Effects of Short-chain Fatty Acids on Inflammatory and Metabolic Parameters in Maintenance Hemodialysis

End-stage renal disease (ESRD) is associated with multiple comorbidities such as cardiovascular disease, anemia, mineral and bone disorders, malnutrition, body wasting, muscle loss (sarcopenia), neurological problems and infections resulting in a poor survival. Important promoters of these obstacles are enhanced generation of reactive oxygen species (ROS), systemic inflammation, acquired immunodeficiency (9, 10) and an impaired glucose and insulin homeostasis (11).

Systemic inflammation and oxidative stress in ESRD are induced by activation of the innate immune system involving monocytes, macrophages, granulocytes and cellular constituents (endothelial cell activation) as well as depletion of natural regulatory T cells that impairs their ability to suppress inflammation .The concomitant reduced humoral immunity is favored by depletion of antigen presenting dendritic cells, a lowered CD44/CD8 T cell ratio, depletion of naïve and central memory T cells, diffuse B cell lymphopenia and an impaired phagocytic ability of monocytes and PMNs (12).

Insulin resistance (IR) participates in the pathogenesis of multiple metabolic and cardiovascular disturbances (13) and is an important factor of the accelerated muscle protein degradation in ESRD (14). Underlying mechanisms of IR are the metabolic inflammation, in particular elevated LPS levels.

In the pathogenesis of the uremic syndrome the altered intestinal function seems to be an important contributor. While the normal gut microbiota plays a prominent role in the maintenance of health and disease prevention, changes of its composition is associated with numerous diseases such as obesity, type 2 diabetes, cardiovascular disturbances and auto-immune diseases. In ESRD metabolic alterations of uremia results in quantitative and qualitative changes of its bacterial flora with an overgrowth of pathobionts (1). Due to concomitant disruption of the intestinal barrier function, noxious luminal products are translocated in the body's internal milieu (Fig.2). The passage includes whole bacteria (going into mesenteric lymph nodes), endotoxins/ lipoproteinlipase (LPS) (cell wall components of the bacteria) and other noxious luminal products which induce a persistent local (gut) and systemic inflammation.The process is intensified by the intestinal generation of several pro-inflammatory uremic toxins such as indoxyl sulfate, p-cresyl sulfate and trimethyamine-N-oxide (2).The accumulation of these compounds correlates with systemic inflammation, protein wasting and accelerated cardiovascular complications in hemodialysis patients (3).

Short-chain fatty acids (SCFA) are produced in the colon and distal small intestine by anaerobic bacteria following fermentation of complex carbohydrates. The 3 major compounds are acetic acid, butyric and propionic acids. SCFA contribute to the health of the gut (microbiome and mucosa) and the host. They have been shown to exert anti-inflammatory, anti-cancer, antibacterial and antidiabetic effects. Lower values and an dysbiotic gut contribute to various diseases such colitis, type 2 diabetes, rheumatoid disease and multiple sclerosis. Supplementation of SCFA exerts anti-inflammatory actions both in intestinal epithelial cells (5) and in the cardiovascular system (6). They also positively influence auto- immune reactions /diseases (7, 8). In particular SCFA enhances formation of regulatory T cells in the colon which are critical for regulating intestinal inflammation (15). Also effector T cells such as Il-10 are implicated (16). Likewise SCFA are involved in the control of body weight and insulin sensitivity (17), cholesterol synthesis (18) and retardation of progressive CKD.

In patients on maintenance hemodialysis (MHD) a diet with a high fiber content, which favors the intestinal SCFA formation (19), lowered the plasma levels of the colon-derived solutes indoxyl sulfate and possibly p-cresol sulfate (20) and reduced inflammation, cardiovascular diseases and all-cause mortality in CKD/ESRD patients (21). However, in ESRD consumption of a fiber rich diet is limited due to the risk of hyperpotassemia. In addition the frequent antibiotic therapy, application of phosphate binder or iron therapy alters the gut microbiome.

The following mechanisms have been proposed for the actions of SCFA: the G‑protein-coupled receptors GPR 41 and GPR 43 (the free fatty acid receptors FFAR 3 and 2 ), GPR 109a, Olfr78 , the inhibition of histone deacetylases (HDAC) and stimulation of histone acetyltransferase (HAT) activity (22, 23).

n this study we want to investigate in MHD patients whether a treatment with SCFA in form of sodium propionate (SP) modulates the systemic inflammation, insulin resistance and accumulation of intestinal uremic toxins. SP is chemically composed by a carboxylic acid moiety and a small hydrocarbon chain with three carbon atoms (black balls), two oxygen (red balls) and the white hydrogen atoms.

SP is involved in most effects of the short chain fatty acids including inhibition of intestinal and hepatocyte lipid synthesis (24), lowering of fasting glycemia (25, 26) and protection against diet-induced obesity ( 27). SP also regulates colonic T-reg cell homeostasis (28) and exerts marked anti-inflammatory actions including intestinal epithelial cells and macrophages (29) as well as in neutrophils, colon cells and colon cultures (30). It improved experimental autoimmune encephalomyelitis (31) and experimental acute renal failure (32). In addition antibacterial effects were documented (33, 34).

The patients under maintenance hemodialysis will receive the food additive sodium propionate with a daily intake of 2 x 500 mg in form of capsules (Propicum) for 12 weeks. The demographic information and the blood chemistry will be collected before the study, after 6, 12 and 16 weeks of drug administration.

The project will last for one year. The planned patient group should comprise of 15 patients on maintenance hemodialysis.