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• W2065457079 abstract "Tubulointerstitial disease plays an important role in the pathophysiology of diabetic kidney disease. To determine whether biomarkers of tubular injury could predict renal outcome and mortality in patients with type 2 diabetes, we measured urinary levels of kidney injury molecule-1 (KIM-1) and glycoprotein non-metastatic melanoma B (Gpnmb), both normalized to the urinary creatinine, in 978 individuals from the Edinburgh Type 2 Diabetes Study. At baseline, 238 patients had an estimated glomerular filtration rate (eGFR) below 60ml/min/1.73m2 while 147 and 15 patients had microalbuminuria or overt proteinuria, respectively. Both the urine KIM-1 and Gpnmb to creatinine ratios correlated with the urinary albumin to creatinine ratio, the duration of diabetes, and the stringency of glycemic control but not with blood pressure or baseline eGFR. Higher ratios of each marker were associated with a faster decline in kidney function during 4 years of follow-up; however, this was not independent of the urinary albumin to creatinine ratio. Higher KIM-1, but not Gpnmb ratios were associated with an increased risk of mortality, but this association was no longer significant after adjustment for other risk factors, in particular albuminuria. Thus, tubular injury in persons with type 2 diabetes may contribute to the decline in kidney function; however, measuring the urinary concentration of these two tubular biomarkers does not confer additional prognostic information beyond established risk factors. Tubulointerstitial disease plays an important role in the pathophysiology of diabetic kidney disease. To determine whether biomarkers of tubular injury could predict renal outcome and mortality in patients with type 2 diabetes, we measured urinary levels of kidney injury molecule-1 (KIM-1) and glycoprotein non-metastatic melanoma B (Gpnmb), both normalized to the urinary creatinine, in 978 individuals from the Edinburgh Type 2 Diabetes Study. At baseline, 238 patients had an estimated glomerular filtration rate (eGFR) below 60ml/min/1.73m2 while 147 and 15 patients had microalbuminuria or overt proteinuria, respectively. Both the urine KIM-1 and Gpnmb to creatinine ratios correlated with the urinary albumin to creatinine ratio, the duration of diabetes, and the stringency of glycemic control but not with blood pressure or baseline eGFR. Higher ratios of each marker were associated with a faster decline in kidney function during 4 years of follow-up; however, this was not independent of the urinary albumin to creatinine ratio. Higher KIM-1, but not Gpnmb ratios were associated with an increased risk of mortality, but this association was no longer significant after adjustment for other risk factors, in particular albuminuria. Thus, tubular injury in persons with type 2 diabetes may contribute to the decline in kidney function; however, measuring the urinary concentration of these two tubular biomarkers does not confer additional prognostic information beyond established risk factors. Diabetic kidney disease remains the single most common cause of end-stage renal failure in the western world, accounting for almost half of incident cases in some countries.1.US Renal Data System USRDS 2009 Annual Data Report: Atlas of Chronic Kidney Disease and End-Stage Renal Disease in the United States, National Institutes of Health, National Institute of Diabetes and Digestive and Kidney Diseases, Bethesda, MD2009Google Scholar This is despite improvements in the management of patients with diabetes, which have resulted in a reduction in the prevalence of overt nephropathy after 30 years of type 1 diabetes to <15% in cohort studies,2.Nathan D.M. Zinman B. Cleary P.A. et al.Modern-day clinical course of type 1 diabetes mellitus after 30 years’ duration: the diabetes control and complications trial/epidemiology of diabetes interventions and complications and Pittsburgh epidemiology of diabetes complications experience (1983-2005).Arch Int Med. 2009; 169: 1307-1316Crossref PubMed Scopus (515) Google Scholar and to as little as 5% in clinical trials.2.Nathan D.M. Zinman B. Cleary P.A. et al.Modern-day clinical course of type 1 diabetes mellitus after 30 years’ duration: the diabetes control and complications trial/epidemiology of diabetes interventions and complications and Pittsburgh epidemiology of diabetes complications experience (1983-2005).Arch Int Med. 2009; 169: 1307-1316Crossref PubMed Scopus (515) Google Scholar,3.de Boer I.H. Sun W. Cleary P.A. et al.Intensive diabetes therapy and glomerular filtration rate in type 1 diabetes.N Engl J Med. 2011; 365: 2366-2376Crossref PubMed Scopus (433) Google Scholar Given that only a small proportion of diabetic patients develop overt nephropathy, development of biomarkers that enable identification of those at greatest risk would be useful in targeting therapeutic interventions. Microalbuminuria indicates the presence of early glomerular disease in patients with diabetes and remains the gold standard for predicting progression to overt nephropathy and end-stage renal failure.4.Karalliedde J. Viberti G. Proteinuria in diabetes: bystander or pathway to cardiorenal disease?.J Am Soc Nephrol. 2010; 21: 2020-2027Crossref PubMed Scopus (59) Google Scholar However, not all patients with microalbuminuria will progress to nephropathy, and indeed many will exhibit regression to normoalbuminuria.5.Perkins B.A. Ficociello L.H. Silva K.H. et al.Regression of microalbuminuria in type 1 diabetes.N Engl J Med. 2003; 348: 2285-2293Crossref PubMed Scopus (652) Google Scholar In type 1 diabetes, it has recently been established that markers of tubular injury, such as urinary kidney injury molecule-1 (KIM-1), are elevated in patients with nephropathy6.Nielsen S.E. Andersen S. Zdunek D. et al.Tubular markers do not predict the decline in glomerular filtration rate in type 1 diabetic patients with overt nephropathy.Kidney Int. 2011; 79: 1113-1118Abstract Full Text Full Text PDF PubMed Scopus (84) Google Scholar, 7.Nielsen S.E. Sugaya T. Hovind P. et al.Urinary liver-type fatty acid-binding protein predicts progression to nephropathy in type 1 diabetic patients.Diabetes Care. 2010; 33: 1320-1324Crossref PubMed Scopus (92) Google Scholar, 8.von Eynatten M. Baumann M. Heemann U. et al.Urinary L-FABP and anaemia: distinct roles of urinary markers in type 2 diabetes.Eur J Clin Invest. 2010; 40: 95-102Crossref PubMed Scopus (38) Google Scholar, 9.Vaidya V.S. Niewczas M.A. Ficociello L.H. et al.Regression of microalbuminuria in type 1 diabetes is associated with lower levels of urinary tubular injury biomarkers, kidney injury molecule-1, and N-acetyl-beta-D-glucosaminidase.Kidney Int. 2011; 79: 464-470Abstract Full Text Full Text PDF PubMed Scopus (163) Google Scholar and that they may predict renal outcome. Urinary KIM-1 concentration correlates with the severity of proteinuria, falls in response to renin–angiotensin system blockade, and predicts decline in renal function, although not independently of proteinuria.10.Nielsen S.E. Hansen H.P. Jensen B.R. et al.Urinary neutrophil gelatinase-associated lipocalin and progression of diabetic nephropathy in type 1 diabetic patients in a four-year follow-up study.Nephron Clin Practice. 2011; 118: c130-c135Crossref PubMed Scopus (20) Google Scholar Furthermore, urinary KIM-1 concentration may predict which patients will exhibit regression of microalbuminuria to normoalbuminuria.9.Vaidya V.S. Niewczas M.A. Ficociello L.H. et al.Regression of microalbuminuria in type 1 diabetes is associated with lower levels of urinary tubular injury biomarkers, kidney injury molecule-1, and N-acetyl-beta-D-glucosaminidase.Kidney Int. 2011; 79: 464-470Abstract Full Text Full Text PDF PubMed Scopus (163) Google Scholar It has yet to be determined whether testing urinary KIM-1 concentration can also identify persons with type 2 diabetes, who are at risk of progressive renal disease. In a small cohort with type 2 diabetes, urinary KIM-1 concentration did not correlate with established risk factors such as proteinuria.8.von Eynatten M. Baumann M. Heemann U. et al.Urinary L-FABP and anaemia: distinct roles of urinary markers in type 2 diabetes.Eur J Clin Invest. 2010; 40: 95-102Crossref PubMed Scopus (38) Google Scholar Furthermore, in comparison with type 1 diabetes, the prevalence of normoalbuminuric kidney disease is greater in patients with type 2 diabetes, as other causes of nephropathy such as renovascular disease, essential hypertension, and cardiovascular disease are relatively more common.11.Krams S.M. Fox C.K. Beatty P.R. et al.Human hepatocytes produce an isoform of FAS that inhibits apoptosis.Transplantation. 1998; 65: 713-721Crossref PubMed Scopus (39) Google Scholar Whether markers of tubular injury can predict prognosis in these normoalbuminuric patients has yet to be investigated. Another potential tubular biomarker is glycoprotein non-metastatic melanoma protein B (Gpnmb), a transmembrane glycoprotein expressed on renal tubular cells and on cells of the monocyte–macrophage lineage. We and others have shown that it is elevated in rodent models of kidney disease including diabetic nephropathy12.Conway B.R. Rennie J. Bailey M.A. et al.Hyperglycemia and Renin-Dependent Hypertension Synergize to Model Diabetic Nephropathy.J Am Soc Nephrol. 2011Google Scholar,13.Patel-Chamberlin M. Wang Y. Satirapoj B. et al.Hematopoietic growth factor inducible neurokinin-1 (Gpnmb/Osteoactivin) is a biomarker of progressive renal injury across species.Kidney Int. 2011; 79: 1138-1148Abstract Full Text Full Text PDF PubMed Scopus (17) Google Scholar Interestingly, it is elevated in mice in the reparative phase following renal ischemia–reperfusion injury, and may promote renal repair, and hence be a marker of tubular regeneration.14.Li B. Castano A.P. Hudson T.E. et al.The melanoma-associated transmembrane glycoprotein Gpnmb controls trafficking of cellular debris for degradation and is essential for tissue repair.Faseb J. 2010; 24: 4767-4781Crossref PubMed Scopus (106) Google Scholar The urinary excretion of Gpnmb is elevated in patients with proteinuric renal disease including diabetic nephropathy; however, the ability of urinary Gpnmb concentration to predict renal outcomes has not been assessed.13.Patel-Chamberlin M. Wang Y. Satirapoj B. et al.Hematopoietic growth factor inducible neurokinin-1 (Gpnmb/Osteoactivin) is a biomarker of progressive renal injury across species.Kidney Int. 2011; 79: 1138-1148Abstract Full Text Full Text PDF PubMed Scopus (17) Google Scholar The aims of the current study were to determine whether urinary KIM-1 and Gpnmb concentrations were elevated in patients with type 2 diabetes who had kidney disease, and whether they could predict progression or regression of microalbuminuria, the rate of decline in renal function, and mortality in 978 patients from the Edinburgh Type 2 Diabetes Study (ET2DS), a large-scale (n=1066), prospective, population-based cohort study of persons with type 2 diabetes aged 60–75 years from South-East Scotland. The baseline characteristics of the study population used in the current analysis (978 participants from the ET2DS with baseline urine available for biomarker measurement) are shown in Table 1. In all, 69% of patients had blood pressures of ≤140/80mmHg, whereas 44% and 83% had HbA1c ≤7.0% and ≤8.0%, respectively. A total of 238 (24%) patients had stage 3–5 chronic kidney disease (CKD), of whom the majority (74%) were normoalbuminuric. Both the urinary KIM-1 (uKIM-1)/Cr and urinary Gpnmb (uGpnmb)/Cr ratios correlated with albumin/creatinine ratio (ACR), and even more strongly with each other (Table 2). In addition, both uKIM-1/Cr and uGpnmb/Cr ratios correlated with the duration of diabetes and with the stringency of glycemic control, but not with blood pressure or baseline estimated glomerular filtration rate (eGFR).Table 1Baseline characteristics of the study populationVariable(n=978)Sex (male)515 (52.6%)Age (years)67.8 (±4.2)Duration of diabetes (years)8.1 (±6.5)HbA1c (%)7.4 (±1.1)Systolic blood pressure (mmHg)133 (±16.4)Diastolic blood pressure (mmHg)69 (±8.9)eGFR (ml/min per 1.73m2)71.9 (±17.8) eGFR ≥60740 (75.7%) eGFR 30–59218 (22.3%) eGFR <3020 (2.0%)Albumin:creatinine ratio (mg/mmol)1.17 (0.77–2.02)aMedians (IQR) and n (%). NormoalbuminuriabNormoalbuminuria (ACR<2.5mg/mmol for males and <3.5mg/mmol for females).816 (83.4%) MicroalbuminuriacMicroalbuminuria (ACR 2.5–30 for males and 3.5–30 for females).147 (15.0%) MacroalbuminuriadMacroalbuminuria (ACR>30mg/mmol).15 (1.5%)Urinary KIM-1:creatinine (ng/mmol)12.6 (7.6–22.8)aMedians (IQR) and n (%).Urinary Gpnmb:creatinine ratio (ng/mmol)32.5 (21.3–50.7)aMedians (IQR) and n (%).Abbreviations: ACR, albumin:creatinine ratio; eGFR, estimated glomerular filtration rate; Gpnmb, glycoprotein non-metastatic melanoma B; IQR, interquartile range; KIM-1, kidney injury molecule-1.Values are means (±s.e.m.).a Medians (IQR) and n (%).b Normoalbuminuria (ACR<2.5mg/mmol for males and <3.5mg/mmol for females).c Microalbuminuria (ACR 2.5–30 for males and 3.5–30 for females).d Macroalbuminuria (ACR>30mg/mmol). Open table in a new tab Table 2Correlation of urinary KIM/creatinine, Gpnmb/creatinine, and albumin/creatinine ratios with each other, and with other baseline clinical variablesLog KIM-1/Cr ratioLog Gpnmb/Cr ratioLog ACRAgeDuration of diabetesHbA1cSystolic BPDiastolic BPBaseline eGFRLog uKIM-1/Cr ratio—0.62 (0.59,0.66) P<0.00010.31 (0.25,0.36) P<0.00010.10 (0.04,0.16) P=0.0020.09 (0.03,0.15) P=0.0040.09 (0.02,0.15) P=0.007n.s.n.s.n.s.Log uGpnmb/Cr ratio0.62 (0.59–0.66) P<0.0001—0.40 (0.35,0.46) P<0.00010.15 (0.09,0.22) P<0.00010.12 (0.05,0.18) P=0.00030.11 (0.04,0.17) P=0.0010.073 (0.00,0.13) P=0.04n.s.n.s.Log ACR0.31 (0.25,0.36) P<0.00010.40 (0.35,0.46) P<0.0001—0.08 (0.02,0.14) P=0.010.15 (0.09,0.20) P<0.00010.17 (0.10,0.22) P<0.00010.134 (0.1,0.22) P<0.0001-0.05 (-0.11,-0.01) P=0.002-0.20 (-0.26,-0.14) P<0.0001Abbreviations: ACR, urinary albumin:creatinine ratio; BP, blood pressure; eGFR, estimated glomerular filtration rate; n.s., not significant; uGpnmb/Cr, urinary glycoprotein non-metastatic melanoma B:creatinine ratio; uKIM-1/Cr, urinary kidney injury molecule-1:creatinine ratio.Values are Pearson's correlation coefficients (95% CI) and corresponding P-values. Open table in a new tab Abbreviations: ACR, albumin:creatinine ratio; eGFR, estimated glomerular filtration rate; Gpnmb, glycoprotein non-metastatic melanoma B; IQR, interquartile range; KIM-1, kidney injury molecule-1. Values are means (±s.e.m.). Abbreviations: ACR, urinary albumin:creatinine ratio; BP, blood pressure; eGFR, estimated glomerular filtration rate; n.s., not significant; uGpnmb/Cr, urinary glycoprotein non-metastatic melanoma B:creatinine ratio; uKIM-1/Cr, urinary kidney injury molecule-1:creatinine ratio. Values are Pearson's correlation coefficients (95% CI) and corresponding P-values. Follow-up ACR data were available for 945 subjects (97% of the study population). During the course of the study, 103 (13%) persons who were normoalbuminuric at baseline developed microalbuminuria or overt proteinuria. When compared with persons with sustained normoalbuminuria, those who subsequently developed microalbuminuria/proteinuria had higher baseline ACR (1.4 vs. 0.99mg/mmol, P<0.0001), but not significantly higher uKIM-1/Cr (11.7 vs. 11.5ng/mmol, P=0.66) or uGpnmb/Cr (33.5 vs. 31.3ng/mmol, P=0.35) ratios. Conversely, 52 (35%) patients who had microalbuminuria at baseline exhibited regression to normoalbuminuria. The median baseline ACR was lower in those who exhibited regression than in those with sustained microalbuminuria (4.10 vs. 6.99mg/mmol, P=0.0001), whereas there were no differences in the median uKIM-1/Cr (22.2 vs. 19.1ng/mmol, P=0.12) or uGpnmb/Cr ratios (43.9 vs. 41.4ng/mmol, P=0.99). Data on renal function during follow-up were available for 965 (99%) patients with a mean follow-up period of 3.6years. The mean (±s.e.m.) rate of decline in renal function during follow-up was -0.35 (±3.87)ml/min per 1.73m2 per year. Lower baseline HbA1c and systolic blood pressure were independently associated with a slower rate of decline in renal function (Table 3), whereas higher baseline ACR was an independent risk factor for a more rapid deterioration in renal function (Table 3, Figure 1). Higher uKIM-1/Cr and uGpnmb/Cr ratios were associated with a faster decline in renal function on univariate analysis (Table 3, Figure 1). However, the association was no longer significant following adjustment for known risk factors, such as baseline ACR, HbA1c, systolic blood pressure, and eGFR (Table 3).Table 3Baseline variables associating with rate of change in renal function by univariate and multivariate analysisUnivariateMultivariateVariableβ (s.e.)P-valueβ (s.e.)P-valueDuration of diabetes-0.06 (0.02)0.001n.s.HbA1c-0.49 (0.11)<0.0001-0.57 (0.14)<0.0001Systolic BP-0.03 (0.01)<0.0001-0.03 (0.01)0.004Baseline eGFR-0.03 (0.01)<0.0001-0.02 (0.07)0.03Log ACR-1.43 (0.30)<0.0001-1.60 (0.40)<0.0001Log uKIM-1/Cr ratio-1.08 (0.31)<0.001n.s.Log uGpnmb/Cr ratio-0.96 (0.45)0.03n.s.Abbreviations: ACR, urinary albumin:creatinine ratio; eGFR, estimated glomerular filtration rate; n.s., not significant; uGpnmb/Cr, urinary glycoprotein non-metastatic melanoma B:creatinine ratio; uKIM-1/Cr, urinary kidney injury molecule-1:creatinine ratio.Values given are regression coefficients (s.e.).Gender, age, duration of diabetes, and baseline HbA1c, systolic and diastolic blood pressure, eGFR, log ACR, log uKIM-1/Cr, log uGpnmb/Cr were entered into the multivariate analysis and serially removed until only those that remained independent risk factors were included in the final analysis. Open table in a new tab Abbreviations: ACR, urinary albumin:creatinine ratio; eGFR, estimated glomerular filtration rate; n.s., not significant; uGpnmb/Cr, urinary glycoprotein non-metastatic melanoma B:creatinine ratio; uKIM-1/Cr, urinary kidney injury molecule-1:creatinine ratio. Values given are regression coefficients (s.e.). Gender, age, duration of diabetes, and baseline HbA1c, systolic and diastolic blood pressure, eGFR, log ACR, log uKIM-1/Cr, log uGpnmb/Cr were entered into the multivariate analysis and serially removed until only those that remained independent risk factors were included in the final analysis. In patients with normoalbuminuria at baseline, a high uKIM-1/Cr ratio, but not uGpnmb/Cr ratio, was associated with a faster decline in renal function (β=0.80, P<0.05), but this was no longer significant following correction for established risk factors such as HbA1c or systolic blood pressure (P>0.05). During the follow-up period, there were 75 deaths, comprising 8% of the study population. Older patients and those with lower baseline eGFR and higher baseline ACR were at increased risk of mortality (Table 4). Although higher uKIM-1/Cr ratios were associated with an increased risk of mortality (P<0.05), the association did not remain statistically significant following adjustment for age, glycemic control, and ACR (P>0.05).Table 4Cox proportional hazards for time to death according to baseline variablesUnivariateMultivariateVariableHazard ratio (95% CI)P-valueHazard ratio (95% CI)P-valueAge (years)1.11 (1.05–1.17)<0.0011.07 (1.01–1.14)0.01Diastolic BP (mmHg)0.97 (0.95–1.00)<0.05n.s.eGFR (ml/min per 1.73m2)0.97 (0.96–0.98)<0.00010.98 (0.97–0.99)<0.01Log ACR2.58 (1.75–3.8)<0.00012.00 (1.33–3.02)<0.001Log uKIM-1/Cr1.68 (1.00–2.83)<0.05n.s.Abbreviations: ACR, urinary albumin:creatinine ratio; CI, confidence intervals; eGFR, estimated glomerular filtration rate; n.s., not significant; uKIM-1/Cr, urinary kidney injury molecule-1:creatinine ratio.Hazard ratios given are for every 1yr increase in age, 1mmHg increase in blood pressure, 1ml/min per 1.73m2 increase in eGFR, unit log increase in ACR or uKIM-1/Cr ratio.Gender, age, duration of diabetes, baseline HbA1c, systolic and diastolic blood pressure, eGFR, log ACR, log uKIM-1/Cr, log uGpnmb/Cr were entered into the multivariate analysis and serially removed until only those that remained independent risk factors for death were included in the final analysis. Open table in a new tab Abbreviations: ACR, urinary albumin:creatinine ratio; CI, confidence intervals; eGFR, estimated glomerular filtration rate; n.s., not significant; uKIM-1/Cr, urinary kidney injury molecule-1:creatinine ratio. Hazard ratios given are for every 1yr increase in age, 1mmHg increase in blood pressure, 1ml/min per 1.73m2 increase in eGFR, unit log increase in ACR or uKIM-1/Cr ratio. Gender, age, duration of diabetes, baseline HbA1c, systolic and diastolic blood pressure, eGFR, log ACR, log uKIM-1/Cr, log uGpnmb/Cr were entered into the multivariate analysis and serially removed until only those that remained independent risk factors for death were included in the final analysis. To our knowledge, this is the first study to examine the role of tubular biomarkers in predicting decline of renal function in patients with type 2 diabetes. The key findings are that uKIM-1/Cr and uGpnmb/Cr ratios correlate with the severity of proteinuria, and that higher uKIM-1/Cr and uGpnmb/Cr ratios are associated with a faster decline in renal function. However, neither uKIM-1/Cr nor Gpnmb/Cr ratios provided additional prognostic information beyond measurement of ACR alone. In addition, higher uKIM-1/Cr, but not Gpnmb/Cr, ratios were associated with an increased risk of mortality, although this association was no longer significant following adjustment for established risk factors. In contrast to a smaller study in persons with incipient type 2 diabetic nephropathy,8.von Eynatten M. Baumann M. Heemann U. et al.Urinary L-FABP and anaemia: distinct roles of urinary markers in type 2 diabetes.Eur J Clin Invest. 2010; 40: 95-102Crossref PubMed Scopus (38) Google Scholar the uKIM-1/Cr ratio correlated strongly with albuminuria, as did the uGpnmb/Cr ratio. This is in keeping with studies in type 1 diabetes,6.Nielsen S.E. Andersen S. Zdunek D. et al.Tubular markers do not predict the decline in glomerular filtration rate in type 1 diabetic patients with overt nephropathy.Kidney Int. 2011; 79: 1113-1118Abstract Full Text Full Text PDF PubMed Scopus (84) Google Scholar,9.Vaidya V.S. Niewczas M.A. Ficociello L.H. et al.Regression of microalbuminuria in type 1 diabetes is associated with lower levels of urinary tubular injury biomarkers, kidney injury molecule-1, and N-acetyl-beta-D-glucosaminidase.Kidney Int. 2011; 79: 464-470Abstract Full Text Full Text PDF PubMed Scopus (163) Google Scholar and suggests that there is a strong interrelationship between disease in the glomerular and tubulointerstitial compartments of the diabetic kidney, although the specific nature of this interaction cannot be determined from observational studies alone. It is possible that excess filtered albuminuria due to glomerular damage may cause tubular injury.15.Tang S.C. Leung J.C. Lai K.N. Diabetic tubulopathy: an emerging entity.Contr Nephrol. 2011; 170: 124-134Crossref PubMed Scopus (92) Google Scholar Conversely, it has been hypothesized that tubular dysfunction may be an initiating factor in diabetic kidney disease, as in normoalbuminuric persons with type 1 diabetes, an elevated urinary concentration of another tubular biomarker, liver-type fatty acid–binding protein, precedes the onset of microalbuminuria.7.Nielsen S.E. Sugaya T. Hovind P. et al.Urinary liver-type fatty acid-binding protein predicts progression to nephropathy in type 1 diabetic patients.Diabetes Care. 2010; 33: 1320-1324Crossref PubMed Scopus (92) Google Scholar Although higher uKIM-1/Cr and Gpnmb/Cr ratios did not predict subsequent microalbuminuria in the current study, it may be that elevated tubular biomarkers in patients with type 2 diabetes and normoalbuminuria represent tubular injury secondary to diseases other than diabetic nephropathy, such as renovascular disease, essential hypertension, or other cardiovascular comorbidities, in which case they would not be expected to precede microalbuminuria. Indeed, although over 20% of the patients in the current study had stage 3–5 CKD, approximately 75% of these remained normoalbuminuric, a proportion similar to that observed previously,16.Kramer H.J. Nguyen Q.D. Curhan G. et al.Renal insufficiency in the absence of albuminuria and retinopathy among adults with type 2 diabetes mellitus.JAMA. 2003; 289: 3273-3277Crossref PubMed Scopus (477) Google Scholar suggesting that classical proteinuric diabetic nephropathy was not responsible for the majority of cases of CKD. We had hypothesized that markers of tubular dysfunction may confer additional diagnostic and prognostic information to ACR for two reasons. First, they reflect damage in the tubulointerstitial rather than the glomerular compartment, and second, they may indicate the presence of active injury rather than structural alterations due to previous insults.17.Mori K. Nakao K. Neutrophil gelatinase-associated lipocalin as the real-time indicator of active kidney damage.Kidney Int. 2007; 71: 967-970Abstract Full Text Full Text PDF PubMed Scopus (330) Google Scholar The elevated uKIM-1/Cr and Gpnmb/Cr ratios in patients with incipient and overt nephropathy suggest that there is ongoing tubular injury. Whether this has a major pathophysiological role in the deterioration of renal function cannot be determined from the current study; however, the fact that higher uKIM-1/Cr and Gpnmb/Cr ratios are associated with a more rapid deterioration in renal function would support this possibility. In addition, it would be consistent with renal biopsy studies in which the degree of tubulointerstitial damage predicted renal outcome.18.Gilbert R.E. Cooper M.E. The tubulointerstitium in progressive diabetic kidney disease: more than an aftermath of glomerular injury?.Kidney Int. 1999; 56: 1627-1637Abstract Full Text Full Text PDF PubMed Scopus (563) Google Scholar The association between higher uKIM-1/Cr or uGpnmb/Cr ratios and a greater rate of decline in renal function did not remain significant following adjustment for standard prognostic markers, in particular albuminuria, as has been observed in type 1 diabetes.6.Nielsen S.E. Andersen S. Zdunek D. et al.Tubular markers do not predict the decline in glomerular filtration rate in type 1 diabetic patients with overt nephropathy.Kidney Int. 2011; 79: 1113-1118Abstract Full Text Full Text PDF PubMed Scopus (84) Google Scholar Similarly, the association between higher uKIM-1/Cr ratios and mortality was not independent of clinical parameters such as glycemic control and ACR. This suggests that tubular biomarkers may not confer any additional prognostic benefit beyond established risk markers, and therefore, screening for tubular biomarkers in persons with type 2 diabetes cannot be recommended for prognostic use in routine clinical practice. Although our results suggest that albuminuria remains the gold standard as a prognostic marker in type 2 diabetes, the majority of patients with declining renal function remained normoalbuminuric. We had hypothesized that these patients may have had predominantly tubulointerstitial disease and that tubular biomarkers could therefore be of prognostic benefit in normoalbuminuric patients. Higher urinary KIM-1/creatinine ratios did indeed predict a faster decline in renal function in this subset of patients; however, the association was weak and was not independent of routine clinical parameters such as systolic blood pressure or HbA1c. Therefore, quantifying uKIM-1 concentration is an unlikely aid in risk stratification in normoalbuminuric patients. Although it has previously been reported that the expression of Gpnmb is increased in rodents and patients with diabetic nephropathy,12.Conway B.R. Rennie J. Bailey M.A. et al.Hyperglycemia and Renin-Dependent Hypertension Synergize to Model Diabetic Nephropathy.J Am Soc Nephrol. 2011Google Scholar,13.Patel-Chamberlin M. Wang Y. Satirapoj B. et al.Hematopoietic growth factor inducible neurokinin-1 (Gpnmb/Osteoactivin) is a biomarker of progressive renal injury across species.Kidney Int. 2011; 79: 1138-1148Abstract Full Text Full Text PDF PubMed Scopus (17) Google Scholar this is the first assessment of its utility as a prognostic biomarker. The uGpnmb/Cr ratio did not predict mortality, and the association between the uGpnmb/Cr ratio and the rate of change in renal function was less strong than for either the uKIM-1/Cr ratio or ACR; therefore, it cannot be recommended as a prognostic marker in type 2 diabetes. It has been suggested that Gpnmb has an important role in tubular regeneration following injury, as tubular Gpnmb expression is not increased immediately following ischemia–reperfusion injury but later during the resolution phase.14.Li B. Castano A.P. Hudson T.E. et al.The melanoma-associated transmembrane glycoprotein Gpnmb controls trafficking of cellular debris for degradation and is essential for tissue repair.Faseb J. 2010; 24: 4767-4781Crossref PubMed Scopus (106) Google Scholar Therefore, in the setting of a chronic disease, such as diabetic kidney disease, the elevated uGpnmb:Cr ratio may reflect an attempt at renal repair amidst ongoing injury. Indeed, a reduced Gpnmb/KIM-1 ratio was associated with a greater rate of decline in renal function (P<0.05, data not shown), suggesting that a failure to mount a regenerative response may confer an increased risk of nephropathy. To our knowledge, this is the largest study of the use of tubular biomarkers in diabetic kidney disease. A major strength of the study is that it is representative of the general population of persons with type 2 diabetes, and therefore, it is applicable to general practice and not subject to referral bias. Conversely, it was not enriched for patients with pre-existing diabetic nephropathy, and it is possible that tubular biomarkers may have greater prognostic utility in this subset of patients, although this was not observed in type 1 diabetes.6.Nielsen S.E. Andersen S. Zdunek D. et al.Tubular markers do not predict the decline in glomerular filtration rate in type 1 diabetic patients with overt nephropathy.Kidney Int. 2011; 79: 1113-1118Abstract Full Text Full Text PDF PubMed Scopus (84) Google Scholar Furthermore, although the follow-up is longer than in previous studies,6.Nielsen S.E. Andersen S. Zdunek D. et al.Tubular markers do not predict the decline in glomerular filtration rate in type 1 diabetic patients with overt nephropathy.Kidney Int. 2011; 79: 1113-1118Abstract Full Text Full Text PDF PubMed Scopus (84) Google Scholar,9.Vaidya V.S. Niewczas M.A. Ficociello L.H. et al.Regression of microalbuminuria in type 1 diabetes is associated with lower levels of urinary tubular injury biomarkers, kidney injury molecule-1, and N-acetyl-beta-D-glucosaminidase.Kidney Int. 2011; 79: 464-470Abstract Full Text Full Text PDF PubMed Scopus (163) Google Scholar in the DCCT trial, it took almost a decade for the effect of intensive glycemic control in preventing a decline in renal function to become apparent;3.de Boer I.H. Sun W. Cleary P.A. et al.Intensive diabetes therapy and glomerular filtration rate in type 1 diabetes.N Engl J Med. 2011; 365: 2366-2376Crossref PubMed Scopus (433) Google Scholar therefore, it may take many years before low-grade tubular injury translates into a decline in eGFR. A further limitation of the current study is that the decline in renal function was calculated from just two estimates of GFR concentration at baseline and after 4 years of follow-up. Some of the fluctuations in eGFR may reflect acute kidney injury at the time of blood sampling or changes in medications between visits. We used the Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) equation rather than Modification of Diet in Renal Disease equation to estimate GFR, as it is more accurate at the higher levels of renal function observed in the majority of patients in the current study.19.Levey A.S. Stevens L.A. Schmid C.H. et al.A new equation to estimate glomerular filtration rate.Ann Int Med. 2009; 150: 604-612Crossref PubMed Scopus (15959) Google Scholar However, both equations are based on serum creatinine measurements, and therefore, fluctuations in CKD-EPI eGFR may be influenced by changes in muscle mass. Inaccuracies in determining the true rate of decline in renal function may underestimate the prognostic performance of a biomarker, as has been observed when using serum creatinine to diagnose acute kidney injury.20.Waikar S.S. Betensky R.A. Emerson S.C. Bonventre J.V. Imperfect gold standards for kidney injury biomarker evaluation.J Am Soc Nephrol. 2012; 23: 13-21Crossref PubMed Scopus (205) Google Scholar However, our results are in keeping with those obtained in type 1 diabetes using serial measurements of ‘true’ GFR as determined by isotope clearance.6.Nielsen S.E. Andersen S. Zdunek D. et al.Tubular markers do not predict the decline in glomerular filtration rate in type 1 diabetic patients with overt nephropathy.Kidney Int. 2011; 79: 1113-1118Abstract Full Text Full Text PDF PubMed Scopus (84) Google Scholar In addition, little is known about the intra-day or day-to-day variability of urinary tubular markers. Although we determined the concentration of tubular markers in a single early morning urine sample, it may be that a 24-h urine collection may provide a more accurate assessment of KIM-1 excretion, although this approach is unlikely to be practical for screening purposes in the context of diabetic clinics. Alternatively, serial spot measurements may overcome problems with variability in a manner similar to microalbuminuria being defined from the results of two out of three successive readings. It is possible that concurrent hypertensive therapy may influence the urinary concentration of tubular biomarkers and hence their prognostic utility; however, including the presence/absence of antihypertensive therapy at baseline as a categorical value into the multivariable analysis had no significant impact on the ability of tubular markers to predict either rate of decline in renal function or mortality (date not shown). In conclusion, in a large, prospective, population-based study, higher uKIM-1/Cr and uGpnmb/Cr ratios were associated with an increased rate of decline in renal function over 4 years of follow-up, suggesting that tubular damage makes an important contribution to kidney dysfunction in patients with type 2 diabetes. In addition, higher uKIM-1/Cr, but not uGpnmb/Cr ratios, conferred a higher risk of mortality. However, neither the uKIM-1/Cr nor the uGpnmb/Cr ratios independently predicted the rate of decline in renal function or mortality after adjusting for classical risk factors, in particular albuminuria, and therefore, routine testing of uKIM-1/Cr or uGpnmb/Cr ratios is unlikely to confer additional prognostic information in patients with type 2 diabetes. The ET2DS is a prospective cohort study of 1066 men and women aged 60–75years with established type 2 diabetes, living in the Lothian region of Central Scotland, and recruited in 2006–2007. The recruitment and baseline examination protocol have been described in detail previously.21.Price J.F. Reynolds R.M. Mitchell R.J. et al.The Edinburgh Type 2 Diabetes Study: study protocol.BMC Endo Disord. 2008; 8: 18Crossref PubMed Scopus (57) Google Scholar Briefly, to establish a representative sample of all persons with type 2 diabetes in the region, patients from the Lothian Diabetes Register recorded as having type 2 diabetes were randomly selected by sex and 5-year age bands to participate in the study. Of the 5454 patients who were invited to participate, 1066 persons were recruited to the study, and these have been previously demonstrated to be representative of all those who were invited to participate and therefore of the target population of older men and women with type 2 diabetes living in the Lothian region.22.Marioni R.E. Strachan M.W. Reynolds R.M. et al.Association between raised inflammatory markers and cognitive decline in elderly people with type 2 diabetes: the Edinburgh Type 2 Diabetes Study.Diabetes. 2010; 59: 710-713Crossref PubMed Scopus (137) Google Scholar A total of 978 (92%) subjects had a baseline urine sample available for testing urinary biomarkers (the remaining samples were not stored following urinary measurements in the baseline clinics), and these subjects formed the study population for the current analysis. The ET2DS was approved by the Lothian Medical Research Ethics Committee. All subjects gave written informed consent, including for follow-up and review of medical records. At the baseline clinic visit, after an overnight fast, all participants underwent venepuncture and provided an early morning urine sample. The physical examination was completed by one of six trained nurses using standardized operating procedures, and included blood pressure measurement using a sphygnomanometer. From the venous blood sample, HbA1c and isotope dilution mass spectrometry–traceable serum creatinine levels were measured according to standard protocols in the Department of Biochemistry, Western General Hospital, Edinburgh, UK. The CKD-EPI equation was used to estimate the GFR, as this has been shown to provide a more accurate estimate than the four-variable Modification of Diet in Renal Disease equation in patients with normal levels of renal function.19.Levey A.S. Stevens L.A. Schmid C.H. et al.A new equation to estimate glomerular filtration rate.Ann Int Med. 2009; 150: 604-612Crossref PubMed Scopus (15959) Google Scholar Albumin and creatinine concentrations were determined using fresh urine samples according to standard protocols in the Department of Biochemistry, Western General Hospital, Edinburgh, UK. Urine was then frozen at -20°C until subsequent measurement of uKIM-1 and uGpnmb concentrations was recorded using the enzyme-linked immunosorbent assay kits (R&D systems, Minneapolis, MN) according to the manufacturer's instructions. The urinary KIM-1 and Gpnmb concentrations were then adjusted for urinary concentration by expressing them relative to the urinary creatinine concentration. Mortality during follow-up was assessed using a combination of relative/family doctor notification and linkage to death records. Subjects who were still alive were invited to re-attend the research clinic 4 years after their initial visit. GFR was again estimated from serum isotope dilution mass spectrometry–traceable creatinine results using the CKD-EPI equation. Of the 978 persons from the ET2DS study population, who had baseline urine samples available for testing urinary biomarkers, 75 (8%) had died within 4 years and 137 (14%) were alive, but did not report for their 4-year follow-up visit. For these 212 (21.7%) persons, the Lothian laboratory database was interrogated to estimate GFR by the CKD-EPI equation, based on the isotope dilution mass spectrometry–traceable serum creatinine concentration at their last outpatient clinic visit. In this way, we excluded rises in creatinine occurring as a result of an acute illness. The rate of change in renal function was determined by subtracting the Year 4 eGFR (or last outpatient eGFR for those not attending the Year 4 visit) from the baseline eGFR, and dividing by the time between measurements. For the entire study population of 978 subjects, the Lothian laboratory database was also used to determine the ACR at each clinic visit during follow-up. New-onset microalbuminuria was defined as occurring in patients who were normoalbuminuric at baseline, but who subsequently had an ACR>2.5 for men or >3.5 for women in at least two of three consecutive readings at any point during the follow-up period. Conversely, regression of microalbuminuria was defined as the presence of microalbuminuria at the baseline research clinic measurement, with a subsequent reversion to normoalbuminuria (ACR≤2.5 for men and ≤3.5 for women on at least two of three consecutive readings) at any time during the follow-up period. Data are expressed as means (±s.d.) and medians (interquartile range), where the data are parametric (age, duration of diabetes, HbA1c, systolic and diastolic blood pressure, baseline, and change in eGFR), and nonparametric (ACR, uKIM-1/Cr, uGpnmb/Cr), respectively. Nonparametric variables were log-transformed before further analyses. Comparison of variables between groups was performed using Student's t-test. The extent of correlation between baseline variables was assessed by Pearson's correlation coefficient. Association between baseline variables and rate of change in renal function was assessed by univariate and multivariate linear regression analysis. The risk of death conferred by baseline variables was determined by Cox proportional hazards. Variables entered into the linear regression and Cox proportional hazards models included sex, age, duration of diabetes, HbA1c, systolic and diastolic blood pressure, baseline eGFR, whether patients were prescribed antihypertensive medications at the time of the baseline measurements, ACR, uKIM-1/Cr, and uGpnmb/Cr, with only the variables that remained independently significant being included in the final multivariate analyses. A P-value of <0.05 was considered significant, and analyses were performed using the GraphPad PRISM (version 4.00 for Windows, GraphPad Software, San Diego, CA) or the Statview (version 5.0 for Windows, SAS Institute, San Francisco, CA) statistical programs. The Edinburgh Type 2 Diabetes Study was supported by a grant from the Medical Research Council UK. We thank the participants and staff of the ET2DS and the staff at the Wellcome Trust Clinical Research Facility, Edinburgh, where the study was performed. BRC was supported by a Transition Fellowship Award from the British Heart Foundation and by a bursary from the Renal Research Fund of the Edinburgh and Lothians Health Foundation." @default.
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