Cysteine and Obesity

Obesity is associated with elevation of circulating levels of several amino acids, but the mechanism of this elevation is unclear. The type of dietary protein influences the risk of obesity, suggesting that specific amino acids could contribute to regulating weight gain. Plasma concentrations of cysteine, but not of other sulfur amino acids, correlate strongly with fat mass and BMI in men and women. High plasma cysteine is also linked to obesity-related disorders such as cardiovascular disease and metabolic syndrome. Several lines of evidence suggest that increased cysteine availability may promote obesity. Evidence from interventions where cysteine-containing, cysteine-producing, or anti-cysteine compounds are administered or restricted supports a causal role for cysteine in regulating body weight. Genetic syndromes characterized by high or low plasma cysteine often exhibit corresponding changes in body weight. Studies on cultured rat adipocytes provide insight into the cellular mechanisms underlying the relation of cysteine with body fat. The prospect of weight control by modulating cysteine intake, synthesis or action is attractive, since cysteine and its precursor, methionine, are ingested in diet and at least one licensed drug reduces cysteine formation. This review summarizes current knowledge about the relationship between cysteine and body weight. Cysteine is a conditionally essential proteinogenic sulfur-containing amino acid. Through its sulfhydryl group reactivity, this amino acid can form disulfide linkages, which in turn control protein structure and stability (1). Nonprotein bound (free) cysteine in plasma often exists as homogeneous (cystine) or mixed (e.g., homocysteine-cysteine) disulfides. Plasma measurements of cysteine are often reported as total cysteine (tCys), which refers to all circulating forms including free, disulfide, and albumin-bound cysteine. Plasma tCys is largely oxidized, while cellular tCys is largely reduced (2). Although cysteine is the limiting precursor of the major intracellular antioxidant glutathione (GSH) (3), not only low plasma tCys, but also high tCys, predicts adverse outcomes, including cardiovascular disease (4). Here, we review the evidence that high tCys may also be causally related to obesity. The cysteine pool is a function of dietary intake, protein turnover, and endogenous synthesis (Figure 1) (2). Cysteine is synthesized by transsulfuration from homocysteine, a product of the essential sulfur amino acid, methionine (1). In the first reaction, catalyzed by cystathionine β-synthase (CBS), homocysteine condenses with serine to form cystathionine, which is cleaved by cystathionase, releasing cysteine. CBS thus catalyzes the first irreversible step that commits homocysteine to transsulfuration and cysteine synthesis (Figure 1) (5). Cysteine: metabolic pathways. Cysteine is a constituent of dietary proteins, a product of turnover of body protein pools, and is synthesized from methionine in the transsulfuration pathway, mainly in the liver. Located at cell membranes, γ-glutamyltransferase (GGT) catalyzes breakdown of glutathione to glutamate and cysteinylglycine, which ultimately releases cysteine, in the γ-glutamyl cycle. Cysteine is also the precursor of coenzyme A, glutathione, and taurine. Dotted arrows indicate pathways with omitted intermediates for purposes of clarity. a.a., amino acid; CBS, cystathionine β-synthase; CDO, cysteine dioxygenase; CGL, cystathionine γ-lyase; GGCS, γ-glutamylcysteine synthase; H2S, hydrogen sulfide; SAM, S-adenosyl methionine; SAH, S-adenosyl homocysteine. Two major factors regulating CBS activity are methionine availability and cellular redox state. S-adenosylmethionine, the methionine product mediating all transmethylation reactions, is an allosteric activator of CBS (5). This regulatory mechanism promotes disposal of excess methionine through irreversible conversion to cysteine (5), and inhibits transsulfuration when methionine supply is limited. Flux through the transsulfuration pathway in the liver is also favored under oxidative stress, which increases the supply cysteine for GSH synthesis (6). It is estimated that ∼50% of the cysteine utilized for hepatic GSH synthesis comes from transsulfuration (6). The enzyme γ-glutamyltransferase (GGT), localized to membranes of certain cell types, catalyzes extracellular GSH cleavage, ultimately releasing cysteine for uptake by cells (Figure 1) (7). The importance of GGT in plasma cysteine homeostasis is highlighted by mouse models (8,9), and one human case report (10), in which genetic GGT deficiency resulted in severe deficiency of plasma cysteine. Cysteine is also the precursor of coenzyme A (11). In conditions of sulfur amino acid excess, cysteine can also be oxidized to inorganic sulfur and pyruvate (11), which can be further used in gluconeogenesis (12). There is evidence that cysteine levels are regulated at the level of cysteine breakdown (13). Cysteine dioxygenase catalyses the first major step in cysteine catabolism and taurine production (Figure 1). Cysteine dioxygenase is markedly upregulated in response to high cysteine or protein availability, thus controlling the conservation or disposal of cysteine, depending on its supply (13). Notably, cysteine dioxygenase shows this powerful cysteine-responsiveness in liver and adipose tissue (13), but not in the kidney, lung, or brain (14), suggesting a relation between cysteine homeostasis and adipose tissue function. This review summarizes available evidence linking cysteine with body weight and obesity. First, epidemiologic studies linking plasma tCys with BMI and body composition are presented. Subsequently genetic syndromes and animal studies supporting a causal role for cysteine in weight regulation are reviewed. Finally conflicting evidence is discussed and a putative mechanism for cysteine action is suggested based on in vitro findings. Several large studies have reported a positive correlation between tCys and BMI in humans, but the association between tCys and BMI was not the main focus of these studies. In 1999, BMI was shown to be a “determinant” of plasma tCys in >16,000 men and women in the Hordaland Homocysteine Study (15). A longitudinal study of a subset of the same cohort, subsequently reported that “change in BMI predicted changes in tCys over time” (16). Two studies investigating tCys as a risk factor for cardiovascular disease recognized this tCys-BMI relationship (4,17). The tCys-BMI correlation was also reported in postmenopausal women (18), metabolic syndrome patients (19), and healthy controls in a breast cancer study (20). These studies did not specify whether tCys was related to fat mass or lean mass. In 2008, we investigated the relation between tCys and body composition measured by dual-energy X-ray absorptiometry, in >5,000 Norwegian subjects from the Hordaland Homocysteine Study (21). There was a positive linear relationship between tCys and fat mass, but no association between tCys and lean mass (21). The association of tCys with fat mass remained robust after adjustment for age, gender, lean mass, and dietary intakes of protein, fat and total energy, and plasma lipid concentrations. Depending on the model, tCys explained 4–8% of the fat mass variability in men and women (21). Subjects in the highest tCys quintile had 6–9 kg higher fat-mass compared to those in the lowest quintile (21). Furthermore, increase in plasma tCys over 6 years was associated with higher fat-mass at follow-up (21). Since this was also a noninterventional study, two possibilities for interpretation of the findings were either that a high cysteine somehow promotes obesity or that obesity influences cysteine turnover, thereby raising plasma tCys. Results from our recent study of changes in plasma sulfur amino acids in super-obese patients undergoing bariatric surgery suggest that body fat mass is not a determinant of plasma tCys concentrations (22). A third possibility is that one or more confounding factor(s) simultaneously increases plasma cysteine and predisposes to obesity, or that cysteine is a marker associated with obesity or obesity-related morbidity. One factor that fulfills the criteria of a confounder in the tCys–fat mass association is GGT enzyme activity. GGT is a recognized marker of obesity (7,23) which also catalyzes breakdown of GSH in the γ-glutamyl cycle, eventually releasing cysteine. Thus it is conceivable that obese individuals, through an unknown mechanism (23), feature increased GGT activity which elevates their tCys. However, in the COMAC cohort of >1,500 men and women from nine European countries, plasma tCys was found to be associated with BMI independent of plasma GGT activity (24). Subjects in the highest quartile of plasma tCys were 3.5 times as likely to be obese, compared to those in the lowest quartile, after adjusting for GGT. These findings are limited by the question of how far plasma GGT activity reflects tissue GGT activity, which is localized to cellular plasma membranes. Nevertheless, in this study the statistical relationships of tCys and GGT with BMI followed different patterns. While plasma GGT showed a strong association with odds of obesity that was weakened by adjustment for obesity-related factors (e.g., serum lipids), tCys was an independent linear predictor of BMI throughout the BMI range and was also independently associated with odds of obesity (24). Diet would be a confounder in the tCys–fat mass association if subjects with high tCys and high-fat mass consume a diet that increases plasma tCys and is simultaneously obesogenic, independent of its effect on cysteine metabolism. In the Hordaland Homocysteine Study, the tCys–fat mass relationship persisted after adjustment for total protein, fat, and energy intakes (21). Cysteine is abundant in whey protein (25) as well as some fruits and vegetables, including red pepper, asparagus, and strawberry (26), which are not recognized to be linked with obesity. There is also some data suggesting that dietary cysteine intake is not a major determinant of plasma tCys. In a recent study, dietary cystine intake was unrelated to plasma tCys in healthy women (20). A similar observation was made in cats fed different levels of cysteine (27), and doubling cystine intake in rats did not raise plasma cystine (28). Even parenteral cysteine administration in neonates failed to raise plasma tCys (29). It remains possible that another dietary pattern, apart from high intake of cysteine-rich foods, could simultaneously promote increase of plasma tCys and of fat mass. One such possibility is high intake of methionine-rich foods, as discussed below (see Dietary methionine restriction section). We investigated whether structurally and metabolically related sulfur amino acids showed a similar relation to BMI as tCys. Plasma tHcy showed a modest positive relation to fat mass in the Hordaland Homocysteine Study, which became negative after adjusting for tCys (21). Despite the association of methionine intake with BMI (30), nonfasting plasma methionine was not related to fat mass (21), and fasting plasma methionine was unrelated to BMI (31). Newgard et al. (32) similarly found no difference in plasma methionine between obese and lean subjects. Plasma cystathionine correlated positively with fat mass and BMI (21,31), but these correlations were weaker than those of tCys, and were attenuated by adjustment for confounders such as serum lipids. Plasma levels of taurine and tGSH, two downstream products of cysteine, were not significantly correlated with BMI (31). Figure 2 shows the estimated differences in body fat percent by plasma sulfur amino acids and triglyceride concentrations in the Hordaland Homocysteine Study. Plasma sulfur amino acids and body fat percent. Estimated differences in body fat % (dose–response curves and 95% confidence interval (CI)) according to plasma concentrations of individual sulfur amino acids (in µmol/l) and triglycerides (in mg/l) after adjustment for age-group and gender by Gaussian generalized additive regression models as described in ref. 21. P values and partial correlation coefficients are from corresponding linear regression analyses. tCys, total cysteine; tHcy, total homocysteine. Statistics for methionine, tHcy, cystathionine, and triglycerides are computed using log-transformed data. Data adapted from the Hordaland Homocysteine Study (21). In summary, only plasma tCys, but not methionine, tHcy, cystathionine, taurine, tGSH, or cysteinylglycine, is a strong independent positive predictor of BMI, fat mass and obesity (21,24,31). The distribution of several amino acids among the different blood compartments is altered in obesity (33). To investigate whether the association of tCys with obesity merely reflects a shift of cysteine from red cells to the plasma in obese individuals, we examined the association of nonprotein bound cysteine in whole blood with BMI. Similar to plasma tCys, free blood cysteine was positively correlated with BMI (cohort described in ref. 34; Figure 3). This may indicate that it is the high plasma cysteine availability in both cells and plasma, rather than a shift in cysteine compartmentation between blood cells and plasma, that is associated with obesity. Whole blood-free cysteine and BMI. Estimated differences in BMI according to concentration of nonprotein bound cysteine in whole blood in a cohort of 877 men and women (described in ref. 34), with adjustment for age and gender. Lower panel is additionally adjusted for free glutathione in whole blood. Free cysteine and glutathione measurements in whole blood were positively associated after adjustment for age and gender (partial r = 0.54, P < 0.001). Obesity is associated with elevation of circulating levels of several amino acids. Branched-chain amino acids, as well as alanine, phenylalanine, and tyrosine, are consistently elevated in overweight and/or obese subjects (32,33,35). Several mechanisms have been postulated to explain plasma amino acid elevation in obesity. In an early study, Holm et al. found strong correlations between plasma amino acid concentrations and lean mass, and concluded that increased lean mass in obesity contributes to the elevated plasma amino acid concentrations (36). Newgard et al. ascribe branched-chain amino acid elevation in obesity to enhanced protein catabolism partly resulting from dietary overload (32). She et al. demonstrated decreased branched-chain amino acid catabolic enzymes in obese rodents (37), and that in humans, these amino acids decrease following gastric bypass surgery, with increase in their metabolizing enzymes in adipose tissue samples (37). Thus branched-chain amino acid elevation in obesity appears to be a consequence or adjunct, rather than cause, of obesity. In contrast, several lines of evidence point to a causal role for cysteine or a product of cysteine, in promoting obesity. If cysteine is a causal determinant of fat mass, then an inborn error characterized by increased cysteine synthesis should be associated with obesity. Conversely, inherited defects of cysteine formation should be associated with a lean phenotype. This is what is observed in the two genetic syndromes, Down's syndrome and the common variant of homocystinuria (Figure 4). Genetic cysteine alterations and body weight. Opposite body weight phenotypes in inherited syndromes cysteine homocystinuria to cystathionine β-synthase decreased cysteine synthesis and a Down's increased cysteine synthesis and Down's syndrome. with Down's syndrome are more overweight and obese compared to other with The of obesity in Down's syndrome is not but been to be to a low metabolic to of the human CBS to this is over in Down's syndrome resulting in markedly elevated plasma tCys which may be linked to the increased of obesity in this to CBS The common type of the inborn error homocystinuria is by genetic defects in the CBS enzyme and is characterized by elevation of plasma and tHcy with decreased cysteine and a range of and Notably, these patients often have low BMI from ref. decreased fat and body weight below the and have been described by et al. as and by the The lean not been reported in the form of homocystinuria by homocysteine defects in which cysteine synthesis is thus low cysteine availability in the of CBS Dietary with cysteine or a cysteine-rich protein been shown to increase weight in and as well as in are to cysteine to methionine, the observed of cysteine on weight be to an increase in the essential amino acid methionine, an cysteine supply may methionine from transsulfuration early as concluded that of of sulfur as methionine, and cysteine in and promoting that the effect was by cysteine or by some of In cysteine to a diet body weight the in dietary protein protein also weight in cancer patients is no evidence that cysteine is the for increased weight gain. The increased weight in these studies not to from increased since intake was often decreased in the group This is in with findings in the Hordaland Homocysteine Study that the correlation of tCys with fat mass independent of energy intake (21). In the methionine of with weight with this dietary methionine restriction in rodents in decreased weight and/or fat mass with increased metabolic The is linked to of hepatic a acid which is a of lipid and energy This is a specific effect of methionine of independent of intake, as by studies In human methionine is mainly from of animal including and In with which are low in methionine are associated in large studies with weight and obesity risk and are also associated with tCys with low methionine are often associated with weight when used for of obesity in ref. Furthermore, weight is observed in on a diet for of energy and protein intakes or increased In methionine intake showed a relation with BMI Thus it is that high methionine intake simultaneously promotes weight and increases plasma tCys by methionine conversion to cysteine. Plasma tCys is decreased in rats suggesting that it may be the decreased cysteine availability which the of methionine restriction on body weight. To the of of cysteine from that of methionine, we investigated whether rats with cysteine would their phenotype. Cysteine the of methionine restriction on and hepatic with corresponding changes in plasma acid and The decrease in serum methionine in rats was not by cysteine This that it is the reduced supply of cysteine that the of methionine with this we have also observed in a dietary mouse that high cystine intake metabolic Several that decrease cysteine turnover or uptake by cells have a negative on body weight. which the cysteine for cysteine uptake by weight as a effect a which the of cysteine from cysteinylglycine, resulting in decreased tCys weight in rats However, is administered as an to the the weight be to its and studies in did not an effect of on body weight decrease in weight is observed in rodents with administration of the which of cysteine from cystathionine A body of also the that cysteine could promote obesity. This mainly a of studies that that cysteine lean mass, and may under certain conditions decrease fat mass. The compounds used in these studies were either cysteine-rich whey protein or the cysteine Data from cysteine-rich protein exists for both and One study found a decrease in body in nine subjects a cysteine-rich whey protein, compared to increase in nine subjects a as by In contrast, a in subjects showed to be to cysteine-rich whey protein in promoting fat mass Furthermore, been found to promote weight in cancer patients A third study was in rats A protein whey increased lean mass and decreased when for compared to whole protein or The cysteine of this study several factors whey protein, and Thus the high cysteine of it is not that body composition changes are a cysteine The of whey protein on body composition have been to its which protein Thus while cysteine-rich protein may have on body composition in some the constituent mediating these is not and the evidence for their on fat mass is and et al. demonstrated that may fat mass in a including human subjects either or for The group = a decrease in fat mass compared to an increase in the group The fat mass in the group was associated with decreased and was to indicate that decreased fat mass by fat mass was also observed in rodents after of of Plasma tCys changes were not reported in the studies. Although is a cysteine of on plasma tCys are to studies have reported either no an increase or a of plasma cysteine levels following administration we that and cysteine may in different on body composition to their redox in relation to lipid In to its on and metabolic in models discussed (see dietary methionine restriction cysteine may also in is the It strongly inhibits the enzyme in and triglyceride and uptake The of cysteine and in relation to and regulation of lipid turnover have been investigated in from these studies may a mechanism for the effect of cysteine, in to intracellular with of which action is when the are and thus by protein The activity of protein on the reduced of its cysteine of which action by protein with factors that production or breakdown protein and is thus to the in These findings are with early in adipocytes that their and other production and that lipid synthesis It is that cysteine and have on in vitro (Figure Cysteine and in relation to for postulated of cysteine and on redox of the a an intracellular which intracellular of including of a of is by the to Cysteine to cystine disulfide is an cysteine and can have on was shown in early studies to in adipocytes that are its production of with sulfhydryl to disulfide A powerful action of cysteine, been observed in cultured rat adipocytes This action was not by to the it persisted when the was by as well as acid synthesis in to The action of cysteine in vitro is by that cysteine in rats In contrast, is a free in The of to and thus been demonstrated in vitro and is suggested by et al. to explain the effect of in fat mass at the of in The cellular of and in are likely to be to redox of these their in relation to Cysteine and had on enzymes in fed a high-fat diet In these a action of cysteine was in to antioxidant of the that the of is for its antioxidant activity This may explain administration been shown under some conditions to decrease fat mass and increases weight in and elevated tCys is associated with obesity in The possible of cysteine in human obesity an role for individual amino acids in weight Studies are under to the relationship in different and question is whether the higher tCys in compared to as reported in one study is linked to the of in the group The findings described in this review other that we can that cysteine is a in the of obesity. some of these studies are under in our but with different contribute to this from different The evidence far linking cysteine with obesity is from epidemiologic data. studies consistently a positive association of plasma tCys and blood-free cysteine with BMI and/or fat mass, which is largely not by or downstream sulfur amino acids. Furthermore, the association appears independent of and plasma Although some of these studies are animal studies cysteine in regulating lipid and energy but a action of cysteine on of a diet is to be demonstrated in and Nevertheless, data from epidemiologic genetic syndromes, animal and in vitro findings suggest that increased cysteine availability may increase body fat, and provide evidence to of this obesity risk We for The study was also by the Norwegian the an and by from the of of the The no of