Hepatic disposal of advanced glycation end products during maturation and aging

Highly oxidized albumin is cleared by liver sinusoidal endothelial cells via the receptors stabilin-1 and -2

Oxidized albumin (oxHSA) is elevated in several pathological conditions, such as decompensated cirrhosis, acute on chronic liver failure and liver mediated renal failure. Patient derived oxidized albumin was previously shown to be an inflammatory mediator, and in normal serum levels of oxHSA are low. The removal from circulation of oxidized albumins is therefore likely required for maintenance of homeostasis. Liver sinusoidal endothelial cells (LSEC) are prominent scavenger cells specialized in removal of macromolecular waste. Given that oxidized albumin is mainly cleared by the liver, we hypothesized the LSEC are the site of uptake in the liver. In vivo oxHSA was cleared rapidly by the liver and distributed to mainly the LSEC. In in vitro studies LSEC endocytosed oxHSA much more than other cell populations isolated from the liver. Furthermore, it was shown that the uptake was mediated by the stabilins, by affinity chromatography-mass spectrometry, inhibiting uptake in LSEC with other stabilin ligands and showing uptake in HEK cells overexpressing stabilin-1 or -2. oxHSA also inhibited the uptake of other stabilin ligands, and a 2-h challenge with 100 µg/mL oxHSA reduced LSEC endocytosis by 60% up to 12 h after. Thus the LSEC and their stabilins mediate clearance of highly oxidized albumin, and oxidized albumin can downregulate their endocytic capacity in turn.

L-theanine protects rat kidney from D-galactose-induced injury via inhibition of the AGEs/RAGE signaling pathway

As the irreversible products of the non-enzymatic reduction of sugars and the amino groups of proteins or peptides, advanced glycation end products (AGEs) are metabolized and excreted via the kidneys. However, if AGEs are not metabolized, they are deposited in the kidneys and bind to AGE receptors (RAGE), which can induce various pathological changes, including oxidative stress, apoptosis, and inflammation. This study used the D-galactose (DG)-induced rat model to explore the potential role and mechanism of L-theanine in inhibiting AGEs/RAGE-related signaling pathways in renal tissues. L-theanine increased the activities of glutathione peroxidase (GSH-Px) and total antioxidant capacity (T-AOC) while downregulating the contents of malondialdehyde (MDA) and AGEs in renal tissues induced by DG (P < 0.05). By inhibiting the upregulation of RAGE protein expression attributed to AGEs accumulation (P < 0.05), L-theanine downregulated phosphorylated nuclear factor (p-NF-κB (p65)), Bax, and cleaved-caspase-3 expression and increased Bcl-2 protein expression (P < 0.05), thereby alleviating the oxidative stress damage and reducing the inflammation and cell injury induced by DG. In addition, the Congo red staining section of renal tissue also showed that the natural product L-theanine can protect against AGEs-induced renal damage in DG-induced rat model.

Non-alcoholic Fatty Liver Disease and Liver Fibrosis during Aging

Non-alcoholic fatty liver disease (NAFLD) and its progressive form non-alcoholic steatohepatitis (NASH) have emerged as the leading causes of chronic liver disease-related mortality. The prevalence of NAFLD/NASH is expected to increase given the epidemics of obesity and type 2 diabetes mellitus. Older patients are disproportionally affected by NASH and related complications such as progressive fibrosis, cirrhosis and hepatocellular carcinoma; however, they are often ineligible for liver transplantation due to their frailty and comorbidities, and effective medical treatments are still lacking. In this review we focused on pathways that are key to the aging process in the liver and perpetuate NAFLD/NASH, leading to fibrosis. In addition, we highlighted recent findings and cross-talks of normal and/or senescent liver cells, dysregulated nutrient sensing, proteostasis and mitochondrial dysfunction in the framework of changing metabolic milieu. Better understanding these pathways during preclinical and clinical studies will be essential to design novel and specific therapeutic strategies to treat NASH in the elderly.

The Scavenger Function of Liver Sinusoidal Endothelial Cells in Health and Disease

The aim of this review is to give an outline of the blood clearance function of the liver sinusoidal endothelial cells (LSECs) in health and disease. Lining the hundreds of millions of hepatic sinusoids in the human liver the LSECs are perfectly located to survey the constituents of the blood. These cells are equipped with high-affinity receptors and an intracellular vesicle transport apparatus, enabling a remarkably efficient machinery for removal of large molecules and nanoparticles from the blood, thus contributing importantly to maintain blood and tissue homeostasis. We describe here central aspects of LSEC signature receptors that enable the cells to recognize and internalize blood-borne waste macromolecules at great speed and high capacity. Notably, this blood clearance system is a silent process, in the sense that it usually neither requires or elicits cell activation or immune responses. Most of our knowledge about LSECs arises from studies in animals, of which mouse and rat make up the great majority, and some species differences relevant for extrapolating from animal models to human are discussed. In the last part of the review, we discuss comparative aspects of the LSEC scavenger functions and specialized scavenger endothelial cells (SECs) in other vascular beds and in different vertebrate classes. In conclusion, the activity of LSECs and other SECs prevent exposure of a great number of waste products to the immune system, and molecules with noxious biological activities are effectively “silenced” by the rapid clearance in LSECs. An undesired consequence of this avid scavenging system is unwanted uptake of nanomedicines and biologics in the cells. As the development of this new generation of therapeutics evolves, there will be a sharp increase in the need to understand the clearance function of LSECs in health and disease. There is still a significant knowledge gap in how the LSEC clearance function is affected in liver disease.

Protein glycation - biomarkers of metabolic dysfunction and early-stage decline in health in the era of precision medicine

Protein glycation provides a biomarker in widespread clinical use, glycated hemoglobin HbA1c (A1C). It is a biomarker for diagnosis of diabetes and prediabetes and of medium-term glycemic control in patients with established diabetes. A1C is an early-stage glycation adduct of hemoglobin with glucose; a fructosamine derivative. Glucose is an amino group-directed glycating agent, modifying N-terminal and lysine sidechain amino groups. A similar fructosamine derivative of serum albumin, glycated albumin (GA), finds use as a biomarker of glycemic control, particularly where there is interference in use of A1C. Later stage adducts, advanced glycation endproducts (AGEs), are formed by the degradation of fructosamines and by the reaction of reactive dicarbonyl metabolites, such as methylglyoxal. Dicarbonyls are arginine-directed glycating agents forming mainly hydroimidazolone AGEs. Glucosepane and pentosidine, an intense fluorophore, are AGE covalent crosslinks. Cellular proteolysis of glycated proteins forms glycated amino acids, which are released into plasma and excreted in urine. Development of diagnostic algorithms by artificial intelligence machine learning is enhancing the applications of glycation biomarkers. Investigational glycation biomarkers are in development for: (i) healthy aging; (ii) risk prediction of vascular complications of diabetes; (iii) diagnosis of autism; and (iv) diagnosis and classification of early-stage arthritis. Protein glycation biomarkers are influenced by heritability, aging, decline in metabolic, vascular, renal and skeletal health, and other factors. They are applicable to populations of differing ethnicities, bridging the gap between genotype and phenotype. They are thereby likely to find continued and expanding clinical use, including in the current era of developing precision medicine, reporting on multiple pathogenic processes and supporting a precision medicine approach.

The melibiose-derived glycation product mimics a unique epitope present in human and animal tissues

Non-enzymatic modification of proteins by carbohydrates, known as glycation, leads to generation of advanced glycation end-products (AGEs). In our study we used in vitro generated AGEs to model glycation in vivo. We discovered in vivo analogs of unusual melibiose-adducts designated MAGEs (mel-derived AGEs) synthesized in vitro under anhydrous conditions with bovine serum albumin and myoglobin. Using nuclear magnetic resonance spectroscopy we have identified MAGEs as a set of isomers, with open-chain and cyclic structures, of the fructosamine moiety. We generated a mouse anti-MAGE monoclonal antibody and show for the first time that the native and previously undescribed analogous glycation product exists in living organisms and is naturally present in tissues of both invertebrates and vertebrates, including humans. We also report MAGE cross-reactive auto-antibodies in patients with diabetes. We anticipate our approach for modeling glycation in vivo will be a foundational methodology in cell biology. Further studies relevant to the discovery of MAGE may contribute to clarifying disease mechanisms and to the development of novel therapeutic options for diabetic complications, neuropathology, and cancer.

LSEC model of aging

Data obtained from genetically modified mouse models suggest a detrimental role for p16High senescent cells in physiological aging and age-related pathologies. Our recent analysis of aging mice revealed a continuous and noticeable accumulation of liver sinusoid endothelial cells (LSECs) expressing numerous senescence markers, including p16. At early stage, senescent LSECs show an enhanced ability to clear macromolecular waste and toxins including oxidized LDL (oxLDL). Later in life, however, the efficiency of this important detoxifying function rapidly declines potentially due to increased endothelial thickness and senescence-induced silencing of scavenger receptors and endocytosis genes. This inability to detoxify toxins and macromolecular waste, which can be further exacerbated by increased intestinal leakiness with age, might be an important contributing factor to animal death. Here, we propose how LSEC senescence could serve as an endogenous clock that ultimately controls longevity and outline some of the possible approaches to extend the lifespan.

Development and Progression of Non-Alcoholic Fatty Liver Disease: The Role of Advanced Glycation End Products

Non-alcoholic fatty liver disease (NAFLD) affects up to 30% of the adult population and is now a major cause of liver disease-related premature illness and deaths in the world. Treatment is largely based on lifestyle modification, which is difficult to achieve in most patients. Progression of simple fatty liver or steatosis to its severe form non-alcoholic steatohepatitis (NASH) and liver fibrosis has been explained by a 'two-hit hypothesis'. Whilst simple steatosis is considered the first hit, its transformation to NASH may be driven by a second hit. Of several factors that constitute the second hit, advanced glycation end products (AGEs), which are formed when reducing-sugars react with proteins or lipids, have been implicated as major candidates that drive steatosis to NASH via the receptor for AGEs (RAGE). Both endogenous and processed food-derived (exogenous) AGEs can activate RAGE, mainly present on Kupffer cells and hepatic stellate cells, thus propagating NAFLD progression. This review focuses on the pathophysiology of NAFLD with special emphasis on the role of food-derived AGEs in NAFLD progression to NASH and liver fibrosis. Moreover, the effect of dietary manipulation to reduce AGE content in food or the therapies targeting AGE/RAGE pathway on disease progression is also discussed.

Novel targets for delaying aging: The importance of the liver and advances in drug delivery

Age-related changes in liver function have a significant impact on systemic aging and susceptibility to age-related diseases. Nutrient sensing pathways have emerged as important targets for the development of drugs that delay aging and the onset age-related diseases. This supports a central role for the hepatic regulation of metabolism in the association between nutrition and aging. Recently, a role for liver sinusoidal endothelial cells (LSECs) in the relationship between aging and metabolism has also been proposed. Age-related loss of fenestrations within LSECs impairs the transfer of substrates (such as lipoproteins and insulin) between sinusoidal blood and hepatocytes, resulting in post-prandial hyperlipidemia and insulin resistance. Targeted drug delivery methods such as nanoparticles and quantum dots will facilitate the direct delivery of drugs that regulate fenestrations in LSECs, providing an innovative approach to ameliorating age-related diseases and increasing healthspan.

Glycation Damage: A Possible Hub for Major Pathophysiological Disorders and Aging

Glycation is both a physiological and pathological process which mainly affects proteins, nucleic acids and lipids. Exogenous and endogenous glycation produces deleterious reactions that take place principally in the extracellular matrix environment or within the cell cytosol and organelles. Advanced glycation end product (AGE) formation begins by the non-enzymatic glycation of free amino groups by sugars and aldehydes which leads to a succession of rearrangements of intermediate compounds and ultimately to irreversibly bound products known as AGEs. Epigenetic factors, oxidative stress, UV and nutrition are important causes of the accumulation of chemically and structurally different AGEs with various biological reactivities. Cross-linked proteins, deriving from the glycation process, present both an altered structure and function. Nucleotides and lipids are particularly vulnerable targets which can in turn favor DNA mutation or a decrease in cell membrane integrity and associated biological pathways respectively. In mitochondria, the consequences of glycation can alter bioenergy production. Under physiological conditions, anti-glycation defenses are sufficient, with proteasomes preventing accumulation of glycated proteins, while lipid turnover clears glycated products and nucleotide excision repair removes glycated nucleotides. If this does not occur, glycation damage accumulates, and pathologies may develop. Glycation-induced biological products are known to be mainly associated with aging, neurodegenerative disorders, diabetes and its complications, atherosclerosis, renal failure, immunological changes, retinopathy, skin photoaging, osteoporosis, and progression of some tumors.

Cathepsin D contributes to the accumulation of advanced glycation end products during photoaging

BACKGROUND

The deposition of advanced glycation end products (AGEs) is accelerated in photoaged skin, but the underlying mechanisms remain elusive. Intracellular degradation has been recently considered to play an important role in AGEs removal. Although lysosomal cathepsin D (CatD), B (CatB), L(CatL) and proteasomes are found to degrade internalized AGEs, it remains unknown which protease degrades internalized AGEs in human dermal fibroblasts (HDFs), and whether a decrease in intracellular degradation contributes to enhanced AGEs deposition in photoaged skin.

OBJECTIVE

This study aims to investigate the specific proteases that contribute to intracellular AGEs degradation in HDFs and regulate AGEs accumulation in photoaged skin.

METHODS

Repetitive UVA irradiation was used to induce primary HDF photoaging in vitro. Uptake and degradation of AGE-BSA were verified and compared between photoaged and non-photoaged fibroblasts with flow cytometry, ELISA and confocal microscopy. Proteasomal and lysosomal activity, expression of CatD, CatB and CatL were also investigated between photoaged and non-photoaged fibroblasts. Further, the effect of protease inhibitors and CatD overexpression via lentiviral transduction on AGE-BSA degradation was analyzed. Finally, the correlation between CatD expression and AGEs accumulation in sun-exposed and sun-protected skin of people from different age was studied with immunohistochemistry.

RESULTS

Fibroblasts underwent photoaging in vitro after repetitive UVA irradiation. AGE-BSA was taken up by both photoaged and non-photoaged fibroblasts, but its degradation was significantly decreased in photoaged cells than that of non-photoaged cells. Although the activity of proteasome, CatB, Cat L and Cat D was significantly reduced in photoaged fibroblasts compared to that of non-photoaged cells, and the expression of CatB, CatL and CatD was profoundly attenuated in photoaged fibroblasts, inhibiting proteasome, CatB and CatL did not affect AGE-BSA degradation in HDFs. In contrast, inhibiting CatD activity dose-dependently decreased AGE-BSA degradation; whereas CatD overexpression significantly increased AGE-BSA degradation. Importantly, AGEs accumulation in photo-damaged skin in vivo was inversely correlated with CatD expression.

CONCLUSION

CatD plays a major role in intracellular AGEs degradation. Decreased CatD expression and activity impairs intracellular AGEs degradation in photoaged fibroblasts, which may contribute to accelerated AGEs deposition in photoaged skin. The present study provides a potentially novel molecular basis for antiphotoaging therapy.

Dietary sugars, not lipids, drive hypothalamic inflammation

Objective

The hypothalamus of hypercaloric diet-induced obese animals is featured by a significant increase of microglial reactivity and its associated cytokine production. However, the role of dietary components, in particular fat and carbohydrate, with respect to the hypothalamic inflammatory response and the consequent impact on hypothalamic control of energy homeostasis is yet not clear.

Methods

We dissected the different effects of high-carbohydrate high-fat (HCHF) diets and low-carbohydrate high-fat (LCHF) diets on hypothalamic inflammatory responses in neurons and non-neuronal cells and tested the hypothesis that HCHF diets induce hypothalamic inflammation via advanced glycation end-products (AGEs) using mice lacking advanced glycation end-products (AGEs) receptor (RAGE) and/or the activated leukocyte cell-adhesion molecule (ALCAM).

Results

We found that consumption of HCHF diets, but not of LCHF diets, increases microgliosis as well as the presence of N(ε)-(Carboxymethyl)-Lysine (CML), a major AGE, in POMC and NPY neurons of the arcuate nucleus. Neuron-secreted CML binds to both RAGE and ALCAM, which are expressed on endothelial cells, microglia, and pericytes. On a HCHF diet, mice lacking the RAGE and ALCAM genes displayed less microglial reactivity and less neovasculature formation in the hypothalamic ARC, and this was associated with significant improvements of metabolic disorders induced by the HCHF diet.

Conclusions

Combined overconsumption of fat and sugar, but not the overconsumption of fat per se, leads to excessive CML production in hypothalamic neurons, which, in turn, stimulates hypothalamic inflammatory responses such as microgliosis and eventually leads to neuronal dysfunction in the control of energy metabolism.

•

HCHF, but not LCHF diets, induce obesity and increase the hypothalamic inflammatory response.

•

A HCHF diet increases N-epsilon-(carboxymethyl)lysine content in hypothalamic neurons in the ARC.

•

Obesity and metabolic symptoms induced by a HCHF diet are improved in mice lacking functional RAGE and ALCAM genes.

•

Lacking RAGE and ALCAM prevents the hypothalamic inflammatory response and angiogenesis that occur on a HCHF diet.

Increases in endothelial caveolin-1 and cavins correlate with cirrhosis progression

BACKGROUND AND AIMS

Caveolin-1 is associated with flat caveolar domains, invaginated smooth plasmalemmal vesicles, and caveolae. Polymerase 1 and transcript release factor (PTRF) (cavin 1) and serum deprivation protein response (SDPR) (cavin 2) are required for the invagination of caveolae, and PRKCDBP (protein kinase C, delta-binding protein; cavin 3) is required for caveolae budding to form caveolar vesicles. To investigate whether cavins are involved in hepatic sinusoidal angiogenesis and remodeling during progression to cirrhosis, normal control liver specimens and early and late cirrhotic liver specimens were studied.

MATERIALS AND METHODS

Cavin-1, cavin-2, and cavin-3 proteins and their gene expression were examined using immunohistochemistry (IHC), Western blotting, and laser capture microdissection (LCM)-polymerase chain reaction (PCR) during progression of cirrhosis caused by hepatitis C. According to the perfusion, fixation methods were designed to reevaluate the precise ultrastructural localizations and changes of cavin-1 and cavin-2 expression on liver sinusoidal endothelial cells (LSECs) facing the sinusoidal blood flow.

RESULTS

For IHC, cavin-1 and cavin-2 expressions were found to be upregulated in small angiogenic LSECs with collagen deposition in the perisinusoidal space as well as in the vascular endothelial cells of the remarkably proliferated portal venules, hepatic arterioles, and arterial capillaries within the fibrotic septa of late-stage cirrhotic liver. Cavin-3 was mainly localized in large vessels, and it was detected only scantly on the central vein and hepatic sinusoids in the control liver. In late-stage cirrhotic liver, the intensity of cavin-3 was enhanced mainly on proliferative large vessels in regenerated nodules and in the peripheral regions of nodules and fibrous septa. On conducting immunoelectron microscopy, in the control liver tissue, cavin-1 was found to be localized on the caveolae of hepatic arterial and portal venous endothelial cells, but it was scantly localized on hepatic sinusoidal lining cells, and cavin-2 was found mainly on vesicles in LSECs. In the cirrhotic liver tissue, aberrant cavin-1 and cavin-2 expressions were observed on caveolae-like structures in LSECs. Significant overexpressions of cavin-1 at the protein and messenger RNA (mRNA) levels in a cirrhotic liver were demonstrated by Western blotting and LCM-PCR.

CONCLUSIONS

Cavin-1 and cavin-2 are strongly expressed within caveolae-like structures and associated vesicles within LSECs of the hepatitis C-related cirrhotic liver. Cavin-1 would play a critical role in regulating aspects of caveolin-1 in LSECs. Moreover, these findings suggest a direct association of cavin-1 and cavin-2 with the process of differentiation and transformation of LSECs inducing hepatic sinusoidal capillarization related to the progression of cirrhosis.

Efficient uptake of blood-borne BK and JC polyomavirus-like particles in endothelial cells of liver sinusoids and renal vasa recta

Liver sinusoidal endothelial cells (LSECs) are specialized scavenger cells that mediate high-capacity clearance of soluble waste macromolecules and colloid material, including blood-borne adenovirus. To explore if LSECs function as a sink for other viruses in blood, we studied the fate of virus-like particles (VLPs) of two ubiquitous human DNA viruses, BK and JC polyomavirus, in mice. Like complete virions, VLPs specifically bind to receptors and enter cells, but unlike complete virions, they cannot replicate. ¹²⁵I-labeled VLPs were used to assess blood decay, organ-, and hepatocellular distribution of ligand, and non-labeled VLPs to examine cellular uptake by immunohisto- and -cytochemistry. BK- and JC-VLPs rapidly distributed to liver, with lesser uptake in kidney and spleen. Liver uptake was predominantly in LSECs. Blood half-life (∼1 min), and tissue distribution of JC-VLPs and two JC-VLP-mutants (L55F and S269F) that lack sialic acid binding affinity, were similar, indicating involvement of non-sialic acid receptors in cellular uptake. Liver uptake was not mediated by scavenger receptors. In spleen, the VLPs localized to the red pulp marginal zone reticuloendothelium, and in kidney to the endothelial lining of vasa recta segments, and the transitional epithelium of renal pelvis. Most VLP-positive vessels in renal medulla did not express PV-1/Meca 32, suggesting location to the non-fenestrated part of vasa recta. The endothelial cells of these vessels also efficiently endocytosed a scavenger receptor ligand, formaldehyde-denatured albumin, suggesting high endocytic activity compared to other renal endothelia. We conclude that LSECs very effectively cleared a large fraction of blood-borne BK- and JC-VLPs, indicating a central role of these cells in early removal of polyomavirus from the circulation. In addition, we report the novel finding that a subpopulation of endothelial cells in kidney, the main organ of polyomavirus persistence, showed selective and rapid uptake of VLPs, suggesting a role in viremic organ tropism.