TGF-beta 1 activates MAP kinase in human mesangial cells: A possible role in collagen expression

Background Although the pathogenic relevance of transforming growth factor- beta (TGF-beta) to glomerular sclerosis has been established: the intracell ular mechanisms by which TGF-beta induces extracellular matrix accumulation are not fully understood. We examined whether the mitogen-activated protei n (MAP) kinase pathway is involved in TGF-beta 1-induced collagen expressio n by cultured human mesangial cells. Methods. The activation of MAP kinase pathways by TGF-beta 1 was assessed b y immunoblot with anti-phospho-ERK or -JNK antibodies and by transfection o f plasmids expressing pathway-specific transcription activators fused to th e DNA-binding domain of GAL4, as well as a GAL4 response element-luciferase reporter gene. The role of MAP kinase was assessed using biochemical inhib itors and transiently expressed dominant negative mutant constructs. The ef fects on TGF-beta 1-induced alpha 1(I) collagen expression were evaluated b y Northern blot and by activation of a transiently transfected alpha 1(I) p romoter-luciferase reporter construct. Results. ERK and JNK phosphorylation occurred 30 minutes and one hour, resp ectively, after TGF-beta 1 treatment. A biochemical blockade of the ERK pat hway inhibited TGF-beta 1-induced alpha 1(I) collagen expression. A dominan t negative mutant of ERK1 but not of JNK decreased alpha 1(I) gene promoter activation. Activation of the TGF-beta-responsive p3TP-Lux-construct was p artially inhibited by cotransfection of an ERK1 dominant negative mutant. Conclusion. These data indicate that MAP kinase pathways can be activated b y TGF-beta 1 in mesangjal cells and that the ERK MAP kinase plays a role in TGF-beta-stimulated collagen I expression. Because we have shown previousl y that SMADs mediate TGF-beta 1-stimulated collagen I expression, our findi ngs raise the possibility of interactions between the MAP kinase and the SM AD pathways.