Amino Acids https://doi.org/10.1007/s00726-018-2602-y ORIGINAL ARTICLE Detection of carnosinase‑1 in urine of healthy individuals and patients with type 2 diabetes: correlation with albuminuria and renal function Angelica Rodriguez‑Niño1 · Christina M. Gant2 · Jana D. Braun1 · Xia Li1 · Shiqi Zhang1 · Thomas Albrecht1 · Jiedong Qiu1 · Stephan J. L. Bakker3 · Gozewijn D. Laverman2 · Bernhard K. Krämer1 · Anna Herold1 · Sibylle J. Hauske1 · Benito A. Yard1 Received: 2 March 2018 / Accepted: 9 June 2018 © Springer-Verlag GmbH Austria, part of Springer Nature 2018 Abstract Low serum carnosinase (CN-1) concentrations are associated with low risk for development of diabetic nephropathy (DN) in patients with type 2 diabetes (T2D). Although CN-1 is expressed in the kidney, urinary CN-1 (CNU) excretion and its pathological relevance in patients with T2D have not been investigated to date. The present study therefore assessed the extent of CNU excretion in healthy subjects (n = 243) and in patients with T2D (n = 361) enrolled in the DIAbetes and LifEstyle Cohort Twente-1 (DIALECT-1) in relation to functional renal parameters. CNU was detected in a high propor- tion of healthy individuals, 180 (74%); median CNU excretion was 0.25 mg/24 h [(IQR 0–0.65 mg/24 h]. In patients with T2D the prevalence and extent of CNU increased in parallel with albuminuria (r = 0.59, p < 0.0001; median CNU 0.1 vs 0.2 vs 1.5 mg/24 h, p < 0.0001; prevalence of CNU 61 vs. 81 vs. 97% p < 0.05 in normo- (n = 241), micro- (n = 80) and macroalbuminuria (n = 40), respectively). Patients with estimated glomerular fltration rate (eGFR) < 30 ml/min/1.73 m2 displayed higher median CNU excretion rates in comparison to patients with preserved eGFR (> 90 ml/min/1.73 m2) (1.36 vs 0.13 mg/24 h, p < 0.05). Backward stepwise multivariate linear regression analysis revealed albuminuria, eGFR and gly- cosuria to be independent factors of CNU excretion rates, all together explaining 37% of variation of CNU excretion rates (R2 = 0.37, p < 0.0001). These results show for the frst time that CN-1 can be detected in urine and warrants prospective studies to assess the relevance of CNU for renal function deterioration in diabetes patients. Keywords Carnosinase-1 · Urine · Diabetic nephropathy · Albuminuria Abbreviations CN-1 Carnosinase-1 DN Diabetic nephropathy eGFR Estimated glomerular fltration rate CNU Urinary carnosinase-1 HbA1c Hemoglobin A1c ESRD End-stage renal disease ACEi/ARB Angiotensin converting enzyme inhibitor/ angiotensin receptor blocker Handling Editor: S. Baba. Introduction Angelica Rodriguez-Niño and Christina M. Gant are equally contributing authors. Diabetic nephropathy (DN) is the leading cause of end-stage * Angelica Rodriguez‑Niño renal disease (ESRD) worldwide and imparts an increased angelica.rodriguez@medma‑uni‑heidelberg.de risk for cardiovascular disease and death in patients with 1 Vth Department of Medicine (Nephrology/Endocrinology/ type 1 and type 2 diabetes (Cusick et al. 2005). Rheumatology), University Medical Center Mannheim, Diabetic nephropathy develops insidiously, starting University of Heidelberg, Heidelberg, Germany with microalbuminuria as an early sign of impaired glo- 2 Department of Internal Medicine and Nephrology, ZGT merular fltration. Accompanied by a gradual decline in Hospital, Almelo, Hengelo, The Netherlands renal function, patients may progress to macroalbuminu- 3 Department of Nephrology, University Medical Center ria, overt proteinuria and ultimately end-stage renal disease Groningen, University of Groningen, Groningen, (ESRD) (Ismail et al. 1999; Campbell et al. 2003). Genetic The Netherlands Vol.:(0123456789)1 3 A. Rodriguez-Niño et al. susceptibility to DN has been suggested in numerous studies investigated possible correlations between urinary CN-1 and (Palmer and Freedman 2012). Amongst these, a trinucleotide renal parameters including albuminuria and renal function. repeat (CTG)n polymorphism in the signal sequence of the CNDP1 gene encoding for the protein carnosinase-1 (CN-1) showed a strong association with DN in patients with T2D Results (Janssen et al. 2005; Freedman et al. 2007; Mooyaart et al. 2009; Ahluwalia et al. 2011; Yadav et al. 2016). The (CTG)n CN‑1 is detectable in urine (CNU) polymorphism is functional since it infuences the efcacy of CN-1 secretion from the hepatic cells into the circulation. We have recently reported the detection of CN-1 in serum or The short allelic variant, i.e., (CTG)5, also referred to as the plasma using an in-house developed ELISA system (Adel- Mannheim allele, was found to be more common in T2D mann et al. 2012). To assess if this ELISA is also suitable patients without nephropathy and is associated with lower for detection of CN-1 in urine, a known concentration of serum CN-1 concentrations and enzymatic activities (Jans- recombinant CN-1 (rCN-1) was spiked into spot urine sam- sen et al. 2005; Riedl et al. 2007). Apart from the (CTG)n ples of healthy individuals or into standard phosphate-buf- genotype, hyperglycemia also enhances CN-1 secretion and ered saline (PBS). By comparing the detected and expected enzymatic activity in vitro and in vivo (Riedl et al. 2010; rCN-1 concentration at diferent dilutions, a spike recovery Peters et al. 2012). of nearly 100% was detected for both matrices. Linearity in CN-1 is the rate limiting enzyme for hydrolysis of the his- the dilution curves was observed in a concentration range tidine-containing dipeptide: carnosine (β-alanyl-L-histidine), between 100 and 1000 ng/ml and was similar for urine and anserine (β-alanyl-N-methyl-histidine) and homocarnosine PBS (Fig. 1a). Since CN-1 in rodents is not a secreted pro- (N-(4-amino-butyryl)-L-histidine). In keeping with a pleth- tein, we further validated the specifcity of the CN-1 ELISA ora of benefcial properties reported for carnosine, e.g., by testing serum and urine of human CNDP1-overexpressing ROS scavenging (Nagasawa et al. 2001; Aydın et al. 2010), transgenic BTBR and wild-type mice. inhibition of AGEs formation (Alhamdani et al. 2007) and While CN-1 was clearly detected in serum and urine of anti-glycemic properties (Sauerhöfer et al. 2007; Albrecht CNDP1 transgenic mice, the CN-1 ELISA was completely et al. 2017), carnosine has widely gained scientifc interest negative on serum and urine of wild-type mice (Fig. 1b). as potential food supplement in T2D patients. Based on the The presence of CN-1 in urine of CNDP1 transgenic mice genetic association between the CNDP1 gene and DN and was further confrmed by Western blotting (Fig. 1c). the benefcial efects of carnosine, the hypothesis has been put forward that low CN-1 concentrations/enzymatic activity CNU in healthy subjects may result in higher renal carnosine concentrations aford- ing protection against hyperglycemia-mediated renal damage Although spiking of rCN-1 in PBS or urine gave similar (Boldyrev et al. 2013). results, we observed in some individuals that the measured Although in humans, CN-1 is predominantly produced CN-1 concentrations were signifcantly higher than expected in the liver from where it is secreted into the circulation, at high rCN-1 dilutions when using urine as diluent. Indeed a number of studies have suggested that CN-1 is also the blanks, i.e., urine without spiked CN-1, showed traces expressed in the kidney, albeit to a much lower extent (Jans- of CN-1 in the samples of these individuals. To substanti- sen et al. 2005; Mooyaart et al. 2009; Peters et al. 2015). In ate these fndings we collected 24-h urine of living kidney the kidney, CN-1 expression was mostly found in distal tubu- donors and assessed if, and to what extent, CN-1 is pre- lar cells and podocytes of healthy individuals, but seemed sent in urine of healthy individuals. In 180 (74%) out of to be reallocated to proximal tubuli in diabetic patients with 243 subjects CN-1 was detected in urine. The median CNU DN (Peters et al. 2015). excretion rate was 0.25 mg/24 h (IQR 0–0.65 mg/24 h). No Based on the current knowledge that CN-1 is expressed diferences were detected between females and males. CN-1 in renal tissue and contains a signal peptide, it is conceiv- enzymatic activity was not detected in any of the urine sam- able that CN-1 can be secreted into the tubular lumen and ples (data not shown). consequently be detected in urine. To test this, we validated our ELISA assay for the detection of CN-1 in urine and CNU in patients with T2D subsequently measured CN-1 in 24-h urine collected from healthy subjects (living kidney donors evaluated prior to CNU was studied in a total of 361 type 2 diabetes patients donation) (n = 243) and patients with T2D enrolled in the (T2D) stratifed for 24-h albumin excretion into normoalbu- DIAbetes and LifEstyle Cohort Twente-1 (DIALECT-1) minuria (n = 241), microalbuminuria (n = 80) or macroalbu- (Gant et al. 2017) (n = 361). In the latter group, we also minuria (n = 40). 1 3 Detection of carnosinase-1 in urine of healthy individuals and patients with type 2 diabetes:… Fig. 1 a rCN-1 was spiked in human urine or PBS. CN-1 was in urine of mice overexpressing human CN-1 (CN1+) (n = 9). Note detected by ELISA in serial dilutions using urine or PBS as dilu- that no human CN-1 was detected in serum or urine of wild-type ent. Detected CN-1 concentrations of rCN-1 spiked in urine were mice (CN1−) (n = 9). c ELISA and Western blotting of CN-1 in urine compared with the expected concentrations to estimate spike recov- obtained from three diferent transgenic mice overexpressing human ery %.