The related proteins p300 and CBP (cAMP-response-element-binding protein (CREB)-binding protein)) are transcriptional co-activators that act with other factors to regulate gene expression and play roles in many cell-differentiation and signal transduction pathways. Both proteins have intrinsic histone-acetyltransferase activity and may act directly on chromatin, of which histone is a component, to facilitate transcription. They are also involved in growth control pathways, as shown by their interaction with the tumour suppressor p53 and the viral oncogenes E1A and SV40 T antigen. Here we report functional differences of p300 and CBP in vivo. We examined their roles during retinoic-acid-induced differentiation, cell-cycle exit and programmed cell death (apoptosis) of embryonal carcinoma F9 cells, using hammerhead ribozymes capable of cleaving either p300 or CBP messenger RNAs. F9 cells expressing a p300-specific ribozyme became resistant to retinoic-acid-induced differentiation, whereas cells expressing a CBP-specific ribozyme were unaffected. Similarly, retinoic-acid-induced transcriptional upregulation of the cell-cycle inhibitor p21Cip1 required normal levels of p300, but not CBP, whereas the reverse was true for p27Kip1. In contrast, both ribozymes blocked retinoic-acid-induced apoptosis, indicating that both co-activators are required for this process. Thus, despite their similarities, p300 and CBP have distinct functions during retinoic-acid-induced differentiation of F9 cells.

Transcription factors carry functional domains, which are often physically distinct, for sequence-specific DNA binding, transcriptional activation and regulatory functions. The transcription factor ATF-2 is a DNA-binding protein that binds to cyclic AMP-response elements (CREs), forms a homodimer or heterodimer with c-Jun, and stimulates CRE-dependent transcription. Here we report that ATF-2 is a histone acetyltransferase (HAT), which specifically acetylates histones H2B and H4 in vitro. Motif A, which is located in the HAT domain, is responsible for the stimulation of CRE-dependent transcription; moreover, in response to ultraviolet irradiation, phosphorylation of ATF-2 is accompanied by enhanced HAT activity of ATF-2 and CRE-dependent transcription. These results indicate that phosphorylation of ATF-2 controls its intrinsic HAT activity and its action on CRE-dependent transcription. ATF-2 may represent a new class of sequence-specific factors, which are able to activate transcription by direct effects on chromatin components.

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The cAMP-response element-binding protein-binding protein (CBP) and p300 are common coactivators for several transcriptional factors. It has been reported that both CBP and p300 are significant for the activation of peroxisome proliferator-activated receptor ␥ (PPAR␥), which is a crucial nuclear receptor in adipogenesis. However, it remains unclear whether CBP and/or p300 is physiologically essential to the activation of PPAR␥ in adipocytes and adipocyte differentiation. In this study, we investigated the physiological significance of CBP/ p300 in NIH3T3 cells transiently expressing PPAR␥ and CBP and in 3T3-L1 preadipocytes stably expressing CBP-or p300-specific ribozymes. In PPAR␥-transfected NIH3T3 cells, induction of expression of PPAR␥ target genes such as adipocyte fatty acid-binding protein (aP2) and lipoprotein lipase (LPL) by adding thiazolidinedione was enhanced, depending on the amount of a CBP expression plasmid transfected. Expression of aP2 and LPL genes, as well as glycerol-3-phosphate dehydrogenase activity and triacylglyceride accumulation after adipogenic induction, was largely suppressed in 3T3-L1 adipocytes expressing either the CBP-or p300-specific active ribozyme, but not in inactive ribozyme-expressing cells. These data suggest that both CBP and p300 are indispensable for the full activation of PPAR␥ and adipocyte differentiation and that CBP and p300 do not mutually complement in the process.