TGF-beta 1 stimulates the release of pre-formed bFGF from renal proximal tubular cells

Background. It is now clear that the progression of renal disease is closel y correlated to the degree of renal interstitial fibrosis. We have previous ly demonstrated that the renal proximal tubular epithelial cell may contrib ute to the fibrotic response by the: generation of profibrotic cytokines. T ransforming growth factor-beta 1 (TGF-beta 1) and basic fibroblast growth f actor (bFGF) are two of a group of profibrotic cytokines that have been ass ociated with the development of renal interstitial fibrosis. In this study, we have examined the influence of TGF-beta 1 on the generation of bFGF by renal tubular epithelial cells. Methods. HK2 cells were grown to confluence and were serum deprived and sti mulated with recombinant TGF-beta 1 under serum-free conditions. Subsequent ly, supernatant. cell-associated, intracellular, and matrix-associated bFGF concentrations were determined by enzyme-linked immunosorbent assay (ELISA ). bFGF mRNA expression was examined by reverse transcription-polymerase ch ain reaction (RT-PCR). Results. The exposure of confluent serum-deprived HK2 cells to TGF-beta 1 l ed to a significant increase in bFGF concentration in the cell culture supe rnatant. Twenty-four hours following the addition of 10 ng/ml TGF-beta 1, t his represented a twofold increase in bFGF concentration (control, 102 pg/m l, N = 24, vs. 202 pg/ml, N = 19, P = 0.0001). Despite the increase in bFGF concentration in the supernatant, there was no change in the expression of bFGF mRNA following the addition of TGF-beta 1 The addition of 10 ng/ml of TGF-beta 1 led to a 30% decrease in the total cell-associated bFGF concent ration (control, 8.51 ng/ml, N = 16, TGF-beta 1. 6.01 ng/ml, N = 13, P = 0. 0042). This decrease in intracellular bFGF was associated with a 15% reduct ion in anti-bFGF antibody binding to fixed permeabilized cells, following t he addition of 10 ng/ml of recombinant TGF-beta 1 (N = 9, P = 0.0007), sugg esting that the mechanism of stimulation of bFGF by TGF-beta 1 involved the release of preformed bFGF from within the cells. In addition, following th e addition of TGF-beta 1, there was a significant dose-dependent decrease i n the amount of bFGF sequestered in the extracellular matrix. At a dose of 10 ng/ml TGF-beta, this represented a greater than sevenfold decrease (N = 9, P = 0.0007) in matrix-bound bFGF, although this represented less than 3% of the total bFGF released into the supernatant. Conclusion. The data presented suggest that the main mechanism by which TGF -beta 1 stimulates bFGF generation by proximal tubular epithelial cells is by stimulation of the secretion of preformed cytokine from within the cells .