Urine levels of TGF-β1 and connective
tissue growth factor increase with the progression of CKD;63–65 however, TGF-β1 is mostly PI3K inhibitor excreted as an inactive complex, which requires brief acidification to permit activation and detection. Some profibrotic molecules that are induced by TGF-β1, such as TGF-β-inducible gene H3 (βig-H3) and plasminogen activator inhibitor-1, are also detectable in urine and can act as surrogate markers of renal TGF-β1 activity. Urine levels of βig-H3 are about approximately 1000 times greater than TGF-β1 in diabetic patients and can be detected before the onset of albuminuria,66 indicating that βig-H3 is an early and sensitive marker of renal fibrosis during diabetes. Urine excretion of plasminogen activator inhibitor-1 has been shown to correlate with renal injury and fibrosis in patients with diabetic nephropathy and progressive chronic glomerulonephritis.67,68 Collagen type IV is a major component of kidney extracellular matrix, which is increased during the progression of renal fibrosis. Urine excretion of collagen IV is elevated in patients with IgA nephropathy and diabetic nephropathy and correlates with declining renal function.69,70 In addition, urine levels of collagen IV correlate Navitoclax ic50 with glomerular matrix accumulation and declining renal function in animal models of kidney disease.71 In contrast, serum levels of collagen IV are not associated with the development
of renal injury or loss of kidney aminophylline function.72 Although reliable ELISA exists for most of the recently described renal biomarkers in serum and urine, this technique is limited to measuring a single marker per assay, which makes assessment of multiple biomarkers time-consuming and expensive. Recently, multiplex assay systems have been developed by Luminex (http://www.luminexcorp.com) and
BD Biosciences (http://www.bdbiosciences.com/reagents/cytometricbeadarray), which uses the principles of both ELISA and flow cytometry to simultaneously quantitate multiple antigens in biological fluids. In the Luminex assays, microspheres with unique spectral signatures are coupled with primary antibodies. The antigens binding to these microspheres are then labelled with biotinylated secondary antibodies and streptavidin coupled to another fluorochrome (phycoerythrin). The microspheres and antigens labelled with phycoerythrin are excited with lasers at different wavelengths and the emission signals are used to identify the antigen and the amount of antigen bound to the microsphere. This technique is theoretically capable of assessing up to 100 different antigens and requires small volumes of biological fluid (30 µL). The Luminex assay system has been used to assess multiple biomarkers in the urine of patients with renal allograft rejection and lupus nephritis.51,73 The advantages and technical considerations for multiplex assays have been recently reviewed by Leng et al.
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