CREB Binding Protein Is a Coactivator for the Androgen Receptor and Mediates Cross-talk with AP-1

Abstract

Androgens are critical in the development and maintenance of the male reproductive system and important in the progression of prostate cancer. The effects of androgens are mediated through the androgen receptor (AR), which is a ligand-modulated transcription factor that belongs to the nuclear receptor superfamily. In addition to its ability to activate transcription from androgen response elements, AR can inhibit activator protein-1 (AP-1) activity, composed of Jun and Fos oncoproteins, in a ligand-dependent manner. Conversely, when activated, AP-1 can block AR activity. We found that CREB (cAMP response element-binding protein) binding protein (CBP) had a direct role in both of these activities of AR. CBP significantly increased the ability of endogenous AR in LNCaP cells to activate transcription from an AR-dependent reporter construct. On the other hand, repression of AR activity by treatment of LNCaP cells with an activator of AP-1 was largely relieved when CBP was ectopically expressed. AR and CBP can physically interact in vitro as was shown in glutathione S-transferase pulldown assays. Whereas both the N terminus and ligand-binding domain of AR can interact with CBP, a short region in the N terminus of CBP is required for these interactions. As opposed to the interaction of CBP with other nuclear receptors studied so far, CBP-AR interactions were not affected by ligand binding to AR in vitro. These data suggest that CBP is a coactivator for AR in vivo and that the transcriptional interference between AR and AP-1 is the result of competition for limiting amounts of CBP in the cell.Androgens have a pivotal role in the development and maintenance of the male reproductive system (1, 2). The actions of androgens are mediated through an intracellular receptor, the androgen receptor (AR), 1 which belongs to the nuclear receptor superfamily (3, 4). Nuclear receptors are ligand-activated transcription factors that possess highly conserved DNA-binding domains and moderately conserved ligand-binding domains (LBDs), whereas they are quite divergent in the N-terminal domain (NTD) (for reviews, see Refs. 3 and 4). Transactivation function of nuclear receptors is primarily mediated by sequences in both the NTD and a short region in the LBD, referred to as the activator function-1 (AF-1) and AF-2 domains, respectively. Recent studies suggest that an interaction between the NTD and the LBD may play a role in the transcriptional activities of some nuclear receptors, including AR (5-7).The activity of nuclear receptors is modulated by interactions with other proteins. These could be mediated through heterodimeric interactions within the nuclear receptor family, such as those between retinoid X receptors and thyroid hormone, retinoic acid, and vitamin D receptors in which the heterodimer has an increased ability to activate transcription (for a review, see Ref. 8). On the other hand, activator protein-1 (AP-1) complexes, composed of either Jun homodimers or JunFos heterodimers (for a review, see Ref. 9), interfere with ligan...

The Transcriptional Co-Activator cAMP Response Element-Binding Protein-Binding Protein Is Expressed in Prostate Cancer and Enhances Androgen- and Anti-Androgen-Induced Androgen Receptor Function

“…11 In this context, it is important to note that the increase in AR activity by ARA70 or CBP is achieved by two different mechanisms: whereas androgens and anti-androgens promote physical interaction between the AR and ARA70, the association between the AR and CBP is not influenced by the ligand. 11,14,15 Our findings showing that AR expression is not altered by anti-androgens in DU-145 cells are consistent with previous data that the levels of immunoreactive ARs are not changed by androgen in cells transfected with AR and CBP cDNA. 15 In LNCaP cells, however, androgenic hormones enhance AR expression by protein stabilization even when neither co-activator is overexpressed.…”

“…14,15 There were differences in the magnitude of AR activity induced by hydroxyflutamide or bicalutamide in the presence of the overexpressed co-activator. Hydroxyflutamide, at a concentration of 5 mol/L, induced 50% of the maximal CAT activity caused by the synthetic androgen R1881, whereas bicalutamide-induced reporter gene activity was 24%.…”

“…12,13 One of the proteins that augments AR activity in a liganddependent manner is CREB (cAMP response elementbinding protein)-binding protein (CBP). 14,15 Moreover, Fu and associates 16 demonstrated that the AR is acetylated by the CBP-related protein p300 and that a p300 mutation results in a reduced ligand-dependent activation of the AR. The CBP-associated factor P/CAF rescued cyclin D1-mediated trans-repression of the AR.…”

Amplification of the androgen receptor may not explain the development of androgen‐independent prostate cancer

“…transcription factors cyclic AMP binding protein and activation transcription factor, by androgen-independent activation of AR [14,15] or via pathways that do not involve AR [16]. The protein kinase C pathway has also been reported to stimulate the expression of androgenregulated genes via AP-1, an AR-independent signal transduction pathway [16,17].…”

Androgen signaling and its interactions with other signaling pathways in prostate cancer

“…(72) Furthermore, there may be molecular interactions between the substrate of JNK, the AP-1 transcription complex, and AR, either directly, or involving cofactors, and these interactions can either be stimulatory or inhibitory. (73)(74)(75) Since there is very little information on JNK expression in human prostate cancer specimens, it is at present not clear in what way JNK affects prostate carcinogenesis.…”