Review
The Impact of Ketone Body Metabolism on Mitochondrial Function and Cardiovascular Diseases
2023 年 30 巻 12 号 p. 1751-1758
詳細
2023 年 30 巻 12 号 p. 1751-1758
Ketone bodies, consisting of beta-hydroxybutyrate, acetoacetate, and acetone, are metabolic byproducts known as energy substrates during fasting. Recent advancements have shed light on the multifaceted effects of ketone body metabolism, which led to increased interest in therapeutic interventions aimed at elevating ketone body levels. However, excessive elevation of ketone body concentration can lead to ketoacidosis, which may have fatal consequences. Therefore, in this review, we aimed to focus on the latest insights on ketone body metabolism, particularly emphasizing its association with mitochondria as the primary site of interaction. Given the distinct separation between ketone body synthesis and breakdown pathways, we provide an overview of each metabolic pathway. Additionally, we discuss the relevance of ketone bodies to conditions such as nonalcoholic fatty liver disease or nonalcoholic steatohepatitis and cardiovascular diseases. Moreover, we explore the utilization of ketone body metabolism, including dietary interventions, in the context of aging, where mitochondrial dysfunction plays a crucial role. Through this review, we aim to present a comprehensive understanding of ketone body metabolism and its intricate relationship with mitochondrial function, spanning the potential implications in various health conditions and the aging process.
Ketone bodies are composed of three metabolites, namely, beta-hydroxybutyrate, acetoacetate, and acetone1). Historically, ketone bodies were considered as intermediate metabolites formed during the beta-oxidation of fatty acids. However, it was later discovered that they are metabolites generated through pathways distinct from fatty acids, thus attracting attention to their physiological functions2). In particular, they are noted to exhibit unique characteristics as compared to other energy substrates, as their blood concentration increases during fasting and they become the primary energy substrate for the brain during periods of starvation. The role of ketone bodies as energy substrates during fasting has been widely recognized3, 4). This utilization as an energy substrate during starvation is not limited to humans but is common among other species as well. Studies using birds have shown that in the initial few days after fasting stimulus, reliance is placed on lipid-derived energy substrates, but subsequently, energy supply is maintained through the elevation of beta-hydroxybutyrate levels5).
Ketone body metabolism has traditionally been considered primarily in terms of its role as an energy substrate. However, in recent years, it has become evident that ketone bodies are also involved in signal transduction and epigenome regulation, highlighting their multifaceted effects1). Furthermore, it has been discovered that ketone body synthesis and degradation take place within the mitochondria, emphasizing the close relationship between ketone body metabolism and mitochondrial function. Therefore, it has become crucial to investigate the connection between ketone body metabolism and mitochondrial function in order to fully understand ketone body metabolism6).
Ketone bodies are known to cause ketosis or ketoacidosis when excessively accumulated, making it a particularly serious complication in individuals with diabetes7, 8). On the other hand, clinical trials aiming to harness the organ-protective effects of mild ketone elevation and the potential for extending healthy lifespan through fasting or calorie restriction-induced ketosis have also gained attention. To properly understand these phenomena, it is crucial to focus on the processes occurring in the mitochondria and gain a comprehensive understanding of the effects brought about by ketone body metabolism9-11).
Ketone Body MetabolismThe most significant characteristic of ketone body metabolism is the clear distinction between synthesizing organs, such as the liver, and consuming organs, such as the heart and brain. These differences are brought about by variations in the expression of rate-limiting enzymes within the mitochondria. Ketogenesis is known to be regulated by the expression of hydroxymethylglutaryl coenzyme A synthase 2 (Hmgcs2), whereas ketone body utilization is controlled by the expression of succinyl-CoA:3-oxoacid CoA-transferase (SCOT) (Figs.1 and 2)12-14).
Ketone bodies are converted into acetyl-CoA and act as energy substrates by entering the TCA cycle.
AcAc, acetoacetate; AcAc-CoA, acetoacetyl-CoA; βOHB, β-hydroxybutyrate; NAD, nicotinamide adenine dinucleotide; SCOT, succinyl-CoA:3-ketoacid CoA-transferase; TCA, tricarboxylic acid cycle
HMG-CoA synthase 2 (Hmgcs2), a mitochondrion-localized protein, serves as the rate-limiting enzyme in ketone body synthesis. AcAc, acetoacetate; AcAc-CoA, acetoacetyl-CoA; βOHB, β-hydroxybutyrate; NAD, nicotinamide adenine dinucleotide
The substrate of ketone bodies is acetyl-CoA. Within the mitochondria, acetyl-CoA is synthesized from molecules to form acetoacetyl-CoA. Subsequently, acetoacetyl-CoA and acetyl-CoA condense through the action of HMGCS2, leading to the synthesis of hydroxymethylglutaryl coenzyme A (HMG-CoA). HMG-CoA is then converted by hydroxymethylglutaryl coenzyme A lyase into acetyl-CoA and acetoacetate. Acetoacetate is further reduced by β-hydroxybutyrate dehydrogenase (BDH1), resulting in the formation of β-hydroxybutyrate (βOHB)15). The reduction reaction from acetoacetyl-CoA to β-hydroxybutyrate (βOHB) involves the concomitant reduction of NADH to NAD+. Unlike HMGCS2, this reaction is reversible, and the equilibrium state is determined by the ratio of acetoacetate to β-hydroxybutyrate and the ratio of NAD+ to NADH. In other words, an increase in the amount of NADH promotes the reduction reaction, while an increase in the amount of NAD+ promotes the oxidation reaction. This equilibrium state regulates the interconversion of acetoacetate and βOHB in ketone body metabolism, adapting to the energy supply demand16). Additionally, a portion of acetoacetate is spontaneously decarboxylated to form acetone, which is then excreted in exhaled breath.
Acetoacetate and βOHB synthesized in the mitochondria are released into the extracellular space and reach the bloodstream through monocarboxylate transporters17). Monocarboxylate transporters play an important role not only in the release of ketone bodies but also in their uptake. Solute carrier family 16a, member 6 (SLC16A6), is a type of monocarboxylate transporter known to be vital in the release of ketone bodies. In a study analyzing a loss-of-function model in zebrafish, it was observed that the accumulation of β-hydroxybutyric acid in the liver led to an exacerbation of hepatosteatosis18). Monocarboxylate transporters also play a crucial role in the uptake of ketone bodies; in fact, genetic analysis of patients experiencing severe ketoacidosis has revealed the presence of a frameshift mutation in the solute carrier family 16a, member 1 (SLC16A1), also known as monocarboxylate transporter 1 (MCT1)19).
Ketone body oxidation occurs not only in the liver but also in skeletal muscles, the brain, and the heart, where it is utilized as an energy substrate20-22). The ketone bodies taken up into cells are oxidized again when they are utilized as energy substrates. The enzymes responsible for the oxidation of these ketone bodies are BDH1 and SCOT. BDH1 also functions in ketone body synthesis, but during the breakdown process, it oxidizes β-hydroxybutyrate to convert it into acetoacetate. Subsequently, through a reaction mediated by SCOT, acetoacetate is converted to acetoacetyl-CoA.
Ketone Body Utilization and Mitochondrial FunctionKetone bodies require reactions within the mitochondria for both synthesis and degradation. Here, we will introduce insights into the utilization of ketone bodies and mitochondrial function, focusing on the data obtained from the human heart (Fig.1). There are various methods to measure the utilization rate of ketone bodies, such as directly measuring ketone bodies in cardiac tissue or analyzing ketone bodies labeled with isotopic carbon23, 24). However, applying these methods to a beating human heart is challenging and not commonly done. One useful method in this regard is the measurement technique utilizing blood sampling obtained during cardiac catheterization. Arterial and venous blood sampling during cardiac catheterization is a method developed to obtain metabolic evidence of myocardial ischemia. Catheters are placed in the aortic root and coronary sinus, respectively, and sampling is performed before and after the induction test for myocardial ischemia to measure the lactate production rate in the event of ischemia25). When myocardial ischemia occurs, anaerobic metabolism is enhanced, which then results in increased lactate production. This can be confirmed by the elevation of lactate concentration in the coronary sinus. However, this sampling method allows for the measurement of various metabolic byproducts other than lactate. By evaluating the difference between arterial and venous blood, it is possible to detect changes in metabolic status.
To elucidate the impact of mitochondrial function on ketone body utilization, we measured ketone bodies along with lactate in patients who have underwent cardiac catheterization for the diagnosis of coronary spastic angina. As a result, we confirmed that the utilization rate of ketone bodies significantly decreases during conditions of myocardial ischemia accompanied by ST elevation and increased lactate production. Furthermore, we observed a prompt recovery of the utilization rate upon alleviation of ischemia26). This finding indicates that aerobic metabolism in mitochondria is essential for the utilization of ketone bodies. It highlights the close relationship between the maintenance of mitochondrial function and the utilization of ketone bodies.
The utilization rate of ketone bodies is known to vary in pathological conditions other than ischemic heart disease. In diabetes, where mitochondrial function is generally believed to decline, the utilization rate of glucose and lactate decreases, while the utilization rate of ketone bodies increases27). In addition, in a study examining the production and consumption of metabolites in the heart using metabolomics analysis, an increased utilization rate of ketone bodies was noted in cases with reduced cardiac contractility28). Both diabetes and heart failure are generally believed to be associated with a decline in mitochondrial function. However, it is interesting to note that unlike cases with significantly enhanced anaerobic metabolism, the utilization rate of ketone bodies increases. In studies comparing the efficiency of oxygen utilization and energy production in the utilization of energy substrates, it is known that ketone bodies produce more ATP than glucose while consuming less oxygen per molecule as compared to the breakdown of fatty acids, making them efficient energy substrates29, 30). Analysis of the changes in handling energy substrates in situations where mild to moderate mitochondrial function is impaired is expected to receive attention in the future.
Ketone Body Production and Mitochondrial FunctionThe synthesis of ketone bodies has been found to have multifaceted roles in addition to its function of supplying ketone bodies to other organs (Fig.2). Hmgcs2 is expressed not only in liver cells but also in the proximal tubules of the kidney and intestinal stem cells. In the intestine, intestinal stem cells expressing Hmgcs2 possess ketone body synthesis capability. They control NOTCH signaling through the inhibitory effect of β-hydroxybutyric acid on histone deacetylase, thereby maintaining stem cell function31). In the kidney, it has been reported that ketone bodies exert a renal protective effect by suppressing excessive activation of mTORC1 in a diabetic nephropathy model32). These results reveal the cellular and organ-protective effects of ketone body synthesis.
In order to elucidate the physiological significance of ketone body synthesis in the liver, we established a knockout mouse model (Hmgcs2 KO) lacking Hmgcs2, which is the rate-limiting enzyme in ketone body synthesis. Interestingly, despite being unable to synthesize ketone bodies, the Hmgcs2 KO mice are deemed viable and exhibit fasting tolerance. Evolutionarily, it has also been revealed that certain species, such as elephants and dolphins, have lost Hmgcs2 during the course of evolution, suggesting that ketone bodies may not be essential as an energy substrate during periods of starvation33).
On the other hand, mice with impaired ketone body synthesis exhibit rapid progression of fatty liver after birth, characterized by typical histological findings of microvesicular hepatosteatosis. Investigations using isolated liver cells have confirmed a decrease in oxygen consumption in Hmgcs2 KO mice, which indicates impaired mitochondrial function in the absence of ketone body synthesis. Ketone body synthesis utilizes acetyl-CoA as a substrate, and the deficiency in ketone body synthesis leads to the accumulation of acetyl-CoA within the cells. As a result, it has been shown that acetylation, a post-translational modification, occurs on mitochondrial proteins, inhibiting their enzymatic activity. The series of research results indicate that ketone body synthesis itself serves to alleviate excessive ketone accumulation within cells and protects mitochondrial function (Fig.3)6).
When ketogenesis is impaired, the substrate acetyl-CoA accumulates. This accumulated acetyl-CoA then leads to ectopic fat deposition through de novo lipogenesis, and within the mitochondria, excessive protein acetylation progresses, causing functional impairment. AcAc, acetoacetate; AcAc-CoA, acetoacetyl-CoA; βOHB, β-hydroxybutyrate; NAD, nicotinamide adenine dinucleotide
The pathogenesis of nonalcoholic fatty liver disease (NAFLD) and nonalcoholic steatohepatitis (NASH) has been found to be closely associated with mitochondrial dysfunction34). In a study comparing the expression patterns of mitochondrial localization genes, it was revealed that the expression of Hmgcs2 was found to decrease in NASH as compared to NAFLD35). In studies using antisense oligonucleotides of Hmgcs2 in adult mice, it has been shown that the progression of NAFLD/NASH induced by a high-fat diet is exacerbated by ketogenesis impairment36, 37). In investigations using Hmgcs2 knockout mice, significant ectopic fat deposition was observed during the juvenile period, even under normal diet conditions, but the disease was alleviated after weaning6). Additionally, while histopathological findings during the juvenile period showed microvesicular ectopic fat deposition, the adult disease model exhibited a characteristic increase in macrovesicular ectopic fat deposition.
Mitochondria have been recognized as potential therapeutic targets in various diseases, including atherosclerosis38). Enhancing ketogenesis has been expected to alleviate excessive de novo lipogenesis and improve mitochondrial protein function. Therefore, therapeutic approaches targeting the enhancement of ketogenesis are also being investigated for the treatment of NAFLD/NASH. Studies using adenovirus vectors have demonstrated that enhancing the expression of Hmgcs2 in the liver leads to a significant reduction in ectopic fat deposition39). Furthermore, dietary interventions known to indirectly induce ketogenesis have been gaining attention for their potential association with NAFLD/NASH pathogenesis40). In humans, practicing alternative day fasting, which involves alternating “fast days” with a calorie intake of 600 kcal/day and “feast days” with unrestricted eating, for a period of 3 months has been shown to lead to weight loss, improvement in fatty liver, as well as improvement in insulin sensitivity and reduction in liver injury markers41).
Ketone Body Metabolism and Cardiovascular DiseasesIn heart failure patients, the blood concentration of ketone bodies increases, and the utilization of ketone bodies within the myocardium also increases23, 28, 42, 43). In studies using mice, it has been demonstrated that cardiac-specific SCOT knockout exacerbates cardiac hypertrophy induced by the transverse aortic constriction (TAC) model due to impaired ketone body utilization in the myocardium44). On the other hand, studies using an overexpression model of BDH1, which promotes the oxidation of ketone bodies, have reported that heart failure is alleviated by treatment with TAC; this suggests the potential of promoting the utilization of ketone bodies as a means of preventing heart failure progression45). Several mechanisms have been reported as the basis for the effects of ketone bodies on heart failure. As an energy substrate, metabolic reprogramming occurs with the progression of heart failure, shifting from fatty acid oxidation to a glycolysis. The addition of ketone bodies suppresses the utilization of glucose derived from heart failure and increases energy production efficiency46). In addition, there have been reports suggesting that β-hydroxybutyrate, among ketone bodies, inhibits the activation of the NLRP3 inflammasome and exerts a cardioprotective effect through its anti-inflammatory properties47).
The cardioprotective effects of ketone body supplementation has been investigated in humans as well10). In a study investigating the short-term effects of intravenous administration of β-hydroxybutyrate in patients with heart failure with reduced ejection fraction, improvements in terms of cardiac output and myocardial external energy efficiency were reported48). However, reports from another group have also indicated that although the utilization rate of ketone bodies increases, it may not lead to improvements in terms of cardiac function49). Further investigation is thus necessary to clarify this matter. To achieve medium- to long-term ketone body supplementation, approaches such as the use of ketone salts and ketone esters and supplementation of medium-chain fatty acids are being explored50). In models of chronic supplementation of ketone esters, it has been demonstrated that the decline in cardiac function induced by TAC is alleviated51).
The cardioprotective effects of ketone body supplementation are not only anticipated for heart failure but also for myocardial ischemia. A report from Japan in 2002 demonstrated the reduction in infarct size with supplementation of β-hydroxybutyrate52). However, there have been lacking subsequent reports on this topic for a long period of time. Indeed, in recent times, the increased ketone body metabolism induced by the administration of sodium–glucose linked transporter inhibitors (SGLT2i) has also become a focus of research. The actual effect of SGLT2 inhibitors on myocardial infarction has been confirmed to reduce the size of the infarction when administered. Similarly, a comparable effect has also been observed with ketone supplementation53).
Ketone Body Metabolism and AgingThe function of mitochondria has been closely related to aging54). In recent years, the beneficial effects of calorie restriction and fasting on extending healthy lifespan have become evident, and there is also growing interest in ketone metabolism55). In this context, it has been demonstrated that a ketogenic diet extends both longevity and healthspan of adult mice56). Furthermore, a ketogenic diet has been shown to improve cognitive function in aging mice57). Recently, investigation using Hmgcs2 KO mice has also been reported in Japan58). In Hmgcs2 KO mice, a reduction in maximum lifespan has been observed, but lifespan extension effects were obtained through administration of 1,3-butanediol, which is the precursor of ketone bodies. On the other hand, in mouse models where 1,3-butanediol or a ketogenic diet was administered from an early stage to wild-type mice, lifespan was shortened as compared to wild-type mice. However, in models where dietary intervention started at 72 weeks of age, the lifespan extension effect of 1,3-butanediol was observed. Investigations using ApoE-deficient mice, which are commonly used as a model for atherosclerosis, showed that both 1,3-butanediol and a ketogenic diet shortened lifespan. These series of results have revealed the importance of endogenous ketone bodies and the differential responsiveness based on the timing of intervention and the pathological condition. Recently, in time-restricted eating, which increases ketone body levels through fasting, reports have shown that it leads to early disease progression in atherosclerosis mouse models, indicating the need for further investigation regarding dietary interventions59).
In this review, we have provided an overview of the multifaceted effects of ketone metabolism and the changes associated with diseases and aging from both the synthesis and breakdown pathways of ketone body metabolism. The ketone body metabolism, which has long been recognized as an energy substrate during periods of fasting, has been shown to have various effects beyond its role as an energy substrate (Fig.4). The impacts of these different effects are yet to be elucidated; thus, further analysis is needed to fully understand the implications of each action. On the other hand, it is evident that excessively high levels of ketone bodies, leading to a condition called ketoacidosis, can have negative effects on the body. Therefore, careful and meticulous research is essential when considering intervention methods related to ketone bodies. This applies not only to pharmacological approaches but also to techniques such as fasting and calorie restriction, which can induce ketone body metabolism. Understanding the appropriate means of regulating ketone body metabolism is deemed crucial in our ongoing pursuit of promoting health and longevity in society.
Ketone body metabolism has diverse functions beyond providing energy substrates; this includes signal transduction, epigenome regulation, and mitochondrial protection.
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