Kerstin Nowotny

Advanced Glycation End Products and Oxidative Stress in Type 2 Diabetes Mellitus

Type 2 diabetes mellitus (T2DM) is a very complex and multifactorial metabolic disease characterized by insulin resistance and β cell failure leading to elevated blood glucose levels. Hyperglycemia is suggested to be the main cause of diabetic complications, which not only decrease life quality and expectancy, but are also becoming a problem regarding the financial burden for health care systems. Therefore, and to counteract the continually increasing prevalence of diabetes, understanding the pathogenesis, the main risk factors, and the underlying molecular mechanisms may establish a basis for prevention and therapy. In this regard, research was performed revealing further evidence that oxidative stress has an important role in hyperglycemia-induced tissue injury as well as in early events relevant for the development of T2DM. The formation of advanced glycation end products (AGEs), a group of modified proteins and/or lipids with damaging potential, is one contributing factor. On the one hand it has been reported that AGEs increase reactive oxygen species formation and impair antioxidant systems, on the other hand the formation of some AGEs is induced per se under oxidative conditions. Thus, AGEs contribute at least partly to chronic stress conditions in diabetes. As AGEs are not only formed endogenously, but also derive from exogenous sources, i.e., food, they have been assumed as risk factors for T2DM. However, the role of AGEs in the pathogenesis of T2DM and diabetic complications—if they are causal or simply an effect—is only partly understood. This review will highlight the involvement of AGEs in the development and progression of T2DM and their role in diabetic complications.

Accumulation of modified proteins and aggregate formation in aging.

Increasing cellular damage during the aging process is considered to be one factor limiting the lifespan of organisms. Besides the DNA and lipids, proteins are frequent targets of non-enzymatic modifications by reactive substances including oxidants and glycating agents. Non-enzymatic protein modifications may alter the protein structure often leading to impaired functionality. Although proteolytic systems ensure the removal of modified proteins, the activity of these proteases was shown to decline during the aging process. The additional age-related increase of reactive compounds as a result of impaired antioxidant systems leads to the accumulation of damaged proteins and the formation of protein aggregates. Both, non-enzymatic modified proteins and protein aggregates impair cellular functions and tissue properties by a variety of mechanisms. This is increasingly important in aging and age-related diseases. In this review, we will give an overview on oxidation and glycation of proteins and the function of modified proteins in aggregate formation. Furthermore, their effects as well as their role in aging and age-related diseases will be highlighted.

Degradation of oxidized and glycoxidized collagen: role of collagen cross-linking.

Skin aging is a multifactorial process leading to structural and physiological changes. Protein modifications are known to play here an important role. Since collagen is the most abundant protein in the extracellular matrix of the skin and due to its slow turnover rates it is a frequent target of modifications by reactive compounds. Using skin biopsies of young and old mice we demonstrated that advanced glycation end products (AGEs), such as argpyrimidine and pentosidine, accumulate in aged skin, whereas protein carbonylation is unchanged. To investigate whether this discrepancy in accumulation is the result of an increased formation or due to reduced degradation we used modified collagen type I in in vitro experiments and tested for proteolytic susceptibility. We were able to show that collagenase is able to degrade oxidized and AGE-modified collagen. However, if collagen is cross-linked heavily, collagenase is unable to degrade the modified collagen. Cross-linking of collagen is preferentially taking place in collagen fibers treated with glycoxidizing agents. In summary, the low presence of oxidized collagen in aged skin seems to be the result of a sufficient degradation by collagenases, whereas the reason of the accumulation of AGE-modified collagen is at least partially an insufficient degradation.

Reprint of "accumulation of modified proteins and aggregate formation in aging".

Increasing cellular damage during the aging process is considered to be one factor limiting the lifespan of organisms. Besides the DNA and lipids, proteins are frequent targets of non-enzymatic modifications by reactive substances including oxidants and glycating agents. Non-enzymatic protein modifications may alter the protein structure often leading to impaired functionality. Although proteolytic systems ensure the removal of modified proteins, the activity of these proteases was shown to decline during the aging process. The additional age-related increase of reactive compounds as a result of impaired antioxidant systems leads to the accumulation of damaged proteins and the formation of protein aggregates. Both, non-enzymatic modified proteins and protein aggregates impair cellular functions and tissue properties by a variety of mechanisms. This is increasingly important in aging and age-related diseases. In this review, we will give an overview on oxidation and glycation of proteins and the function of modified proteins in aggregate formation. Furthermore, their effects as well as their role in aging and age-related diseases will be highlighted.

Age-related oxidative changes in pancreatic islets are predominantly located in the vascular system

Aged tissues usually show a decreased regenerative capacity accompanied by a decline in functionality. During aging pancreatic islets also undergo several morphological and metabolic changes. Besides proliferative and regenerative limitations, endocrine cells lose their secretory capacity, contributing to a decline in functional islet mass and a deregulated glucose homeostasis. This is linked to several features of aging, such as induction of cellular senescence or the formation of modified proteins, such as advanced glycation end products (AGEs) - the latter mainly examined in relation to hyperglycemia and in disease models. However, age-related changes of endocrine islets under normoglycemic and non-pathologic conditions are poorly investigated. Therefore, a characterization of pancreatic tissue sections as wells as plasma samples of wild-type mice (C57BL/6J) at various age groups (2.5, 5, 10, 15, 21 months) was performed. Our findings reveal that mice at older age are able to secret sufficient amounts of insulin to maintain normoglycemia. During aging the pancreatic islet area increased and the islet size doubled in 21 months old mice when compared to 2.5 months old mice, whereas the islet number was unchanged. This was accompanied by an age-dependent decrease in Ki-67 levels and pancreatic duodenal homeobox-1 (PDX-1), indicating a decline in proliferative and regenerative capacity of pancreatic islets with advancing age. In contrast, the number of p16Ink4a-positive nuclei within the islets was elevated starting from 10 months of age. Interestingly, AGEs accumulated exclusively in the islet blood vessels of old mice associated with increased amounts of inflammatory markers, such as the inducible nitric oxide synthase (iNOS) and 3-nitrotyrosine (3-NT). In summary, the age-related increase in islet size and area was associated with the induction of senescence, accompanied by an accumulation of non-enzymatically modified proteins in the islet vascular system.

Impaired proteostasis during skeletal muscle aging

Abstract Aging is a complex phenomenon that has detrimental effects on tissue homeostasis. The skeletal muscle is one of the earliest tissues to be affected and to manifest age‐related changes such as functional impairment and the loss of mass. Common to these alterations and to most of tissues during aging is the disruption of the proteostasis network by detrimental changes in the ubiquitin‐proteasomal system (UPS) and the autophagy‐lysosomal system (ALS). In fact, during aging the accumulation of protein aggregates, a process mainly driven by increased levels of oxidative stress, has been observed, clearly demonstrating UPS and ALS dysregulation. Since the UPS and ALS are the two most important pathways for the removal of misfolded and aggregated proteins and also of damaged organelles, we provide here an overview on the current knowledge regarding the connection between the loss of proteostasis and skeletal muscle functional impairment and also how redox regulation can play a role during aging. Therefore, this review serves for a better understanding of skeletal muscle aging in regard to the loss of proteostasis and how redox regulation can impact its function and maintenance. Graphical abstract Fernando, Drescher, Nowotny, Grune and Castro Figure. No Caption available. HighlightsProteasomal and lysosomal systems play a pivotal role in proteostasis maintenance during muscle aging.Skeletal muscle aging is affected by increased levels of oxidative stress.Oxidative stress disrupts cellular redox signaling affecting both proteolytic systems.

Oxidants produced by methylglyoxal-modified collagen trigger ER stress and apoptosis in skin fibroblasts

ABSTRACT Methylglyoxal (MG), a highly reactive dicarbonyl, interacts with proteins to form advanced glycation end products (AGEs). AGEs include a variety of compounds which were shown to have damaging potential and to accumulate in the course of different conditions such as diabetes mellitus and aging. After confirming collagen as a main target for MG modifications in vivo within the extracellular matrix, we show here that MG‐collagen disrupts fibroblast redox homeostasis and induces endoplasmic reticulum (ER) stress and apoptosis. In particular, MG‐collagen‐induced apoptosis is associated with the activation of the PERK‐eIF2&agr; pathway and caspase‐12. MG‐collagen contributes to altered redox homeostasis by directly generating hydrogen peroxide and oxygen‐derived free radicals. The induction of ER stress in human fibroblasts was confirmed using collagen extracts isolated from old mice in which MG‐derived AGEs were enriched. In conclusion, MG‐derived AGEs represent one factor contributing to diminished fibroblast function during aging. HIGHLIGHTSMG‐derived AGEs are increased in aged murine decellularized tissue and collagen.MG‐collagen‐induced apoptosis is mediated by activation of caspase‐12 and PERK‐eIF2&agr; pathway.MG‐collagen impairs redox homeostasis by the generation of oxidants.Old collagen induces ER stress in primary human fibroblasts.

Dietary advanced glycation end products and their relevance for human health

Due to their bioactivity and harmful potential, advanced glycation end products (AGEs) are discussed to affect human health. AGEs are compounds formed endogenously in the human body andexogenously, especially, in foods while thermal processing. In contrast to endogenous AGEs, dietary AGEs are formed in much higher extent. However, their risk potential is also depending on absorption, distribution, metabolism and elimination. For over 10 years an intense debate on the risk of dietary AGEs on human health is going on. On the one hand, studies provided evidence that dietary AGEs contribute to clinical outcomes. On the other hand, human studies failed to observe any association. Because it was not possible to draw a final conclusion, the call for new interdisciplinary approaches arose. In this review, we will give an overview on the current state of scientific knowledge in this field. In particular, we focus on (I) the occurrence of AGEs in foods and the daily uptake of AGEs, (II) contribution to endogenous levels and (III) the effect on health-/disease-related biomarkers in humans.

Mitochondrial dynamics impairment leads to adipogenesis failure

The white adipose tissue (WAT) is crucial for maintaining metabolism homeostasis by storing lipids and/or by secreting adipokines. A healthy WAT relies on continuous renewal, through a multi-step process called adipogenesis. Upon failure, the risk for lipotoxicity across the organism is higher and insulin resistance can arise in several tissues. A likely contributor for adipogenesis failure is mitochondrial dysfunction and subsequently cellular redox changes. Using 3T3-L1 cells, we have found that exposing them to hyperoxia (as a source of oxidative stress) for 8 days led to adipogenesis impairment, confirmed by oil red staining and transcription factors qPCRs. In our model, oxidative stressed preadipocytes exhibited mitochondrial dynamics diminishment (live cell imaging), Reactive Oxygen Species (ROS) production (fluorescence probe), less mitochondrial mass (WB and qPCR), smaller ATP/ADP ratio (HPLC) and less respiration capacity (Seahorse). In fact, challenging cells with antimycin, under normoxic conditions, showed similar features to hyperoxic treated cells in what adipogenesis is concerned. Recently, we found NFkB to be activated during the 8 days of hyperoxia in contrast to NFkB levels of differentiating cells which decreased after 2 days. We are convinced that mitochondrial dysfunction during aging/oxidative stress activates NFkB, blocking the main transcription factors and impairing adipogenesis.