Glycation


Here’s the latest in our (infrequent and irregular) series of “review roundups” — links, without extensive further comment, to the reviews I found most intriguing over the past few weeks. For the previous foray into the secondary literature, see here.

Remember, each Review Roundup is guaranteed to contain at least one link to a review you will find highly educational, or your money back.

Autophagy:

Chaperones:

Evolution:

Glycation:

Immunology:

Mitochondria:

Neurodegeneration:

Resveratrol:

Senescence:

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Advanced glycation endproducts (AGEs) have been implicated in age-related disease and aging itself, convincingly enough that significant effort has gone into finding compounds that can “break” or reverse them.

Among many unresolved questions: Are specific proteins AGEylated, and if so, which proteins are being modified within cells? Unterluggauer et al. report that in both senescent fibroblasts and senescent endothelial cells, the chaperone Hsc70 is modified by AGE:

Identification of Hsc70 as target for AGE modification in senescent human fibroblasts

Cellular senescence is known as a potent mechanism of tumor suppression, and cellular senescence in vitro also reflects at least some features of aging in vivo. The Free Radical Theory of aging suggests that reactive oxygen species are important causative agents of aging and cellular senescence. Besides damage of nucleic acids and lipids, also oxidative modifications of proteins have been described as potential causative events in the senescence response. However, the identity of protein targets for post-translational modifications in senescent cells has remained unclear. In the present communication, we analyzed the occurrence of oxidative posttranslational modifications in senescent human endothelial cells and dermal fibroblasts. We found a significant increase in the level of protein carbonyls and AGE modification with senescence in both cell types. Using 2D-Gel electrophoresis and Western Blot we found that heat shock cognate protein 70 is a bona fide target for AGE modification in human fibroblasts.

If a major chaperone is modified by AGEs, and the modification is deleterious to its function, this could dramatically decrease the efficiency of protein folding, which (given that chaperones are proteins too) could dramatically decrease the number of chaperones, which…

Garbage catastrophe, anyone? (Then again: senescent cells persist in culture for a long time, implying that they have some way to deal with the admittedly hypothetical protein-folding death spiral implied in the previous paragraph.)

A new and as-yet-unpublished study proposes a causative link between advanced glycation endproducts (AGEs) and DNA damage, potentially explaining the link between diabetes (where frequent blood glucose spikes cause accelerated formation of AGEs) and male infertility. The authors of the study focus on sperm, but the result suggests that AGEs may play a role DNA damage in other tissues as well. Since DNA damage is widely believed to play a causative role in the aging process, follow-up experiments could provide the long-awaited smoking gun connecting AGEs to fundamental mechanisms of aging — and breathe new life into the longstanding search for a clinical use for AGE-breaking compounds like Synvista‘s alagebrium.

(Hat tip to Longevity Meme.)

We usually discuss advanced glycation endproducts (AGEs) in the context of slow, lifelong accumulation of nonenzymatic reaction between protein and sugars. Until recently, I hadn’t realized that the diet is also a major source of AGE and AGE-related damage. I learned about the extrinsic source of AGEs from a recent study that showed diets lower in AGE (i.e., products of the Maillard reaction — outside the body, usually the heat-catalyzed reaction of hexose and pentoses with muscle proteins in cooked meat) alleviate both cardiac and kidney fibrosis.

On a related note, it turns out that most renoprotective measures taken to prevent damage to diabetic kidneys have something in common — surprisingly, it’s not that they lower blood sugar, rather but that they retard or prevent the formation of AGEs in the kidney, suggesting a causative role for AGEs at least in diabetes-related nephropathy.

How might AGEs cause age-related damage in the kidney and other tissues? One mechanism might involve AGE-induced production of matrix metallproteases (MMPs) by fibroblasts. MMPs are known to break down the extracellular matrix, weakening the structural integrity of a tissue and also promoting metastatic invasion by cancer cells.

Nonenzymatic carbohydrate modification of proteins results in the formation advanced glycation endproducts (AGEs), which can interfere with protein function. These molecules accumulate over the course of aging, in part because their unusual chemistry makes them difficult to clear. In the retina, accumulation of AGE and concomitant activation of the AGE receptor (RAGE) have been implicated in retinal aging and age-related macular degeneration (AMD), the primary cause of industrial blindness in the industrialized world.

In the lens of the eye, however, the story is more complex. A specific type of AGE, resulting from reaction with the abundant small carbohydrate methylglyoxal (MGO), can actually improve the function of a major lens chaperone that prevents cataracts. From Nagaraj et al.:

The Other Side of the Maillard Reaction

The Maillard reaction plays an important role in eye lens aging and cataract formation. Methylglyoxal (MGO) is a metabolic dicarbonyl compound present in the lens. It reacts with arginine residues in lens proteins to form advanced glycation end products (AGEs), such as hydroimidazolones and argpyrimidine. -Crystallin, comprising A- and B-crystallin, is a major protein of the lens and it functions as a chaperone protein. We have found that upon reaction with MGO, human A-crystallin becomes a more effective chaperone. Modification of specific arginine residues to AGEs appears to be the reason. Mutation of these arginine residues to alanine mirrors the effect of MGO, suggesting neutralization of the positive charge on arginine residues as a cause for improved chaperone function. Reaction with MGO also blocks the loss of the chaperone function of A-crystallin caused by nonenzymatic glycation by ascorbate and ribose. These findings suggest that low levels of MGO might help the lens remain transparent during aging.

Not all AGE-ylation of crystallins improve function, so there’s something special about the MGO modification — among other things, it prevents more deleterious modifications by other carbs.

One wonders whether instead of coming up with ways to break AGEs, we should be looking for ways to “pre-AGE” all vulnerable proteins with small carbohydrates whose adducts preserve (or, as is the case here, even improve) protein function.

Another thought: If we do identify good AGE-breakers, we’d better make sure they either target all AGEs equally, or hit deleterious AGEs harder. Otherwise, we could end up in a situation where we’re removing smaller, relatively innocuous modifications (e.g., by MGO) and making proteins more vulnerable to larger, more insidious ones (e.g., by ribose and ascorbate).

Advanced glycation endproducts (AGEs) are the result of nonenzymatic condensation reactions between sugars and proteins. AGEs accumulate in multiple tissues over the course of aging, and they have been implicated in a variety of age-related diseases. The data connecting them to aging isn’t terribly strong, in part because it’s hard to prevent them from forming and thereby observe aging and disease progression in their absence. Most of the evidence that they’re involved is circumstantial, derived from observations of diseases where they accumulate prematurely (e.g., diabetes, where the “sugar spikes” that result from impaired glucose homeostasis cause increased AGE formation).

A mechanistic understanding of the effect of AGE on tissues and cells, then, would go a long way toward boosting the argument that these compounds are causative in aging, rather than merely a harmless epiphenomenon. To that end, Molinari et al. studied the effect of AGEs on gene expression in fibroblasts (mesenchymal cells that provide support and structure to tissues throughout the body, especially the skin):

Effect of advanced glycation endproducts on gene expression profiles of human dermal fibroblasts

The Maillard reaction and its end products, AGE-s (Advanced Glycation End products) are rightly considered as one of the important mechanisms of post-translational tissue modifications with aging. We studied the effect of two AGE-products prepared by the glycation of lysozyme and of BSA, on the expression profile of a large number of genes potentially involved in the above mentioned effects of AGE-s. The two AGE-products were added to human skin fibroblasts and gene expression profiles investigated using microarrays. Among the large number of genes monitored the expression of 16 genes was modified by each AGE-preparations, half of them only by both of them. Out of these 16 genes, 12 were more strongly affected, again not all the same for both preparations. Both of them upregulated MMP and serpin-expression and downregulated some of the collagen-chain coding genes, as well as the cadherin- and fibronectin genes. The BSA-AGE preparation downregulated 10 of the 12 genes strongly affected, only the serpin-1 and MMP-9 genes were upregulated. The lysozyme-AGE preparation upregulated selectively the genes coding for acid phosphatase (ACP), integrin chain α5 (ITGA5) and thrombospondin (THBS) which were unaffected by the BSA-AGE preparation. It was shown previously that the lysozyme-AGE strongly increased the rate of proliferation and also cell death, much more than the BSA-AGE preparation. These differences between these two AGE-preparations tested suggest the possibility of different receptor-mediated transmission pathways activated by these two preparations. Most of the gene-expression modifications are in agreement with biological effects of Maillard products, especially interference with normal tissue structure and increased tissue destruction.

The authors exposed fibroblasts to two types of AGE-modified (AGE-ylated?) proteins, which had overlapping but non-identical effects on gene expression. The common features of the response to the two proteins are most intriguing, however: increased transcription of matrix metalloproteases (MMPs), which break down the extracellular matrix (ECM), and decreased transcription of ECM components like collagen and fibronectin. Taken together, these effects would result in a net weakening of the ECM, which in turn would have profoundly negative effects on organ function, ranging from skin wrinkling to cardiomyopathy.

On another note: increased MMPs and ECM breakdown are hallmarks of fibroblast senescence, which is usually associated with DNA damage or telomere shortening — could AGEs be stimulating premature senescence, either by damaging DNA or via some other pathway?

Skin wrinkling, one of the most conspicuous signs of aging, is the result of breakdown of structural molecules such as collagen and elastin. In our lab and others, we’re accumulating evidence that proteases secreted by senescent fibroblasts are to blame.

Therefore, my ears always prick up when I hear reports like a recent one from Kim and Chung, who describe a plant alkaloid (berberine) that can reduce the production of matrix metalloproteases by dermal fibroblasts in response to UV irradiation. Is the compound decreasing senescence in response to DNA damage — possibly allowing mutated cells to escape cell cycle arrest — or is it instead diminishing the senescence-associated secretion of degradative enzymes? If the latter is true, the protein targeted by berberine will be an excellent target for pharmaceutical development.

Also on a plant-related note, another group has identified a botanical compound that slows the production of advanced glycation endproducts (AGEs); we’ve seen similar results from corn silk extracts. It does make one wonder whether an anti-aging pharmacopoeia is just sitting there in nature, waiting to be discovered.

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