Urinary measurement of transforming growth factor-β and type IV collagen as new markers of renal injury: application in diabetic nephropathy

Abstract

Abstract Urinary samples were concentrated rapidly and efficiently and were used to develop several protein assays that may be of value in monitoring individuals with progressive renal disorders. Transforming growth factor-β1 (TGF-β1) and retinol binding protein (RBP) were measured with modification of commercially available methods used to assay serum specimens; type 3 collagen (T3C) was measured with a new immunonephelometric assay. The precision characteristics of these assays are comparable with those reported for microalbuminuria. The clinical utility of measuring a panel of these markers was demonstrated in urine samples from 16 control subjects and from 46 individuals with insulin-dependent diabetes mellitus (IDDM) with various albumin excretion rates (AERs). TGF-β1 and T3C were used as markers of cytokine expression and of the renal fibrogenic process, whereas RBP excretion served as a marker of tubular injury or dysfunction. Compared with controls, T3C excretion was significantly increased in 18 normoalbuminuric and further increased in 13 microalbuminuric (AER 20 ≤ 200 μg/min) IDDM subjects. RBP excretion was increased in macroalbuminuric IDDM subjects (AER >200 μg/min, overt nephropathy). Significant correlations were also found between AER and RBP in all but macroalbuminuric individuals, whereas TGF-β1 correlated with T3C excretion in controls and in normoalbuminuric diabetic subjects. Urinary RBP but not AER was an excellent predictor of diabetic nephropathy as defined by serum creatinine (P = 0.0001). This underscores the importance of an early tubulopathy in the subsequent development of glomerulopathy and overt nephropathy. The data suggest that longitudinal monitoring of a panel of urinary markers such as that used in the current study may better define their relevance in progressive glomerulosclerosis and may also provide greater insight into the mechanisms underlying such process.