Hematopoietic stem cells (HSC) are tightly regulated through, as yet, undefined mechanisms that balance self-renewal and differentiation. We have identified a role for the transcriptional coactivators CREB-binding protein (CBP) and p300 in such HSC fate decisions. A full dose of CBP, but not p300, is crucial for HSC self-renewal. Conversely, p300, but not CBP, is essential for proper hematopoietic differentiation. Furthermore, in chimeric mice, hematologic malignancies emerged from both CBP ؊/؊ and p300 ؊/؊ cell populations. Thus, CBP and p300 play essential but distinct roles in maintaining normal hematopoiesis, and, in mice, both are required for preventing hematologic tumorigenesis.

Angiotensin II (Ang II) stimulates norepinephrine transporter (NET) and tyrosine hydroxylase (TH) in the neurons, but the signal transduction mechanism of this neuromodulation is not understood. Treatment of neuronal cultures of hypothalamusbrainstem with Ang II resulted in a time-and dose-dependent activation of Ras, Raf-1, and mitogen-activated protein kinase. This activation was mediated by the interaction of Ang II with the AT 1 receptor subtype and was associated with the redistribution of AT 1 receptor with Ras and Raf-1 on the neuronal membrane. Treatment with antisense oligonucleotide (AON) to mitogen-activated protein kinase decreased mitogen-activated protein kinase immunoreactivity by 70% and attenuated Ang II stimulation of c-fos, NET, and TH mRNA levels. This demonstrates that induction of these genes requires mitogenactivated protein kinase activation by Ang II. In contrast, AON to mitogen-activated protein kinase failed to inhibit Ang II stimulation of plasminogen activator inhibitor-1 mRNA levels. These results suggest that AT 1 receptors are coupled to a Ras-Raf-1 mitogen-activated protein kinase signal transduction pathway that is responsible for stimulation of NET and TH, two neuromodulatory actions of Ang II in the brain.

The androgen receptor, like other nuclear receptors, activates target genes by binding to hormone-responsive enhancers. Here we demonstrate that androgen induces robust recruitment of androgen receptor, members of the p160 coactivator family, and CREB-binding protein͞p300 specifically at the distant enhancer of prostate-specific antigen (PSA) gene. Unexpectedly, we found that RNA polymerase II (Pol II) is directly recruited to the enhancer in a hormone-dependent manner, independent of the proximal promoter, and that the isolated PSA enhancer can mediate efficient androgen induction of transcription. Inhibition of the Pol II carboxyl-terminal domain kinase activity with low concentrations of flavopiridol blocks Pol II transfer from the enhancer to the promoter and selectively abolishes PSA induction by androgen. Moreover, elevated levels of the p160 coactivator ACTR͞ AIB1 increase both androgen-dependent and -independent PSA expression, by facilitating Pol II recruitment to the enhancer. These results support a model in which nuclear receptors and their coactivators mediate hormone induction by serving as a staging platform for Pol II recruitment.A ndrogen is a key regulator of cell growth and differentiation in male sexual development and function as well as in the progression of prostate cancer. These hormonal effects are mediated by the androgen receptor (AR), a member of the nuclear receptor superfamily, which consists of liganddependent transcription factors (1). Recently, a growing number of nuclear proteins have been found to associate with AR and are postulated to mediate transcriptional control by the receptor (2, 3). Among them are members of the p160 coactivator family, including SRC-1, TIF2͞GRIP1, and ACTR (AIB1͞RAC3͞ TRAM1͞pCIP). The p160 coactivators associate with nuclear receptors in a hormone-dependent fashion primarily through the central receptor-interaction domain that harbors several LXXLL motifs.Although p160 coactivators possess intrinsic histone acetylase (HAT) activities, they are also capable of recruiting other HAT proteins such as CREB-binding protein (CBP), p300, and PCAF (4, 5), and the nuclear protein arginine methyltransferases CARM1 and PRMT (6, 7). Both the acetylases and methylases can regulate transcription by modifying nucleosomal and nonnucleosomal nuclear proteins (7,8). However, how the p160 coactivators promote RNA polymerase II (Pol II) transcription is still not well understood. Domain mapping experiments suggest that transactivation by the p160 coactivators is largely attributable to their recruitment of CBP͞p300 and the HAT activities (4, 9-12). Although earlier experiments indicate that CBP͞p300 may interact directly with the Pol II complex (13,14), this notion lost currency with the discovery of the importance of the HAT activity in transcriptional activation and the more recent demonstration that the simple recruitment and assembly of Pol II machinery in mammalian cells is not sufficient for productive transcription (15, 16).Previously, we and others demonstrated th...