Hybrid hepatocyte: A newly identified player for regeneration in hepatic injuries

Epigenetic regulation in liver regeneration

The liver is considered unique in its enormous capacity for regeneration and self-repair. In contrast to other regenerative organs (i.e., skin, skeletal muscle, and intestine), whether the adult liver contains a defined department of stem cells is still controversial. In order to compensate for the massive loss of hepatocytes following liver injury, the liver processes a precisely controlled transcriptional reprogram that can trigger cell proliferation and cell-fate switch. Epigenetic events are thought to regulate the organization of chromatin architecture and gene transcription during the liver regenerative process. In this review, we will summarize how changes to the chromatin by epigenetic modifiers are translated into cell fate transitions to restore liver homeostasis during liver regeneration.

Improving the repopulation capacity of elderly human hepatocytes by decoding aging-associated hepatocyte plasticity

BACKGROUND AND AIMS

The loss of liver regenerative capacity is the most dramatic age-associated alteration. Because of an incomplete mechanistic understanding of the liver aging process, a successful therapeutic strategy to improve liver regeneration in the elderly has not been developed so far. Hepatocyte plasticity is a principal mechanism for producing new hepatocytes and cholangiocytes during regeneration. This study aims to promote the repopulation capacity of elderly hepatocytes by decoding the underlying mechanism about the regulation of aging on human hepatocyte plasticity.

APPROACH AND RESULTS

To understand the age-related mechanisms, we established a hepatocyte aging model from human-induced pluripotent stem cells and developed a method for ex vivo characterization of hepatocyte plasticity. We found that hepatocyte plasticity was gradually diminished with aging, and the impaired plasticity was caused by age-induced histone hypoacetylation. Notably, selective inhibition of histone deacetylases could markedly restore aging-impaired plasticity. Based on these findings, we successfully improved the plasticity of elderly primary human hepatocytes that enhanced their repopulation capacity in the liver injury model.

CONCLUSIONS

This study suggests that age-induced histone hypoacetylation impairs hepatocyte plasticity, and hepatocyte plasticity might be a therapeutic target for promoting the regenerative capacity of the elderly liver.

Progress in human liver organoids

Understanding the development, regeneration, and disorders of the liver is the major goal in liver biology. Current mechanistic knowledge of human livers has been largely derived from mouse models and cell lines, which fall short in recapitulating the features of human liver cells or the structures and functions of human livers. Organoids as an in vitro system hold the promise to generate organ-like tissues in a dish. Recent advances in human liver organoids also facilitate the understanding of the biology and diseases in this complex organ. Here we review the progress in human liver organoids, mainly focusing on the methods to generate liver organoids, their applications, and possible future directions.

Cell Sources and Influencing Factors of Liver Regeneration: A Review

Liver regeneration (LR) is a set of complicated mechanisms between cells and molecules in which the processes of initiation, maintenance, and termination of liver repair are regulated. Although LR has been studied extensively, there are still numerous challenges in gaining its full understanding. Cells for LR have a wide range of sources and the feature of plasticity, and regeneration patterns are not the same under different conditions. Many patients undergoing partial hepatectomy develop cirrhosis or steatosis. The changes of LR in these cases are not clear. Many types of cells participate in LR. Hepatocytes, biliary epithelial cells, hepatic progenitor cells, and human liver stem cells can serve as the cell sources for LR. However, different types and degrees of damage trigger the response from the most suitable cells. Exploring the cell sources of LR is of great significance for accelerating recovery of liver function under different pathological patterns and developing a cell therapy strategy to cope with the shortage of donors for liver transplantation. In clinical practice, the background of the liver influences regeneration. Fibrosis and steatosis create different LR microenvironments and signal molecule interaction patterns. In addition, factors such as partial hepatectomy, aging, platelets, nerves, hormones, bile acids, and gut microbiota are widely involved in this process. Understanding the influencing factors of LR has practical value for individualized treatment of patients with liver diseases. In this review, we have summarized recent studies and proposed our views. We discuss cell sources and the influential factors on LR to help in solving clinical problems.

MiR-126 Regulates Properties of SOX9+ Liver Progenitor Cells during Liver Repair by Targeting Hoxb6

Liver progenitor cells (LPCs) have a remarkable contribution to the hepatocytes and ductal cells when normal hepatocyte proliferation is severely impaired. As a biomarker for LPCs, Sry-box 9 (Sox9) plays critical roles in liver homeostasis and repair in response to injury. However, the regulation mechanism of Sox9 in liver physiological and pathological state remains unknown. In this study, we found that miR-126 positively regulated the expression of Sox9, the proliferation and differentiation of SOX9⁺ LPCs by suppressing the translation of homeobox b6 (Hoxb6). As a transcription factor, HOXB6 directly binds to the promoter of Sox9 to inhibit Sox9 expression, resulting in the destruction of the properties of SOX9⁺ LPCs in CCl₄-induced liver injury. These findings revealed the role of miR-126 in regulating SOX9⁺ LPCs fate by targeting Hoxb6 in liver injury repair. Our findings suggest the potential role of miR-126 as a nucleic acid therapy drug target for liver failure.

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miR-126 promotes Sox9 expression and maintains SOX9⁺ LPCs in adult mouse livers

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HOXB6 suppresses properties of SOX9⁺ LPCs in chronic liver injury model

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HOXB6 negatively regulates Sox9 trans-activity

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miR-126 regulates properties of SOX9⁺ LPCs by targeting Hoxb6

Pregnane X Receptor Regulates Liver Size and Liver Cell Fate by Yes-Associated Protein Activation in Mice

Activation of pregnane X receptor (PXR), a nuclear receptor that controls xenobiotic and endobiotic metabolism, is known to induce liver enlargement, but the molecular signals and cell types responding to PXR-induced hepatomegaly remain unknown. In this study, the effect of PXR activation on liver enlargement and cell change was evaluated in several strains of genetically modified mice and animal models. Lineage labeling using AAV-Tbg-Cre-treated Rosa26^EYFP mice or Sox9-Cre^ERT , Rosa26^EYFP mice was performed and Pxr-null mice or AAV Yap short hairpin RNA (shRNA)-treated mice were used to confirm the role of PXR or yes-associated protein (YAP). Treatment with selective PXR activators induced liver enlargement and accelerated regeneration in wild-type (WT) and PXR-humanized mice, but not in Pxr-null mice, by increase of cell size, induction of a regenerative hybrid hepatocyte (HybHP) reprogramming, and promotion of hepatocyte and HybHP proliferation. Mechanistically, PXR interacted with YAP and PXR activation induced nuclear translocation of YAP. Blockade of YAP abolished PXR-induced liver enlargement in mice. Conclusion: These findings revealed a function of PXR in enlarging liver size and changing liver cell fate by activation of the YAP signaling pathway. These results have implications for understanding the physiological functions of PXR and suggest the potential for manipulation of liver size and liver cell fate.

In Vitro Expansion of Primary Human Hepatocytes with Efficient Liver Repopulation Capacity

Transplantation of human hepatocytes (HHs) holds significant potential for treating liver diseases. However, the supply of transplantable HHs is severely constrained by limited donor availability and compromised capacity for in vitro expansion. In response to chronic injury, some HHs are reprogrammed into proliferative cells that express both hepatocyte and progenitor markers, suggesting exploitable strategies for expanding HHs in vitro. Here, we report defined medium conditions that allow 10,000-fold expansion of HHs. These proliferating HHs are bi-phenotypic, partially retaining hepatic features while gaining expression of progenitor-associated genes. Importantly, these cells engraft into injured mouse liver at a level comparable to primary HHs, and they undergo maturation following transplantation in vivo or differentiation in vitro. Thus, this study provides a protocol that enables large-scale expansion of transplantable HHs, which could be further developed for modeling and treating human liver disease.

Maturity of associating liver partition and portal vein ligation for staged hepatectomy-derived liver regeneration in a rat model

AIM

To establish a rat model for evaluating the maturity of liver regeneration derived from associating liver partition and portal vein ligation for staged hepatectomy (ALPPS).

METHODS

In the present study, ALPPS, partial hepatecotmy (PHx), and sham rat models were established initially, which were validated by significant increase of proliferative markers including Ki-67, proliferating cell nuclear antigen, and cyclin D1. In the setting of accelerated proliferation in volume at the second and fifth day after ALPPS, the characteristics of newborn hepatocytes, as well as specific markers of progenitor hepatic cell, were identified. Afterwards, the detection of liver function followed by cluster analysis of functional gene expression were performed to evaluate the maturity.

RESULTS

Compared with PHx and sham groups, the proliferation of FLR was significantly higher in ALPPS group (P = 0.023 and 0.001 at second day, P = 0.034 and P < 0.001 at fifth day after stage I). Meanwhile, the increased expression of proliferative markers including Ki-67, proliferating cell nuclear antigen, and cyclin D1 verified the accelerated liver regeneration derived from ALPPS procedure. However, ALPPS-induced Sox9 positive hepatocytes significantly increased beyond the portal triad, which indicated the progenitor hepatic cell was potentially involved. And the characteristics of ALPPS-induced hepatocytes indicated the lower expression of hepatocyte nuclear factor 4 and anti-tryptase in early proliferative stage. Both suggested the immaturity of ALPPS-derived liver regeneration. Additionally, the detection of liver function and functional genes expression confirmed the immaturity of renascent hepatocytes derived in early stage of ALPPS-derived liver regeneration.

CONCLUSION

Our study revealed the immaturity of ALPPS-derived proliferation in early regenerative response, which indicated that the volumetric assessment overestimated the functional proliferation. This could be convincing evidence that the stage II of ALPPS should be performed prudently in patients with marginally adequate FLR, as the ALPPS-derived proliferation in volume lags behind the functional regeneration.

Fortilin: A Potential Target for the Prevention and Treatment of Human Diseases

Fortilin is a highly conserved 172-amino-acid polypeptide found in the cytosol, nucleus, mitochondria, extracellular space, and circulating blood. It is a multifunctional protein that protects cells against apoptosis, promotes cell growth and cell cycle progression, binds calcium (Ca²⁺) and has antipathogen activities. Its role in the pathogenesis of human and animal diseases is also diverse. Fortilin facilitates the development of atherosclerosis, contributes to both systemic and pulmonary arterial hypertension, participates in the development of cancers, and worsens diabetic nephropathy. It is important for the adaptive expansion of pancreatic β-cells in response to obesity and increased insulin requirement, for the regeneration of liver after hepatectomy, and for protection of the liver against alcohol- and ER stress-induced injury. Fortilin is a viable surrogate marker for in vivo apoptosis, and it plays a key role in embryo and organ development in vertebrates. In fish and shrimp, fortilin participates in host defense against bacterial and viral pathogens. Further translational research could prove fortilin to be a viable molecular target for treatment of various human diseases including and not limited to atherosclerosis, hypertension, certain tumors, diabetes mellitus, diabetic nephropathy, hepatic injury, and aberrant immunity and host defense.