Hypoxia stimulates proximal tubular cell matrix production via a TGF-β₁-independent mechanism. Tubulointerstitial fibrosis is characterized by tubular basement membrane thickening and accumulation of interstitial extracellular matrix (ECM). Since chronic low-grade hypoxia has been implicated in the pathogenesis of fibrosis and proximal tubular epithelial cells (PTE) are sensitive to oxygen deprivation, we hypothesized that hypoxia may stimulate ECM accumulation. In human PTE, hypoxia (1% 0₂, 24 hr) increased total collagen production (15%), decreased MMP-2 activity (55% ± 13%; control = 100%) and increased tissue inhibitor of metalloproteinase-1 (TIMP-1) protein. Collagen IV mRNA levels decreased while collagen I mRNA increased, suggesting induction of interstitial collagen. Hypoxia-induced changes persisted on re-oxygenation with increased expression of TIMP mRNAs. A potential mediator for these effects is transforming growth factor-β₁ (TGF-β₁, a major pro-fibrogenic factor produced by PTE. Although hypoxia stimulated TGF-β production (2- to 3-fold), neutralizing anti-TGF-β₁ antibody did not abolish the hypoxia-induced changes in gelatinase activity, TIMP-1, collagen IV or collagen I mRNA expression, implying that TGF-β₁ is not the mediator. Furthermore, exogenous TGF-β₁ (0 to 10 ng/ml) did not mimic hypoxia, as it stimulated MMP-2 activity and increased the expression of collagen IV, collagen I and TIMP-1 mRNA. The data suggest that hypoxia may be an important pro-fibrogenic stimulus independent of TGF-β₁.