The Polyploid State Restricts Hepatocyte Proliferation and Liver Regeneration in Mice

Abstract

The liver contains a mixture of hepatocytes with diploid or polyploid (tetraploid, octaploid, etc.) nuclear content. Polyploid hepatocytes are commonly found in adult mammals, representing ~90% of the entire hepatic pool in rodents. The cellular and molecular mechanisms that regulate polyploidization have been well characterized; however, it is unclear whether diploid and polyploid hepatocytes function similarly in multiple contexts. Answering this question has been challenging because proliferating hepatocytes can increase or decrease ploidy, and animal models with healthy diploid‐only livers have not been available. Mice lacking E2f7 and E2f8 in the liver (liver‐specific E2f7/E2f8 knockout; LKO) were recently reported to have a polyploidization defect, but were otherwise healthy. Herein, livers from LKO mice were rigorously characterized, demonstrating a 20‐fold increase in diploid hepatocytes and maintenance of the diploid state even after extensive proliferation. Livers from LKO mice maintained normal function, but became highly tumorigenic when challenged with tumor‐promoting stimuli, suggesting that tumors in LKO mice were driven, at least in part, by diploid hepatocytes capable of rapid proliferation. Indeed, hepatocytes from LKO mice proliferate faster and out‐compete control hepatocytes, especially in competitive repopulation studies. In addition, diploid or polyploid hepatocytes from wild‐type (WT) mice were examined to eliminate potentially confounding effects associated with E2f7/E2f8 deficiency. WT diploid cells also showed a proliferative advantage, entering and progressing through the cell cycle faster than polyploid cells, both in vitro and during liver regeneration (LR). Diploid and polyploid hepatocytes responded similarly to hepatic mitogens, indicating that proliferation kinetics are unrelated to differential response to growth stimuli. Conclusion: Diploid hepatocytes proliferate faster than polyploids, suggesting that the polyploid state functions as a growth suppressor to restrict proliferation by the majority of hepatocytes.

Surviving Acute Organ Failure: Cell Polyploidization and Progenitor Proliferation

“…Indeed, in mutagen-and high-fat-induced mouse models, tumorigenesis was accelerated by proliferation of diploid hepatocytes [92], and accordingly, human data show that, for example, hepatocarcinomas are predominantly diploid [93][94][95]. Consistently, in mouse models of a stable polyploidy knockout, hepatocyte progenitors proliferate faster than in wild-type mice and become highly tumorigenic when challenged with tumor-promoting stimuli, suggesting that tumors are driven by rapid progenitor proliferation [20]. Preliminary evidence indicates that renal tumors, such as angiomyolipoma [96], papillary carcinoma, and clear cell carcinoma, may derive from tubular progenitors [97,98].…”

“…Indeed, cytoskeleton structure [16,17] and cell polarity [18] can be reiteratively lost during mitosis and cytokinesis, correlating with transient adhesion disengagement and dramatic deformation of organism morphology [18]. However, evolution has selected alternative solutions and minimized the mitotic ability of differentiated parenchymal cells [19,20]. In recent years, evolving evidence has revealed that two main response programs sustain residual functional performance and enforce recovery upon acute organ injury.…”

“…Accordingly, blocking mitosis through the conditional knockout of CDK1 in a mouse liver did not impair recovery of liver function mediated by polyploidization upon 70% partial hepatectomy, indicating that polyploid cells are crucial players in supporting and maintaining almost normal liver function [42]. In contrast, by using a model of polyploidy knockout (E2F7/E2F8 knockout mice) with 70% partial hepatectomy, Wilkinson et al demonstrated that a true mitotic cell division occurs and is driven mostly by a small population of diploid hepatocyte progenitors, representing about 10% of all hepatocytes [20]. Moreover, proliferation of hepatocyte progenitors occurs sooner and faster than polyploidization in differentiated hepatocytes and is responsible for the true structural recovery of the organ [20].…”

“…In contrast, by using a model of polyploidy knockout (E2F7/E2F8 knockout mice) with 70% partial hepatectomy, Wilkinson et al demonstrated that a true mitotic cell division occurs and is driven mostly by a small population of diploid hepatocyte progenitors, representing about 10% of all hepatocytes [20]. Moreover, proliferation of hepatocyte progenitors occurs sooner and faster than polyploidization in differentiated hepatocytes and is responsible for the true structural recovery of the organ [20]. Consistently, hepatocyte progenitors also clonally expand after acute toxic injury by CCl 4 injection in order to maintain liver mass [38].…”

Cardiac interstitial tetraploid cells that escape replicative senescence are found in rodents but not large mammals

“…Examples include 1) the diploid population undergoing endoreplication or endomitosis to become tetraploid, 2) indirect influence of tetraploids to inhibit diploid proliferation through cell-cell communication, possibly via secreted factors, or 3) greater adaptive capacity of the tetraploid population to the stress of in vitro expansion, leading to diminished competitive capability of the diploids. Indeed, tetraploid cCICs and CSCs may inhibit diploid proliferation through cell-cell communication, as was recently discovered between tetraploid and diploid hepatocytes to maintain stability and prevent tumor formation 62,63 . Although we cannot exclude fusion as a possible mechanism 64 , no evidence of fusion has been observed in time-lapse recordings of CSC cultures (unpublished observation).…”