Background
Autosomal dominant polycystic kidney disease (ADPKD) is the most prevalent inherited form of kidney disease and the fourth leading cause of end-stage renal disease (ESRD) in the US [
1,
2]. ADPKD occurs in one of every 400 live births and is characterized by bilateral, progressive enlargement of focal cysts in all nephron segments. Onset and morbidity often occurs in children and adolescents and leads to numerous complications including hypertension, pain, hematuria, proteinuria, kidney stones and declining renal function. Direct medical costs for dialysis and transplantation to treat ADPKD are estimated at 1.5 billion U.S. dollars per year [
2]. Current therapies are directed at controlling hypertension, pain and the complications of chronic kidney disease but do not prevent progression of ADPKD [
3].
Endothelin-1 (ET-1), an endothelium-derived vasoconstrictor and pro-fibrotic peptide [
4‐
6], has emerged as a possible therapeutic target for inhibiting cyst growth and interstitial fibrosis in ADPKD. Transgenic overexpression of ET-1 in mice causes extensive cyst formation in the kidney with glomerular and peritubular fibrosis and monocyte infiltration without increasing blood pressure [
7]. Renal expression of ET-1 is robustly elevated in murine and rat models of PKD and is associated with cyst formation and the development of hypertension [
8‐
10]. In humans with ADPKD, immunoreactive ET-1 has been localized in cyst epithelia, mesangial cells and vascular smooth muscle cells [
11]. Compared to healthy age-matched controls, ET
A mRNA is 5-10-fold higher in ADPKD cystic kidneys. In a small cohort of patients with all forms of PKD (
n = 20, ESCAPE Trial Group), urine ET-1 excretion was elevated compared to control subjects without chronic kidney disease [
12].
Synthesis of ET-1 by kidney cells is the primary source of urinary ET-1; < 1.0 % of ET-1 in urine derives from the filtered load [
13‐
17]. Thus urinary ET- is a non-invasive surrogate for ET-1 in kidney tissue that avoids the risk and cost associated with immunohistochemical analysis in percutaneous renal biopsy. Here, we investigated the hypothesis that elevated levels of urine ET-1 are associated with decreased eGFR and increased total kidney volume in a pilot study of outpatients with ADPKD.
Discussion
Abundant preclinical data suggest that elevated ET-1 promotes cyst growth and renal fibrosis in ADPKD [
7‐
10]. Here we report that increased ET-1 was associated with decreased eGFR and may be correlated with elevated TKV in patients with ADPKD. ET-1 correlated with a marker of tubular damage in ADPKD, NAGase, but not with systolic or diastolic blood pressure. Taken together with data from experimental models, these results support the hypothesis that ET-1 is elevated in patients with ADPKD is association with renal insufficiency.
Grenda et al. [
12] reported a 1.6-fold increase in urinary ET-1 excretion compared to controls in 20 children (age 10.2
± 3.6 years) with polycystic kidney disease. In the present study, we significantly expand on these results by showing that urine ET-1 is elevated 3.2-fold in adults with ADPKD compared to age- and gender-matched controls. Urinary excretion of ET-1 in ADPKD was significantly and negatively associated with eGFR (
r = −0.480,
P = 0.026), and appeared to be positively associated with increased excretion of urine albumin, consistent with a role in progressive renal injury and dysfunction. Of note, urine ET-1 did not correlate with hypertension, which further supports results in experimental models demonstrating a blood pressure-independent action of ET-1 in the pathogenesis of ADPKD [
7,
10].
Elevated ET-1 has correlated with scarring and chronic disease in a variety of preclinical models of kidney disease [
10,
11,
29,
30], thus we asked whether ET-1 correlates with urine NAGase in ADPKD. ET-1 correlated strongly with NAGase (
r = 0. 687,
P = 0.001). These findings may be significant because ADPKD is characterized histologically by injury to tubules that often precedes a decline in GFR [
1]. NAGase, distributed mainly in lysozymes of the proximal tubule and cyst-lining epithelial cells in ADPKD patients [
26,
31], is shed into urine upon damage to tubular epithelial cells [
27]. In a cross-sectional analysis, urine NAGase was 7.8-fold higher in 102 participants with ADPKD compared to 102 age- and sex-matched healthy controls [
32]. Moreover, urine NAGase predicted patients with progressive decline of GFR (area under the receiver operating characteristic curve = 0.794) in a 1-year prospective cohort study of 270 patients with ADPKD [
31]. Consistent with previous studies by Meijer et al. [
32] and Park et al. [
31], we also observed that NAGase correlated inversely with eGFR and positively with total kidney volume. Collectively, these data are consistent with a role for ET-1 as a mediator of tubular damage in ADPKD.
Progression of ADPKD stems primarily from formation and growth of renal cysts [
33,
34]. Cyst formation, due to localized epithelial cell proliferation, results in renal function decline secondary to obstruction of the tubules in which cysts grow. Cyst growth is also thought to be responsible for subsequent upstream blockage, downstream tubular atrophy and, eventually, glomerulosclerosis. Thus, while cyst formation is an early indicator of disease progression, in the later stages, fibrosis is the dominant factor for renal decline [
34]. Current therapies are directed at controlling hypertension, pain and the complications of chronic kidney disease but do not prevent progression of ADPKD [
3]. In addition, standard measures of renal disease progression, including estimated or measured glomerular filtration rates (GFR), do not show changes until significant damage has occurred. Thus, there is an urgent need for non-invasive biomarkers to identify ADPKD patients at high risk for progression before significant renal insufficiency occurs. Those patients, at an early stage of disease, could then be targeted for therapies as new approaches emerge. It is also possible that ET-1 may be a therapeutic target in ADPKD. In a previous study, an ET
B receptor antagonist (A-192621) accelerated disease progression in a murine model of ADPKD [
35]. However, recent studies demonstrate differential effects of ET
A and ET
B receptors in progression of kidney disease and favor a reno-protective effect of ET
A receptor antagonists [
36], so studies are necessary to determine efficacy of ET-1 receptor blockers in ADPKD.
Conclusions
Urine ET-1 is associated inversely with eGFR independent of age, sex and blood pressure in a pilot, cross-sectional study. Potential significance of our findings are that ET-1 may be a novel therapeutic target for slowing progression of kidney disease in ADPKD. Development of orally-active receptor antagonists for ET
A and/or ET
B receptor is in early stages, but these drugs have been approved recently for treatment of pulmonary artery hypertension and scleroderma-related digital ulcers. Multiple clinical studies point to efficacy of ET
A receptor antagonists in diabetic and non-diabetic chronic kidney disease [
37,
38]. To our knowledge ET
A or ET
B antagonists have not been studied in trials of ADPKD. Another possibility is that elevated urinary excretion of ET-1 may be a biomarker of early renal injury. Our findings suggest that additional studies of ET-1 and ET-1 receptor antagonists in ADPKD, particularly longitudinal trials, may be warranted.
Competing interests
The authors declare that they have no competing interests.
Authors’ contributions
RR, LL, BB, BV, RC and KD recruited participants. PV and KH analyzed MRI images. ASD and MSS performed statistical analysis, and all authors wrote the manuscript and provided critical review. All authors read and approved the final manuscript.