Hallmarks of Aging in the Liver Hunt, Nicholas J.; Kang, Sun Woo (Sophie); Lockwood, Glen P. ...
Computational and Structural Biotechnology Journal,
01/2019, Letnik:
17
Journal Article
Recenzirano
Odprti dostop
While the liver demonstrates remarkable resilience during aging, there is growing evidence that it undergoes all the cellular hallmarks of aging, which increases the risk of liver and systemic ...disease. The aging process in the liver is driven by alterations of the genome and epigenome that contribute to dysregulation of mitochondrial function and nutrient sensing pathways, leading to cellular senescence and low-grade inflammation. These changes promote multiple phenotypic changes in all liver cells (hepatocytes, liver sinusoidal endothelial, hepatic stellate and Küpffer cells) and impairment of hepatic function. In particular, age-related changes in the liver sinusoidal endothelial cells are a significant but under-recognized risk factor for the development of age-related cardiometabolic disease.
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•Liver aging is driven by transcription and metabolic epigenome alterations.•This leads to cellular senescence and low-grade inflammation.•Hepatocyte, sinusoidal endothelial, stellate and Küpffer cells undergoes the hallmarks of aging.•Each cell type demonstrates phenotypical cellular changes with age.
Abstract
Age-related changes in the liver sinusoidal endothelium, particularly the reduction in fenestrations, contribute to insulin resistance in old age. Metformin impacts on the aging process and ...improves insulin resistance. Therefore, the effects of metformin on the liver sinusoidal endothelium were studied. Metformin increased fenestrations in liver sinusoidal endothelial cells isolated from both young and old mice. Mice administered metformin in the diet for 12 months had increased fenestrations and this was associated with lower insulin levels. The effect of metformin on fenestrations was blocked by inhibitors of AMP-activated protein kinase (AMPK), endothelial nitric oxide synthase, and myosin light chain kinase phosphorylation. Metformin led to increased transgelin expression and structural changes in the actin cytoskeleton but had no effect on lactate production. Metformin also generated fenestration-like structures in SK-Hep1 cells, a liver endothelial cell line, and this was associated with increased ATP, cGMP, and mitochondrial activity. In conclusion, metformin ameliorates age-related changes in the liver sinusoidal endothelial cell via AMPK and endothelial nitric oxide pathways, which might promote insulin sensitivity in the liver, particularly in old age.
Mitochondrial damage is the major factor underlying drug-induced liver disease but whether conditions that thwart mitochondrial injury can prevent or reverse drug-induced liver damage is unclear. A ...key molecule regulating mitochondria quality control is AMP activated kinase (AMPK). When activated, AMPK causes mitochondria to elongate/fuse and proliferate, with mitochondria now producing more ATP and less reactive oxygen species. Autophagy is also triggered, a process capable of removing damaged/defective mitochondria. To explore whether AMPK activation could potentially prevent or reverse the effects of drug-induced mitochondrial and hepatocellular damage, we added an AMPK activator to collagen sandwich cultures of rat and human hepatocytes exposed to the hepatotoxic drugs, acetaminophen or diclofenac. In the absence of AMPK activation, the drugs caused hepatocytes to lose polarized morphology and have significantly decreased ATP levels and viability. At the subcellular level, mitochondria underwent fragmentation and had decreased membrane potential due to decreased expression of the mitochondrial fusion proteins Mfn1, 2 and/or Opa1. Adding AICAR, a specific AMPK activator, at the time of drug exposure prevented and reversed these effects. The mitochondria became highly fused and ATP production increased, and hepatocytes maintained polarized morphology. In exploring the mechanism responsible for this preventive and reversal effect, we found that AMPK activation prevented drug-mediated decreases in Mfn1, 2 and Opa1. AMPK activation also stimulated autophagy/mitophagy, most significantly in acetaminophen-treated cells. These results suggest that activation of AMPK prevents/reverses drug-induced mitochondrial and hepatocellular damage through regulation of mitochondrial fusion and autophagy, making it a potentially valuable approach for treatment of drug-induced liver injury.
Quantum dots (QDs) are used for imaging and transport of therapeutics. Here we demonstrate rapid absorption across the small intestine and targeted delivery of QDs with bound materials to the liver ...sinusoidal endothelial cells (LSECs) or hepatocytes in vitro and in vivo following oral administration. QDs were radiolabeled with 3H-oleic acid, with a fluorescent tag or 14C-metformin placed within a drug binding site. Three different biopolymer shell coatings were compared (formaldehyde-treated serum albumin (FSA), gelatin, heparin). Passage across the small intestine into mesenteric veins is mediated by clathrin endocytosis and micropinocytosis. 60% of an oral dose of QDs was rapidly distributed to the liver within 30 min, and this increased to 85% with FSA biopolymer coating. Uptake into LSECs also increased 3-fold with FSA coating, while uptake into hepatocytes was increased from 40% to 85% with gelatin biopolymer coating. Localization of QDs to LSECs was confirmed with immunofluorescence and transmission electron microscopy. 85% of QDs were cleared within 24 h of administration. The bioavailability of 14C-metformin 2 h post-ingestion was increased 5-fold by conjugation with QD-FSA, while uptake of metformin into LSECs was improved 50-fold by using these QDs. Endocytosis of QDs by SK-Hep1 cells (an LSEC immortal cell line) was via clathrin- and caveolae-mediated pathways with QDs taken up into lysosomes. In conclusion, we have shown high specificity targeting of the LSEC or hepatocytes after oral administration of QDs coated with a biopolymer layer of FSA or gelatin, which improved the bioavailability and delivery of metformin to LSECs.
In the liver, mitochondria are exposed to different concentrations of nutrients due to their spatial positioning across the periportal and pericentral axis. How the mitochondria sense and integrate ...these signals to respond and maintain homeostasis is not known. Here, we combine intravital microscopy, spatial proteomics, and functional assessment to investigate mitochondrial heterogeneity in the context of liver zonation. We find that periportal and pericentral mitochondria are morphologically and functionally distinct; beta-oxidation is elevated in periportal regions, while lipid synthesis is predominant in the pericentral mitochondria. In addition, comparative phosphoproteomics reveals spatially distinct patterns of mitochondrial composition and potential regulation via phosphorylation. Acute pharmacological modulation of nutrient sensing through AMPK and mTOR shifts mitochondrial phenotypes in the periportal and pericentral regions, linking nutrient gradients across the lobule and mitochondrial heterogeneity. This study highlights the role of protein phosphorylation in mitochondrial structure, function, and overall homeostasis in hepatic metabolic zonation. These findings have important implications for liver physiology and disease.
Hepatocyte polarization is energy dependent. The establishment of polarization in collagen sandwich culture of hepatocytes requires utilization of lipid droplets and mitochondrial β-oxidation to ...supply ATP. Multiple cellular pathways are involved in lipid droplet homeostasis; however, mechanistic insights of how hepatocytes utilize lipid droplets during polarization remain elusive. The current study investigated the effects of various pathways involved in lipid droplet homeostasis on bioenergetics during hepatocyte polarization. The results showed that hepatocytes were dependent on lipolysis of lipid droplets to release fatty acids for β-oxidation. Inhibition of lipolysis significantly decreased cellular fatty acid and ATP levels and inhibited hepatocyte polarization, revealing that lipolysis was an important mechanism for providing energy for hepatocyte polarization. The results also demonstrated that autophagic degradation of lipid droplets (lipophagy) was not essential for breaking down lipid droplets. Conversely, autophagy contributed to lipid droplet formation and played a key role in sustaining lipid droplet stores for energy production. In addition, cholesterol biosynthesis/cholesterol esterification and de novo fatty acid synthesis also contributed to maintaining lipid droplet stores for bioenergetics during hepatocyte polarization. In summary, multiple cellular pathways are coordinated to maintain lipid droplet homeostasis and sustain fatty acid β-oxidation during hepatocyte polarization.
Tumor heterogeneity is a hallmark of cancer but a challenging problem to dissect mechanistically. Less recognized is that cells within normal tissues are also remarkably diverse. Hepatocytes are a ...great example because their spatial positioning and the local microenvironment govern their genetic heterogeneity. Recent studies show that primary liver tumors display heterogeneity similar to that observed in the normal tissue providing clues to the cellular precursor of the tumor and how variations in the lobule microenvironment support tumor formation and aggressiveness. Identifying the principles that control cellular diversity in a healthy liver may highlight potential mechanisms driving hepatic tumor heterogeneity.
The clearance of peripheral beta amyloid (Aβ) is a potential target for the treatment of Alzheimer’s disease (AD). The liver has been implicated in the elimination of Aβ from the peripheral ...circulation. Here, the single-pass uptake of Aβ40 in perfused livers from young and old rats (6 to 10 rats per group) was investigated with the multiple indicator dilution technique. Aβ40 had volumes of distribution between those of the vascular marker Evans Blue and the extracellular marker sucrose. The hepatic extraction of Aβ40 was negligible, explained in part by the small permeability surface area products consistent with a high endothelial barrier to liver uptake. There were no substantial effects of age on any of these results. In vitro experiments with isolated hepatocytes and liver sinusoidal endothelial cells showed only very small amounts of Aβ uptake consistent with low intrinsic clearance. These results indicate that the hepatic clearance of Aβ is capacity-limited, explained by the low-permeability surface area products and hepatocyte uptake. However, this does not preclude an effect of aging in longer-term in vivo studies where age-related changes in liver blood flow and protein binding influence liver clearance.
As important as the fasting response is for survival, an inability to shut it down once nutrients become available can lead to exacerbated disease and severe wasting. The liver is central to ...transitions between feeding and fasting states, with glucagon being a key initiator of the hepatic fasting response. However, the precise mechanisms controlling fasting are not well defined. One potential mediator of these transitions is liver kinase B1 (LKB1), given its role in nutrient sensing. Here, we show LKB1 knockout mice have a severe wasting and prolonged fasting phenotype despite increased food intake. By applying RNA sequencing and intravital microscopy, we show that loss of LKB1 leads to a dramatic reprogramming of the hepatic lobule through robust up‐regulation of periportal genes and functions. This is likely mediated through the opposing effect that LKB1 has on glucagon pathways and gene expression. Conclusion: Our findings show that LKB1 acts as a brake to the glucagon‐mediated fasting response, resulting in “periportalization” of the hepatic lobule and whole‐body metabolic inefficiency. These findings reveal a mechanism by which hepatic metabolic compartmentalization is regulated by nutrient‐sensing.
The regulatory mechanisms behind glucagon‐mediated fasting are not fully understood. In this study, we identify Liver Kinase B1 (LKB1) as a critical inhibitor of glucagon‐mediated fasting and a key regulator of the hepatic lobule. These findings further our understanding of the regulation of the fasting responses and what factors shape liver zonation.
Epithelial cells in the skin and other tissues rely on signals from their environment to maintain homeostasis and respond to injury, and GPCRs play a critical role in this communication. A better ...understanding of the GPCRs expressed in epithelial cells will contribute to understanding the relationship between cells and their niche and could lead to developing new therapies to modulate cell fate. This study used human primary keratinocytes as a model to investigate the specific GPCRs regulating epithelial cell proliferation and differentiation. We identified 3 key receptors-HCAR3, LTB4R, and GPR137-and found that knockdown of these receptors led to changes in numerous gene networks that are important for maintaining cell identity and promoting proliferation while inhibiting differentiation. Our study also revealed that the metabolite receptor HCAR3 regulates keratinocyte migration and cellular metabolism. Knockdown of HCAR3 led to reduced keratinocyte migration and respiration, which could be attributed to altered metabolite use and aberrant mitochondrial morphology caused by the absence of the receptor. This study contributes to understanding the complex interplay between GPCR signaling and epithelial cell fate decisions.
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