Adiponectin is present in the urine in its native conformation, and specifically reduces the secretion of MCP-1 by proximal tubular cells.

AIM: To determine whether adiponectin detected in urine is present in its native form and if adiponectin receptors (AdipoR) present and functional in proximal tubular (HK-2) cells. BACKGROUND: Adiponectin is a protein with anti-inflammatory, anti-atherogenic and insulin-sensitizing properties. It has previously been detected antigenically in the urine in several forms of renal disease. METHODS: We compared the isoform distribution of urinary adiponectin in patients with proteinuric and non-proteinuric renal disease with that of matched controls using chromatography and enzyme-linked immunosorbent assay. We examined whether AdipoR were present in HK-2 cells by real-time reverse transcription polymerase chain reaction. Their functionality was investigated by determining the effect of recombinant adiponectin on adenosine monophosphate-activated protein kinase phosphorylation using western blotting, and on the secretion of monocyte chemotactic protein-1 and C3 using enzyme-linked immunosorbent assays. RESULTS: Adiponectin in the urine is physiologically intact and largely present as the low molecular weight isoform. Subjects with urinary protein >150 mg/L excreted significantly more adiponectin and its high and low molecular weight isoforms than those with <150 mg/L. mRNA for AdipoR were present in HK-2 cells, with levels of mRNA for AdipoR1 being 20 times greater than those for AdipoR2. Ligation of AdipoR on proximal tubular cells increased phosphorylation of adenosine monophosphate-activated protein kinase, and downregulated the secretion of the inflammatory cytokine monocyte chemotactic protein-1, but not of C3. CONCLUSION: Physiologically relevant isoforms of adiponectin are present in the urine of normal subjects and those with proteinuria. In addition, functional receptors for adiponectin are present in HK-2 cells. Abnormal levels of adiponectin in the urine may therefore activate these receptors, potentially resulting in anti-inflammatory activity.

Review article: Adiponectin: its role in kidney disease.

The multifactorial glycoprotein, adiponectin has demonstrable insulin-sensitizing, anti-atherogenic and anti-inflammatory properties. However, despite the prevalence of both insulin-resistance and vascular disease in patients with end-stage kidney disease, levels of adiponectin are high. Adiponectin circulates in different sizes (the high-molecular-weight (HMW) isoform is thought to be the most insulin-sensitizing type) and binds to two receptors, adiponectin receptors (AdipoR) 1 and 2. The adiponectin/receptor system appears to be upregulated in end-stage kidney disease possibly as an appropriate counter-regulatory response to the uraemic milieu. In contrast, adiponectin and its HMW isoform, AdipoR mRNA expression on peripheral blood mononuclear cells decrease after kidney transplantation, likely secondary to immunosuppression and/or an improvement in glomerular filtration rate and the uraemic environment. Adiponectin has also been detected in the urine of patients with proteinuric kidney disease. The presence of AdipoR on an immortal cell line of proximal tubular epithelial cells (HK-2) and an increased amount of intact HMW isoform in the urine of patients with various forms of proteinuria lead us to speculate about the potential role of urinary adiponectin. This review will also discuss the structure and function of adiponectin and its potential relevance to patients with kidney disease and the different factors that may influence the metabolism of this protein in kidney failure.

The effect of renal transplantation on adiponectin and its isoforms and receptors.

Insulin resistance (IR) and other proatherogenic risk factors associated with end-stage kidney disease (ESKD) are improved by renal transplantation. Adiponectin is a protein with insulin-sensitizing, anti-inflammatory, and antiatherogenic properties. It exists in several isoforms, but the high molecular weight (HMW) isoform correlates best with insulin sensitivity. Paradoxically, the levels of this protein and its HMW isoform are increased in ESKD. We measured the homeostasis model assessment for insulin resistance (HOMA-IR), plasma adiponectin and its isoforms, and messenger RNA for adiponectin receptors (AdipoR1 and AdipoR2) on peripheral blood mononuclear cells in 54 stable transplant recipients, 50 patients established on hemodialysis, and 52 controls; groups were matched for body mass index and sex. HOMA-IR values were significantly higher in patients with ESKD compared with controls (P < .0005) and transplant patients (P < .05) but there was no difference between the latter 2 groups. Adiponectin levels were also higher in patients with ESKD compared with controls (P < .0005), and although levels were lower in the transplant group, they remained higher than in controls (P < .0001). However, although the absolute amount of HMW isoform in transplant patients remained higher than in controls (P < .0001), the proportion was similar, and less than in patients with ESKD (P < .005). The absolute amount of the HMW isoform correlated with superior metabolic indices in all 3 cohorts. AdipoR1 and AdipoR2 messenger RNA levels after transplantation were significantly lower than those of ESKD subjects (P < .0001, P < .01), but transplant patients had less AdipoR1 than controls, although their AdipoR2 levels were higher. AdipoR1 correlated with AdipoR2 in all 3 cohorts. We conclude that HOMA-IR was lower in the transplant group compared with the group on hemodialysis and this coincided with lower total adiponectin levels and absolute amount of the HMW isoform and AdipoR on peripheral blood mononuclear cells. Lower AdipoR after transplantation may be secondary to immunosuppression and/or an improvement in glomerular filtration rate and the uremic milieu.

Up-regulation of adiponectin, its isoforms and receptors in end-stage kidney disease.

BACKGROUND: Despite the favourable effects of adiponectin on the vasculature and insulin resistance (IR), levels are increased in patients with end-stage kidney disease (ESKD), in whom both IR and atherosclerosis are prevalent. METHODS: To investigate this paradox, we examined the distribution of adiponectin isoforms, the expression of adiponectin receptor (AdipoR) mRNA on peripheral blood mononuclear cells (PBMC) in 41 patients with ESKD on haemodialysis and 41 matched controls, and its function by adenosine monophosphate-activated protein kinase (AMPK) phosphorylation of AdipoR on PBMC. We also compared the expression of AdipoR on PBMC with that on muscle and subcutaneous and visceral fat in 10 patients undergoing elective cholecystectomy. RESULTS: The proportion of the high molecular weight (HMW) isoform of adiponectin was increased in the dialysis group (P = 0.001), even though these patients were significantly insulin resistant compared with controls (P = 0.006). AdipoR1 and AdipoR2 on PBMC were also increased in patients with ESKD (P < 0.05 and P = 0.007, respectively), but levels did not correlate with IR, the HMW isoform or other anthropometric measurements. There was a strong correlation between AdipoR1 and AdipoR2 on PBMC in ESKD and in subcutaneous and visceral fat in controls. However, there was no relationship between AdipoR in PBMC, muscle or visceral fat. Phosphorylation of AMPK by recombinant adiponectin showed that AdipoR on PBMC, from controls and ESKD patients, were equally functional. CONCLUSIONS: IR in ESKD is not explained by the change in isoformic distribution, or by AdipoR down-regulation or dysfunction. Rather, this receptor-ligand axis is up-regulated and may be a beneficial response to the inflammatory milieu of ESKD.