Cell types

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Liver Cell Types

Strategic placement in the venous drainage of the intestine (for absorption of nutrients) and its unique structure, consisting of single-layered cell plates in direct contact with the blood system, both enable the liver to perform its diverse tasks. The liver does this with 3 main cell types: liver parenchymal cells (hepatocytes), cholangiocytes, and sinusoid-lining cells.

Although histologically hepatocytes appear identical, they perform very different and often opposing metabolic functions depending on their location within the lobule - a phenomenon known as liver zonation. For instance, periportal hepatocytes primarily engage in gluconeogenesis and fatty acid oxidation, while their pericentral counterparts specialize in glycolysis and lipogenesis. This sophisticated spatial division of labor allows the liver to perform competing metabolic processes simultaneously with remarkable efficiency.

It is well established that chylomicron remnant (dietary) vitamin A (There are two types of vitamin A that are found in the diet ) is taken up from the circulation by hepatocytes, but more than 80 % of the vitamin A in the liver is stored in hepatic stellate cells (HSC). It presently is not known how vitamin A is transferred from hepatocytes to HSCs for storage. It may take up to 2 years for signs of deficiency to appear.

Hepatocytes accumulate iron via receptor-mediated endocytosis, accepting it mainly in the form of circulating transferrin or ferritin; additionally there is a constant stream of ferritin from Kupffer cells, as they ingest effete red cell. 

Hepatocyte play a critical role in glucose metabolism and can store or produce glucose depending on the requirement. Glucose storage by the liver is particularly important to glucose homeostasis, and hepatocytes can store ∼100 g (or 400 kilocalories) within branched chains of glycogen polysaccharides (Wasserman, 2009). 

Hepatocyte replacement occurs relatively slowly; the average life span of adult hepatocytes ranges from 200 to 300 days. A number of different hypotheses have been proposed to explain hepatocyte turnover. 

The liver is the primary site for RBP4 synthesis, with hepatocytes being the main cell type responsible for its production. RBP4 is essential for delivering retinol to tissues where it is needed for various biological processes, such as vision, cell growth, and immune function. While hepatocytes are the primary source, RBP4 can also be produced in other tissues like adipose tissue, but the liver remains the dominant source for circulating RBP4. Obesity and insulin resistance are associated with elevated RBP4 levels in the blood.     

Protein metabolism

About 170mg/g of wet liver weight is the total amount of proteins in the liver. Amino acids are absorbed via the portal veins or by local build-up through transamination from ketones. Some of these proteins have a local structural, binding or enzymatic role (such as ligandin, ferritin and liver cytochrome P450). The other, second part, is transported to the blood system or the bile. These export proteins include albumin (hypoalbuminemia occurs in liver failure), all blood clotting proteins (with factor VIII mainly built up in endothelial cells), the glyco- and lipoproteins.

Normal liver function also includes the breakdown of proteins, whereby the resulting amino acids are deaminated and ammonia is formed as a reaction product. In the case of impaired liver function, ammonia cannot be efficiently further metabolized, which can lead to hepatic encephalopathy.

Carbohydrate metabolism

Simple sugars, monosaccharides, are absorbed. Molecules that are not burned directly are converted into glycogen that is stored in the hepatocytes. This glycogen (as a reserve energy storage) is released (broken down) as glucose under endocrine control by glycogenolytic enzymes in the liver.

Starting from amino acids and breakdown molecules from glucose combustion (glycolysis), such as lactic acid and pyruvic acid, glucose can be formed again (gluconeogenesis). In acute liver failure, both hypoglycemia and lactic acidosis are on the radar in this way. An adult person has an average of approximately 300 grams of glycogen, of which 70 grams is stored in the liver and available for immediate use. The reserve is muscle glycogen. Fasting for 24 hours completely depletes the liver glycogen reserve, because the human organism has an unceasingly large need for glucose. The brain uses an average of 120 grams of glucose per day. When the glycogen reserve is used up, the body turns to gluconeogenesis.

Lipid metabolism

Hepatocytes regulate an important part of triglyceride synthesis. The fatty acids required for this come completely or partly from the ingested food and/or completely or partly from the adipose tissue. A small part, mainly saturated fatty acids, is formed from acetate. Fatty acids are incorporated in hepatocytes and phospholipids, esterified with cholesterol, under the influence of the enzyme lecithin-cholesterol acyltransferase or oxidized to ketones or carbon dioxide. Under normal circumstances, lipids make up approximately 5 percent of the liver weight. An excess of lipids leads to pathology steatosis.

Approximately 80 percent of the endogenous cholesterol synthesis (1 to 2 g/24 h, with 500 mg directly from ingested food) is synthesized in the liver from acetate. A key role in this reaction process is reserved for the enzyme b-hydroxy-methylglutaryl reductase (HMG reductase), which catalyzes the formation of mevalonate. Medically, inhibitors of this enzyme are used in the pathology hypercholesterolemia. Cholesterol is secreted in the bile under optimal conditions or incorporated into lipoproteins and transported to the plasma.

In addition, cholesterol is also broken down in the hepatocytes. Cholesterol is broken down into bile acids. This process is catalyzed by the enzyme 7alpha-hydroxylase.

Vitamins

Hepatocytes are also a storage site for ferritin iron and a range of vitamins (A, D, E, K, B12). Only when vitamin K is available can hepatocytes regulate the synthesis of the prothrombin complex (clotting factors II, VII, IX and X).

Growing evidence of vitamin K’s importance in human health beyond blood coagulation and bone health necessitates its further research. Two groups of vitamin K occur in nature; phylloquinone (PK), which is primarily produced by plants, and menaquinones (MKs), which are of bacterial and animal origin (Shearer & Newman, 2008). Bacteria in the human gut also produce MKs, in particular the long-chained MK-6, MK-7, MK-8, MK-10 and MK-11 (Shearer & Newman, 2008). Vitamin K vitamers act as co-factors, activating the carboxylation activity of the vitamin K-dependent enzyme; ɣ-glutamyl carboxylase (GGCX). GGCX will ɣ-carboxylate specific peptide-bound glutamate (Glu) residues of proteins into ɣ-carboxyglutamate (Gla) residues (Shearer and Newman, 2008, Tie and Stafford, 2016). GGCX is primarily associated with the liver but is present in all human tissue, suggesting that the need for Gla-containing proteins is widely spread in the body (Tie & Stafford, 2016). Besides PK’s role in the coagulation of blood, it has been established that MKs play a role in maintaining healthy bone tissue, and that especially MK-7 is a crucial factor in preventing vascular calcification (Halder et al., 2019, Turck et al., 2017). Further, it has been suggested that MKs have anti-cancer effects, immunosuppressive effects and prevent the risk of type 2 diabetes (Halder et al., 2019, Turck et al., 2017). 

There is a necessity to study both PK and MKs further. At present, only limited data of the content of vitamin K, especially the MKs, in our food is available. 

Phylloquinone improves endothelial function, inhibits cellular senescence, and vascular inflammation 

Vitamin K (VK) is a ligand of the pregnane X receptor (PXR), which plays a critical role in the detoxification of xenobiotics and metabolism of bile acids. VK1 may reduce the risk of death in patients with chronic liver failure. Current insights show that the liver requires vitamin K as a coenzyme. It is involved in the carboxylation of the coagulation factors II, VII, IX, X, making them biologically active. A vitamin K deficiency leads to a decrease in the functional factors II, VII, IX, X. The speed at which this occurs depends on the half-life of the various factors. Factor VII disappears first due to its very short half-life. Since vitamin K is a fat-soluble vitamin, the presence of bile salts is necessary for its presumed absorption via the intestine. One motivation for this is the fact that if the discharge of bile salts to the intestine is obstructed, a vitamin K deficiency occurs. A vitamin K deficiency can also occur in the event of digestive problems and the use of (broad-spectrum) antibiotics. Vitamin K deficiency may also be a relevant issue in newborns, where it can cause bleeding.

Laboratory tests show that the PT and APTT are prolonged. In the early stages of the deficiency, the rapid decrease in factor VII can prolong the PT, while the APTT is still normal. In the case of a proven vitamin K deficiency, with or without bleeding, the dosage today depends on the severity of the deficiency, the age and the cause. It is now customary that vitamin K is preferably given orally. In biliary congestion it is given parenterally, because the absorption of vitamin K via the intestine is then disturbed by the lack of necessary bile salts.

Liver recolonization models have demonstrated that hepatocytes have an unlimited regenerative capacity. However, in normal liver, cell turnover is very slow. All zones of the resting liver lobules have been equally implicated in the maintenance of hepatocyte and cholangiocyte populations in normal liver. 

Cholangiocytes, the lining epithelial cells in bile ducts, are an important subset of liver cells. They are activated by endogenous and exogenous stimuli and are involved in the modification of bile volume and composition. They are also involved in damaging and repairing the liver. Cholangiocytes have many functions including bile production. They are also involved in transport processes that regulate the volume and composition of bile. Cholangiocytes undergo proliferation and cell death under a variety of conditions. Cholangiocytes have functional and morphological heterogenecity. The immunobiology of cholangiocytes is important, particularly for understanding biliary disease. Secretion of different proinflammatory mediators, cytokines, and chemokines suggests the major role that cholangiocytes play in inflammatory reactions. Furthermore, paracrine secretion of growth factors and peptides mediates extensive cross-talk with other liver cells, including hepatocytes, stellate cells, stem cells, subepithelial myofibroblasts, endothelial cells, and inflammatory cells. Cholangiopathy refers to a category of chronic liver diseases whose primary disease target is the cholangiocyte. Cholangiopathy usually results in end-stage liver disease requiring liver transplant. 

Cholangiocytes line a complex network of interconnecting tubes extending from the Canals of Hering in the liver to the duodenum. In humans, the total length of this network is estimated to be ~1.25 miles (2 km)1. As with other epithelial cells, cholangiocytes are polarized with distinct apical and basolateral plasma membrane domains and multiple transport functions, many relevant to bile formation. Although cholangiocytes comprise a minority cell population in the liver, they are critical in bile generation, a life-sustaining function of the liver2. Bile is a secretory fluid product of the hepatobiliary system containing a variety of components, including bile acids, electrolytes, lipids, proteins and endobiotic and xenobiotic compounds. These factors contribute to health by aiding digestion, maintaining the enterohepatic circulation, helping to eliminate unwanted compounds from the body and an additional communication tool of the human body.

The continuous and extensive network of these cells within and outside the liver results in considerable heterogeneity in cholangiocyte function along the biliary tract. The blood supply to cholangiocytes originates from the hepatic artery and forms a peribiliary plexus (PBP) consisting of a 3D network of blood vessels of homogeneous diameter surrounding bile ducts3. The intimate anatomic association of the PBP with cholangiocytes enables crosstalk that probably both helps regulate normal cholangiocyte function and is associated with cholangiocyte malfunction in disease4. Under healthy circumstances, cholangiocytes have major physiological functions: bile is modified within the ductal lumen via activities at their apical plasma membrane domain; they form a barrier to potentially damaging molecules and microorganisms in bile via their tight junctions and immunoglobulin A (IgA) secretion; and they enable access to the immune and vascular systems via their basolateral plasma membrane domain. These complex processes are regulated by extracellular signals (for example, peptides, nucleotides, hormones and neurotransmitters), biliary constituents (such as bile acids, glucose and vesicles) and physical forces (including flow and pressure) that are reflected in various intracellular pathways, relying mostly on cAMP and Ca2+ signalling as second messengers. 

Sinusoids are the canals formed by the plates of hepatocytes. They are approximately 8-10 μm in diameter and comparable with the diameter of normal capillaries. They are orientated in a radial direction in the lobule. Sinusoids are lined with endothelial cells and Kupffer cells, which have a phagocytic function.

Plasma and proteins migrate through these lining cells via so-called fenestrations (100-150 nm) into the Space of Disse, where direct contact with the hepatocytes occurs and uptake of nutrients and oxygen by the hepatocytes takes place. On the opposite side of the hepatocyte plates are the bile canaliculi situated (1 μm diameter). Bile produced by the hepatocytes empties in these bile canaliculi and is transported back towards the portal canal into bile ductiles and bile ducts, and finally to the main bile duct and gallbladder to become available for digestive processes in the intestine. The direction of bile flow is opposite to the direction of the blood flow through the sinusoids.

Kupffer cells (KCs) are resident macrophages in the liver. Recent studies have revealed that KCs are closely related to inflammatory liver diseases, including nonalcoholic liver diseases (NAFLD). Similar to general macrophages, KCs show several different phenotypes according to their environment. Activated KCs are involved in either proinflammatory responses or anti-inflammatory responses. 

They have a high phagocytic competency, allowing them to remove the foreign medium, for example, viruses and bacteria, apoptotic cells, and cellular debris. Accordingly, KCs in the liver sinusoids serve a crucial role as gatekeepers in the hepatic immune system. KCs play a central role in immunity, tissue injury, and repair in the liver. KCs generate various inflammatory mediators containing cytokines, prostaglandins, and reactive oxygen species mainly through NADPH-oxidase or inducible NO-synthase (iNOS) activities. There is no doubt that KC dysfunction contributes to the pathogenesis of nonalcoholic fatty liver diseases (NAFLD). Nevertheless, the role of KCs in regulating liver metabolism and the incidence of metabolic disease remains unknown.

KCs can be important as a Therapeutic Target for Inflammatory Diseases. Nutrients absorbed from the small intestine flow through the portal vein or lymph vessels to the liver, where they are metabolized as an energy source. An excessive influx of nutrients can impose excessive stress on the liver by promoting the production of reactive oxygen species and waste products during metabolism. In addition, various infectious microorganisms and wastes, including hepatitis viruses, enter the liver through the circulatory system, causing damage to the liver. The liver is equipped with a hepatic immune system to counter these factors. KCs reside in sinusoids and monitor blood influx to the liver. KCs have also been reported to be involved in lipid metabolism and iron metabolism derived from aged red blood cells. KCs are supposed to be directly involved in various liver diseases, including infectious diseases, lifestyle-induced alcoholic and nonalcoholic liver diseases, autoimmune diseases, and drug-induced diseases. In each case, the inflammatory response progresses as the disease progresses, and in some cases, the disease becomes more severe, leading to cirrhosis and hepatocellular carcinoma (HCC). Some studies have reported the involvement of KCs in these diseases. Based on these studies, there is no doubt about the importance of KCs as a therapeutic target for the inflammatory diseases.

Under physiological conditions, KCs are the first innate immune cells and protect the liver from bacterial infections. Under pathological conditions, they are activated by different components and can differentiate into M1-like (classical) or M2-like (alternative) macrophages. The metabolism of classical or alternative activated Kupffer cells will determine their functions in liver damage. KCs are the first immune cells in the liver that come in contact with the gut bacteria and gut bacterial endotoxins and microbial debris derived from the gastrointestinal tract that have been transported to the liver via the portal vein. They also play an essential role in the host defense and participate in the metabolism of multiple compounds such as protein complexes, small particles, and lipids, and in removing apoptotic cells from the circulation. Consequently, modifications or alterations of KC functions are associated with various liver diseases: viral hepatitis, steatohepatitis, alcoholic liver disease, intrahepatic cholestasis, activation or rejection of the liver during liver transplantation and liver fibrosis.

Life span and renewal of Kupffer cells in liver

Little is known concerning the life span and the renewal mechanisms of KCs. The calculated life span of mammalian KCs was determined to be 3.8 days; however, experimental data showed a longer life span. Iin transplanted human livers, donor KCs persisted for up to one year.

In relation to paracrine factors involved in circuits of intercellular communication, the existence of a hepatic oxygen sensor located in the Kupffer cell has been postulated. According to this postulate the oxygen metabolism of the liver parenchymal cells could be under the control of the Kupffer cells. 

The interference with the mitochondrial electron flow by some biomolecules released from the activated Kupffer cell, such as tumour necrosis factor, interleukins, eicosanoids, etc., would increase the rate of generation of reactive oxygen species by the inhibited mitochondrial respiratory chain. 

The content of vitamin A per Kupffer cell was found to be less than one-tenth that of the average liver cell, and the Kupffer cell population was calculated to contain less than 4% of the total vitamin, the rest being present in the hepatocytes. 

Studies have shown that liver storage is positively correlated with total body status and can range from 40% to 90% of total-body vitamin A. 

Primarily synthesized in the liver, Retinol Binding Protein (RBP) is not exclusively produced there. Other tissues, including the kidney, testis, retinal pigment epithelium, and choroid plexus of the brain, also synthesize it. RBP's primary function is to transport retinol (vitamin A) from the liver to other tissues in the body. 

Liver sinusoidal endothelial cells (LSECs) line the liver sinusoids and separate passenger leukocytes in the sinusoidal lumen from hepatocytes. LSECs further act as a platform for adhesion of various liver-resident immune cell populations such as Kupffer cells, innate lymphoid cells or liver dendritic cells. 

Liver sinusoidal endothelial cells (LSECs) that line the hepatic sinusoids have important physiological roles and mediate the filtration and scavenger functions of the liver. In addition to having an extraordinary scavenger function, LSECs possess potent immune functions, serving as sentinel cells to detect microbial infection through pattern recognition receptor activation and as antigen (cross)-presenting cells. Liver sinusoidal endothelial cells (LSECs) that line the hepatic sinusoids have important physiological roles and mediate the filtration and scavenger functions of the liver. 

The immune function of LSECs complements conventional immune-activating mechanisms to accommodate optimal immune surveillance against infectious microorganisms while preserving the integrity of the liver as a metabolic organ. 

Major functions of LSECs include (1) elimination of macromolecules and small particulates from the blood, (2) an immunological role, (3) LSEC interactions with tumor metastases, (4) LSECs as determinants of hepatic fibrosis, and (5) LSECs as drivers of liver regeneration. 

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