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As several studies indirectly suggest that inhibiting the intracellular breakdown of cyclic nucleotides may inhibit fibrogenesis, this study used membrane permeable cyclic nucleotide analogues to examine the role of cAMP and cGMP signaling pathways in the regulation of renal fibroblast function. Fibroblasts were isolated by explant outgrowth culture of rat kidneys post unilateral ureteric obstruction. Subcultured cells were exposed to 10– 1,000 µM of the cyclic nucleotide analogues 8-bromo-cAMP (8br-cAMP) and 8-bromo-cGMP (8br-cGMP). Functional parameters examined included mitogenesis (thymidine incorporation), collagen synthesis (proline incorporation), myofibroblast differentiation (Western blotting for α-smooth muscle actin; α-SMA) and expression of CTGF (Northern blotting), a TGF-β1-driven immediate early response gene. Serum-stimulated mitogenesis was decreased 27 ± 4% by 100 µM 8br-cAMP (p < 0.01), 49 ± 6% by 1,000 µM 8br-cAMP (p < 0.001) and 43 ± 7% by 1,000 µM 8br-cGMP (p < 0.01). 1,000 µM 8br-cAMP and 8br-cGMP reduced basal collagen synthesis by 80 ± 5 and 60 ± 21% respectively (both p < 0.05). Maximum dose of 8br-cAMP but not 8br-cGMP inhibited basal expression of the differentiation marker α-SMA by 43 ± 33 (p < 0.05), resulted in a more rounded cell morphology and reduced expression of CTGF by 39 ± 24% (p < 0.05). Measurement of mitochondrial activity confirmed that effects were independent of cell toxicity. In conclusion, cyclic nucleotides inhibit fibrogenesis in vitro. Strategies which elevate intracellular cyclic nucleotide concentrations may therefore be therapeutically valuable in preventing the proliferation and activation of fibroblasts in progressive renal disease.

Background. Renal fibroblasts are important effector cells in tubulointerstitial fibrosis, with experimental antifibrotic strategies focusing on the functional downregulation of these cells. Several experimental models of fibrosis have provided evidence for the effectiveness of the polypeptide hormone relaxin as a potential antifibrotic agent. This study was conducted to further elucidate the antifibrotic mechanisms of relaxin on renal fibroblasts in vitro. Methods. Rat cortical fibroblasts were obtained from outgrowth culture of renal tissue isolated from kidneys 3 days post-unilateral ureteric obstruction and constituted 100% of cells studied. A relaxin radio-receptor assay was used to establish binding of relaxin to renal fibroblasts in vitro. Functional studies then examined the effects of H2 relaxin (0, 1, 10 and 100 ng/ml) on fibroblast kinetics, expression of alpha-smooth muscle actin (-SMA), total collagen synthesis, collagenase production and collagen-I lattice contraction. CTGF mRNA expression was also measured by northern analysis. Results. H2 relaxin bound with high affinity to rat renal fibroblasts, but receptor numbers were low. Consistent with its previously reported bimodal effect, transforming growth factor (TGF-1) reduced fibroblast proliferation, an effect abrogated by H2 relaxin. Fibroblasts exposed to H2 relaxin (100 ng/ml) for 24 h demonstrated decreased immunostaining for -SMA and reduced -SMA protein expression compared with controls. There was a trend for a relaxin-mediated reduction in total collagen synthesis and 1(I) mRNA expression with large dose-related increases in collagenase protein expression being observed. TGF-1-stimulated collagen-I lattice contraction was significantly inhibited following co-incubation with 100 ng/ml relaxin. Incremental doses of H2 relaxin had no significant effect on CTGF mRNA expression. Conclusions. The findings of this study suggest that the antifibrotic effects of relaxin involve down-regulation of fibroblast activity, increase in collagenase synthesis and restructuring of collagen-I lattices, which are consistent with its known physiological role of matrix remodelling. Although there appears to be an interaction between TGF-1 and H2 relaxin, this does not appear to involve a reduction in CTGF mRNA expression.

Interstitial myofibroblasts (MF) are cells with features of both smooth muscle cells and fibroblasts. They have been universally recognized in situations of tubulointerstitial injury, where their presence has been shown to be a marker of disease progression. The objective of this study was to determine if functions of MF relevant to fibrogenesis can be modified in vitro by the phosphodiesterase inhibitor pentoxifylline (PTX). MF were obtained from sub-culture of normal rat kidney explant outgrowths maintained in DMEM + 20% fetal calf serum (FCS), supplemented with antibiotics. Cells were characterized on the basis of growth characteristics and immunohistochemistry. MF constituted >95% of cells at passage 3. Cell culture media was supplemented with the potential antagonist PTX alone (0, 1, 10, 100 μg/ml) and in combination with TGFβ1 (5 ng/ml). Population kinetics, proliferation and collagen production were determined from cell growth, [3H]thymidine incorporation and [3H]proline incorporation in collagenous proteins, respectively. Both serum-stimulated population growth and proliferation were reduced in a linear fashion by 1, 10 and 100 μg/ml PTX (all p < 0.05 versus 0 μg/ml). Effect of PTX on cell population growth was however reversible when PTX was removed. Basal collagen secretion was decreased by PTX at 10 and 100 μg/ml (p < 0.05 versus 0 μg/ml), although cell layer collagen remained unchanged. Collagen production (secreted and cell layer) was augmented by 5 ng/ml TGFβ1. These effects on collagen production were partially reduced when 100 μg/ml PTX was added. The authors conclude that myofibroblast function can be altered with agonists/antagonists. Attempts to down-regulate fibrogenic functions of MF may therefore offer a valuable therapeutic strategy.

Background: Generation of thrombin occurs in response to parenchymal injury. Thrombin not only converts plasma fibrinogen into an insoluble fibrin clot, but also potentially augments inflammation through receptor-mediated activity. This study examines whether thrombin may potentially exacerbate fibrosis by upregulating the function of interstitial fibroblasts in vitro. Methods: Fibroblasts were isolated by explant outgrowth culture of rat kidneys. Subcultured cells were grown in DMEM+10% FCS supplemented with 0.1–0.5 U/ml thrombin. Functional parameters examined included kinetics (thymidine incorporation and change in cell number), differentiation (Western blotting for α-smooth muscle actin; αSMA), expression of procollagen α1(I) (Northern blotting) and contraction of collagen I lattices. RT-PCR was used to characterise expression of protease-activated receptors (PAR) previously implicated in thrombin’s cellular effects. Results: Cell population growth was increased 66 ± 41 and 47 ± 41% by 0.1 and 0.5 U/ml thrombin respectively (both p < 0.05 vs. basal). Likewise, 0.5 U/ml thrombin increased corrected procollagen α1(I) expression 2.4-fold (p < 0.05 vs. basal) and exacerbated the ability of fibroblasts to contract collagen matrix (p < 0.05 vs. basal). These effects were not associated with any change in expression of the myofibroblast marker αSMA. Effects on cell number were inhibited by treatment with (D)-Phe-Pro-Arg-chloromethylketone HCl (PPACK) suggesting that functional effects were mediated by serine protease activity. PAR-1 was the only fully functional known thrombin receptor expressed by these cells. Conclusion: Thrombin is a potential unrecognised fibroblast agonist in renal disease. Further studies of thrombin and its receptors may yield valuable insights into the pathogenesis of interstitial fibrosis.

Fibroblasts are universally recognized in situations of tubulointerstitial injury, where their presence has been shown to be a marker of disease progression. The objective of this study was to determine whether the functions of fibroblasts relevant to fibrogenesis can be modified in vitro with dipyridamole. Cells were obtained from obstructed rat renal tissue and characterized on the basis of immunohistochemical findings. Fibroblasts constituted all of the cells from passage 3. Functional parameters were measured in cells cultured with 1, 5, and 50 micromol/L dipyridamole and compared to basal parameters of cells grown in Dulbecco's modified Eagle's medium plus 10% fetal calf serum (control). Northern-blot analysis indicated that dipyridamole decreased procollagen alpha1(I) messenger ribonucleic acid expression (P.05, 50 micromol/L vs control), results that were reflected in a reduction in total collagen secretion as measured on the basis of hydroxyproline incorporation (P.001, 50 micromol/L vs control). Mitogenesis, as measured on the basis of incorporation of tritiated thymidine, was decreased in a dose-dependent fashion by dipyridamole. Likewise, 50 micromol/L dipyridamole reduced cell-population growth to 16.8% +/- 2.1% of basal growth over 3 days (P.001 vs control). Effects of dipyridamole on population growth were prevented by coincubation with a protein kinase G inhibitor peptide (P.001 vs 50 micromol/L dipyridamole; P = not significant vs control). No such effect was observed with inhibitors for protein kinase A (H-89) and protein kinase C (bisindolylmaleimide I). Consistent with a protein kinase G-dependent mechanism, immunofluorescence staining indicated that dipyridamole increased basal expression of the inducible form of nitric oxide synthase. In conclusion, the results of this study demonstrate that at clinically relevant concentrations, dipyridamole inhibits profibrotic activities of renal fibroblasts. Effects on mitogenesis are mediated through a cyclic guanosine monophosphate-protein kinase G effector pathway.

Interstitial fibrosis is recognised as the best histological predictor of progressive renal disease. Myofibroblasts contribute to this process through several functions including hyperproliferation, collagen and collagenase synthesis and reorganisation of extracellular matrix. Recent limited in vitro studies suggest that 3-hydroxy-3-methylglutaryl-coenzyme A (HMG CoA) reductase inhibitors may reduce renal injury not only through their lipid-lowering effects but also by antagonising myofibroblast function. This study therefore examined the effects of lovastatin on the above interstitial myofibroblast behaviours in vitro. Primary cultures of rat renal cortical myofibroblasts were grown by explantation and characterised by immunohistochemistry. Dose response effects of lovastatin (0, 15, 30 µM) in DMEM and 10% FCS were examined on myofibroblast kinetics, total collagen synthesis, collagen I lattice contraction and actin filament rearrangement. Lovastatin decreased myofibroblast proliferation and growth. Likewise, collagen I lattice contraction and actin filament rearrangement were partially inhibited when lovastatin was added at 30 µM. In addition, lovastatin decreased both collagen and collagenase synthesis. Our results suggest that myofibroblast function may be downregulated by lovastatin in vitro. Although a decrease in myofibroblast activity may offer potential benefit in the prevention of progressive scarring, further studies will be necessary to determine the relative importance of these functions.