Chronic hypoxia as a mechanism of progression of chronic kidney diseases: from hypothesis to novel therapeutics.

In chronic kidney disease, functional impairment correlates with tubulointerstitial fibrosis characterised by inflammation, accumulation of extracellular matrix, tubular atrophy and rarefaction of peritubular capillaries. Loss of the microvasculature implies a hypoxic milieu and suggested an important role for hypoxia when the "chronic hypoxia hypothesis" was proposed a decade ago as an explanation for the progressive nature of fibrosis. Recent data in man provide evidence of decreased renal oxygenation in chronic kidney disease while more direct support for a causal role comes from data in rodent models showing that the decline in renal oxygenation precedes matrix accumulation, suggesting hypoxia may both initiate and promote the fibrotic response. Indeed, in vitro studies show that hypoxia can induce pro-fibrotic changes in tubulointerstitial cells. Additional postulated roles for hypoxia in chronic kidney disease are the sustaining of the inflammatory response, the recruitment, retention and differentiation towards a pro-fibrotic phenotype of circulating progenitor cells and the alteration of the function of intrinsic stem cell populations. Given that accumulating data suggests that chronic hypoxia is a final common pathway to end-stage renal disease, therapeutic strategies that target hypoxia may be of benefit in retarding progression. Normalisation of microvascular tone, administration of pro-angiogenic factors to restore microvasculature integrity, activation of hypoxia-inducible transcription factors and hypoxia-mediated targeting and mobilisation of progenitor cells are all potential targets for future therapy. The limited success of existing strategies in retarding chronic kidney disease mandates that these new avenues of treatment be explored.