Neonatal nicotine exposure affects adult rat hepatic pathways involved in endoplasmic reticulum stress and macroautophagy in a sex-dependent manner

Nonalcoholic fatty liver disease (NAFLD) involves changes in hepatic pathways, as lipogenesis, oxidative stress, endoplasmic reticulum (ER) stress, and macroautophagy. Maternal nicotine exposure exclusively during lactation leads to fatty liver (steatosis) only in the adult male offspring, not in females. Therefore, our hypothesis is that neonatal exposure to nicotine sex-dependently affects the signaling pathways involved in hepatic homeostasis of the offspring, explaining the hepatic lipid accumulation phenotype only in males. For this, between postnatal days 2 and 16, Wistar rat dams were implanted with osmotic minipumps, which released nicotine (NIC; 6 mg/Kg/day) or vehicle. The livers of offspring were evaluated at postnatal day 180. Only the male offspring that had been exposed to nicotine neonatally showed increased protein expression of markers of unfolded protein response (UPR), highlighting the presence of ER stress, as well as disruption of the activation of the macroautophagy repair pathway. These animals also had increased expression of diacylglycerol O-acyltransferase 1 and 4-hydroxynonenal, suggesting increased triglyceride esterification and oxidative stress. These parameters were not altered in the female offspring that had been neonatally exposed to nicotine, however they exhibited increased phospho adenosine monophosphate-activated protein kinase pAMPK expression, possibly as a protective mechanism. Thus, the disturbance in the hepatic homeostasis by UPR, macroautophagy, and oxidative stress modifications seem to be the molecular mechanisms underlying the liver steatosis in the adult male offspring of the nicotine-programming model. This highlights the importance of maternal smoking cessation during breastfeeding to decrease the risk of NAFLD development, especially in males.

HDAC3 Controls Liver Homeostasis More by Facilitating Deoxyribonucleic Acid Damage Repair than by Regulating Transcription in Hepatocytes

By controlling DNA damage repair and regulating gene transcription, the critical epigenetic regulator histone deacetylase 3 (HDAC3) plays pivotal roles in liver cancer and liver regeneration; however, the role of HDAC3 in liver homeostasis has not been fully elucidated. In this study, we found that HDAC3-deficient livers developed a defective morphology and metabolism with an increasing degree of DNA damage in the hepatocytes along the portal-central axis of the lobule. Most strikingly, in the Alb-CreERT:Hdac3-/- mice, it was demonstrated that HDAC3 ablation did not impair liver homeostasis in terms of histologic characteristics, function, proliferation, or gene profiles prior to the profound accumulation of DNA damage. Next, we identified that the hepatocytes in the portal area, which carried less DNA damage than those in the central area, repopulated the hepatic lobule by active regeneration and movement toward the center. As a result, the liver became more viable after each surgery. Furthermore, in vivo tracing of keratin-19-expressing hepatic progenitor cells, which lacked HDAC3, showed that the hepatic progenitor cells gave rise to newly generated periportal hepatocytes. In hepatocellular carcinoma, HDAC3 deficiency impaired DNA damage response and enhanced radiotherapy sensitivity in vitro and in vivo. Taken together, we demonstrated that HDAC3 deficiency interferes with liver homeostasis, which is more dependent on the accumulation of DNA damage in hepatocytes than on transcriptional dysregulation. Our findings support the hypothesis that selective HDAC3 inhibition has the potential to augment the effect of chemoradiotherapy aimed at inducing DNA damage in cancer therapy.

Liver macrophages in health and disease

Single-cell and spatial transcriptomic technologies have revealed an underappreciated heterogeneity of liver macrophages. This has led us to rethink the involvement of macrophages in liver homeostasis and disease. Identification of conserved gene signatures within these cells across species and diseases is enabling the correct identification of specific macrophage subsets and the generation of more specific tools to track and study the functions of these cells. Here, we discuss what is currently known about the definitions of these different macrophage populations, the markers that can be used to identify them, how they are wired within the liver, and their functional specializations in health and disease.

Creation of a Single Cell RNASeq Meta-Atlas to Define Human Liver Immune Homeostasis

The liver is unique in both its ability to maintain immune homeostasis and in its potential for immune tolerance following solid organ transplantation. Single-cell RNA sequencing (scRNA seq) is a powerful approach to generate highly dimensional transcriptome data to understand cellular phenotypes. However, when scRNA data is produced by different groups, with different data models, different standards, and samples processed in different ways, it can be challenging to draw meaningful conclusions from the aggregated data. The goal of this study was to establish a method to combine ‘human liver’ scRNA seq datasets by 1) characterizing the heterogeneity between studies and 2) using the meta-atlas to define the dominant phenotypes across immune cell subpopulations in healthy human liver. Publicly available scRNA seq data generated from liver samples obtained from a combined total of 17 patients and ~32,000 cells were analyzed. Liver-specific immune cells (CD45+) were extracted from each dataset, and immune cell subpopulations (myeloid cells, NK and T cells, plasma cells, and B cells) were examined using dimensionality reduction (UMAP), differential gene expression, and ingenuity pathway analysis. All datasets co-clustered, but cell proportions differed between studies. Gene expression correlation demonstrated similarity across all studies, and canonical pathways that differed between datasets were related to cell stress and oxidative phosphorylation rather than immune-related function. Next, a meta-atlas was generated via data integration and compared against PBMC data to define gene signatures for each hepatic immune subpopulation. This analysis defined key features of hepatic immune homeostasis, with decreased expression across immunologic pathways and enhancement of pathways involved with cell death. This method for meta-analysis of scRNA seq data provides a novel approach to broadly define the features of human liver immune homeostasis. Specific pathways and cellular phenotypes described in this human liver immune meta-atlas provide a critical reference point for further study of immune mediated disease processes within the liver.

Identification of circRNAs in the Liver of Whitespotted Bamboo Shark (Chiloscyllium plagiosum)

Whitespotted bamboo shark (Chiloscyllium plagiosum), a member of the cartilaginous fish family, has an extremely large liver and demonstrates a strong regeneration ability and immune regulation. Circular RNAs (circRNAs) is an important class of non-coding RNAs. Increasing evidences suggest that circRNAs are a kind of potential regulators. Recently, researchers have isolated and identified different circRNAs from various species, while few reports were on the circRNAs of C. plagiosum. In this study, we have identified a total of 4,558 circRNAs in the liver of C. plagiosum. This finding suggests that circRNAs are not evenly distributed in the chromosomes and follow the GT-AG rule during cyclization. Alternative back-splicing might exist in shark circRNAs as shown by the authenticity identification of predicted circRNAs. The binding strength of circRNAs (<2,000 bp) and the detected miRNAs in shark liver were simultaneously analyzed to construct an mRNA–miRNA–circRNA network for the Glutathione S-transferase P1 gene, and the circRNA authenticity was simultaneously verified. Our data provide not only novel insights into the rich existence of circRNAs in marine animals, but also a basis for characterizing functions of identified circRNAs in the liver homeostasis of C. plagiosum.

AXIN2+ Pericentral Hepatocytes Have Limited Contributions to Liver Homeostasis and Regeneration

The existence of specialized liver stem cell populations, including AXIN2⁺ pericentral hepatocytes, that safeguard homeostasis and repair has been controversial. Here, using AXIN2 lineage tracing in BAC-transgenic mice, we confirm the regenerative potential of intestinal stem cells (ISCs) but find limited roles for pericentral hepatocytes in liver parenchyma homeostasis. Liver regrowth following partial hepatectomy is enabled by proliferation of hepatocytes throughout the liver, rather than by a pericentral population. Periportal hepatocyte injury triggers local repair as well as auxiliary proliferation in all liver zones. DTA-mediated ablation of AXIN2⁺ pericentral hepatocytes transiently disrupts this zone, which is reestablished by conversion of pericentral vein-juxtaposed glutamine synthetase (GS)^- hepatocytes into GS⁺ hepatocytes and by compensatory proliferation of hepatocytes across liver zones. These findings show hepatocytes throughout the liver can upregulate AXIN2 and LGR5 after injury and contribute to liver regeneration on demand, without zonal dominance by a putative pericentral stem cell population.