Fight Aging!

Below you will find a paper on advanced glycation end-products (AGEs) and their contribution to degenerative aging, with a focus on the vascular system. AGEs are the result of various chemical reactions involving sugars, many of which occur both inside the body in the normal course of metabolic operations and outside the body during food preparation. There are many different types of AGEs, most well-known to chemists, but the tools for working with AGEs and their precursors in a biological context are not that great in comparison to the rest of the biochemistry field, and as a consequence their study has lagged. You will still find many papers hedging or being carefully agnostic on the question of which AGEs are important in mammalian aging, and on their relative amounts and relevant mechanisms. That is the case here.

As is true of mitochondrial damage, there are two quite distinct ways of looking at AGEs in aging, two collections of processes, one of which offers worse prospects when it comes to producing gains in health and longevity, but is nonetheless the point of greatest mainstream research focus. This is, sadly, the story for much of the young field of rejuvenation research. We have a lot of work left to accomplish when it comes to driving the necessary interest and funding to make meaningful progress towards addressing the causes of aging. Not least of this is the matter of steering more of the research community in the right direction, towards high-yield rather than low-yield strategies.

The first, minority area of AGE research is focused on persistent cross-links formed by AGEs in the extracellular matrix. AGEs can bind to the matrix proteins, linking them together. Since the physical properties of tissue are determined by the arrangement of these proteins and their ability to move relative to one another, this process of cross-linking can corrode elasticity and strength. It makes bone brittle, skin wrinkled, and blood vessels stiff. The last of those starts the cascade leading to hypertension, heart disease, and death: even if there were no other processes in aging, it would kill you eventually.

Few AGEs are persistent, however. Most are like the sugars that formed them: here today, gone tomorrow, with current levels very dependent on the diet. It is thought that glucosepane makes up the overwhelming majority of all persistent cross-links in human tissues, but there is next to no funding for research into this compound and its role in aging. The SENS Research Foundation has been funding near all of the work aimed at the production of necessary scientific infrastructure and then AGE-breaker pharmaceuticals that can destroy cross-links. This is hopefully nearing fruition. That this is just a single type of target makes it a very promising line of work, hopefully soon to join senolytics as a rejuvenation therapy moving from theory to practice. Reversing blood vessel stiffness alone would have a profound effect on health for older individuals.

The second, majority area of AGE research is more focused on chronic inflammation, metabolic dysfunction characteristic of type 2 diabetes and obesity, and dietary intake of sugars and AGEs. It encompasses the short-lived AGEs, which are a much larger fraction of the AGE population than the rare, persistent AGEs that linger to accumulate slowly in tissues. A primary point of interest here is the interaction between AGEs and RAGE, the receptor for AGEs. RAGE is a mediator of inflammation, and high levels of AGEs trigger it relentlessly; it also has a number of other roles that are poorly cataloged, but the general consensus is that these probably don't benefit from continual activation either. This is thought to be one of the ways in which diabetes and excess fat tissue result in high degrees of chronic inflammation, and all of the downstream consequences of that inflammation. That means higher rates of age-related disease, greater immune dysfunction, more tissue damage, and so forth. At least until quite late in old age, much of this is the result of lifestyle choices, and can be avoided or reversed through choice in diet and weight.

The paper here is focused on a future of therapies that interfere in the interaction between AGEs and RAGE. The author dismisses AGE-breakers due to past failures, which I think is an incorrect conclusion. The past failures were failures because they targeted forms of AGE that are noteworthy in short-lived rodent species, but unfortunately not all that relevant in humans. The types of AGE that cause pathology vary considerably by species and species life span. No past initiative ever targeted glucosepane. Meanwhile, efforts to target the AGE / RAGE interaction are probably best thought of as belonging to the same class of strategy as other attempts to block the inflammatory consequences of fat tissue or diabetes. It is an effort to compensate for a problem, not repair the cause of the problem.

The AGE-RAGE Axis: Implications for Age-Associated Arterial Diseases

Changes in the components of large arteries due to advancing age have been described in humans and animals. Age-associated blood vessel remodeling includes such features as dilation of the lumina, intimal and medial thickening, changes in the extracellular matrix (ECM), and augmented stiffness. In addition to these structural changes, other mechanisms contribute to the overall consequences of aging to the arterial wall, including such phenomena as inflammation, endothelial dysfunction, and oxidative stress. Fibroblasts and smooth muscle cells (SMC) contribute to aging in the vasculature, in part by increasing ECM; macrophages contribute by increasing inflammatory factors that have a wide range of possible consequences. These pathobiological events adversely affect the vessel wall and all of its components, potentially contributing to arterial aging.

It has been shown that the aged human arterial wall exhibits a more proinflammatory signature, with increased expression and activity of matrix metalloproteinases (MMPs) and chemokines. Atop these considerations is the effect of co-morbid conditions in aging, which may augment production of inflammatory mediators and exacerbate the impact of arterial aging, examples of which include diabetes mellitus (types 1 or 2 or the rarer forms of diabetes); chronic renal disease; and chronic immune/inflammatory disorders.

Advanced glycation endproducts (AGEs) are a diverse group of macromolecules and at least 20 different specific AGEs have been described to date. Among the major groups of AGEs are carboxymethyl lysine (CML), carboxyethyl lysine (CEL), pentosidine, glucosepane, methylglyoxal lysine dimer, glyoxal lysine dimer, and glycolic acid lysine amide. AGEs form throughout life via the process of non-enzymatic glycation of proteins and lipids, and this process is accelerated during hyperglycemia, oxidative stress, aging, advanced renal disease, and inflammation. Humans and animals are also exposed to exogenous sources of AGEs ingested through food-derived AGEs and tobacco products. It has been shown that restriction in dietary AGE intake may increase the lifespan in animals.

AGEs accumulate in aging tissues and on vulnerable plasma proteins. Higher levels of circulating AGEs have been linked to chronic diseases in aging subjects. The accumulation of AGEs is increased and accelerated in hypertensive subjects and is also associated with diabetes. In fact, aged subjects, even though healthy, may have higher AGE accumulation compared to younger subjects with diabetes and its complications, thus underscoring that AGE production and accumulation accompanies the normal aging process. Therefore, multiple factors such as the rate of accumulation of AGE ligand, the absolute concentration of the ligand, and individual susceptibility to AGE formation may be important in determining an individual's AGE burden.

Numerous studies have confirmed the correlation between AGE accumulation and increased artery stiffness. Arterial stiffness is associated with greater risk for aging-associated cardio- and cerebrovascular diseases and mortality. AGE accumulation causes upregulation of inflammation and destruction of collagen and elastin, along with other proteins of the ECM. It is noteworthy that despite testing of multiple classes of anti-AGE agents, none have obtained, at least to date, approval for anti-AGE indications. Although there are many possible reasons for this, we propose that one reason is that solely targeting AGEs fails to capture the pathobiological effects of distinct RAGE ligands. Therefore, it is not surprising that attempts are underway to directly target RAGE as a therapeutic strategy.

RAGE is expressed on a number of important cell types implicated in arterial aging and vascular pathology. Once AGEs are formed, albeit by diverse intrinsic and environmentally-triggered mechanisms, their interaction with RAGE on endothelial cells, SMCs, and immune cells such as macrophages, results in upregulation of inflammatory and oxidative stress-provoking factors, thereby providing a mechanism to link AGE-RAGE to arterial aging and its consequences.

Approaches to limit RAGE ligand AGEs have been accompanied by efforts to block RAGE itself and these have been tested in vitro and in vivo; in addition, human clinical trial testing is also underway. In vitro, pre-treatment of AGE-stimulated endothelial cells with anti-RAGE antibodies or anti-oxidants blocked cellular perturbation. Another RAGE blocking agent currently in Phase III clinical trials in Alzheimer's disease is the small molecule Azeliragon, which inhibits the receptor for advanced glycation endproducts and prevents RAGE ligands from interacting with RAGE. Certainly, more research is required to understand the entire scope of RAGE signaling and the extent to which blocking AGEs/RAGE interaction may intercept the full pathobiology of RAGE activation.