Hypoxia promotes fibrogenesis in human renal fibroblasts

Background. The mechanisms underlying progressive renal fibrosis are unknow n, but the common association of fibrosis and microvascular loss suggests t hat hypoxia per se may be a fibrogenic stimulus. Methods. To determine whether human renal fibroblasts (HRFs), the primary m atrix-producing cells in the tubulointerstitium, possess oxygen-sensitive r esponses relevant to fibrogenesis, cells were exposed to 1% O-2 in vitro. Results. Hypoxia simultaneously stimulated extracellular matrix synthesis a nd suppressed turnover with increased production of collagen alpha1(I) (Col l-I), decreased expression of collagenase, and increased tissue inhibitor o f metalloproteinase (TIMP)-1. These effects are time dependent, require new RNA and protein synthesis, and are specific to hypoxia. The changes in Col l-I and TIMP-1 gene expression involve a heme-protein O-2 sensor and protei n kinase- and tyrosine kinase-mediated signaling. Although hypoxia induced transforming growth factor-beta1 (TGF-beta1), neutralizing anti-TGF-beta1-a ntibody did not block hypoxia-induced Coll-I and TIMP-1 mRNA expression. Fu rthermore, hypoxic-cell conditioned-medium had no effect on the expression of these mRNAs in naive fibroblasts, suggesting direct effects on gene tran scription. Transient transfections identified a hypoxia response element (H RE) in the TIMP-1 promoter and demonstrated HIF-1-dependent promoter activa tion by decreased ambient pO(2). Conclusions. These data suggest that hypoxia co-ordinately up-regulates mat rix production and decreases turnover in renal fibroblasts. The results sup port a role for hypoxia in the pathogenesis of fibrosis and provide evidenc e for novel, direct hypoxic effects on the expression of genes involved in fibrogenesis.