Paracrine stimulation of human renal fibroblasts by proximal tubule cells

Background. Interstitial fibrosis strongly predicts the degree and progression of renal failure in human renal disorders. Since active fibrosis lends to initially occur in a peritubular distribution, the possibility that human proximal tubule cells (PTC) relay fibrogenic signals to neighboring cortical fibroblasts was examined in vitro. Methods. Cell proliferation (cell counts and thymidine incorporation). total collagen synthesis (proline incorporation), matrix metalloproteinase (MMP) activity (gelatin zymography), and autocrine secretion of insulin-like growth factor-I (IGF-I) were measured in primary cultures of human cortical fibroblasts cocultured with PTC or exposed to PTC-conditioned media (PTC-CM). Results. Cell numbers and thymidine incorporation rates were increased in cortical fibroblasts cocultured with PTC (136.4 +/- 7.3% and 119.3 +/- 8.2% of control values, respectively, P < 0.05) or incubated in PTC-CM (114.0 +/- 5.9%, P < 0.05 acid 146.7 +/- 13.3% P < 0.05, respectively). PTC-CM stimulated cortical fibroblast collagen synthesis (13.5 +/- 1.0% vs. 10.8 +/- 0.7%, respectively, N = 24, P < 0.05) and MMP-2 and MMP-9 secretion. Cortical fibroblast secretion of IGF-I binding protein-3 (IGFBP-3), which in turn modulates the autocrine and paracrine actions of IGF-I, was enhanced in the presence of PTC-CM compared with control (1162.2 +/- 94.2 vs. 969.1 +/- 58.9 ng/mg protein/day, P < 0.05), but no change was observed in cortical fibroblast secretion of IGFBP-2 (260.9 +/- 35.5 vs. 290.9 +/- 36.6 ng/mg protein/day, P = NS) or IGF-I (56.7 +/- 6.6 vs. 57.0 +/- 6.8 ng/mg protein/day, P = NS). Human PTC secreted transforming growth factor-beta(1) (TGF-beta(1)) and the AB heterodimer of platelet-derived growth factor (PDGF-AB) in a time-dependent fashion and the augmentation of cortical fibroblasts mitogenesis, collagen synthesis and IGFBP-3 secretion induced by PTC-CM was replicated by exogenous TGF-beta(1) and PDGF. Furthermore, the stimulatory effects of PTC on cortical fibroblasts were potentiated in transiently acidified PTC-CM (which activated latent TGF-beta(1)), and were abrogated by neutralizing antibodies specifically directed against TGF-beta(1) and PDGF-AB. Cortical fibroblasts in turn released a soluble factor(s) into cortical fibroblast-conditioned media that reciprocally stimulated PDGF-AB production by PTC (4.79 +/- 1.55 vs. 0.78 +/- .06 ng/mg protein/day, P < 0.05). Conclusions. PTC modulate the biological behavior of neighboring cortical fibroblasts in the human kidney through paracrine mechanisms, which include the production and release of PDGF-AB and TGF-beta(1). Renal insults that result in proximal tubule injury may perturb this paracrine interaction, thereby culminating in excessive fibroblast proliferation and interstitial fibrosis.