DIFFERENTIAL RESPONSE OF ACTIVATED VERSUS NONACTIVATED RENAL FIBROBLASTS TO TUBULAR EPITHELIAL-CELLS - A MODEL OF INITIATION AND PROGRESSION OF FIBROSIS

Background/Aims: The mechanisms of initiation and maintenance of renal fibrosis remain obscure, but one hypothesis highlights the importance of tubular epithelial cell-interstitial fibroblast interactions and s uggests that tubular injury may be a precipitating factor. The aims of the study were to examine the effects of factors of proximal tubular origin on renal fibroblasts expressing different levels of smooth musc le actin (SMA; a putative marker of fibroblast activation) and to exam ine the modulation of SMA by extracellular matrix (ECM) proteins and t ransforming growth factor beta 1 (TGF-beta(1)), a major profibrotic cy tokine. Methods: Primary cultures of rat cortical fibroblasts (CF) and the rat kidney fibroblast cell line NRK-49F were (1) cultured on diff erent ECM proteins; (2) treated with medium conditioned by rat proxima l tubular epithelial cells (PTE), and (3) treated with TGF-beta(1). SM A protein expression was examined by immunocytochemistry, while expres sion of MMP-2, MMP-3, TIMP-1, and collagen I mRNA was determined by No rthern blot analysis or reverse-transcriptase polymerase chain reactio n. Results: In cells with low basal levels of SMA (CF), serum was the most potent inducer of increased SMA expression, although ECM proteins also modulated expression. With high basal levels of SMA expression ( NRK), ECM proteins alone had little or no effect, but acted synergisti cally with serum to stimulate expression. In CF, PTE-conditioned mediu m (CM) had no effect on SMA and TIMP mRNA levels, but suppressed expre ssion of MMP mRNAs. In NRK-49F, PTE-CM stimulated SMA and TIMP-1 mRNA levels, but had no effect on MMP mRNA levels. Although TGF-beta(1) mod ulated some cellular responses in NRK-49F, neutralizing antibody studi es showed it was not the main mediator of the PTE-CM-induced effects. Conclusions: These data suggest (1) that in renal fibroblasts SMA expr ession can be modulated by both serum and ECM proteins and (2) that PT E induce a fibrogenic phenotype in both non-activated (low SMA) and ac tivated (high SMA) fibroblasts via different mechanisms.