HYPOXIA STIMULATES PROXIMAL TUBULAR CELL-MATRIX PRODUCTION VIA A TGF-BETA(1)-INDEPENDENT MECHANISM

Tubulointerstitial fibrosis is characterized by tubular basement membr ane thickening and accumulation of interstitial extracellular matrix ( ECM). Since chronic low-grade hypoxia has been implicated in the patho genesis of fibrosis and proximal tubular epithelial cells (PTE) are se nsitive to oxygen deprivation, we hypothesized that hypoxia may stimul ate ECM accumulation. In human PTE, hypoxia (1% O-2, 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. Hypoxi a-induced changes persisted on re-oxygenation with increased expressio n of TIMP mRNAs. A potential mediator for these effects is transformin g growth factor-beta(1) (TGF-beta(1)), a major pro-fibrogenic factor p roduced by PTE. Although hypoxia stimulated TGF-beta production (2- to 3-fold), neutralizing anti-TGF-beta(1) antibody did not abolish the h ypoxia-induced changes in gelatinase activity, TIMP-1, collagen IV or collagen I mRNA expression, implying that TGF-beta(1) is not the media tor. Furthermore, exogenous TGF-beta(1) (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 hypo xia may be an important pro-fibrogenic stimulus independent of TGF-bet a(1).