Transforming Growth Factor-beta Suppresses Macrophage-induced Mesangial Cell Fibronectin Expression

Background: We have previously shown that macrophages are able to promote prosclerotic responses in rat mesangial cells. Th2-type cytokines, including interleukin-10 (IL-10), IL-13, and IL-4 as well as transforming growth factor-beta (TGF-beta), are known to have suppressive effects on various aspects of macrophage function. In the current study, we investigated the effect of TGF-beta pretreatment on the ability of macrophages to induce fibronectin expression.

Results: Conditioned medium from TGF-beta pretreated macrophages (MPCM(TGF)) induced lower fibronectin levels in mesangial cells in both the secreted and cell-associated forms, compared with conditioned medium from standard macrophages (MPCM) (5.5 +/- 0.2 vs. 3.4 +/- 0.3 and 4.05 +/- 0.45 vs. 2.3 +/- 0.2-fold increase over medium alone for MPCM versus MPCM(TGF) in supernatants and cell lysates, respectively). Northern blot analysis demonstrated that fibronectin message was marginally reduced to 0.88 +/- 0.04 (P < 0.03 vs. MPCM, N = 3) of MPCM-induced levels. However, mesangial cell transin mRNA levels induced in response to MPCM(TGF) were 2.29 +/- 0.47-fold greater than those induced by standard MPCM (P = 0.03 vs. MPCM, N = 4). TIMP-1 mRNA levels were also increased in response to MPCM(TGF), but only by 1.43 +/- 0.1-fold (P = 0.02 vs. MPCM, N = 5). Casein-FITC digestion studies confirmed that MPCM(TGF) stimulated more mesangial cell caseinolytic activity than did MPCM. In addition, MPCM-mediated up-regulation of mesangial cell TGF-beta mRNA and protein expression was significantly reduced in response to conditioned medium from macrophages pretreated with TGF-beta.

Conclusion: This study suggests that TGF-beta is able to regulate negatively the profibrotic effects of macrophages on mesangial cells by both enhancing matrix degradation and reducing synthesis.