Free fatty acids are a new marker of insulin resistance and ischemia
"Laboratory. Journal for Doctors", 2008, in print.
Velkov V.V.,
CJSC "DEACON", 142290, Pushchino, Moscow region, 5 Nauki Ave.
Obesity is one of the main causes predisposing to the development of metabolic syndrome (MS), insulin resistance (IR), leading to the pandemic spread of type 2 diabetes mellitus (DM2) and, as a result, to cardiovascular diseases (CVD). MS is sometimes even called the "plague of the XXI century". In the USA, for example, MS is present in 22.8% of men and 22.6% of women. At the same time, it is diagnosed in 4.6% of people with normal weight, in 22.4% of overweight people and in 59.6% of obese people (1).
It could be assumed that the metabolic syndrome develops in people whose economic status allows excessive food intake. In fact, in the USA, a large-scale study (4,978 men and 2,035 women, aged 39 to 63 years old, were examined) found an inverse relationship between a person's position on the socio-economic ladder and the probability of having MS. The authors concluded that "the development of MS is a biological mechanism that leads to "social inequality" in the distribution of coronary risk among people." As is known, people with high socio-economic status have a significantly lower risk, according to statistics (2,3).
It seems that it is childhood spent in poverty that leads to obesity in adulthood. This was established when 4,774 men and 2,206 women born in 1930-1953 were studied. It turned out that high coronary risks (elevated triglyceride levels and low HDL-C levels) are more associated with a low socio-economic status that took place in adulthood than with a "poor childhood". Nevertheless, it was those individuals whose fathers belonged to lower classes who were predominantly overweight in adulthood. It is assumed that "low socio-economic status begins to increase coronary risks from childhood and increases sharply when it decreases in adulthood" (4).
Determining the risks associated with obesity and leading to severe vascular complications in specific individuals is a very urgent task. At the moment, the best solution to this problem is to measure the concentration of free fatty acids in the blood.
Free (or non-esterified) fatty acids (FFA) are formed as a result of hydrolysis of triglycerides contained in adipose tissues. Plasma fatty acids are either esterified and in this case are mostly bound to albumin or are not esterified and are in a free state. In plasma, FFA is found in the concentration range from 100 mmol/l to 1 mmol / l and their level strongly depends on the time of day. After each meal, the level of FFA in plasma decreases, since insulin suppresses lipolysis in fat cells, as a result of which FFA is formed. At night, the concentration of FFA in plasma increases. Almost all other tissues, in particular skeletal muscles, "adjust" to these normal daily fluctuations in FFA levels, which "switch" from glucose utilization (during the day) to FFA consumption (at night).
The ability of skeletal muscles (and other tissues) to adjust their metabolism to the currently dominant substrate is commonly referred to as "good metabolic health" or "metabolic flexibility". It is clear that good "metabolic health" is associated with normal insulin sensitivity (5). But there is one fabric that "runs on fuel" of the LPG both day and night.
Free fatty acids are the main metabolic resource for the myocardiumIn the myocardium, FFA is rapidly metabolized due to beta-oxidation in the mitochondria and provides the heart with 65 to 70% ATP.
The remaining 20-25% of ATP is obtained by the myocardium due to glycolysis. Despite the fact that FFA is the most preferred "fuel" for the heart, its "burning" is a very costly process: the oxidation of 1 mole of FFA requires more oxygen than the oxidation of 1 mole of glucose. And although these oxygen needs are normally met, with its deficiency (ischemia), the consumption of FFA decreases, their plasma level increases almost immediately, which leads to the most serious pathological consequences (6,7). But more on that later.
The first danger associated with elevated levels of FFA is insulin resistance (IR). Numerous results of recent studies clearly indicate that elevated levels of FFA caused by an excessive amount of adipose tissue are, if not the first, then at least one of the main causes of IR. It has been repeatedly and reliably shown that the majority of patients suffering from obesity, metabolic syndrome and type 2 diabetes mellitus (DM2) have elevated levels of FFA, which leads to IR of many tissues – fat, muscle, liver, as well as endothelial cells (8-10).
Violation of FFA metabolism is a key event leading to IR
With obesity, excessive amounts of FFA enter the bloodstream. The future will depend on which tissues, not intended for their storage, will accumulate FFA. If they accumulate in skeletal muscles, this will lead to IR, if in the liver, to dyslipidemia. At first, as a rule, IR develops, then, with its aggravation, ischemic heart disease – CHD (10).
The development of IR at excessive concentrations of FFA occurs in at least two different ways.
1. The glucose level rises. Chronically high levels of FFA have a so-called lipotoxic effect on beta cells of the pancreas (11). This is aggravated by the fact that the increased flow of FFA into the liver, especially due to lipolysis of visceral fat, increases endogenous glucose synthesis in the liver [12].
2. The actual IR is developing. A large mass of adipocytes synthesizes increased amounts of pro-inflammatory cytokines, which leads to a chronic inflammatory process that: a) disrupts the pathway of insulin signal transmission and, b) damages mitochondrial functions, which disrupts glucose homeostasis. In particular, IL-6 and TNF-alpha secreted by fat cells make IR heavier, and angiotensin II secreted increases blood pressure and promotes the development of atherosclerosis (8,12,13).
However, according to recent studies, the disruption of the insulin signal transmission pathway is mainly due to the pathological metabolism of FFA in skeletal muscle cells that "cannot cope" with their utilization when FFA is in excess. Indeed, the local accumulation within skeletal muscles of such FFA metabolites as ceramides, glycerol or acyl-CoA leads to a violation of the transmission of the insulin signal and, thereby, to a violation of glucose transport.
Ceramides consist of a sphingoid base (sphingosine) and a fatty acid residue connected by an amide bond. Ceramide– an intermediate in the biosynthesis of sphingolmyelin, is formed by the interaction of sphingosine with acyl-CoA. The most important role of ceramide is to participate as a messenger in the signaling pathway of sphingomyelin and in the regulation of cellular processes such as differentiation, proliferation and apoptosis (programmed cell death). Excessive amounts of ceramide inhibit the insulin signal transduction pathway by inhibiting the phosphorylation of Akt/PKB protein kinase and block the translocation of Akt/PKB from the cytoplasm to the plasma membrane, which then inactivates the insulin signal transduction pathway. Indeed, in obese individuals suffering from IR, ceramide levels in skeletal muscles are increased by 2 times. This increase is associated with high concentrations of FFA in plasma and with a decrease in the intensity of phosphorylation of Akt protein kinase (14).
To prove that it is the elevated levels of ceramide that lead to a violation of glucose homeostasis, experiments were conducted with animals and pharmacological agents. Indeed, a decrease in ceramide levels (due to inhibition of its synthesis) improves glucose homeostasis in insulin-resistant transgenic mice with obesity and DM (15,16).
But what can these important discoveries provide for today's routine laboratory practice?
FFA is an independent predictor of impaired glucose toleranceIn a prospective study, 3,671 individuals with initially normal glucose tolerance (TG) were observed for 5 years. During this time, 418 individuals developed a TG disorder.
It was found that the violation of TG was associated with high levels of fasting FFA in individuals with initially normal TG. The authors believe that "elevated levels of FFA are predictors of the development of TG disorders, independent of IR or glucose secretion disorders" (17).
Increased fasting levels of FFA – risk factor of DM 2
In another prospective study, 580 individuals with diagnosed DM2 and 556 individuals of the control group were observed for 9 years. As it turned out, the levels of FFA were directly proportional to: 1) body mass index, 2) waist size, 3) pulse rate, 4) plasma triglyceride levels and 5) inflammation indicators determined by 6 markers of inflammation (18).
IR caused by high levels of FFA further increases the concentration of FFA in plasmaInsulin-resistant fat cells have been found to secrete elevated levels of FFA.
This, in fact, allows us to consider elevated levels of FFA as a marker of IR. Indeed, with IR, the level of FFA in hepatocytes increases, because, in them: 1) de novo lipogenesis increases, 2) esterification of FFA exceeds their oxidation, 3) esterified LC are stored in the form of triglycerides or directed to the synthesis of X-VLDL (rich in triglycerides), 4) insulin-regulated mobilization of triglycerides decreases.
Insulin-resistant adipocytes intensively break down the triglycerides contained in them and release the FFA formed from them into the bloodstream (both with and without obesity). The flow of FFA from fat cells increases and, moreover, FFA also leaves the VLDL and plasma chylomicrons and is partially directed through the bloodstream to other organs, and partially back to the liver, where it again turns into triglycerides. There is a "pumping" of the liver with FFA and triglycerides. This has the most serious consequences (7,8,19).
Elevated levels of FFA lead to dyslipidemia and atherogenesisThat's how it happens.
1) From the liver, high levels of X-VLDL are secreted into plasma, where due to lipolysis from X-VLDL, FFA and highly atherogenic remnant (residual) particles of lipoproteins rich in triglycerides are formed.
2) From plasma, FFA and remnant particles are reabsorbed by the liver, which further increases the level of FFA in hepatocytes and further stimulates the synthesis of X-VLDL.
3) In the liver, with a high level of VLDL-C and a normal level of CETP protein (cholesterol ester transfer protein), a cholesterol ester carrier, triglycerides from VLDL–C pass into HDL-C, and cholesterol from HDL-C passes into VLDL-C. As a result, the following are formed: a) cholesterol-rich very atherogenic remnant particles of X-VLDL and b) X-HDL containing a lot of triglycerides and little cholesterol.
4) Such HDL-C particles lose triglycerides (under the action of hepatic lipase) and their main apolipoprotein Apo A1. As a result, the level of antiterogenic HDL-C decreases.
5) At high levels of VLDL-C (rich in triglycerides), CETP transfers triglycerides from VLDL-C to LDL-C, and cholesterol from LDL–C to VLDL-C.
6) Triglyceride-rich X-LDL due to the activity of hepatic or lipoprotein lipase lose triglycerides, decrease in size and become very atherogenic small dense particles of X-LDL.
Thus, elevated levels of FFA lead to a decrease in the level of "anti-atherogenic" HDL-C, the formation of extremely atherogenic small dense LDL-C particles and an increase in plasma triglyceride levels [19].
But there is another way in which high levels of FFA cause atherogenesis. This way is more direct and shorter. The increased level of FFA in IR causes supersynthesis of reactive oxygen species in the mitochondria of macrovascular endothelial cells, which leads to the oxidation of LDL-C and modification of HDL-C. This induces an inflammatory process in the walls of blood vessels, the formation and accumulation of cholesterol plaques and, as a result, ischemia [20].
Ischemia further worsens the already severe pathological situationIschemia caused by high levels of FFA further increases the level of FFA in blood plasma.
As mentioned, in the myocardium, FFA due to beta-oxidation in the mitochondria are rapidly metabolized and give 65-70% ATP (glycolysis – 20-25% ATP). Fundamentally, the oxidation of 1 mole of FFA requires more oxygen than the oxidation of 1 mole of glucose. Normally, these needs are met, but with ischemia with the metabolism of FFA in the myocardium, severe pathological changes occur that lead to the most serious consequences, namely:
– in ischemic and cardiomyopathic conditions, glucose utilization prevails over the use of FFA,
– with ischemia, the metabolism of FFA becomes pathological, lactate and hydrogen ions are formed inside the ischemic cells,
– this leads to: a) degradation of myocardial contractility, b) diastolic dysfunction and, c) reduces the arrhythmogenic threshold of cardiomyocytes (7.21).
In general, plasma levels of FFA, regardless of other parameters, are associated with diastolic dysfunction of the left ventricle [22].
Elevated levels of FFA are the earliest marker of ischemiaIn a prospective study, 2103 men who initially did not have coronary heart disease were observed for 5 years.
During this period, 144 of them developed coronary heart disease. As it turned out, increased fasting levels of FFA (3rd term) were associated with a 2-fold increase in the risk of coronary heart disease (compared with the lower term) (23).
In another study, 30 patients admitted to the emergency department with acute coronary syndrome (heart pain for 12 hours) were observed, in whom troponin I (TnI) and FFA levels were measured. 9 individuals were diagnosed with myocardial infarction (MI). Within 24 hours after admission, TnI levels increased in all 9 individuals with MI. At the same time, in each of the 9 cases of increased TnI, the levels of FFA were also elevated. Upon admission, 28 out of 30 patients (93%) had high concentrations of FFA.In all 9 who were diagnosed with MI, the levels of FFA at admission were increased (100%). The authors conclude: "With ischemia, FFA increases regardless of the presence or absence of myocardial necrosis tested by TnI" (24).
Similar results were obtained in a study in which FFA levels were measured after coronary angioplasty. As is known, immediately after such an operation, a transient increase in the severity of ischemia occurs. Is it possible to register and quantify it by changing the levels of FCS? In 22 patients undergoing percutaneous transluminal angioplasty, plasma levels of FFA were measured 5 minutes before and 30 minutes after surgery. In all patients, postoperative levels of FFA were higher than preoperative. So, after the operation, the level of FFA reached 103 nM (14 times higher than normal – 7.5 nM). And although all 22 patients had high levels of FFA, only 11 had an ischemic change in the ST segment of the cardiogram after surgery. In such patients, the levels of FFA were significantly higher than in patients without ST segment elevation (47 nM). The authors believe that "an increase in serum FFA level reflects ischemia caused by angioplasty; determination of FFA level is a more sensitive indicator of the degree of ischemia than electrocardiographic measurement" (25).
In patients without ischemia, high levels of FFA are often associated with a complex of premature ventricular contractions, which ultimately increases the risk of CVD. With ischemia, the concentration of FFA increases sharply and has a proarrhythmic effect, causing tachyarrhythmia (frequent irregular contractions of the ventricles). In general, elevated levels of FFA are an indicator of the severity of ischemia in "troponin–negative" patients (23-25). However, elevated levels of FFA are not only an indicator of ischemia.
Elevated levels of FFA predict sudden death3315 persons who underwent coronary angiography were observed for 6.85 years.
Coronary artery diseases were diagnosed in 2,231 patients. Sudden death occurred in 165 patients. Elevated levels of FFA were associated with an increased risk of sudden death (26).
So, an increase in the level of FFA drags the patient into a spiral of pathological events:
1. FFA accumulates in adipose tissues, the concentration of FFA in the blood increases.
2. Accumulation of FFA in skeletal muscles not intended for storage of FFA,
leads to pathological metabolism of FFA, the intermediates of which cause IR.
3. IR further increases the concentration of FFA in the blood and in the liver.
4. An increase in the concentration of FFA in the liver disrupts cholesterol metabolism and leads to coronary heart disease.
5. Ischemia further increases the level of FFA.
6. See No. 1.
Timely determination of the concentration of FFA in the blood and appropriate therapy can be the beginning of a way out of this vicious circle.
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