Emerging roles of steroid receptor coactivators in stromal cell responses.

Tissue parenchyma is the functional unit of an organ and all of the remaining cells within that organ collectively make up the tissue stroma. The stroma includes fibroblasts, endothelial cells, immune cells, and nerves. Interactions between stromal and epithelial cells are essential for tissue development and healing after injury. These interactions are governed by growth factors, inflammatory cytokines and hormone signaling cascades. The steroid receptor coactivator (SRC) family of proteins includes three transcriptional coactivators that facilitate the assembly of multi-protein complexes to induce gene expression in response to activation of many cellular transcription factor signaling cascades. They are ubiquitously expressed and are especially critical for the developmental function of steroid hormone responsive tissues. The SRCs are overexpressed in multiple cancers including breast, ovarian, prostate and endometrial cancers. In this review, we focus on the role of the SRCs in regulating the functions of stromal cell components responsible for angiogenesis, inflammation and cell differentiation.

A steroid receptor coactivator stimulator (MCB-613) attenuates adverse remodeling after myocardial infarction.

Progressive remodeling of the heart, resulting in cardiomyocyte (CM) loss and increased inflammation, fibrosis, and a progressive decrease in cardiac function, are hallmarks of myocardial infarction (MI)-induced heart failure. We show that MCB-613, a potent small molecule stimulator of steroid receptor coactivators (SRCs) attenuates pathological remodeling post-MI. MCB-613 decreases infarct size, apoptosis, hypertrophy, and fibrosis while maintaining significant cardiac function. MCB-613, when given within hours post MI, induces lasting protection from adverse remodeling concomitant with: 1) inhibition of macrophage inflammatory signaling and interleukin 1 (IL-1) signaling, which attenuates the acute inflammatory response, 2) attenuation of fibroblast differentiation, and 3) promotion of Tsc22d3-expressing macrophages-all of which may limit inflammatory damage. SRC stimulation with MCB-613 (and derivatives) is a potential therapeutic approach for inhibiting cardiac dysfunction after MI.

Proteomic profiling identifies key coactivators utilized by mutant ERα proteins as potential new therapeutic targets.

Approximately 75% of breast cancers are estrogen receptor alpha (ERα)-positive and are treatable with endocrine therapies, but often patients develop lethal resistant disease. Frequent mutations (10-40%) in the ligand-binding domain (LBD) codons in the gene encoding ERα (ESR1) have been identified, resulting in ligand-independent, constitutively active receptors. In addition, ESR1 chromosomal translocations can occur, resulting in fusion proteins that lack the LBD and are entirely unresponsive to all endocrine treatments. Thus, identifying coactivators that bind to these mutant ERα proteins may offer new therapeutic targets for endocrine-resistant cancer. To define coactivator candidate targets, a proteomics approach was performed profiling proteins recruited to the two most common ERα LBD mutants, Y537S and D538G, and an ESR1-YAP1 fusion protein. These mutants displayed enhanced coactivator interactions as compared to unliganded wild-type ERα. Inhibition of these coactivators decreased the ability of ESR1 mutants to activate transcription and promote breast cancer growth in vitro and in vivo. Thus, we have identified specific coactivators that may be useful as targets for endocrine-resistant breast cancers.

Acetylation on histone H3 lysine 9 mediates a switch from transcription initiation to elongation.

The transition from transcription initiation to elongation is a key regulatory step in gene expression, which requires RNA polymerase II (pol II) to escape promoter proximal pausing on chromatin. Although elongation factors promote pause release leading to transcription elongation, the role of epigenetic modifications during this critical transition step is poorly understood. Two histone marks on histone H3, lysine 4 trimethylation (H3K4me3) and lysine 9 acetylation (H3K9ac), co-localize on active gene promoters and are associated with active transcription. H3K4me3 can promote transcription initiation, yet the functional role of H3K9ac is much less understood. We hypothesized that H3K9ac may function downstream of transcription initiation by recruiting proteins important for the next step of transcription. Here, we describe a functional role for H3K9ac in promoting pol II pause release by directly recruiting the super elongation complex (SEC) to chromatin. H3K9ac serves as a substrate for direct binding of the SEC, as does acetylation of histone H4 lysine 5 to a lesser extent. Furthermore, lysine 9 on histone H3 is necessary for maximal pol II pause release through SEC action, and loss of H3K9ac increases the pol II pausing index on a subset of genes in HeLa cells. At select gene promoters, H3K9ac loss or SEC depletion reduces gene expression and increases paused pol II occupancy. We therefore propose that an ordered histone code can promote progression through the transcription cycle, providing new mechanistic insight indicating that SEC recruitment to certain acetylated histones on a subset of genes stimulates the subsequent release of paused pol II needed for transcription elongation.

Targeting SRC Coactivators Blocks the Tumor-Initiating Capacity of Cancer Stem-like Cells.

Tumor-initiating cells (TIC) represent cancer stem-like cell (CSC) subpopulations within tumors that are thought to give rise to recurrent cancer after therapy. Identifying key regulators of TIC/CSC maintenance is essential for the development of therapeutics designed to limit recurrence. The steroid receptor coactivator 3 (SRC-3) is overexpressed in a wide range of cancers, driving tumor initiation, cell proliferation, and metastasis. Here we report that SRC-3 supports the TIC/CSC state and induces an epithelial-to-mesenchymal transition (EMT) by driving expression of the master EMT regulators and stem cell markers. We also show that inhibition of SRC-3 and SRC-1 with SI-2, a second-generation SRC-3/SRC-1 small-molecule inhibitor, targets the CSC/TIC population both in vitro and in vivo Collectively, these results identify SRC coactivators as regulators of stem-like capacity in cancer cells and that these coactivators can serve as potential therapeutic targets to prevent the recurrence of cancer. Cancer Res; 77(16); 4293-304. ©2017 AACR.

Steroid receptor coactivators present a unique opportunity for drug development in hormone-dependent cancers.

Steroid receptor coactivators (SRCs) are essential regulators of nuclear hormone receptor function. SRCs coactivate transcription mediated by hormone stimulation of nuclear receptors and other transcription factors and have essential functions in human physiology and health. The SRCs are over expressed in a number of cancers such as breast, prostate, endometrial and pancreatic cancers where they promote tumor growth, invasion, metastasis and chemo-resistance. With their multiple roles in cancer, the SRCs are promising targets for the development of small molecule agents that can interfere with their function. For instance, perturbing SRC function with small molecule inhibitors and stimulators has been shown to be effective in reducing tumor growth in vivo. These early studies demonstrate that targeting the SRCs might prove effective for cancer treatment and more effort should be made to realize the untapped potential of developing drugs designed to target these coactivators.

Maf1 is a novel target of PTEN and PI3K signaling that negatively regulates oncogenesis and lipid metabolism.

Maf1 was initially identified as a transcriptional repressor of RNA pol III-transcribed genes, yet little is known about its other potential target genes or its biological function. Here, we show that Maf1 is a key downstream target of PTEN that drives both its tumor suppressor and metabolic functions. Maf1 expression is diminished with loss of PTEN in both mouse models and human cancers. Consistent with its role as a tumor suppressor, Maf1 reduces anchorage-independent growth and tumor formation in mice. PTEN-mediated changes in Maf1 expression are mediated by PTEN acting on PI3K/AKT/FoxO1 signaling, revealing a new pathway that regulates RNA pol III-dependent genes. This regulatory event is biologically relevant as diet-induced PI3K activation reduces Maf1 expression in mouse liver. We further identify lipogenic enzymes as a new class of Maf1-regulated genes whereby Maf1 occupancy at the FASN promoter opposes SREBP1c-mediated transcription activation. Consistent with these findings, Maf1 inhibits intracellular lipid accumulation and increasing Maf1 expression in mouse liver abrogates diet-mediated induction of lipogenic enzymes and triglycerides. Together, these results establish a new biological role for Maf1 as a downstream effector of PTEN/PI3K signaling and reveal that Maf1 is a key element by which this pathway co-regulates lipid metabolism and oncogenesis.

Covalent small ubiquitin-like modifier (SUMO) modification of Maf1 protein controls RNA polymerase III-dependent transcription repression.

RNA polymerase (pol) III transcribes genes that determine biosynthetic capacity. Induction of these genes is required for oncogenic transformation. The transcriptional repressor, Maf1, plays a central role in the repression of these and other genes that promote oncogenesis. Our studies identify an important new role for SUMOylation in repressing RNA pol III-dependent transcription. We show that a key mechanism by which this occurs is through small ubiquitin-like modifier (SUMO) modification of Maf1 by both SUMO1 and SUMO2. Mutation of each lysine residue revealed that Lys-35 is the major SUMOylation site on Maf1 and that the deSUMOylase, SENP1, is responsible for controlling Maf1K35 SUMOylation. SUMOylation of Maf1 is unaffected by rapamycin inhibition of mammalian target of rapamycin (mTOR) and mTOR-dependent Maf1 phosphorylation. By preventing SUMOylation at Lys-35, Maf1 is impaired in its ability to both repress transcription and suppress colony growth. Although SUMOylation does not alter Maf1 subcellular localization, Maf1K35R is defective in its ability to associate with RNA pol III. This impairs Maf1 recruitment to tRNA gene promoters and its ability to facilitate the dissociation of RNA pol III from these promoters. These studies identify a novel role for SUMOylation in controlling Maf1 and RNA pol III-mediated transcription. Given the emerging roles of SENP1, Maf1, and RNA pol III transcription in oncogenesis, our studies support the idea that deSUMOylation of Maf1 and induction of its gene targets play a critical role in cancer development.