Hepatokines and Non-Alcoholic Fatty Liver Disease

Hepatokines and Non-Alcoholic Fatty Liver Disease: Comparison

Please note this is a comparison between Version 1 by Yoon Mee Yang and Version 2 by Peter Tang.

Hepatokines are hormone-like proteins secreted by hepatocytes, and a number of these have been associated with extra-hepatic metabolic regulation. Mounting evidence has revealed that the secretory profiles of hepatokines are significantly altered in non-alcoholic fatty liver disease (NAFLD), the most common hepatic manifestation, which frequently precedes other metabolic disorders, including insulin resistance and type 2 diabetes.

inter-organ communication
NAFLD
organokine
liver
metabolism
ANGPTL
Fetuin

1. Introduction

The global prevalence of obesity has been increasing over several decades, reaching epidemic levels, and thus, raising serious public health concerns [1]. Obesity greatly increases the risk of metabolic syndrome, including insulin resistance, type 2 diabetes, fatty liver disease, cardiovascular disease, and neurodegenerative conditions, thereby significantly contributing to greater morbidity and mortality [1]. Moreover, obesity-induced chronic inflammation, particularly in the liver, adipose tissue, and skeletal muscle, plays a crucial role in the development of local and systemic insulin resistance, simultaneously inducing a range of metabolic disturbances in multiple organs via inter-organ crosstalk ^[2][3][2,3]. Given the concurrent detrimental influence of obesity on various organs, obesity-driven metabolic disorders are largely attributed to dysregulated multi-directional interactions between organ metabolism. In other words, inter-organ communication via autocrine, paracrine, and endocrine signals regulates systemic energy homeostasis, which can be disturbed by a disequilibrium between energy intake and expenditure (e.g., obesity, hepatic steatosis, etc.).

Non-alcoholic fatty liver disease (NAFLD) refers to the spectrum of chronic liver disease progression in the absence of excessive alcohol consumption, ranging from simple hepatic steatosis to severe pathological conditions, including non-alcoholic steatohepatitis (NASH), fibrosis, cirrhosis, and hepatocellular carcinoma ^[4][5][6][7][4,5,6,7]. Hepatic steatosis is characterized by the accumulation of ectopic lipids in hepatocytes by more than 5% of total liver mass, exhibiting mostly benign and/or mild clinical symptoms. However, this becomes problematic after developing NASH with the signs of hepatocellular damage, inflammation and fibrotic changes, favoring its progression towards more debilitating conditions. In parallel, emerging evidence has also revealed that patients with NAFLD have greater susceptibility to various infectious diseases, including bacterial pneumonia, Helicobacter pylori infection, urinary tract infection, Clostridium difficile colitis, and coronavirus disease 2019 (COVID-19) in close association with low-grade chronic inflammation, impaired innate immunity, and/or vitamin D deficiency ^[8][9][8,9]. NAFLD serves as an early hepatic manifestation that is primarily responsible for the progression of a variety of metabolic disorders ^[4][5][6][7][4,5,6,7], implying the multifactorial features of NAFLD and its close link with other comorbidities. Given the heterogenous pathogenesis of NAFLD and its increasing prevalence, leading international societies of hepatology have recently proposed new nomenclature ‘metabolic dysfunction-associated fatty liver disease’ (MAFLD) in the replacement of NAFLD, emphasizing more on its function in metabolic dysregulation, in order to better reflect the current knowledge, as well as its diverse etiologies of metabolic liver disease ^[10][11][10,11].

Recent advances in comprehensive genetic, transcriptomic, and proteomic technologies have provided insights into the role of the liver as an endocrine organ central to metabolism. Moreover, growing evidence has revealed that the liver mediates metabolic regulation through the release of various secretory factors, including hepatokines ^{[12][13][14][15]}[12,13,14,15]. Hepatokines are hormone-like proteins primarily secreted by hepatocytes, with the hepatokine secretory profile known to be markedly disturbed in NAFLD. In fact, NAFLD frequently precedes dysfunction in other organs during the pathogenesis of systemic metabolic diseases. Therefore, it is highly likely that altered hepatokine secretion at NAFLD onset will significantly impair the inter-organ signaling crosstalk, which may trigger the progression of complex and multifaceted metabolic dysregulation.

2. Role of the Liver in Metabolism under Normal Physiology

The liver contributes to the maintenance of systemic metabolism by controlling complex pathways that coordinate nutrient intake and energy expenditure. Blood flows into the liver from the heart through the hepatic artery (~25% of total blood volume) or from the gastrointestinal tract via the portal vein (~75%), with most of the nutrients (e.g., glucose and lipids) absorbed in the intestines which are delivered prior to their entry into systemic circulation. These anatomical and structural features enable the liver to sense and promptly respond to changes in nutrient availability ^[16][17][16,17].

The liver serves as a key regulator of glucose metabolism by orchestrating hepatic glucose production and glycogen storage. In a postprandial state, hepatic glucose uptake is upregulated in response to elevated plasma glucose and insulin levels. Then, the absorbed glucose is stored in the liver as glycogen or is utilized for fatty acid synthesis (i.e., de novo lipogenesis). Under the influence of insulin secreted in response to elevated blood glucose, hepatic glucose production and glycogenolysis are suppressed in order to normalize blood glucose concentrations ^[13][14][13,14]. During the fasting state, the liver upregulates blood glucose levels by stimulating hepatic glucose production and glycogen breakdown via transcriptional and non-transcriptional mechanisms ^[13][18][19][13,18,19], thus increasing the supply of glucose as a major energy source to non-hepatic peripheral tissues (e.g., brain, adipose tissue, skeletal muscle). The liver also plays a key role in lipid metabolism in response to nutrient availability, as well as insulin. Following a meal, insulin facilitates hepatic fatty acid uptake, the synthesis and storage of triglycerides (TG) via the utilization of dietary fatty acids, and promotes de novo lipogenesis in the liver [20], favoring long-term lipid storage. However, under fasting, wherein glucose availability is low, the liver carries out lipid oxidation and acutely produces ketone bodies, which serve as an alternative fuel source for non-hepatic tissues in order to meet energy demands ^[20][21][20,21]. As the liver encounters repetitive fasting-feeding transitions throughout the life cycle of living organisms, the hepatic regulation of metabolism, including energy production, expenditure, storage, and redistribution, is critical for maintaining systemic energy homeostasis under normal physiology.

3. Overview of Hepatokines

The liver regulates physiology and metabolism via the production and secretion of various plasma proteins, including albumin, coagulation factors, complement factors, transport proteins, and other factors [14]. Although the liver has long been recognized as a secretory organ, recent advances in mass spectrometry-based quantitative proteomics have enabled researchers to identify ~10,200 proteins produced in the human liver [22], with up to 40% of hepatic transcripts encoding secretory proteins [23]. Similarly, quantitative analysis of the mouse liver and plasma proteome identified 7099 and 4727 proteins, respectively, with ~25% of these overlapping with plasma proteins, suggestive of their secretion ^[12][24][12,24]. As mentioned above, the liver is directly connected to systemic circulation, receiving a substantial amount of blood from the heart and gastrointestinal tract, exchanging nutrients and other substances within the sinusoids, and then draining blood via the hepatic vein and inferior vena cava towards the heart for recirculation throughout the body [14]. Considering its secretory capacity as an endocrine organ, it is plausible to expect that the liver plays a fundamental role in inter-organ crosstalk through the release of secretory proteins, including hepatokines.

Analogous to adipose tissue-derived adipokines and skeletal muscle-derived myokines, hepatokines are a class of organokines, meaning a group of secretory proteins that are exclusively produced by the parenchymal cell type of respective tissue. The significance of hepatokines in the regulation of various biological processes in autocrine, paracrine, and endocrine fashion has been recently highlighted ^[13][25][13,25]. Given the functional features of hepatocytes, which constitute ~80% of the volume and ~70% of the total cell number within the whole liver, several studies have shown that normal mouse hepatocytes release more than 500 secretory proteins, which may or may not contain an N-terminal secretory peptide ^[12][26][12,26]. Since emerging evidence has demonstrated that factors secreted from hepatocytes actively mediate metabolic regulation between the liver and other organs, hepatokines have drawn increasing attention due to their capacity for metabolic regulation, making them novel targets for the modulation of energy homeostasis and the treatment of metabolic disorders ^{[12][13][14][15]}[12,13,14,15]. In this review, we summarized target organs or cells of important hepatokines and their biological functions (Table 1).

Table 1. Target organs or cells of hepatokines and their biological functions.

Hepatokines	Target Organs or Cells	Biological Functions
ANGPTL3	WAT, muscle, liver	Suppressed LPL and endothelial lipase	Increased plasma TG and FFA	Increased VLDL-TG secretion (liver)		Increased uptake of VLDL-TGs (WAT)		Decreased glucose uptake (WAT)		Promoted lipogenesis and inflammatory response (liver)		^[15]^[27]^[28]^[29]^[30]^[31]^[32]^[33]	[15,27,28,29,30,31,32,33]
ANGPTL4	WAT, vascular endothelial cells	Inhibited LPL activity	Increased plasma TG levels	NAFLDIncreased adipocyte lipolysis		Suppressed hepatic glucose production		^[15]^[34]^[35]^[36]^[37]^[38]^[39]	[15,34,35,36,37,38,39]
ANGPTL6	skeletal muscle, WAT, liver	Enhanced insulin signaling (skeletal muscle)	Inhibited gluconeogenic pathway (liver)	Increased mitochondrial oxygen consumption (WAT)		^[40]^[41]^[42]^[43]	[40,41,42,43]
ANGPTL8	hepatocytes, adipocytes	Improved insulin signaling and suppressed gluconeogenic gene expression (liver)	Suppressed lipolysis (hepatocyte, adipocyte)	Promoted lipogenesis (liver)		^[44]^[45]^[46]^[47]^[48]	[44,45,46,47,48]
Fetuin-A	liver, WAT, skeletal muscle, monocytes	Blocked insulin signaling through inhibition of insulin receptor tyrosine kinase (liver, WAT, skeletal muscle)	Provoked inflammatory response (monocytes, adipocytes)	Inhibited adiponectin production		^[13]^[15]^[49]^[50]^[51]^[52]^[53]^[54]^[55]	[13,15,49,50,51,52,53,54,55]
Fetuin-B	hepatocytes, myotubes	Induced insulin resistance (hepatocytes, myotubes)	Promoted lipogenesis (hepatocytes)	^[26]^[56]	[26,56]
FGF21	WAT/BAT, liver, skeletal muscle, pancreas, CNS	Promoted glucose uptake (adipocytes)	Stimulated thermogenesis (BAT)	Enhanced insulin secretion (pancreatic β cells)		Increased fatty acid oxidation and insulin sensitivity (liver, skeletal muscle)		Reduced NAFLD		Decreased VLDL uptake and lipogenesis (liver)		Decreased alcohol and sugar intake		Increased energy expenditure and decreased body weight (CNS)	^[57]^[58]^[59]^[60]	[57,58,59,60]
Selenoprotein P	liver, skeletal muscle	Inhibited hepatic glucose production	Decreased glucose uptake (skeletal muscle)	^[12]^[61]^[62]^[63]^[64]	[12,61,62,63,64]
LECT2	liver, skeletal muscle	Increased M1/M2 ratio and hepatic inflammation (liver)	Development of insulin resistance (skeletal muscle)	Promoted lipid accumulation (liver)		^[65]^[66]	[65,66]
Follistatin	pituitary, skeletal muscle, liver, skeletal muscle, WAT, BAT	Inhibition of FSH production (pituitary)	Suppressed skeletal muscle growth via antagonizing myostatin	Promoted insulin resistance (liver, skeletal muscle, WAT)		Increased glucose and FFA uptake after exercise training (skeletal muscle)		Induced differentiation of brown adipocytes		Promoted thermogenesis (BAT)		^[14]^[15]^[^]^[⁷¹^]	[14,15^67][68][69,67^],68^[,69⁷⁰,70,71]
Hepassocin	liver, skeletal muscle, WAT	Promoted insulin resistance	NAFLD	Adipogenesis (WAT)		^[72]^[73]^[74]^[75]	[72,73,74,75]
RBP4	various peripheral tissues including retina	Increased lipolysis in adipocytes	Promoted hepatic mitochondrial dysfunction and hepatic steatosisSerum RBP4 levels were associated with insulin resistance and components of metabolic syndrome in humans	Depending on the source of RBP4 (hepatocytes or adipocytes), the effect of RBP4 is controversial.		- RBP4 treatment increased PEPCK (liver) and impaired insulin signaling (muscle and adipocytes).		- No effect of liver-secreted RBP4 on glucose homeostasis in mice		^[76]^[77]^[78]^[79]^[80]^[81]^[82]	[76,77,78,79,80,81,82]
SMOC1	liver, skeletal muscle, etc.	Improved glycemic control via inhibiting gluconeogenesis and glucose output (liver)	[83]
GDF15	adipose tissue, skeletal muscle, liver, brain, heart, kidney	Anorexia	Increased energy metabolism (liver, muscle, adipose tissue) and lowered body weight	Stimulated thermogenic and lipolytic genes (BAT, WAT)		Improved glucose tolerance and insulin sensitivity		Prevented liver steatosis in HFD-fed mice		^[84]^[85]^[86]^[87]^[88]^[89]^[90]	[84,85,86,87,88,89,90]

ANGPTL: Antiopoietin-like proteins; BAT: Brown adipose tissue; CNS: Central nervous system; FFA: Free fatty acid; FSH: Follicle-stimulating hormone; GDF15: Growth differentiation factor 15; HFD: High fat diet; LECT2: Leukocyte cell-derived chemotaxin 2; LPL: Lipoprotein lipase; RBP4: Retinol binding protein 4; SMOC1: SPARC-related modular calcium-binding protein 1; TG: Triglyceride; VLDL: Very low-density lipoprotein; WAT: White adipose tissue.

4. Hepatokines and NAFLD

It has been established that NAFLD is strongly associated with other metabolic comorbidities, including obesity, type 2 diabetes, dyslipidemia, cardiovascular disease, colonic diverticulosis, and neurodegenerative conditions ^{[1][4][5][6][7][14][91]}[1,4,5,6,7,14,91]. Hepatic steatosis, which refers to a pathological state of the liver characterized by an accumulation of lipid content at over ~5% of the total organ weight, is closely associated with insulin resistance in multiple organs, as supported by several studies demonstrating impaired insulin action in both lean and non-diabetic obese individuals ^{[14][92][93][94]}[14,92,93,94]. Moreover, hepatic steatosis usually develops prior to the accumulation of lipid in skeletal muscle, macrophage-driven inflammation, extrahepatic insulin resistance, and hyperglycemia ^[14][16][95][14,16,95], suggestive of its potential as an early indicator of systemic metabolic dysregulation. Interestingly, similar to adipose tissue or skeletal muscle where the secretion of respective organokines is altered via overnutrition, hepatic gene expression and protein content are also regulated in response to caloric overload, and these changes have been revealed to be strongly associated with the onset of insulin resistance and type 2 diabetes ^{[26][96][97][98]}[26,96,97,98]. Many of the liver-secreted proteins that are upregulated in the plasma of subjects with type 2 diabetes were capable of inducing insulin resistance, supportive of the pathophysiological role of hepatokines in metabolic dysregulation [99]. Recent studies have provided valuable insight into the impact of ectopic fat accumulation in hepatocytes on alterations in the hepatic proteome, with suppressed protein synthesis observed in the livers of obese mice [100]. Furthermore, metabolic remodeling was mediated via changes in the translation/secretion processes or post-translational modifications, with minor changes at the transcriptional level ^[12][26][12,26]. In line with this notion, approximately 20% of proteins with an N-terminal signal peptide were differentially secreted from mouse steatotic hepatocytes compared to normal hepatocytes, and some of these proteins were found to induce insulin resistance and pro-inflammatory signaling [26]. The above-described findings indicate that a substantial fraction of the hepatic proteome is heavily distributed during NAFLD progression, which may significantly impair energy homeostasis and systemic metabolism. However, relatively a small number of hepatokines have been identified, with a few of their metabolic functions and related regulatory mechanism(s) in NAFLD progression investigated.