By Jessica Lu
Hepatocytes are the major cell type in the human liver, making up 70% of all liver cells. They are main contributors to liver functions such as metabolic homeostasis, synthesis, storage, distribution, and detoxification of xenobiotic compounds (Wang et al., 2017). Unlike typical cells which are diploid, mature mammalian hepatocytes are polyploid (Wang et al., 2017). However, the biological significance of polyploidy in hepatocytes is still uncertain (Zhang et al., 2019). Various hypotheses suggest it may economise energy resources, enhance metabolic function or protect against oxidative stress (Gentric & Desdouets, 2014).
Hepatocyte polyploidisation increases with age (Gentric & Desdouets, 2014; Wang et al., 2017). In the newborn liver, all hepatocytes are diploid. Polyploidisation first begins 3 weeks after birth, mainly due to failed cytokinesis (Wang et al., 2017). Successful cytokinesis generates two diploid hepatocytes, whereas incomplete cytokinesis generates a tetraploid hepatocyte with two diploid nuclei. This binucleated tetraploid hepatocyte can undergo complete mitosis to generate two mononucleated tetraploid cells. However, if the cytokinesis is again unsuccessful in mitosis of the binucleated tetraploid hepatocyte, then a binucleated octoploid hepatocyte is generated (Wang et al., 2017). In young individuals, the proportion of polyploid cells remains low. For example, in 20-year old adult humans, the relative number of polyploid hepatocytes generally does not exceed 15%. However, there is a second wave of polyploidisation during ageing as 42% of hepatocytes are polyploid in an 80-year old adult (Kudryavtsev et al., 1993; Wang et al., 2017). Apart from ageing, radiation, oxidative stress and injury can all stimulate hepatocyte polyploidisation (Wang et al., 2017).
The cytokinesis failure which causes polyploidy is reported to be regulated by the insulin levels via the phosphoinositide 3-kinase (PI3K)-protein kinase B (Akt)-cytoskeleton regulation pathway. Reduced insulin levels give rise to fewer binucleated tetraploid hepatocytes, whereas increased insulin levels increase the number of binucleated tetraploid hepatocytes (Wang et al., 2017). Other factors that may contribute include E2F transcription factors. Inducing the expression of E2F target genes promotes cytokinesis, resulting in fewer polyploid cells. (Gentric & Desdouets, 2014; Wang et al., 2017). In mouse models, silencing of other factors such as Skp2, Ccne2, p21, p53, pRb, survivin, Ssu72, and nucleotide excision repair gene ERCC1 that regulate the cell cycle also increase liver polyploidisation (Wang et al., 2017).
One idea is that liver polyploidisation is associated with terminal differentiation and cellular senescence (Gentric & Desdouets, 2014). The liver has the ability to regenerate, however, this ability is decreased in older animals that have higher proportions of polyploid hepatocytes (Wang et al., 2017). Older mice with senescence tend to have increased polyploid hepatocytes, also, in vitro the replicative ability of diploid hepatocytes appears to be higher than for polyploid hepatocytes (Gentric & Desdouets, 2014). However, this theory that polyploidisation is related to senescence is disputed by several groups, as tetraploid hepatocytes appear to be highly regenerative after partial hepatectomy. Additionally, E2f8−/− mice have livers mainly composed of diploid hepatocytes, however, their livers have no significant differences in regenerative capacity compared to wild-type livers (Gentric & Desdouets, 2014).
There are several theories for why polyploidy may be beneficial in hepatocytes. One possibility is that polyploidy saves energy compared to hepatocyte proliferation. There is some evidence to support this, since the rat suckling-weaning period is both linked to changes in hepatocyte polyploidisation and high-energy consumption (Gentric & Desdouets, 2014). Furthermore, there is a tendency of the polyploidy liver to produce more energy anaerobically and obtain ATP from carbohydrates rather than fatty acids (Wang et al., 2017). It has been suggested that polyploidy switches the liver to an economy saving mode to perform specific liver functions, rather than investing energy into cell division (Wang et al., 2017).
In addition, it is possible that polyploidy in hepatocytes enhances their function (Gentric & Desdouets, 2014). Hepatocytes play an important role in metabolism. For example, polyploidy may allow two- or four-fold increased expression of certain genes, improving certain metabolic functions (Gentric & Desdouets, 2014; Zhang et al., 2019). Despite this hypothesis, there appears to be no major differences in gene expression between mice hepatocytes of different ploidy according to microarray analysis. Although 50 candidate genes are differently expressed in tetraploid hepatocytes compared to diploid hepatocytes, the changes are less than two-fold either way (Lu et al., 2007). Even though polyploidy may not majorly change levels of gene expression, polyploid hepatocytes appear to exhibit less transcriptional noise compared to diploid hepatocytes, suggesting polyploidy may still be beneficial by allowing tighter control of gene expression, (Zhang et al., 2019).
One final explanation of hepatocyte polyploidy is that polyploidisation is a protective response against oxidative stress and genotoxic damage, particularly important because the primary role of the liver is to metabolise and eliminate toxic compounds (Gentric & Desdouets, 2014). Because polyploid cells have more copies of genes, they have more of a buffer before a critical gene is damaged (Wang et al., 2017). This is supported because polyploidy appears to be inversely correlated with tumorigenesis (Zhang et al., 2018). In rats treated with the carcinogens diethylnitrosamine and 2-acetyl-aminofluorene), only the diploid cell population increases at different stages of hepatocyte transformation, suggesting that polyploid cells are protected (Wang et al., 2017). However, other evidence suggests hepatocyte polyploidy may actually increase the risk of hepatocellular cancer (HCC) formation (Müller, May & Bird, 2021).
Overall, many questions regarding hepatocyte polyploidy are still unanswered, mainly regarding its physiological significance. This is an intriguing question, particularly due to the rarity of polyploidy in animals (Zhang et al., 2019). Better understanding could potentially develop therapies which manipulate polyploidy to treat liver cancer or age-dependent diseases (Wang et al., 2017).
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Kudryavtsev, B. N., Kudryavtseva, M. V., Sakuta, G. A. & Stein, G. I. (1993) Human hepatocyte polyploidization kinetics in the course of life cycle. Virchows Archiv.B, Cell Pathology Including Molecular Pathology. 64 (6), 387-393. Available from: doi: 10.1007/BF02915139.
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Zhang, S., Zhou, K., Luo, X., Li, L., Tu, H., Sehgal, A., Nguyen, L., Zhang, Y., Gopal, P., Tarlow, B., Siegwart, D. & Zhu, H. (2018) The polyploid state plays a tumor suppressive role in the liver. Developmental Cell. 44 (4), 447-459.e5. Available from: doi: 10.1016/j.devcel.2018.01.010.