Abstract
Background and Aims
Indole-3-acetic acid (IAA, also called auxin) is a protein-bound indolic uremic toxin deriving from tryptophan metabolism by the intestinal bacteria. Previous studies have shown that increased IAA is associated with enhanced tissue factor synthesis in endothelial and peripheral blood mononuclear cells, oxidative stress and endothelial inflammation with resulting higher risk of thrombotic events and both cardiovascular and all-cause mortality. An emerging biomarker of cardiovascular disease is the monocyte to high-density lipoprotein (HDL) ratio (MHR). Its prognostic value is related to the ability of monocytes to release several cytokines involved in inflammation and atherogenesis and to the protective role of HDL through removal of cholesterol from peripheral tissues and suppression of both monocyte progenitor cell proliferation and differentiation and monocyte activation. In this single-centre cross-sectional observational study, we investigated the potential association of IAA with MHR and other markers of cardiovascular risk in a cohort of patients with CKD and evaluated the effect of a single midweek dialysis session with AFB (Acetate-free Biofiltration) technique on IAA serum concentrations.
Method
We enrolled 61 non-dialysis CKD adult patients and 6 dialysis patients treated with AFB technique. IAA levels were measured using an enzyme-linked immunosorbent assay (ELISA) kit (Cat. number abx150354; Abbexa Ltd, Cambridge, UK). Post-dialysis IAA levels were corrected for haemoconcentration.
Results
In the whole cohort of 67 patients, IAA was directly related to creatinine (ρ = 0.247; P = 0.0441), potassium (r = 0.2871; P = 0.0185), Ca x P product (ρ = 0.256; P = 0.0365) and MHR (ρ = 0.321; P = 0.0082).
After adjustment for creatinine, the correlation between IAA and potassium became not significant (r = 0.1968; P = 0.1133). Stratifying patients according to the history of cardiovascular disease, in the 40 patients with previous cardiovascular events IAA levels correlated significantly with uric acid (r = 0.3952; P = 0.0116) and MHR (ρ = 0.380; P = 0.0157).
In the remaining 27 patients without history of cardiovascular disease, IAA only correlated with potassium (r = 0.3912; P=0.0481) and, though borderline significantly, with creatinine (ρ = 0.349; P = 0.0805). To assess whether IAA would independently predict MHR values, we evaluated potential correlations of MHR with risk factors for cardiovascular disease. MHR was related with fibrinogen (ρ = 0.426; P = 0.0010), arterial hypertension (ρ = 0.274; P = 0.0251), C-reactive protein (ρ = 0.332; P = 0.0061), gender (ρ = -0.375; P = 0.0017; 0 = male, 1 = female), and CKD stage (ρ = 0.260; P = 0.0337). A multiple regression analysis identified IAA as an independent predictor of MHR. Lastly, IAA levels were higher in dialysis patients compared to non-dialysis CKD patients (97.44 ± 21.58 versus 65.08 ± 24.38 ng/ml respectively; P = 0.0026) and it was significantly removed by a single AFB session (97.44 ± 21.58 versus 54.59 ± 21.74 ng/ml; P = 0.0028) with a reduction ratio of 43.80 ± 17.47%.
Conclusion
This study shows a statistically significant association between IAA and MHR. Based on previous experimental studies, such relationship could be explained by the activation of the transcription factor aryl hydrocarbon receptor. Indeed, IAA is a potent ligand of aryl hydrocarbon receptor and the latter has proinflammatory and proatherogenic activities and can reduce HDL levels. Moreover, AFB efficiently removes IAA during a single dialysis session. Prospective studies with appropriate sample size and sufficiently long period of observation are required to evaluate if decreasing IAA levels, through targeted therapeutic strategies in non dialysis CKD patients or by optimization of dialysis techniques and prescriptions in patients receiving renal replacement therapy, may reduce MHR levels and cardiovascular events and improve clinical outcomes and survival.
Functional Characterization and Evolutionary History of Two Aryl Hydrocarbon Receptor Isoforms (AhR1 and AhR2) from Avian Species