Albumin Signals the Coming of Age of Proteinuric Nephropathy

The great majority of progressive renal diseases are associated with the development of a common renal histological lesion known as the end-stage kidney, characterized by prominent interstitial inflammation and scarring, together with fibrosis and gross tubular atrophy. Declining renal function correlates extremely closely with these developing changes in the renal tubulointerstitium (¹), and these observations have led to a hypothesis that suggests that, whatever the initiating renal insult, a common pathophysiological process is precipitated, which culminates in the development of the end-stage kidney. Investigators have therefore sought to identify factors common to the majority of renal diseases that may be responsible for the development of this pathological lesion. Proteinuria, manifest predominantly as albuminuria, is a poor prognostic sign and an almost universal finding in patients with progressive renal diseases. It has been postulated therefore that excess protein exerts a toxic effect on the proximal tubular epithelial cell, damaging the cells and thus initiating interstitial inflammation and scarring (²).

Protein-mediated, particularly albumin-mediated, effects on proximal tubular cells have been subject to intense study. As well as illuminating our understanding of renal disease mechanisms, the results of these studies have changed our perception of albumin from that of an intrinsically uninteresting molecule, at least in a pathophysiological sense, to that of a key mediator of signaling and phenotype in the proximal tubule. In this issue of JASN, Nakajima et al (³) challenge our preconceptions by demonstrating activation of the signal transducer and activator of transcription (STAT) proteins by albumin in proximal tubular cells. The study also reveals how information derived from genomic studies may be interpreted and rationally translated into significant functional information of direct relevance to cell biology. These observations are important because STAT protein activation is involved in a wide array of signaling pathways and provides a universal paradigm for signaling from cytokine receptors as well as being a module used by many growth factor receptors (⁴). The data from Nakajima et al (³) therefore suggest that albumin may stimulate proximal tubular cells in the manner of a cytokine.

Understanding the mechanism of protein, especially albumin, interactions with the proximal tubular cells is clearly key to understanding how cell activation may occur. Appreciable quantities of albumin enter the proximal tubule in normal individuals. After filtration, albumin binds to the giant receptors megalin and cubilin in proximal tubular cells (⁵), although the relative paucity of urinary albumin in kidney-specific megalin knockout mice (⁶) suggests other receptors may be involved. Binding is followed by endocytosis, and these processes are accompanied by activation of numerous enzymes, particularly kinases, involved in intracellular signaling such as extracellular signal-regulated mitogen-activated protein kinase, phosphatidylinositol 3-kinase, p70^S6 kinase, c-jun terminal kinase (JNK), and protein kinase C (^7–10). How albumin, sometimes at very low concentrations, transduces these signals is not well understood. Some may be mediated via megalin (¹¹), and others may require internalization and degradation of albumin with the generation of other intermediates such as reactive oxygen species.

Stimulation of extracellular signaling pathways by albumin results in transcription factor activation. These include the STAT proteins as currently described by Nakajima et al(³) and NF-κB as previously demonstrated by several authors (^12,13). The net result of proximal tubular cell exposure to albumin is the production of a cocktail of chemoattractants, pro-fibrotic agents, matrix proteins, and vasoconstrictive agents by PTEC. Specific examples include MCP-1, RANTES, IL-8, platelet-derived growth factor, TGF-β, endothelin, fibronectin, and collagen (^10,12–19). This complex mixture of pro-inflammatory substances and growth factors attracts macrophages into the tubulointerstitium and may induce changes in proximal tubular cell growth characteristics and/or precipitate epithelial-mesenchymal transition with transdifferentiation of proximal tubular cells into myofibroblasts (^20,21).

The data from Nakajima et al (³) therefore adds to a growing body of in vitro and in vivo evidence that strongly implicates proteinuria as a crucial factor in the pathogenesis of chronic renal injury. Furthermore exciting new data contribute a fresh dimension to the story by incriminating albumin as a central mediator of fluid retention in nephrosis. Both in nephrotic rats and in cultured proximal tubular cells, albumin per se is able to positively regulate activity of the Na⁺/H⁺ exchanger 3 (NHE-3) through effects on NHE-3 protein abundance and trafficking (²²). One potential consequence of these changes is enhancement of Na⁺ reabsorption with attendant expansion of the extracellular volume and possibly hypertension.

How does identification of these signaling effects of albumin and other proteins in the proximal tubule help us? Certainly the complexity of the pathways involved provides the possibility of multiple avenues for intervention and modulation using agonists and inhibitors. Comparative studies in other cell types may also be helpful. A trickle of publications is beginning to describe albumin signaling in other cells. For instance in chronic lymphocytic leukemia cells, albumin accumulation in intracellular vesicles is associated with activation of Akt signaling and protection of cells from apoptosis (²³). Albumin also regulates activity of heterotrimeric GTP-binding proteins by controlling nucleotide dissociation from α subunits, together with their intrinsic GTPase activity (²⁴). Understanding how nonrenal cells, such as blood cells or endothelial cells, deal with obligate exposure to high concentrations of albumin may aid our comprehension of the threshold at which the normal albumin contact with proximal tubular cells that occurs due to healthy glomerular filtration becomes maladaptive in disease.

Although controversial until recently, it is becoming increasingly difficult to reasonably challenge the hypothesis of proteinuria-induced renal injury. The available information predominantly points to a major pathophysiological role for tubular protein in progressive renal disease. Whilst other factors such as renal hypoxia undoubtedly also contribute, the concept of proteinuric nephropathy is coming of age.