Renal fibrosis: an update

Tubulointerstitial fibrosis invariably accompanies the course of chronic renal failure towards end-stage renal disease. Tubular epithelial cells, the predominant cell type in the tubulointerstitium, are increasingly being recognized for playing a dominant role as mediators of renal fibrogenesis. Tubular epithelial cells become activated either by the glomerular ultrafiltrate from their apical side or by mononuclear cells from their basolateral side. They initiate the scarring process by secreting chemokines, which in return attract mononuclear cells as well as growth factors that stimulate interstitial fibroblasts. In later phases of renal fibrogenesis, cellular changes of tubular epithelial cells contribute to the chronic impairment of renal function. Whereas tubular epithelial cells react by proliferation or hypertrophy to initial stimuli, they may undergo apoptosis or transdifferentiate into fibroblasts, and thus contribute to tubular atrophy in later stages of progressive renal disease. Resident interstitial fibroblasts are also important in renal fibrogenesis, and recent research has demonstrated that these cells are much more heterogeneous than expected. Cytokines such as fibroblast growth factor type 2 and epithelial growth factor have been shown to be pro-fibrogenic, whereas hepatocyte growth factor and bone morphogenic protein type 7 may inhibit fibrogenesis. Despite recent progress, further research is mandatory for a better understanding and the development of novel therapeutic approaches.