Reactive carbonyl compounds as uremic toxins

Active vitamin D treatment in CKD patients raises serum sclerostin and this effect is modified by circulating pentosidine levels

BACKGROUND AND AIMS

1,25(OH)₂Vitamin D increases the expression of the sclerostin gene. Whether vitamin D receptor activation (VDRA) influences serum sclerostin in CKD and whether compounds interfering with VDRA like Advanced Glycosylation End Products (AGEs) may alter the sclerostin response to VDRA is unknown.

METHODS AND RESULTS

Eighty-eight stage G3-4 CKD patients randomly received 2 μg paricalcitol (PCT)/day (n = 44) or placebo (n = 44) for 12 weeks. Sclerostin, a major AGE compound like pentosidine, and bone mineral disorder biomarkers were measured at baseline, at 12 weeks and 2 weeks after stopping the treatments. At baseline, in the whole study population sclerostin correlated with male gender (P = 0.002), BMI (P < 0.001), waist circumference (P < 0.001), serum pentosidine (P = 0.002) and to a weaker extent, with diabetes (P = 0.04), 1,25(OH)₂Vitamin D (r = 0.22, P = 0.04) and serum phosphate (r = -0.26, P = 0.01). Sclerostin increased during PCT treatment (average + 15.7 pg/ml, 95% CI: -3.0 to +34.3) but not during placebo (P = 0.03) and the PCT effect was abolished 2 weeks after stopping this drug. The increase in sclerostin levels induced by PCT was modified by prevailing pentosidine levels (P = 0.01) and was abolished by statistical adjustment for simultaneous changes in PTH but not by FGF23 changes.

CONCLUSIONS

VDRA by paricalcitol causes a moderate increase in serum sclerostin in CKD patients. Such an effect is abolished by adjustment for PTH, suggesting that it may serve to counter PTH suppression. The sclerostin rise by PCT is attenuated by pentosidine, an observation in keeping with in vitro studies showing that AGEs alter the functioning of the VDRA.

Nutrition and muscle catabolism in maintenance hemodialysis: does feeding make muscle cells selective self-eaters?

Efforts to build muscle by increased protein feeding in hemodialysis patients have been thwarted by parallel increases in both muscle protein synthesis and degradation. The evidence suggests that muscle cells replace older proteins in response to feeding rather than using new proteins to drive muscle cell hypertrophy. This review presents the hypothesis that protein feeding provides an opportunity for muscle to accelerate proteolysis of proteins that have been damaged by oxidation, nitrosylation, and/or glycosylation and to replace damaged mitochondria that contribute to oxidative stress. Increases in proteolysis with feeding are driven by insulin resistance and the increased oxidative stress of mitochondrial respiration. Oxidized proteins and organelles are excellent substrates for degradation by the proteasome, macroautophagy, and chaperone-mediated autophagy: these systems of proteolysis seem to be activated by oxydatiative stress. Replacement of oxidized and other damaged proteins may be a benefit of protein feeding in hemodialysis, but alternative strategies, including exercise, will be required to build muscle.

Impact of glucose levels on advanced glycation end products in hemodialysis

The current obesity epidemic throughout the western world has resulted in a considerable increase in the condition Type II diabetes mellitus. Recently, the World Health Organization has predicted that the global prevalence of Type II will increase from 175 million patients in 2003 to over 350 million by 2030. One of the major consequences of this disorder is renal failure, which presents itself as chronic kidney disease, and can progress to end-stage renal disease. Once diagnosed, patients are generally treated using dialysis due to a shortage of kidney donors. The fundamental process of dialysis still requires improvement because the survival rate of these patients is relatively poor. This has resulted in considerable research into improvements in hemodialysis membranes, and the challenge to find more suitable marker(s) in assessing the efficacy of the dialysis process. A class of compounds highlighted as a possible accumulative toxin is advanced glycation end products or AGEs. This is an article regarding the impact of hemodialysis and hemodiafiltration on glucose and AGE levels within the body and the consequences of a chronic hyperglycemic condition. It also highlights the negative aspects of using dextrose in conventional dialysis solutions (an area that has already been identified by peritoneal dialysis clinicians as problematic). The review concludes by suggesting several possible topics of future research.

In vitro evidence for immune activating effect of specific AGE structures retained in uremia

BACKGROUND

Advanced glycation end-products (AGEs) have been identified to be accumulated in blood and tissues of patients with end-stage renal disease (ESRD). AGEs have been shown to modulate immune competent cell activities and in this way they may contribute to the progression of atherosclerosis. All studies in this context have been performed, however, with generated mix of glycation compounds, and not with structures similar to those encountered in uremia. In the present study, the immunologic effect of specific AGE compounds, known to be retained in uremia, has been evaluated.

METHODS

Four albumin preparations, modified chemically at lysine or arginine residues, respectively, to contain N-epsilon-carboxymethyllysine (CML albumin), N-epsilon-carboxyethyllysine (CEL albumin), glyoxal-induced imidazolinones (Arg I albumin) or methylglyoxal-induced imidazolinones (Arg II albumin) were applied. Their effect on chemiluminescence production, CD14 expression, and the DNA synthesis of calcitriol-differentiated HL-60 (monocyte/macrophage phenotype) was studied.

RESULTS

The phorbol 12-myristate 13-acetate (PMA)-stimulated chemiluminescence production of the calcitriol differentiated HL-60 cells was enhanced in the presence of CEL albumin (44.1 +/- 18.5 vs. 64.7 +/- 28.1 counts 10(3)/30 min) (P < 0.05), Arg I albumin (46.4 +/- 18.8 vs. 66.1 +/- 32.6 counts 10(3)/30 min) (P < 0.05) and CML albumin (41.9 +/- 25.5 vs. 60.9 +/- 5.5 counts 10(3)/30 min) (P= 0.0625) pointing to an increase in free radical production. The latter AGE compounds also significantly increased the calcitriol-induced CD14 expression on HL-60 cells (1675 +/- 796 vs. 2075 +/- 1044; 768 +/- 143 vs. 890 +/- 150; 647 +/- 63 vs. 716 +/- 69 mean fluorescence intensity) (P < 0.05, respectively) pointing to an increase in expression of the lipopolysaccharide (LPS) receptor. Finally, the DNA synthesis of the calcitriol-differentiated HL-60 cells was enhanced in the presence of Arg I albumin [34.5 +/- 4.6 vs. 27.7 +/- 9.7% 5-bromo-2'-deoxyuridine (BrdU)-positive cells] (P < 0.05) resulting in an increased cell proliferation.

CONCLUSION

Genuine AGE compounds, as they are encountered in the uremic condition, activate leukocyte response, and hence could play a role in uremia related atherogenesis.