Identification of zinc finger proteins bound to a silencer region in the rat glutathione transferase P gene

Modulation of transforming growth factor beta (TGFbeta)/Smad transcriptional responses through targeted degradation of TGFbeta-inducible early gene-1 by human seven in absentia homologue

Transforming growth factor beta (TGFbeta)-inducible early gene-1 (TIEG1) is a Krüppel-like transcription factor that is rapidly induced upon TGFbeta treatment. TIEG1 promotes TGFbeta/Smad signaling by down-regulating negative feedback through the inhibitory Smad7. In this report, we describe the identification of an E3 ubiquitin ligase, Seven in Absentia homologue-1 (SIAH1), as a TIEG1-interacting protein. We show that TIEG1 and SIAH1 interact through an amino-terminal domain of TIEG1. Co-expression of SIAH1 results in proteasomal degradation of TIEG1 but not of the related factor TIEG2. Importantly, co-expression of SIAH1 completely reverses repression of Smad7 promoter activity by TIEG1. Furthermore, overexpression of a dominant negative SIAH1 stabilizes TIEG1 and synergizes with TIEG1 to enhance TGFbeta/Smad-dependent transcriptional activation. These findings suggest a novel mechanism whereby the ability of TGFbeta to modulate gene transcription may be regulated by proteasomal degradation of the downstream effector TIEG1 through the SIAH pathway. In this manner, turnover of TIEG1 may serve to limit the duration and/or magnitude of TGFbeta responses.

Functional analysis of zinc finger proteins that bind to the silencer element in the glutathione transferase P gene

Glutathione transferase P (GST-P) gene expression is repressed in normal rats but markedly promoted during the early stage of chemical hepatocarcinogenesis. We have previously identified a silencer region in this gene promoter. The silencer is composed of several cis-elements to which at least three proteins (Silencer factor-A, -B, and -C: SF-A, SF-B, and SF-C) are known to bind. We cloned and characterized the nuclear factor 1 family and the CCAAT/enhancer-binding protein family as SF-A and SF-B, respectively. Recently, zinc finger proteins as candidates for SF-C, which binds to GST-P silencer 2 (GPS2), were isolated. These proteins include four Krüppel-like proteins (BTEB2, EZF, LKLF, and TIEG1) and other factors containing multiple zinc finger motifs (TFIIIA and MZFP). In the present study, we found that the zinc finger proteins showed the same DNA-binding affinities to GPS2. Moreover, transfection analyses revealed that BTEB2, EZF, and TIEGI repressed the GST-P promoter activity. Therefore, these three factors might contribute to the repression of the GST-P gene expression in normal rat liver.