Background: The best characterized signaling pathway employed by transforming growth factor-beta (TGF-beta) is the Smad pathway; however, its role in matrix production in mesangial cells is unclear. We focused on Smad4, as Smad4 is essential for the activation of Smad-dependent target genes.
Methods: To investigate the function of Smad4 in extracellular matrix (ECM) production, we generated several stably transfected mesangial cell lines (MMC) that have a deletion in the linker region (Smad4 Delta M4: Delta 275-322) or have a deletion in MH1 of Smad4 (Smad4N4: Delta 1-136). The ECM genes, alpha1 type I collagen (COL1A1), plasminogen activator inhibitor-1 (PAI-1) and fibronectin (FN) were assessed in wild-type mesangial cells and stably transfected Smad4-DN cell lines in the absence and presence of TGF-beta.
Results: As compared to wild-type MMC that had a 10.8-fold stimulation of TGF-beta-induced p3TP-Lux activity, MMC stably transfected with Smad4 Delta M4 and Smad4N4 had only a 2.0-fold and 1.3-fold stimulation, respectively, indicating that they had dominant-negative effects on TGF-beta signaling. Basal and TGF-beta-induced COL1A1 expression in Smad4 dominant-negative cells were dramatically reduced to very low levels. The early (2 hours) TGF-beta-induced PAI-1 mRNA expression was inhibited; however, the sustained (24 to 48 hours) TGF-beta-induced expression was not affected in Smad4 dominant-negative cells. For FN, TGF-beta-induced expression was maintained in Smad4-dominant negative cells.
Conclusion: These results indicate that Smad4 is essential for basal and TGF-beta-induced COL1A1 expression, and contributes to the early, but not sustained TGF-beta-induced PAI-1 expression in mesangial cells. However, TGF-beta-induced FN expression is independent of Smad4. In conclusion, Smad4 has a discriminate effect in mediating specific ECM molecules stimulated by TGF-beta in mesangial cells.