Alan R. Hipkiss

Chapter 3 Carnosine and Its Possible Roles in Nutrition and Health

The dipeptide carnosine has been observed to exert antiaging activity at cellular and whole animal levels. This review discusses the possible mechanisms by which carnosine may exert antiaging action and considers whether the dipeptide could be beneficial to humans. Carnosines possible biological activities include scavenger of reactive oxygen species (ROS) and reactive nitrogen species (RNS), chelator of zinc and copper ions, and antiglycating and anticross-linking activities. Carnosines ability to react with deleterious aldehydes such as malondialdehyde, methylglyoxal, hydroxynonenal, and acetaldehyde may also contribute to its protective functions. Physiologically carnosine may help to suppress some secondary complications of diabetes, and the deleterious consequences of ischemic-reperfusion injury, most likely due to antioxidation and carbonyl-scavenging functions. Other, and much more speculative, possible functions of carnosine considered include transglutaminase inhibition, stimulation of proteolysis mediated via effects on proteasome activity or induction of protease and stress-protein gene expression, upregulation of corticosteroid synthesis, stimulation of protein repair, and effects on ADP-ribose metabolism associated with sirtuin and poly-ADP-ribose polymerase (PARP) activities. Evidence for carnosines possible protective action against secondary diabetic complications, neurodegeneration, cancer, and other age-related pathologies is briefly discussed.

On the enigma of carnosine's anti-ageing actions

Carnosine (beta-alanyl-L-histidine) has described as a forgotten and enigmatic dipeptide. Carnosines enigma is particularly exemplified by its apparent anti-ageing actions; it suppresses cultured human fibroblast senescence and delays ageing in senescence-accelerated mice and Drosophila, but the mechanisms responsible remain uncertain. In addition to carnosines well-documented anti-oxidant, anti-glycating, aldehyde-scavenging and toxic metal-ion chelating properties, its ability to influence the metabolism of altered polypeptides, whose accumulation characterises the senescent phenotype, should also be considered. When added to cultured cells, carnosine was found in a recent study to suppress phosphorylation of the translational initiation factor eIF4E resulting in decreased translation frequency of certain mRNA species. Mutations in the gene coding for eIF4E in nematodes extend organism lifespan, hence carnosines anti-ageing effects may be a consequence of decreased error-protein synthesis which in turn lowers formation of protein carbonyls and increases protease availability for degradation of polypeptides altered postsynthetically. Other studies have revealed carnosine-induced upregulation of stress protein expression and nitric oxide synthesis, both of which may stimulate proteasomal elimination of altered proteins. Some anti-convulsants can enhance nematode longevity and suppress the effects of a protein repair defect in mice, and as carnosine exerts anti-convulsant effects in rodents, it is speculated that the dipeptide may participate in the repair of protein isoaspartyl groups. These new observations only add to the enigma of carnosines real in vivo functions. More experimentation is clearly required.

Energy metabolism, proteotoxic stress and age-related dysfunction – Protection by carnosine

This review will discuss the relationship between energy metabolism, protein dysfunction and the causation and modulation of age-related proteotoxicity and disease. It is proposed that excessive glycolysis, rather than aerobic (mitochondrial) activity, could be causal to proteotoxic stress and age-related pathology, due to the generation of endogenous glycating metabolites: the deleterious role of methylglyoxal (MG) is emphasized. It is suggested that TOR inhibition, exercise, fasting and increased mitochondrial activity suppress formation of MG (and other deleterious low molecular weight carbonyl compounds) which could control onset and progression of proteostatic dysfunction. Possible mechanisms by which the endogenous dipeptide, carnosine, which, by way of its putative aldehyde-scavenging activity, may control age-related proteotoxicity, cellular dysfunction and pathology, including cancer, are also considered. Whether carnosine could be regarded as a rapamycin mimic is briefly discussed.

Carnosine and cancer: a perspective

The application of carnosine in medicine has been discussed since several years, but many claims of therapeutic effects have not been substantiated by rigorous experimental examination. In the present perspective, a possible use of carnosine as an anti-neoplastic therapeutic, especially for the treatment of malignant brain tumours such as glioblastoma is discussed. Possible mechanisms by which carnosine may perform its anti-tumourigenic effects are outlined and its expected bioavailability and possible negative and positive side effects are considered. Finally, alternative strategies are examined such as treatment with other dipeptides or β-alanine.

Carnosine: can understanding its actions on energy metabolism and protein homeostasis inform its therapeutic potential?

The dipeptide carnosine (β-alanyl-L-histidine) has contrasting but beneficial effects on cellular activity. It delays cellular senescence and rejuvenates cultured senescent mammalian cells. However, it also inhibits the growth of cultured tumour cells. Based on studies in several organisms, we speculate that carnosine exerts these apparently opposing actions by affecting energy metabolism and/or protein homeostasis (proteostasis). Specific effects on energy metabolism include the dipeptide’s influence on cellular ATP concentrations. Carnosine’s ability to reduce the formation of altered proteins (typically adducts of methylglyoxal) and enhance proteolysis of aberrant polypeptides is indicative of its influence on proteostasis. Furthermore these dual actions might provide a rationale for the use of carnosine in the treatment or prevention of diverse age-related conditions where energy metabolism or proteostasis are compromised. These include cancer, Alzheimers disease, Parkinsons disease and the complications of type-2 diabetes (nephropathy, cataracts, stroke and pain), which might all benefit from knowledge of carnosine’s mode of action on human cells.

Aging, Proteotoxicity, Mitochondria, Glycation, NAD and Carnosine: Possible Inter-Relationships and Resolution of the Oxygen Paradox.

It is suggested that NAD+ availability strongly affects cellular aging and organism lifespan: low NAD+ availability increases intracellular levels of glycolytic triose phosphates (glyceraldehyde-3-phosphate and dihydroxyacetone-phosphate) which, if not further metabolized, decompose spontaneously into methylglyoxal (MG), a glycating agent and source of protein and mitochondrial dysfunction and reactive oxygen species (ROS). MG-damaged proteins and other aberrant polypeptides can induce ROS generation, promote mitochondrial dysfunction and inhibit proteasomal activity. Upregulation of mitogenesis and mitochondrial activity by increased aerobic exercise, or dietary manipulation, helps to maintain NAD+availability and thereby decreases MG-induced proteotoxicity. These proposals can explain the apparent paradox whereby aging is seemingly caused by increased ROS-mediated macromolecular damage but is ameliorated by increased aerobic activity. It is also suggested that increasing mitochondrial activity decreases ROS generation, while excess numbers of inactive mitochondria are deleterious due to increased ROS generation. The muscle- and brain-associated dipeptide, carnosine, is an intracellular buffer which can delay senescence in cultured human fibroblasts and delay aging in senescence-accelerated mice. Carnosines ability to react with MG and possibly other deleterious carbonyl compounds, and scavenge various ROS, may account for its protective ability towards ischemia and ageing.

Physiological and therapeutic effects of carnosine on cardiometabolic risk and disease.

Obesity, type 2 diabetes (T2DM) and cardiovascular disease (CVD) are the most common preventable causes of morbidity and mortality worldwide. They represent major public health threat to our society. Increasing prevalence of obesity and T2DM contributes to escalating morbidity and mortality from CVD and stroke. Carnosine (β-alanyl-l-histidine) is a dipeptide with anti-inflammatory, antioxidant, anti-glycation, anti-ischaemic and chelating roles and is available as an over-the-counter food supplement. Animal evidence suggests that carnosine may offer many promising therapeutic benefits for multiple chronic diseases due to these properties. Carnosine, traditionally used in exercise physiology to increase exercise performance, has potential preventative and therapeutic benefits in obesity, insulin resistance, T2DM and diabetic microvascular and macrovascular complications (CVD and stroke) as well as number of neurological and mental health conditions. However, relatively little evidence is available in humans. Thus, future studies should focus on well-designed clinical trials to confirm or refute a potential role of carnosine in the prevention and treatment of chronic diseases in humans, in addition to advancing knowledge from the basic science and animal studies.

Mitochondrial dysfunction, proteotoxicity, and aging: causes or effects, and the possible impact of NAD+-controlled protein glycation.

Aging is frequently characterized by the accumulation of altered proteins and dysfunctional mitochondria. This review discusses possible causes of these effects, their interdependence and the impact of energy metabolism on proteostasis, especially formation and elimination of altered proteins. It is suggested NAD+ to some degree regulates formation of aberrant proteins and generation of oxygen free-radicals and reactive oxygen species (ROS), because when NAD+ is limiting, glycolytic triose phosphates spontaneously decompose into methylglyoxal (MG), a highly deleterious glycating agent and ROS inducer. That NAD+ has stimulatory effects on stress protein expression and autophagy, while mitochondria regenerate NAD+ from NADH, further integrates energy metabolism into proteostasis. It is suggested that, as altered proteins can deleteriously interact with mitochondria, changes in synthesis, or elimination, of cytosolic error-proteins will affect mitochondrial activity. It is also suggested that functional mitochondria are essentially antiaging agents, while their dysfunction or inactivity accelerate ROS formation and aging. These proposals may also help explain the oxygen paradox that while ROS may be causal to aging, increased mitochondrial activity (i.e., oxygen utilization) suppresses aging and much associated pathology. Increased synthesis of glutathione, humanin, and mitochondrial chaperone proteins are other additional consequences of increased mitogenesis and which would help ensure proteostasis.

Aging risk factors and Parkinson's disease: contrasting roles of common dietary constituents.

Aging is a Parkinsons disease (PD) risk factor. It is suggested here that certain dietary components may either contribute to or ameliorate PD risk. There is evidence, which indicates that excessive carbohydrate (glucose or fructose) catabolism is a cause of mitochondrial dysfunction in PD, one consequence is increased production of methylglyoxal (MG). However, other dietary components (carnosine and certain plant extracts) not only scavenge MG but can also influence some of the biochemical events (signal transduction, stress protein synthesis, glycation, and toxin generation) associated with PD pathology. As double blind, placebo-controlled carnosine supplementation studies have revealed beneficial outcomes in humans, it is suggested that MG scavengers such as carnosine be further explored for their therapeutic potential toward PD.

Can the beneficial effects of methionine restriction in rats be explained in part by decreased methylglyoxal generation resulting from suppressed carbohydrate metabolism

The mechanisms by which dietary restriction of the amino acid methionine exerts beneficial effects on oxidative damage towards rat liver mitochondria are discussed. It is suggested that methionine restriction decreases amino acid utilization in protein synthesis which, by decreasing synthesis of non-essential amino acids from carbohydrate precursors, also decreases formation of the highly deleterious glycolytic by-product methylglyoxal, a well-recognised source of age-related damage including formation of reactive oxygen species, mitochondrial dysfunction and proteotoxicity. Additionally, decreased protein synthesis will lower the error-protein load which the protein quality system (proteasomal and autophagic) must deal with to maintain proteostasis.