Basic fibroblast growth factor expression is increased in human renal fibrogenesis and may mediate autocrine fibroblast proliferation

Background. Interstitial fibroblasts play a critical role in renal fibrogen esis, and autocrine proliferation of these cells may account for continuous matrix synthesis. Basic fibroblast growth factor (FGF-2) is mitogenic for most cells and exerts intracrine, autocrine, and paracrine effects on epith elial and mesenchymal cells. The aims of the present studies were to locali ze and quantitate the expression of FGF-2 in normal and pathologic human ki dneys and to study the in vitro effects of FGF-2 on proliferation, differen tiation, and matrix production of isolated cortical kidney fibroblasts. Methods. FGF-2 protein expression was localized by immunofluoresence double labelings in normal and fibrotic human kidneys. Subsequently, interstitial FGF-2 labeling was determined semiquantitatively in 8 normal kidneys and 3 9 kidneys with variable degrees of interstitial fibrosis and was correlated with the morphometrically determined interstitial cortical volume. In addi tion, FGF-2 expression was quantitated by immunoblot analysis in three norm al and six fibrotic kidneys. FGF-2 mRNA was localized by in situ hybridizat ions. Seven primary cortical fibroblast lines were established, and express ion of FGF-2 and FGF receptor-1 (FGFR-1) were examined. The effects of FGF- 2 on cell proliferation were determined by bromo deoxyuridine incorporation and cell counts, those on differentiation into myofibroblasts by staining for or-smooth muscle actin, and those on matrix synthesis by enzyme-linked immunosorbent assay for collagen type I and fibronectin. Finally, prolifera tive activity in vivo was evaluated by expression of MIB-1 (Ki-67 antigen). Results. In normal kidneys, FGF 2 expression was confined to glomerular, va scular, and a few tubular as well as interstitial fibroblast-like cells. Th e expression of FGF-2 protein was increased in human kidneys, with tubuloin terstitial scarring correlating with the degree of interstitial fibrosis (r = 0.84, P < 0.01). Immunoblot analyses confirmed a significant increase in FGF-2 protein expression in kidneys with interstitial scarring. In situ hy bridization studies demonstrated low-level detection of FGF-2 mRNA in norma l kidneys. However, FGF-2 mRNA expression was robustly up-regulated in inte rstitial and tubular cells in end-stage kidneys, indicating that these cell s are the source of excess FGF-2 protein. Primary cortical fibroblasts expr ess FGF-2 and FGFR-1 in vitro. FGF-2 induced a robust growth response in th ese cells that could be blocked specifically by a neutralizing FGF-2 antibo dy. Interestingly, the addition of the neutralizing antibody alone did redu ce basal proliferation up to 31.5%. In addition, FGF-2 induced expression o f or-smooth muscle actin up to 1.6-fold, but no significant effect was obse rved on the synthesis of collagen type I and fibronectin. Finally, staining for MIB-1 revealed a good correlation of interstitial FGF-2 positivity wit h interstitial and tubular proliferative activity (r = 0.71, P < 0.01 for i nterstitial proliferation, N = 30). Conclusions. Interstitial FGF-2 protein and mRNA expression correlate with interstitial scarring. FGF-2 is a strong mitogen for cortical kidney fibrob lasts and may promote autocrine fibroblast growth. Expression of FGF-2 corr elates with intersti tial and tubular proliferation in vive.