Hepatocyte

Function

Protein synthesis

The hepatocyte is a cell in the body that manufactures serum albumin, fibrinogen, and the prothrombin group of clotting factors (except for Factors 3 and 4).

It is the main site for the synthesis of lipoproteins, ceruloplasmin, transferrin, complement, and glycoproteins. Hepatocytes manufacture their own structural proteins and intracellular enzymes.

Synthesis of proteins is by the rough endoplasmic reticulum (RER), and both the rough and smooth endoplasmic reticulum (SER) are involved in secretion of the proteins formed.

The endoplasmic reticulum (ER) is involved in conjugation of proteins to lipid and carbohydrate moieties synthesized by, or modified within, the hepatocytes.

Proteins produced by hepatocytes that function as hormones are known as hepatokines.

Carbohydrate metabolism

The liver forms fatty acids from carbohydrates and synthesizes triglycerides from fatty acids and glycerol. Hepatocytes also synthesize apoproteins with which they then assemble and export lipoproteins (VLDL, HDL).

The liver is also the main site in the body for gluconeogenesis, the formation of carbohydrates from precursors such as alanine, glycerol, and oxaloacetate.

Lipid metabolism

The liver receives many lipids from the systemic circulation and metabolizes chylomicron remnants. It also synthesizes cholesterol from acetate and further synthesizes bile salts. The liver is the sole site of bile salts formation.

Detoxification

Hepatocytes contain abundant endoplasmic reticulum.

Hepatocytes have the ability to metabolize, detoxify, and inactivate exogenous compounds such as drugs (see drug metabolism), insecticides, and endogenous compounds such as steroids.

The drainage of the intestinal venous blood into the liver requires efficient detoxification of miscellaneous absorbed substances to maintain homeostasis and protect the body against ingested toxins.

One of the detoxifying functions of hepatocytes is to modify ammonia into urea for excretion. Hepatic encephalopathy occurs when the liver fails to perform this function, causing the built-up ammonia to interfere with the functions of the brain.

Renewal, regeneration and aging

Hepatocytes are able to self-renew. Homeostatic self-renewal is mostly carried out by diploid cells (71%/year), but binucleated double-diploid cells (10%/year) and tetraploid cells (1%/year) also contribute. The diploid and binucleated cells have higher annual birth rates in younger people, settling into a stable value around adulthood. Even in an 80-year-old person, the average age of the diploid cell population does not exceed one year and the other cell types 5 years. There is limited switching between these three cell types.

The human and rodent livers are also famously able to regenerate after damage and/or partial excision: it expands to around its original size without retaining the original shape. As in self-renewal, regeneration is mostly carried out by the diploid hepatocyte population, with the addition of some small contribution from cholangiocytes converting to hepatocytes. When damage happens, a migratory population of ANXA2⁺ hepatocytes leads the charge in the necrotic region to close the wound as the sheets of regular hepatocytes move and expand behind them. Wound closure predates most of the proliferation activity.

As mammalian liver cells age, damages in their DNA increase in prevalence. A review of the literature indicated that in mouse liver cells DNA damages (single-strand breaks, oxidized bases and 7-methylguanine) increase with age. Also, in rat liver, DNA single- and double-strand breaks, oxidized bases, and methylated bases increase with age; and in rabbit liver, cross-linked bases increase with age. Liver cells depend on DNA repair pathways that specifically protect the transcribed compartment of the genome to promote sustained functionality and cell preservation with age.

Society and culture

Use in research

Primary hepatocytes are commonly used in cell biological and biopharmaceutical research. In vitro model systems based on hepatocytes have been of great help to better understand the role of hepatocytes in (patho)physiological processes of the liver. In addition, pharmaceutical industry has heavily relied on the use of hepatocytes in suspension or culture to explore mechanisms of drug metabolism and even predict in vivo drug metabolism. For these purposes, hepatocytes are usually isolated from animal or human whole liver or liver tissue by collagenase digestion, which is a two-step process. In the first step, the liver is placed in an isotonic solution, in which calcium is removed to disrupt cell-cell tight junctions by the use of a calcium chelating agent. Next, a solution containing collagenase is added to separate the hepatocytes from the liver stroma. This process creates a suspension of hepatocytes, which can be seeded in multi-well plates and cultured for many days or even weeks. For optimal results, culture plates should first be coated with an extracellular matrix (e.g. collagen, Matrigel) to promote hepatocyte attachment (typically within 1-3 hr after seeding) and maintenance of the hepatic phenotype. In addition, and overlay with an additional layer of extracellular matrix is often performed to establish a sandwich culture of hepatocytes. The application of a sandwich configuration supports prolonged maintenance of hepatocytes in culture. Freshly-isolated hepatocytes that are not used immediately can be cryopreserved and stored. They do not proliferate in culture. Hepatocytes are intensely sensitive to damage during the cycles of cryopreservation including freezing and thawing. Even after the addition of classical cryoprotectants there is still damage done while being cryopreserved. Nevertheless, recent cryopreservation and resuscitation protocols support application of cryopreserved hepatocytes for most biopharmaceutical applications.

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