The streaming liver IV: DNA content of the hepatocyte increases with its age

Polyploidization of Hepatocytes: Insights into the Pathogenesis of Liver Diseases

Polyploidization is a process by which cells are induced to possess more than two sets of chromosomes. Although polyploidization is not frequent in mammals, it is closely associated with development and differentiation of specific tissues and organs. The liver is one of the mammalian organs that displays ploidy dynamics in physiological homeostasis during its development. The ratio of polyploid hepatocytes increases significantly in response to hepatic injury from aging, viral infection, iron overload, surgical resection, or metabolic overload, such as that from non-alcoholic fatty liver diseases (NAFLDs). One of the unique features of NAFLD is the marked heterogeneity of hepatocyte nuclear size, which is strongly associated with an adverse liver-related outcome, such as hepatocellular carcinoma, liver transplantation, and liver-related death. Thus, hepatic polyploidization has been suggested as a potential driver in the progression of NAFLDs that are involved in the control of the multiple pathogenicity of the diseases. However, the importance of polyploidy in diverse pathophysiological contexts remains elusive. Recently, several studies reported successful improvement of symptoms of NAFLDs by reducing pathological polyploidy or by controlling cell cycle progression in animal models, suggesting that better understanding the mechanisms of pathological hepatic polyploidy may provide insights into the treatment of hepatic disorders.

Single-nucleus RNA-seq2 reveals functional crosstalk between liver zonation and ploidy

Single-cell RNA-seq reveals the role of pathogenic cell populations in development and progression of chronic diseases. In order to expand our knowledge on cellular heterogeneity, we have developed a single-nucleus RNA-seq2 method tailored for the comprehensive analysis of the nuclear transcriptome from frozen tissues, allowing the dissection of all cell types present in the liver, regardless of cell size or cellular fragility. We use this approach to characterize the transcriptional profile of individual hepatocytes with different levels of ploidy, and have discovered that ploidy states are associated with different metabolic potential, and gene expression in tetraploid mononucleated hepatocytes is conditioned by their position within the hepatic lobule. Our work reveals a remarkable crosstalk between gene dosage and spatial distribution of hepatocytes.

The Polyploid State Plays a Tumor-Suppressive Role in the Liver

Most cells in the liver are polyploid, but the functional role of polyploidy is unknown. Polyploidization occurs through cytokinesis failure and endoreduplication around the time of weaning. To interrogate polyploidy while avoiding irreversible manipulations of essential cell-cycle genes, we developed orthogonal mouse models to transiently and potently alter liver ploidy. Premature weaning, as well as knockdown of E2f8 or Anln, allowed us to toggle between diploid and polyploid states. While there was no detectable impact of ploidy alterations on liver function, metabolism, or regeneration, mice with more polyploid hepatocytes suppressed tumorigenesis and mice with more diploid hepatocytes accelerated tumorigenesis in mutagen- and high-fat-induced models. Mechanistically, the diploid state was more susceptible to Cas9-mediated tumor-suppressor loss but was similarly susceptible to MYC oncogene activation, indicating that polyploidy differentially protected the liver from distinct genomic aberrations. This suggests that polyploidy evolved in part to prevent malignant outcomes of liver injury.

Hepatocyte polyploidization and its association with pathophysiological processes

A characteristic cellular feature of the mammalian liver is the progressive polyploidization of the hepatocytes, where individual cells acquire more than two sets of chromosomes. Polyploidization results from cytokinesis failure that takes place progressively during the course of postnatal development. The proportion of polyploidy also increases with the aging process or with cellular stress such as surgical resection, toxic stimulation, metabolic overload, or oxidative damage, to involve as much as 90% of the hepatocytes in mice and 40% in humans. Hepatocyte polyploidization is generally considered an indicator of terminal differentiation and cellular senescence, and related to the dysfunction of insulin and p53/p21 signaling pathways. Interestingly, the high prevalence of hepatocyte polyploidization in the aged mouse liver can be reversed when the senescent hepatocytes are serially transplanted into young mouse livers. Here we review the current knowledge on the mechanism of hepatocytes polyploidization during postnatal growth, aging, and liver diseases. The biologic significance of polyploidization in senescent reversal, within the context of new ways to think of liver aging and liver diseases is considered.

Sox9 and programming of liver and pancreatic progenitors

Recent advances in developmental biology have greatly expanded our understanding of progenitor cell programming and the fundamental roles that Sox9 plays in liver and pancreas organogenesis. In the last 2 years, several studies have dissected the behavior of the Sox9+ duct cells in adult organs, but conflicting results have left unanswered the long-standing question of whether physiologically functioning progenitors exist in adult liver and pancreas. On the other hand, increasing evidence suggests that duct cells function as progenitors in the tissue restoration process after injury, during which embryonic programs are sometimes reactivated. This article discusses the role of Sox9 in programming liver and pancreatic progenitors as well as controversies in the field.

The stem cells of the liver ? a selective review

The current status of the much-debated question of the still-hypothetical stem cells of the liver is reviewed, with an emphasis on their role in hepatocarcinogenesis. The widely held view of the primacy of the hepatocyte, notably of the mononuclear diploid type, in this process--the "hepatocytic theory"--has been compared with variants of the "stem cell hypothesis" based on the "non-parenchymal epithelial cells" of the liver--the "oval" or biliary ductular cells, the "nondescript periductular" cells and the "primitive" bipotential epithelial cells. An attempt has been made to concentrate mainly on the more recent publications, in an effort to balance the conflicting opinions expressed by comparing results obtained by the newer procedures currently in use. Despite some interesting and relevant findings it appears that the evidence in favour of the stem-cell hypothesis is still circumstantial and that the hepatocytic theory has not been invalidated. Presumably the question of the hepatic stem cells will be answered when the riddle of hepatocarcinogenesis has been solved.

Streaming liver. VIII: Cell production rates following partial hepatectomy

Twenty-four young, female, random-bred rats weighing 250 g were partially hepatectomized and killed in groups of four animals at the following times: 1 h and 1, 2, 3, 7 and 14 days. One hour before killing, each rat was injected i.p. with 0.5 microCi [3H]-thymidine, specific activity 5 Ci/mmol/g body weight. Livers were processed histologically and dipped into liquid emulsion for autoradiography. Twenty-four hours after partial hepatectomy, hepatocyte and littoral labelling indices rose, reaching on the third day respective peak values of 3.7%, and 15.4%, whereupon they declined, remaining slightly above pre-treatment level. Labelling indices served for cell production estimates. On day 3 the hepatocyte labelling index rose 26-fold. At the same time hepatocytes doubled their ploidy, indicating that half of the observed L.I. increase was directed to DNA accumulation and not to cell division. The hepatocyte production rate therefore increased 13-fold (or 1300%). The acinus diameter increased 15%, and cell density declined 5%, so that the acinus capacity to retain cells increased only 5%. Since the acinus did not enlarge proportionally to cell production, it is concluded that 95% of newly formed cells were eliminated. Partial hepatectomy thus triggers two processes: an acute process lasting about a week marked by massive and rapid cell turnover during which most newly formed cells are eliminated; and a second, more protracted process which serves for liver mass restoration. It is proposed that partial hepatectomy induces an acute shortage of a hitherto unknown metabolite that is produced by newly formed cells immediately after hepatectomy.

Structural and functional aspects of regeneration of human liver

Regeneration of human liver was long suspected to occur. It was proven in animals 100 years ago but could not be demonstrated in man until liver biopsy and modern hepatic tests became available. Structural changes in the regenerating liver mainly concern the arrangement of liver cell plates and the size and appearance of hepatocytic nuclei. A return to normalcy in test results depends on the factors responsible for regeneration since various test results change at different rates. Mass, estimated by imaging procedures, is restored parallel with the return of function. Shape is not restored but the pressure of neighboring organs and structures molds the growing remnant so that it almost resembles the original. Factors regulating regeneration in man are beginning to be recognized as they have been in animals. The hope is that regeneration can be accelerated or that cells can be transplanted to replace those lost.

Congenital hepatoblastoma and Beckwith-Wiedemann syndrome: a case study including DNA ploidy profiles of tumor and adrenal cytomegaly

A case of fatal congenital hepatoblastoma is described in which the autopsy provided the first evidence of Beckwith-Wiedemann syndrome. Aneuploid quantitative DNA patterns were found by image analysis of the tumor and the cytomegalic adrenal gland.

Streaming liver. VII: DNA turnover in acinus zone-3

Livers of four young, male, random-bred rats weighing 250 g were fixed in formalin and stained with Feulgen. In each section, 50 hepatocytes were randomly selected and their distance from the nearest terminal hepatic vein was measured with an eye-piece micrometer. Distance was expressed in two units: 1. micrometer and, 2. cell location, or the cell number separating a sampled cell from the terminal hepatic rim. The sections were scanned with an image cytometer. The tissue was magnified x 400 and digitized into 512 x 512 pixels. The software determined the nuclear boundary and estimated its area and optical absorbance, which was expressed in relative DNA units. One unit equals the absorbance of a diploid lymphocyte. Nuclear area and optical absorbance measurements were made up to 220 microns distance from the terminal hepatic vein which roughly covers zones 2 and 3. Previously we have shown that in zones 1 and 2 hepatocyte nucleus enlarges and becomes polyploid. The present study demonstrates that in zone-3 this trend is reversed, nuclear area and DNA absorbance decline. The average hepatocyte advances 2 microns daily. In zone-1 it accumulates daily 0.005 DNA absorbance units and in zone-3 it loses daily 0.24 DNA absorbance units. It is proposed that in zone-3 DNA leaves the intact cell in a physiological way. DNA accumulation and polyploid are regarded here as forms of gene amplification which may be reversed, so that under these circumstances amplified DNA may be "deamplified" and the excess DNA leaves the cell.