Cited2 is a transcription factor without typical DNA binding domains. Cited2 interacts with cAMP-responsive element-binding protein-binding protein (CBP)/p300, TFAP2, Lhx2, and nuclear receptors, such as peroxisome proliferator-activated receptor and estrogen receptor to function as a transcriptional modulator. Overexpression of Cited2 in Rat1 cells leads to tumor formation in nude mice, suggesting that Cited2 is a transforming gene. Through microarray analysis, Cited2 was found to be down-regulated by transforming growth factor 1 (TGF-) in various cell lines. In this study, we confirmed that both mRNA and protein levels of Cited2 are down-regulated in MDA-MB-231 breast cancer cells. Overexpression of Smad7 or knockdown of Smad4 in MDA-MB-231 cells showed that the Smad pathway is involved in the down-regulation of Cited2. Based on nuclear run-on analysis and Cited2 promoter/reporter assay, Cited2 transcription was not affected by TGF-, supporting that down-regulation of Cited2 by TGF- is most likely through post-transcriptional regulation. By using transcriptional inhibitors, we demonstrated that the turnover of Cited2 transcripts appears to be accelerated during TGF- stimulation. Pharmacologic inhibition of translation with cycloheximide attenuated Cited2 down-regulation by TGF-. We examined the expression of recombinant Cited2 gene introduced into MDA-MB-231 cells by stable transfection, and we found that mRNA containing the Cited2 protein-coding region controlled by a heterologous promoter indeed responds to TGF--mediated down-regulation. Study from Cited2 deletion mutants showed that the C-terminal conserved region of Cited2 coding sequence is essential for the down-regulation. This is the first demonstration that TGF--mediated down-regulation of Cited2 is post-transcriptional, through the Smad pathway, and requires the presence of its coding sequence.
Cited2 (cAMP-responsive element-binding protein-binding protein (CBP)2 /p300-interacting transactivators with glutamic acid and aspartic acid-rich tail) is one of the founding members of transcriptional activators, previously named melanocyte-specific gene-related gene (MRG)1/p35srj (1-4). The members in this family function as transcriptional modulators through interaction with the p300/CBP complex (5-7). Cited2 interacts with Lhx2 to enhance the recruitment of CBP/ p300 and the TATA-binding protein leading to transcription of glycoprotein hormone ␣ subunit genes (5). On the other hand, Cited2 competes with HIF-1␣ for binding to the CH1 domain of p300, and functions as a negative modulator in the hypoxia signaling pathway (2). In addition, Cited2 interacts with TFAP2 (8) and nuclear receptors such as estrogen receptor (9) and peroxisome proliferator-activated receptor (10) to function as a transcriptional coactivator. Cited2 Ϫ/Ϫ mouse embryos die during mid-gestation with profound developmental abnormalities, including cardiac malformations, exencephaly, and adrenal agenesis (11-13). Cited2 acts upstream of Nodal, a member of the TGF- superfamily, an...