The CREB-binding protein (CBP) and p300 are two paralogous lysine acetyltransferases (KATs) that were discovered in the 1980s–1990s. Since their discovery, CBP/p300 have emerged as important regulatory proteins due to their ability to acetylate histone and non-histone proteins to modulate transcription. Work in the last 20 years has firmly established CBP/p300 as critical regulators for nuclear hormone signaling pathways, which drive tumor growth in several cancer types. Indeed, CBP/p300 are critical co-activators for the androgen receptor (AR) and estrogen receptor (ER) signaling in prostate and breast cancer, respectively. The AR and ER are stimulated by sex hormones and function as transcription factors to regulate genes involved in cell cycle progression, metabolism, and other cellular functions that contribute to oncogenesis. Recent structural studies of the AR/p300 and ER/p300 complexes have provided critical insights into the mechanism by which p300 interacts with and activates AR- and ER-mediated transcription. Breast and prostate cancer rank the first and forth respectively in cancer diagnoses worldwide and effective treatments are urgently needed. Recent efforts have identified specific and potent CBP/p300 inhibitors that target the acetyltransferase activity and the acetytllysine-binding bromodomain (BD) of CBP/p300. These compounds inhibit AR signaling and tumor growth in prostate cancer. CBP/p300 inhibitors may also be applicable for treating breast and other hormone-dependent cancers. Here we provide an in-depth account of the critical roles of CBP/p300 in regulating the AR and ER signaling pathways and discuss the potential of CBP/p300 inhibitors for treating prostate and breast cancer.

Estrogen receptor alpha (ER) is the oncogenic driver for ER+ breast cancer (BC). ER antagonists are the standard-of-care treatment for ER+ BC; however, primary and acquired resistance to these agents is common. CBP and p300 are critical ER co-activators and their acetyltransferase (KAT) domain and acetyl-lysine binding bromodomain (BD) represent tractable drug targets, but whether CBP/p300 inhibitors can effectively suppress ER signaling remains unclear. We report that the CBP/p300 KAT inhibitor A-485 and the BD inhibitor GNE-049 downregulate ER, attenuate estrogen-induced c-Myc and Cyclin D1 expression, and inhibit growth of ER+ BC cells through inducing senescence. Microarray and RNA-seq analysis demonstrates that A-485 or EP300 (encoding p300) knockdown globally inhibits expression of estrogen-regulated genes, confirming that ER inhibition is an on-target effect of A-485. Using ChIP-seq, we report that A-485 suppresses H3K27 acetylation in the enhancers of ER target genes (including MYC and CCND1) and this correlates with their decreased expression, providing a mechanism underlying how CBP/p300 inhibition downregulates ER gene network. Together, our results provide a preclinical proof-of-concept that CBP/p300 represent promising therapeutic targets in ER+ BC for inhibiting ER signaling.

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De novo lipogenesis produces lipids for membrane biosynthesis and cell signaling. Elevated lipogenesis is a major metabolic feature in cancer cells. In breast and other cancer types, genes involved in lipogenesis are highly upregulated, but the mechanisms that control their expression remain poorly understood. DAXX modulates gene expression through binding to diverse transcription factors although the functional impact of these diverse interactions remains to be defined. Our recent analysis indicates that DAXX is overexpressed in diverse cancer types. However, mechanisms underlying DAXX’s oncogenic function remains elusive. Using global integrated transcriptomic and lipidomic analyses, we show that DAXX plays a key role in lipid metabolism. DAXX depletion attenuates, while its overexpression enhances, lipogenic gene expression, lipid synthesis and tumor growth. Mechanistically, DAXX interacts with SREBP1 and SREBP2 and activates SREBP-mediated transcription. DAXX associates with lipogenic gene promoters through SREBPs. Underscoring the critical roles for the DAXX-SREBP interaction for lipogenesis, SREBP2 knockdown attenuates tumor growth in cells with DAXX overexpression, and a DAXX mutant unable to bind SREBPs are incapable of promoting lipogenesis and tumor growth. Our results identify the DAXX-SREBP axis as an important pathway for tumorigenesis.

Cancer cells exhibit elevated lipid synthesis. In breast and other cancer types, genes involved in lipid production are highly upregulated, but the mechanisms that control their expression remain poorly understood. Using integrated transcriptomic, lipidomic, and molecular studies, here we report that DAXX is a regulator of oncogenic lipogenesis. DAXX depletion attenuates, while its overexpression enhances, lipogenic gene expression, lipogenesis, and tumor growth. Mechanistically, DAXX interacts with SREBP1 and SREBP2 and activates SREBP-mediated transcription. DAXX associates with lipogenic gene promoters through SREBPs. Underscoring the critical roles for the DAXX-SREBP interaction for lipogenesis, SREBP2 knockdown attenuates tumor growth in cells with DAXX overexpression, and DAXX mutants unable to bind SREBP1/2 have weakened activity in promoting lipogenesis and tumor growth. Remarkably, a DAXX mutant deficient of SUMO-binding fails to activate SREBP1/2 and lipogenesis due to impaired SREBP binding and chromatin recruitment and is defective of stimulating tumorigenesis. Hence, DAXX’s SUMO-binding activity is critical to oncogenic lipogenesis. Notably, a peptide corresponding to DAXX’s C-terminal SUMO-interacting motif (SIM2) is cell-membrane permeable, disrupts the DAXX-SREBP1/2 interactions, and inhibits lipogenesis and tumor growth. These results establish DAXX as a regulator of lipogenesis and a potential therapeutic target for cancer therapy.

CBP/p300 are two paralogous lysine acetyltransferases that acetylate protein substrates including histones, and serve as transcriptional co-activators for numerous signaling pathways involved in tumorigenesis and cancer progression. Recently, evidence has emerged that p300 regulates the transcription of genes involved in lipogenesis in cancers. Dysregulated lipogenesis and increased de novo production of fatty acids (FA) represent a major metabolic shift in cancer. De novo FA synthesis in cancer is suggested to be a source of lipids for membrane biogenesis, a reservoir of energy, and a source of pro-survival signaling molecules. Therefore, de novo FA synthesis has been linked to survival and increased proliferation of cancer cells, representing a potential area for therapeutic intervention. While emerging evidence indicates CBP/p300 may play a key role in regulating lipogenesis, little is known about the mechanism of how these proteins control lipid production in different cancers. Colorectal cancer (CRC) is a commonly diagnosed human cancer and is a major cause of cancer-related mortality. Key regulators for de novo FA synthesis, such as FASN, have been reported to be elevated in colorectal cancer. We analyzed publically available data sets and report that in colon adenocarcinoma, the mRNA levels of CBP/p300 positively correlate with the expression of genes involved in lipogenesis, such as FASN, SREBP1 and SREBP2. Furthermore, our analysis shows that EP300 expression is negatively correlated with patient survival. Strikingly, our preliminary data suggests that HDAC inhibitors may regulate p300 activity and could serve as a therapeutic approach for suppressing lipogenesis. We are currently utilizing small molecule inhibitors, genetic manipulation of the CREBBP/EP300 genes, and biochemical experiments to investigate the role of CBP/p300 in de novo FA synthesis in colorectal cancer. We hypothesize that inhibition of CBP/p300 could be an attractive therapeutic option for inhibiting tumor growth through downregulating de novo FA synthesis in colorectal cancer. (Supported by grants from James and Esther King Biomedical Research Program and Bankhead-Coly Cancer Research Program, Florida Department of Health.) Citation Format: Aaron Waddell, Iqbal Mahmud, Daiqing Liao. Role of acetyltransferases CBP and p300 in de novo fatty acid synthesis in colorectal cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 1473.

Luminal breast cancer represents approximately two thirds of all breast cancer cases and is characterized by the expression of hormone receptors, such as the estrogen receptor alpha (ER). ERα is a member of the steroid nuclear receptor family and is involved in a hormonal signaling pathway that drives tumor growth through upregulation of genes involved in cell cycle progression, such as MYC and CCND1. Antagonists of ER function are the standard of care treatment for ER+ luminal breast cancer. However, resistance to ER antagonists is common in advanced breast cancer cases. Therefore, there is an urgent need to investigate new therapeutics that can be utilized to treat tumors that are resistant to traditional therapies. p300/CBP are two paralogous acetyltransferases that catalyze histone 3, lysine 18 and 27 acetylation (H3K18ac and H3K27ac) at promoters and enhancers to promote gene expression. ER mediated transcription is critically reliant upon the recruitment of co-activators, including p300/CBP. However, the mechanistic role of p300/CBP catalytic activity in globally regulating ER mediated transcription remains poorly understood. Inhibition of p300/CBP, as a critical co-activator of ER, potentially represents an applicable strategy for inhibiting ER function in luminal breast tumors. Importantly, the catalytic histone acetyltransferase (HAT) domain of p300/CBP is under intense investigation as a target of small molecule therapeutics in cancer. We analyzed publicly available data and report that p300/CBP are upregulated at the mRNA level in breast cancer, including the luminal subtypes, and that their expression negatively correlates with patient survival. We also found that pharmacologic inhibition of p300/CBP HAT activity in ER+ cell lines potently suppresses ER mediated transcription, downregulates ER, c-Myc and Cyclin D1 protein levels and prevents estrogen induced growth in vitro. Studies are underway to understand how p300/CBP regulates ER signaling using genetic, pharmacologic, and biochemical methods to assess the therapeutic effects of pharmacologic p300/CBP inhibition on ER+ breast cancer. (Supported by Bankhead-Coley Research Program, and James and Esther King Biomedical Research Program, Florida Department of Health) Citation Format: Aaron Waddell, Iqbal Mahmud, Guimei Tian, Daiqing Liao. Targeting the estrogen receptor pathway in luminal breast cancer through inhibition of p300/CBP [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 5214.

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