Expression and purification of recombinant human progesterone receptor in baculovirus and bacterial systems

A Novel IMP1 Inhibitor, BTYNB, Targets c-Myc and Inhibits Melanoma and Ovarian Cancer Cell Proliferation

The oncofetal mRNA-binding protein, IMP1 or insulin-like growth factor-2 mRNA-binding protein 2 (IGF2BP1), binds to and stabilizes c-Myc, β-TrCP1, and other oncogenic mRNAs, leading to increased expression of the proteins encoded by its target mRNAs. IMP1 is frequently overexpressed in cancer and is strongly correlated with a poor prognosis and reduced survival in melanoma, ovarian, breast, colon, and lung cancer. While IMP1 is an attractive anticancer drug target, there are no small molecule inhibitors of IMP1. A fluorescence anisotropy-based assay was used to screen 160,000 small molecules for their ability to inhibit IMP1 binding to fluorescein-labeled c-Myc mRNA. The small molecule, BTYNB, was identified as a potent and selective inhibitor of IMP1 binding to c-Myc mRNA. In cells, BTYNB downregulates several mRNA transcripts regulated by IMP1. BTYNB destabilizes c-Myc mRNA, resulting in downregulation of c-Myc mRNA and protein. BTYNB downregulates β-TrCP1 mRNA and reduces activation of nuclear transcriptional factors-kappa B (NF-κB). The oncogenic translation regulator, eEF2, emerged as a new IMP1 target mRNA, enabling BTYNB to inhibit tumor cell protein synthesis. BTYNB potently inhibited proliferation of IMP1-containing ovarian cancer and melanoma cells with no effect in IMP1-negative cells. Overexpression of IMP1 reversed BTYNB inhibition of cell proliferation. BTYNB completely blocked anchorage-independent growth of melanoma and ovarian cancer cells in colony formation assays. With its ability to target c-Myc and to inhibit proliferation of difficult-to-target melanomas and ovarian cancer cells, and with its unique mode of action, BTYNB is a promising small molecule for further therapeutic evaluation and mechanistic studies.

High-throughput fluorescence anisotropy screen for inhibitors of the oncogenic mRNA binding protein, IMP-1

Cancer cell proliferation is regulated by oncogenes, such as c-Myc. An alternative approach to directly targeting individual oncogenes is to target IMP-1, an oncofetal protein that binds to and stabilizes messenger RNAs (mRNAs), leading to elevated expression of c-Myc and other oncogenes. Expression of IMP-1 is tightly correlated with a poor prognosis and reduced survival in ovarian, lung, and colon cancer. Small-molecule inhibitors of IMP-1 have not been reported. We established a fluorescence anisotropy/polarization microplate assay (FAMA) for analyzing binding of IMP-1 to a fluorescein-labeled 93 nucleotide c-Myc mRNA target (flMyc), developed the assay as a highly robust (Z' factor = 0.60) FAMA-based high-throughput screen for inhibitors of binding of IMP-1 to flMyc, and carried out a successful pilot screen of 17,600 small molecules. Our studies support rapidly filtering out toxic nonspecific inhibitors using an early cell-based assay in control cells lacking the target protein. The physiologic importance of verified hits from the in vitro high-throughput screen was demonstrated by identification of the first small-molecule IMP-1 inhibitor, a lead compound that selectively inhibits proliferation of IMP-1-positive cancer cells with very little or no effect on proliferation of IMP-1-negative cells.

ERα-associated protein networks

Estrogen receptor α (ERα) is a ligand-activated transcription factor that, upon binding hormone, interacts with specific recognition sequences in DNA. An extensive body of literature has documented the association of individual regulatory proteins with ERα. It has recently become apparent that, instead of simply recruiting individual proteins, ERα recruits interconnected networks of proteins with discrete activities that play crucial roles in maintaining the structure and function of the receptor, stabilizing the receptor-DNA interaction, influencing estrogen-responsive gene expression, and repairing misfolded proteins and damaged DNA. Together these studies suggest that the DNA-bound ERα serves as a nucleating factor for the recruitment of protein complexes involved in key processes including the oxidative stress response, DNA repair, and transcription regulation.

A new small molecule inhibitor of estrogen receptor alpha binding to estrogen response elements blocks estrogen-dependent growth of cancer cells

Estrogen receptor alpha (ERalpha) plays an important role in several human cancers. Most current ERalpha antagonists bind in the receptor ligand binding pocket and compete for binding with estrogenic ligands. Instead of the traditional approach of targeting estrogen binding to ER, we describe a strategy using a high throughput fluorescence anisotropy microplate assay to identify small molecule inhibitors of ERalpha binding to consensus estrogen response element (cERE) DNA. We identified small molecule inhibitors of ERalpha binding to the fluorescein-labeled (fl)cERE and evaluated their specificity, potency, and efficacy. One small molecule, theophylline, 8-[(benzylthio)methyl]-(7CI,8CI) (TPBM), inhibited ERalpha binding to the flcERE (IC(50) approximately 3 microm) and inhibited ERalpha-mediated transcription of a stably transfected ERE-containing reporter gene. Inhibition by TPBM was ER-specific, because progesterone and glucocorticoid receptor transcriptional activity were not significantly inhibited. In tamoxifen-resistant breast cancer cells that overexpress ERalpha, TPBM inhibited 17beta-estradiol (E(2))-ERalpha (IC(50) 9 microm) and 4-hydroxytamoxifen-ERalpha-mediated gene expression. Chromatin immunoprecipitation showed TPBM reduced E(2).ERalpha recruitment to an endogenous estrogen-responsive gene. TPBM inhibited E(2)-dependent growth of ERalpha-positive cancer cells (IC(50) of 5 microm). TPBM is not toxic to cells and does not affect estrogen-independent cell growth. TPBM acts outside of the ER ligand binding pocket, does not act by chelating the zinc in ER zinc fingers, and differs from known ERalpha inhibitors. Using a simple high throughput screen for inhibitors of ERalpha binding to the cERE, a small molecule inhibitor has been identified that selectively inhibits ERalpha-mediated gene expression and estrogen-dependent growth of cancer cells.

Arsenic disruption of steroid receptor gene activation: Complex dose-response effects are shared by several steroid receptors

Chronic intake of arsenic (As) has been associated with increased risk of cancer, diabetes, developmental and reproductive problems, and cardiovascular disease. Recent studies suggest increased health risks with drinking water levels as low as 5-10 ppb. We previously reported that As disrupts glucocorticoid receptor (GR) mediated transcription in a very complex fashion. Low As levels (0.1-0.7 microM) stimulated transcription, whereas slightly higher levels (1-3 microM) were inhibitory. The DNA binding domain (DBD) was the minimal region of GR required for the response to As. Mutations in the DBD that alter the conformation of the dimerization domain (D-loop) to a DNA-bound GR conformation abolished the stimulatory effect and enhanced the inhibitory response to As. Here we report that receptors for progesterone (PR) and mineralocorticoids display a complex As response similar to that of the GR, suggesting a common mechanism for this effect. The complex response to As is not due to altered steroid or receptor levels. Moreover, a well-characterized GR dimerization mutant displayed a wild-type biphasic response to As for several divergent reporter genes, suggesting that dimerization is not critical for the response to As. Fluorescence polarization studies with purified PR and GR demonstrated that the specific PR/GR-DNA interaction is not altered in the presence of As. These results indicate that the numerous and diverse human health effects associated with As exposure may be mediated, at least in part, through its ability to simultaneously disrupt multiple hormone receptor systems.

Regulation of the amino-terminal transcription activation domain of progesterone receptor by a cofactor-induced protein folding mechanism

We previously identified a small basic leucine zipper (bZIP) protein, Jun dimerization protein 2 (JDP-2), that acts as a coregulator of the N-terminal transcriptional activation domain of progesterone receptor (PR). We show here that JDP-2, through interaction with the DNA binding domain (DBD), induces or stabilizes structure in the N-terminal domain in a manner that correlates with JDP-2 stimulation of transcriptional activity. Circular dichroism spectroscopy experiments showed that JDP-2 interaction caused a significant increase in overall helical content of a two-domain PR polypeptide containing the N-terminal domain and DBD and that the change in structure resides primarily in the N-terminal domain. Thermal melt curves showed that the JDP-2/PR complex is significantly more stable than either protein alone, and partial proteolysis confirmed that JDP-2 interaction alters conformation of the N-terminal domain of PR. Functional analysis of N-terminal domain mutants and receptor chimeras provides evidence that the stimulatory effect of JDP-2 on transcriptional activity of PR is mediated through an interdomain communication between the DBD and the N-terminal domain and that transcriptional activity and functional response to JDP-2 are mediated by multiple elements of the N-terminal domain as opposed to a discrete region.

Fluorescence anisotropy microplate assay for analysis of steroid receptor-DNA interactions

To analyze the interactions of steroid/nuclear hormone receptors with their DNA response elements, we used ultra low-volume microplates to develop a simple and rapid fluorescence anisotropy assay. The novel fluorescence anisotropy microplate assay (FAMA) was applied to the binding of estrogen and progesterone receptors (ER and PR, respectively) to their respective DNA response elements. The FAMA offers exceptional flexibility in its ability to test a variety of binding conditions and DNA response elements in real time. This assay can differentiate between, and quantitate, sequence-specific and nonspecific binding of receptors to DNA and offers the possibility of true solution analysis of the interaction of coregulators with the estrogen response element (ERE)-ER complex. To test suitability for screening large compound libraries, we demonstrated that the FAMA generates stable signals for more than 4 hours, is insensitive to inhibition by dimethyl sulfoxide (DMSO), and works well in 384-well plates. We analyzed inhibition of receptor-DNA interaction by several zinc chelators and demonstrated zinc dependence and a generally higher sensitivity to inhibition for PR-progesterone response element (PRE) interactions than for ER-ERE interactions. The FAMA is the first system suitable for screening large compound libraries to identify novel compounds that antagonize (or stimulate) binding of steroid receptors to their DNA response elements.