Connective tissue growth factor gene regulation. Requirements for its induction by transforming growth factor-beta 2 in fibroblasts.

In skin, the profibrotic protein connective tissue growth factor (CTGF) is not normally expressed. However, when skin cells are exposed to transforming growth factor-beta (TGF-beta), CTGF is induced in fibroblasts but not in epithelial cells. We have begun to investigate the requirements for the fibroblast-selective induction of CTGF by TGF-beta. Previously we found that this response was Smad-dependent. Now we show that protein kinase C and Ras/MEK/ERK are necessary for the TGF-beta induction of the CTGF promoter but not of a generic Smad-responsive promoter (SBE-lux). Induction of the CTGF promoter is antagonized by c-Jun or by MEKK1, suggesting that a proper balance between the Ras/MEK/ERK and JNK MAPK cascades is necessary for TGF-beta induction of CTGF. We identify the minimal CTGF promoter element necessary and sufficient to confer TGF-beta responsiveness to a heterologous promoter and show that a tandem repeat of a consensus transcription enhancer factor binding element, 5'-GAGGAATGG-3', is necessary for this induction. This element has not been previously shown to play a role in TGF-beta induction of gene expression in fibroblasts. Gel shift analysis shows that this sequence binds nuclear factors that are greatly enriched in fibroblasts relative to epithelial cells. Thus Smads, Ras/MEK/ERK, protein kinase C, and fibroblast-enriched factors that bind GAGGAATGG act together to drive the TGF-beta-mediated induction of CTGF in fibroblasts.