Differential response of activated versus non-activated renal fibroblasts to tubular epithelial cells: a model of initiation and progression of fibrosis?

Objective: 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 suggests 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 muscle actin (SMA; a putative marker of fibroblast activation) and to examine the modulation of SMA by extracellular matrix (ECM) proteins and transforming growth factor beta 1 (TGF-beta 1), a major profibrotic cytokine.

Methods: Primary cultures of rat cortical fibroblasts (CF) and the rat kidney fibroblast cell line NRK-49F were (1) cultured on different ECM proteins; (2) treated with medium conditioned by rat proximal tubular epithelial cells (PTE), and (3) treated with TGF-beta 1. SMA protein expression was examined by immunocytochemistry, while expression of MMP-2, MMP-3, TIMP-1, and collagen I mRNA was determined by Northern blot analysis or reverse-transcriptase polymerase chain reaction.

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 synergistically with serum to stimulate expression. In CF, PTE-conditioned medium (CM) had no effect on SMA and TIMP mRNA levels, but suppressed expression 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 modulated some cellular responses in NRK-49F, neutralizing antibody studies showed it was not the main mediator of the PTE-CM-induced effects.

Conclusions: These data suggest (1) that in renal fibroblasts SMA expression can be modulated by both serum and ECM proteins and (2) that PTE induce a fibrogenic phenotype in both non-activated (low SMA) and activated (high SMA) fibroblasts via different mechanisms.