Although MA has been proposed to be an early predictive biomarker of DN, it is clear that in the majority of patients MA can regress to normoalbuminuria and in a minority it progresses to proteinuria.5
Although MA has generally been attributed to glomerular injury, nephrotoxicity studies in animals reveal that albuminuria is a sensitive marker of early tubular toxicity.12
It is therefore possible that the early MA observed in many patients with type 1 diabetes may be partially due to tubular injury resulting from hyperglycemia and other metabolic factors. The degree of tubular injury may be associated with more favorable albuminuria outcome. We evaluated biomarkers that specifically measure tubular cell injury with the hypothesis that this injury may have a role in the development and progression/regression of MA in type 1 diabetes.
Using the clinical and albuminuria data from the Second Joslin Study, we found, in a cross-sectional comparison, that urinary levels of tubular injury biomarkers KIM-1 and NAG were significantly elevated in patients with type 1 diabetes and MA in comparison with diabetics with normoalbuminuria and nondiabetic healthy controls. Low urinary KIM-1 and NAG at baseline were strongly associated with regression of MA during a 2-year follow-up and the effect was independent of clinical characteristics.
To the best of our knowledge this is the first prospective study showing the association between urinary excretion of very specific tubular injury biomarkers and time-dependent changes in MA in patients with type 1 diabetes. Our findings provide strong evidence that tubular injury is an important component of the natural history of MA in type 1 diabetes. Less injury to the proximal tubule, as reflected by lower levels of urinary KIM-1 and NAG, is associated with regression of MA independently from glycemic control, or BP, or treatment with ACE inhibitors. There has been a great deal of focus on the role of proteinuria and progression of tubulointerstitial damage,27
and it is well appreciated that tubulointerstitial scarring is the best predictor of renal outcome in diabetic and nondiabetic renal disease. Tubulointerstitial disease has been proposed to be secondary to both enhanced protein uptake by proximal tubule cells with lysosomal rupture resulting in direct tubule toxicity as well as cytokines and chemokines generated by the proximal tubule after albumin uptake, which enhance the inflammatory response and activate fibrotic processes in the interstitial compartment.28
It is possible, however, that proximal tubule injury is primary rather than secondary in the development of MA. In an attempt to identify early urinary biomarkers of nephrotoxicity the Predictive Safety Testing Consortium tested many proximal tubule toxins in rats and found that MA was a excellent biomarker for tubular injury.12
The fact that cultured proximal29
cells respond directly to high levels of glucose with production of profibrotic mediators also supports the concept that tubulointerstitial disease may be primary rather than secondary in DM. Although there is some controversy over how much protein is filtered in a normal kidney,31
it is generally agreed upon that there is some normally filtered albumin that is reabsorbed by the proximal tubule. If this reabsorption is impaired, we would expect to see MA; therefore, the earliest kidney lesion in type 1 DM may be tubular injury, not glomerular injury.
We propose an important role for urinary NAG and KIM-1 for the diagnosis and monitoring of the course of renal disease in DM. As a lysosomal enzyme, urinary NAG elevation would be expected to be enhanced with proximal tubule injury although it may also be increased because of enhanced lysosomal activity without injury, per se.23
NAG has been extensively studied in both the adult and pediatric populations and has proven to be a sensitive, persistent, and robust indicator of proximal tubule injury.23
Many studies have established that the KIM-1 ectodomain serves as a very reliable biomarker of kidney injury both in rodents and in humans in which the data suggest it is not only a sensitive indicator of injury but can be a predictive biomarker of outcome.6,18,23,32
We have reported recently an algorithm using four biomarkers, including KIM-1 and NAG, to identify optimally acute kidney injury in hospitalized patients.23
IL-6, IL-8, IP-10, and MCP-1 have been previously implicated in the development of DN, renal function decline, and chronic inflammation.9,33
Although elevated levels of markers in urine indicative of low-level inflammation were specific for early progressive renal function decline, none of them was associated with MA in a previous study.9
Consistent with these prior results the present study shows that patients with MA had significantly higher levels of IL-6, IP-10, and MCP-1 levels when compared with patients with normoalbuminuria. In no case, however, were changes in one of these urinary cytokine levels associated with progression or regression of MA.
One of the limitations of this study is the small number of patients who had MA progression, thereby preventing adequate evaluation of biomarkers associated with MA progression. In addition, the follow-up period was only 2 years and hence too short to adequately determine the potential of urinary KIM-1 and NAG to predict permanent regression or progression of MA. Because this study was focused on the relationship between proximal tubule injury and MA early in type 1 DM, it does not establish the relationship of urinary tubular injury biomarkers with changes in glomerular filtration rate over time. It is possible that the findings in patients with advanced proteinuria may be different, and findings in type 1 DM may not be generalized to patients with type 2 DM and MA. Nevertheless this study provides important new insight into the importance of proximal tubule injury with associated MA early in the course of type 1 DM and identifies proximal tubule injury biomarkers that, when at low levels are associated with reduction in albuminuria over a 2-year follow-up period. It is possible, for example, that MA in the absence of significant tubule injury may inflict a functional state that is more readily reversible. This may relate to reversible changes in glomerular hemodynamics, for example.
In summary, the results of our study show that low urinary levels of KIM-1 and NAG are associated with regression of MA in type 1 diabetes and suggest that tubular dysfunction is a critical component of the early course of DN.