Proteinuria: detection and role in native renal disease progression

The presence of albuminuria or proteinuria constitutes a sign of kidney damage and, together with the estimation of glomerular filtration rate, is based on the evaluation of chronic kidney disease. Proteinuria is a strong marker for progression of chronic kidney disease, and it is also a marker of increased cardiovascular morbimortality. Filtration of albumin by the glomerulus is followed by tubular reabsorption, and thus, the resulting albuminuria reflects the combined contribution of these 2 processes. Dysfunction of both processes may result in increased excretion of albumin, and both glomerular injury and tubular impairment have been involved in the initial events leading to proteinuria. Independently of the underlying causes, chronic proteinuric glomerulopathies have in common the sustained or permanent loss of selectivity of the glomerular barrier to protein filtration. The integrity of the glomerular filtration barrier depends on its 3-layer structure (the endothelium, the glomerular basement membrane, and the podocytes). Increased intraglomerular hydraulic pressure or damage to glomerular filtration barrier may elicit glomerular or overload proteinuria. The mechanisms underlying glomerular disease are very variable and include infiltration of inflammatory cells, proliferation of glomerular cells, and malfunction of podocyte-associated molecules such as nephrin or podocin. Albumin is filtered by the glomeruli and reabsorbed by the proximal tubular cells by receptor-mediated endocytosis. Internalization by endocytosis is followed by transport into lysosomes for degradation. The multiligand receptors megalin and cubilin are responsible for the constitutive uptake in this mechanism. Albumin and its ligands induce expression of inflammatory and fibrogenic mediators resulting in inflammation and fibrosis resulting in the loss of renal function as a result of tubular proteinuria. TGF-β, which may be induced by albumin exposure, may also act in a feedback mechanism increasing albumin filtration and at the same time inhibiting megalin- and cubilin-mediated albumin endocytosis, leading to increased albuminuria. Urinary proteins themselves may elicit proinflammatory and profibrotic effects that directly contribute to chronic tubulointerstitial damage. Multiple pathways are involved, including induction of tubular chemokine expression, cytokines, monocyte chemotactic proteins, different growth factors, and complement activation, which lead to inflammatory cell infiltration in the interstitium and sustained fibrogenesis. This tubulointerstitial injury is one of the key factors that induce the renal damage progression. Therefore, high-grade proteinuria is an independent mediator of progressive kidney damage. Glomerular lesions and their effects on the renal tubules appear to provide a critical link between proteinuria and tubulointerstitial injury, although several other mechanisms have also been involved. Injury is transmitted to the interstitium favoring the self-destruction of nephrons and finally of the kidney structure.