acyl-CoA must now be transported into the mitochondria through the outer mitochondrial membrane and is done so by carnitine palmitoyltransferase-I (CPT-I) where it becomes fatty acyl-carnitine. The fatty acyl-carnitine is then transported across the inner membrane into the mitochondrial matrix by carnitine acyl-translocase (CAT) and converted back to fatty acyl-CoA by palmitoyltransferase-II (CPT-II) where it is now ready for oxidation.

Beta oxidation

Beta oxidation is the degradation of fatty acids by removing two carbons at a time. It is the primary pathway for catabolism of fatty acids and takes place in the mitochondrial matrix of tissues such as the liver, muscle, and adipose. Two-carbon fragments are successively removed from the carboxyl end of the fatty acyl-CoA, producing NADH, FADH, and Acetyl CoA, which is used in the TCA cycle to make ATP. Fatty acids with odd numbers of carbon ultimately yield one mole of propionyl-CoA, which is converted to succinyl CoA so that it is usable in the TCA cycle. Beta oxidation is also important as the primary regulator of movement through the pyruvate dehydrogenase (PDH) complex. When rates of fatty acid oxidation are high, PDH activity decreases, which limits glycolysis, which is significant because patients with a deficiency in fatty acid oxidation have a compensatory increase in glucose oxidation and impaired gluconeogenesis.

Ketone synthesis

Ketone levels are low during normal feeding and physiological status. They are used by the heart and skeletal muscles to preserve the limited glucose for use by the brain and erythrocytes. During the fasting state, fatty acids are oxidized in the liver to acetyl CoA, which converts to the ketone bodies acetoacetate and beta-hydroxybutyrate. These high levels of ketones also inhibit PDH activity and fatty acid oxidation, to conserve glucose and permit entry into the brain where they can serve as sources for energy. Normally during a fast, muscle metabolizes ketone bodies as rapidly as the liver releases them preventing their accumulation in the blood. If ketones increase sufficiently in the blood, this can result in ketoacidosis, which is especially prevalent in people with type I diabetes and require close monitoring.

Testing

There are currently several strategies in place to estimate lipolysis and these generally fall into two categories: non-activity-based methods and activity-based methods. The non-activity-based methods involve determining the quantity of the associated enzymes and regulatory proteins. The activity-based methods involve measuring the activity of the associated enzymes directly.[2]

Over the last several years, new and updated information has come to light, and the opinions of lipolysis have changed. It is now known that the measurement of mRNA or protein expression used in the non-activity-based methods is often not enough to estimate the capacity of lipolysis. A combination of methods is necessary.[2]

Pathophysiology

Neutral lipid storage disease with myopathy (NLSDM) – a rare inherited disorder arising from mutations in the ATGL gene, which results in systemic TAG accumulation, myopathy, cardiac abnormalities, and hepatomegaly.[6]

Chanarin-Dorfman syndrome or NLSD with ichthyosis (NLSD-I) results from mutations in CGI-58, the activator of ATGL. They also exhibit systemic TAG accumulation, mild myopathy, and hepatomegaly but also present with ichthyosis, which is a skin disorder characterized by dry, thickened, scaly skin.[6]

Familial partial lipodystrophy (FPLD) type 4 is associated with a mutation in the PLIN1 gene coding for perilipin 1. It is characterized phenotypically by loss of subcutaneous fat from the extremities. Histologically, the six patients with this mutation have small adipocytes with increased macrophage infiltration and abundant fibrosis.[7]

Familial partial lipodystrophy (FPLD) type 6 occurs due to a mutation in the LIPE gene coding for hormone-sensitive lipase. It is characterized by abnormal subcutaneous fat distribution, and thus the complications commonly associated with it it. These include dysregulated lipolysis, insulin resistance, diabetes mellitus, increased fat storage in bodily organs, and dyslipidemia; others may even develop muscular dystrophy as indicated by elevated serum creatine phosphokinase.[8]

There are many disorders of fat metabolism that present with serious and specific characteristics but are not discussed here as they are beyond the scope of lipolysis, specifically. These include, but are not limited to, fatty acid oxidation disorders (FAODs) such as MCAD deficiency or primary carnitine deficiency and peroxisomal disorders such as Zellweger syndrome and adrenoleukodystrophy.

Clinical Significance

Alterations in lipolysis are often associated with obesity. These changes include increased basal rates of lipolysis, which may promote the development of insulin resistance and also diminished responsiveness to stimulated lipolysis.[9] The combination of enhanced lipolysis and impaired lipogenesis ultimately promotes insulin resistance due to the release of cytokines and lipid metabolites. Furthermore, adipose tissue of insulin-resistant people displays a lack of proteins involved in mitochondrial function. Mitochondria-derived energy sources function in lipogenesis in adipose tissue.[10] 

Obesity is characterized primarily by an excess of WAT due to hypertrophy of adipocytes that results from increased TAG storage. Obesity is a rampant health problem across the world due to its association with several disorders, including insulin resistance, type II diabetes, hypertension, and atherosclerosis.