The primary finding of this study is that high plasma levels of S100A12 are independently associated with low mid-thigh muscle mass and low subcutaneous fat mass in HD patients, even after adjusting for potential confounding variables. To our knowledge, this study is the first to determine the role that the RAGE system plays in the nutritional status of patients with advanced CKD.
Advanced CKD patients often suffer from nutritional problems that are associated with increased morbidity and mortality . PEW is a term that has been proposed to describe the state of decreased body stores of protein and energy (i.e., muscle and fat mass) that occurs in CKD. In fact, HD patients have been reported to exhibit lower BMIs than age- and sex-matched control subjects from the general population . Based on this observation, several studies have shown that increased BMI contributes to survival advantages in dialysis patients [8, 9]. Because increased BMI has been associated with an increased risk of cardiovascular disease and all-cause mortality in the general population , this opposite relationship that is observed in dialysis patients is known as “risk factor paradox” or “reverse epidemiology” [29, 30].
In dialysis patients, increased serum Cr levels have been associated with improved survival, whereas lower serum Cr levels have been associated with increased mortality [31–33]. This finding suggests that low serum levels of Cr as a proxy for low muscle mass could be associated with adverse outcomes . Carrero et al.  also reported that muscle wasting measured by subjective global assessment (SGA) is associated with the increased mortality. These observations suggest that muscle mass is an important predictor of mortality in HD patients. However, increased fat mass has been associated with a lower risk of mortality and a reduced risk of hospitalization in HD patients . Moreover, Kakiya et al.  showed that a decrease in body fat is associated with an increased risk of death in these patients. Currently, it remains to be determined whether the survival advantage associated with higher BMI in dialysis patients is caused by increases in muscle mass, fat mass, or both. One reason that this question remains unanswered is because BMI cannot differentiate between weight changes caused by muscle mass alterations and weight changes resulting from fat mass or water weight . Previously, Beddhu et al.  attempted to investigate this issue using 24-hour urinary creatinine excretion as a surrogate for muscle mass. Based on their analysis, Beddhu and colleagues hypothesized that muscle mass might be a more important contributor to this survival advantage than fat mass. However, additional studies that directly measure muscle mass and fat mass are required to clarify this issue.
AGEs are generated as a result of chronic hyperglycemia and enhanced oxidative stress [38, 39]. AGEs were initially thought to be the primary active ligands for their receptors, such as RAGE, but several new ligands, including the high-mobility group box proteins, S100 proteins, and amyloid fibrils, have been recently identified . The binding of these ligands to RAGE induces oxidative stress, inflammation, and extracellular matrix accumulation [39, 41]. Because the plasma AGE level changes that are observed in CKD patients are relatively modest , ligands other than AGEs for RAGE may be more important in developing RAGE-mediated complications in CKD. For example, S100A12, which is also known as EN-RAGE, has been identified as an interesting pro-inflammatory ligand for RAGE that triggers the RAGE pathway. S100A12 activates key inflammatory signals, such as nuclear factor-κB (NF-κB), and it stimulates the production of pro-inflammatory cytokines, such as IL-1β and TNF-α [18, 20]. In contrast, sRAGE, which acts as a decoy receptor for RAGE ligands, suppresses RAGE-mediated inflammatory responses . Recently, several studies have focused on the pivotal role of RAGE signaling in patients with CKD [15, 21–23]. For example, sRAGE is negatively associated with systemic inflammation and with carotid intima-media thickness in PD patients . In addition, S100A12 has been shown to predict the cardiovascular and all-cause mortality in HD patients [22, 23]. In the present study, S100A12 was shown to be significantly and negatively associated with muscle mass and fat mass by both univariate and multivariate analysis, whereas no such relationships were observed for sRAGE. Thus, our study reveals the potential value of S100A12 as a predictor of nutritional status and provides clinical evidence regarding its possible role in the development of PEW in HD patients. In contrast, our data indicate that sRAGE levels are likely of limited clinical value in identifying PEW patients, although this possibility should be tested in a specifically designed clinical trial.
Another key finding from the present study is that the negative associations of S100A12 with muscle mass and fat mass both still remain significant even after adjusting for systemic inflammation (Table 3). As reported previously, systemic inflammation can cause a decrease in muscle mass because pro-inflammatory cytokines can contribute to anorexia, inhibit protein synthesis and promote catabolism . Adipose tissue is a well-known source of pro-inflammatory cytokines, and obese CKD patients have actually been shown to exhibit higher levels of inflammation [43, 44]. These findings highlight the difficulty of using inflammation as the sole explanation for the observed reductions of muscle and fat mass. Therefore, to clarify the influence of S100A12 on PEW in dialysis patients, we tried to adjust for the levels of systemic inflammation. Our analysis revealed that S100A12 is a direct predictor of muscle and fat mass independent of systemic inflammation. Previously, Hofmann et al.  demonstrated that S100A12 reduces cellular proliferation and increases H2O2 production via the NADPH oxidase system. Together, our results suggest that high levels of plasma S100A12 are associated with PEW via non-inflammatory mechanisms, such as oxidative stress. Further basic studies are warranted to clarify the precise role of this interesting ligand in determining nutritional status.
Our study has several limitations. First, due to the cross-sectional study design, a longitudinal causal relationship cannot be established between the changes in plasma S100A12 levels and alterations in muscle and/or fat mass. Second, because of the relatively small number of patients in our cohort, the generalizability of our conclusions remains unclear, and our data should be confirmed by larger studies.