LDB1-mediated transcriptional complexes are sensitive to islet stress

Excess nutrients and proinflammatory cytokines impart stresses on pancreatic islet β-cells that, if unchecked, can lead to cellular dysfunction and/or death. Among these stress-induced effects is loss of key β-cell transcriptional regulator mRNA and protein levels required for β-cell function. Previously, our lab and others reported that LIM-domain complexes comprised the LDB1 transcriptional co-regulator and Islet-1 (ISL1) transcription factor are required for islet β-cell development, maturation, and function. The LDB1:ISL1 complex directly occupies and regulates key β-cell genes, including MafA, Pdx1, and Slc2a2, to maintain β-cell identity and function. Given the importance of LDB1:ISL1 complexes, we hypothesized that LDB1 and/or ISL1 levels, like other transcriptional regulators, are sensitive to β-cell nutrient and cytokine stresses, likely contributing to β-cell (dys)function under various stimuli. We tested this by treating β-cell lines or primary mouse islets with elevating glucose concentrations, palmitate, or a cytokine cocktail of IL-1β, TNFα, and IFNγ. We indeed observed that LDB1 mRNA and/or protein levels were reduced upon palmitate and cytokine (cocktail or singly) incubation. Conversely, acute high glucose treatment of β-cells did not impair LDB1 or ISL1 levels, but increased LDB1:ISL1 interactions. These observations suggest that LDB1:ISL1 complex formation is sensitive to β-cell stresses and that targeting and/or stabilizing this complex may rescue lost β-cell gene expression to preserve cellular function.

The Ldb1 transcriptional co-regulator is required for establishment and maintenance of the pancreatic endocrine lineage

Pancreatic islet cell development is regulated by transcription factors (TFs) that mediate embryonic progenitor differentiation toward mature endocrine cells. Prior studies from our lab and others showed that the islet-enriched TF, Islet-1 (Isl1), interacts with the broadly-expressed transcriptional co-regulator, Ldb1, to regulate islet cell maturation and postnhyperatal function (by embryonic day (E)18.5). However, Ldb1 is expressed in the developing pancreas prior to Isl1 expression, notably in multipotent progenitor cells (MPCs) marked by Pdx1 and endocrine progenitors (EPs) expressing Neurogenin-3 (Ngn3). MPCs give rise to the endocrine and exocrine pancreas, while Ngn3⁺ EPs specify pancreatic islet endocrine cells. We hypothesized that Ldb1 is required for progenitor identity in MPC and EP populations during development to impact islet appearance and function. To test this, we generated a whole-pancreas Ldb1 knockout, termed Ldb1^ΔPanc , and observed severe developmental and postnatal pancreas defects including disorganized progenitor pools, a significant reduction of Ngn3-expressing EPs, Pdx1^HI β-cells, and early hormone⁺ cells. Ldb1^ΔPanc neonates presented with severe hyperglycemia, hypoinsulinemia, and drastically reduced hormone expression in islets, yet no change in total pancreas mass. This supports the endocrine-specific actions of Ldb1. Considering this, we also developed an endocrine-enriched model of Ldb1 loss, termed Ldb1^ΔEndo . We observed similar dysglycemia in this model, as well as a loss of islet identity markers. Through in vitro and in vivo chromatin immunoprecipitation experiments, we found that Ldb1 occupies key Pdx1 and Ngn3 promoter domains. Our findings provide insight into novel regulation of endocrine cell differentiation that may be vital toward improving cell-based diabetes therapies.

The DUX4 protein is a co-repressor of the progesterone and glucocorticoid nuclear receptors

DUX4 is a transcription factor required during early embryonic development in placental mammals. In this work we provide evidence that DUX4 is a co-repressor of nuclear receptors (NRs) of progesterone (PR) and glucocorticoids (GR). The DUX4 C-ter and N-ter regions, including the nuclear localization signals and homeodomain motifs, contribute to the corepressor activity of DUX4 on PR and GR. Immunoprecipitation studies, using total protein extracts of cells expressing tagged versions of DUX4 and GR, support that these proteins are physically associated. Our studies suggest that DUX4 could modulate gene expression by coregulating the activity of hormone NRs. This is the first report highlighting a potential endocrine role for DUX4.

Mediator of DNA Damage Checkpoint 1 (MDC1) Is a Novel Estrogen Receptor Coregulator in Invasive Lobular Carcinoma of the Breast

Invasive lobular carcinoma (ILC) is the most common special histologic subtype of breast cancer, and nearly all ILC tumors express estrogen receptor alpha (ER). However, clinical and laboratory data suggest ILC are strongly estrogen-driven but not equally antiestrogen-sensitive. We hypothesized ILC-specific ER coregulators mediate ER functions and antiestrogen resistance in ILC, and profiled ER-associated proteins by mass spectrometry. Three ER+ ILC cell lines (MDA MB 134VI, SUM44PE, and BCK4) were compared with ER+ invasive ductal carcinoma (IDC) line data, and we examined whether siRNA of identified proteins suppressed ER-driven proliferation in ILC cells. This identified mediator of DNA damage checkpoint 1 (MDC1), a tumor suppressor in DNA damage response (DDR), as a novel ER coregulator in ILC. We confirmed ER:MDC1 interaction was specific to ILC versus IDC cells, and found MDC1 knockdown suppressed ILC cell proliferation and tamoxifen resistance. Using RNA-sequencing, we found in ILC cells MDC1 knockdown broadly dysregulates the ER transcriptome, with ER:MDC1 target genes enriched for promoter hormone response elements. Importantly, our data are inconsistent with MDC1 tumor suppressor functions in DDR, but suggest a novel oncogenic role for MDC1 as an ER coregulator. Supporting this, in breast tumor tissue microarrays, MDC1 protein was frequently low or absent in IDC, but MDC1 loss was rare in ER+ ILC. ER:MDC1 interaction and MDC1 coregulator functions may underlie ER function in ILC and serve as targets to overcome antiestrogen resistance in ILC. IMPLICATIONS: MDC1 has novel ER coregulator activity in ILC, which may underlie ILC-specific ER functions, estrogen response, and antiestrogen resistance.

Chromatin Modifiers in Transcriptional Regulation: New Findings and Prospects

Histone-modifying and remodeling complexes are considered the main coregulators that affect transcription by changing the chromatin structure. Coordinated action by these complexes is essential for the transcriptional activation of any eukaryotic gene. In this review, we discuss current trends in the study of histone modifiers and chromatin remodelers, including the functional impact of transcriptional proteins/ complexes i.e., "pioneers"; remodeling and modification of non-histone proteins by transcriptional complexes; the supplementary functions of the non-catalytic subunits of remodelers, and the participation of histone modifiers in the "pause" of RNA polymerase II. The review also includes a scheme illustrating the mechanisms of recruitment of the main classes of remodelers and chromatin modifiers to various sites in the genome and their functional activities.

ASH2L is involved in promotion of endometrial cancer progression via upregulation of PAX2 transcription

Absent, small or homeotic 2‐like protein (ASH2L) is a core component of a multimeric histone methyltransferase complex that is involved in the maintenance of active transcription, participating in several cancers, however the biological function and molecular mechanism of ASH2L in endometrial cancer (ECa) are largely unknown. Endometrial cancer is a common malignant tumor in women and the incidence of this cancer is on the rise. Estrogen‐ERα signaling, as an oncogenic pathway, plays a crucial role in endometrial carcinogenesis. Therefore, further exploration of the molecular mechanisms around ERα‐mediated gene transcription in ECa would be helpful to the understanding of tumor development and to finding a new therapeutic target for ECa. Here, our study demonstrated that ASH2L was highly expressed in ECa samples, and higher expression of ASH2L was positively correlated with a poor prognosis. Moreover, we identified that ASH2L associated with ERα and that knockdown of ASH2L resulted in decreased expression of a subset of the estrogen‐induced target genes, including paired box 2 (PAX2), an oncogenic gene in ECa. ASH2L was recruited to cis‐regulatory elements in PAX2, thereby altering histone H3K4me3 and H3K27me3 levels, to enhance ERα‐mediated transactivation. Finally, depletion of ASH2L suppressed endometrial cancer cell proliferation and migration. Our findings suggest that ASH2L participates in the promotion of ECa progression, if not totally at least partially, via upregulation of PAX2 transcription.

Nuclear receptor co-repressor RIP140 regulates diurnal expression of cytochrome P450 2b10 in mouse liver

Elucidating the mechanisms for circadian expression of drug-metabolizing enzymes is essential for a better understanding of dosing time-dependent drug metabolism and pharmacokinetics. CYP2B6 (Cyp2b10 in mice) is an important enzyme responsible for metabolism and detoxification of approximately 10% of drugs. Here, we aimed to investigate a potential role of nuclear receptor co-repressor RIP140 in circadian regulation of Cyp2b10 in mice.We first uncovered diurnal rhythmicity in hepatic RIP140 mRNA and protein with peak values at ZT10 (ZT, zeitgeber time). RIP140 ablation up-regulated Cyp2b10 expression and blunted its rhythm in mice and in AML-12 cells. Consistent with a negative regulatory effect, overexpression of RIP140 inhibited Cyp2b10 promoter activity and reduced cellular Cyp2b10 expression.Furthermore, RIP140 suppressed Car- and Pxr-mediated transactivation of Cyp2b10, and the suppressive effects were attenuated when the RIP140 gene was silenced. Chromatin immunoprecipitation assays revealed that recruitment of RIP140 protein to the Cyp2b10 promoter was circadian time-dependent in wild-type mice. More extensive recruitment was observed at ZT10 than at ZT2 consistent with the rhythmic pattern of RIP140 protein. However, the time-dependency of RIP140 recruitment was lost in RIP140^-/- mice.Additionally, we identified a D-box and a RORE cis-element in RIP140 promoter. D-box- and RORE-acting clock components such as Dbp, E4bp4, Rev-erbα/β and Rorα transcriptionally regulated RIP140, potentially accounting for its rhythmic expression.In conclusion, RIP140 regulates diurnal expression of Cyp2b10 in mouse liver through periodical repression of Car- and Pxr-mediated transactivation. This co-regulator-driven mechanism represents a novel source of diurnal rhythmicity in drug-metabolizing enzymes.

Neurofibromin Is an Estrogen Receptor-α Transcriptional Co-repressor in Breast Cancer

We report that neurofibromin, a tumor suppressor and Ras-GAP (GTPase-activating protein), is also an estrogen receptor-α (ER) transcriptional co-repressor through leucine/isoleucine-rich motifs that are functionally independent of GAP activity. GAP activity, in turn, does not affect ER binding. Consequently, neurofibromin depletion causes estradiol hypersensitivity and tamoxifen agonism, explaining the poor prognosis associated with neurofibromin loss in endocrine therapy-treated ER⁺ breast cancer. Neurofibromin-deficient ER⁺ breast cancer cells initially retain sensitivity to selective ER degraders (SERDs). However, Ras activation does play a role in acquired SERD resistance, which can be reversed upon MEK inhibitor addition, and SERD/MEK inhibitor combinations induce tumor regression. Thus, neurofibromin is a dual repressor for both Ras and ER signaling, and co-targeting may treat neurofibromin-deficient ER⁺ breast tumors.

The nuclear receptor superfamily: A structural perspective

Nuclear receptors (NRs) are a family of transcription factors that regulate numerous physiological processes such as metabolism, reproduction, inflammation, as well as the circadian rhythm. NRs sense changes in lipid metabolite levels to drive differential gene expression, producing distinct physiologic effects. This is an allosteric process whereby binding a cognate ligand and specific DNA sequences drives the recruitment of diverse transcriptional co-regulators at chromatin and ultimately transactivation or transrepression of target genes. Dysregulation of NR signaling leads to various malignances, metabolic disorders, and inflammatory disease. Given their important role in physiology and ability to respond to small lipophilic ligands, NRs have emerged as valuable therapeutic targets. Here, we summarize and discuss the recent progress on understanding the complex mechanism of action of NRs, primarily from a structural perspective. Finally, we suggest future studies to improve our understanding of NR signaling and better design drugs by integrating multiple structural and biophysical approaches.

New Insights into the Binding Mechanism of Co-regulator BUD31 to AR AF2 Site: Structural Determination and Analysis of the Mutation Effect

Introduction

Androgen Receptor (AR) plays a pivotal role in the development of male sex and contributes to prostate cancer growth. Different from other nuclear receptors that bind to the co-regulator LxxLL motif in coregulator peptide interaction, the AR Ligand Binding Domain (LBD) prefers to bind to the FxxLF motif. BUD31, a novel co-regulator with FxxLF motif, has been demonstrated to suppress wild-type and mutated AR-mediated prostate cancer growth.

Methods

To find out the interaction mechanisms of BUD31 with WT/T877A/W741L AR complex, molecular dynamics simulations were employed to study the complex BUD31 and WT/mutant ARs. The molecular mechanics Poisson-Boltzmann surface area (MM-PBSA) results demonstrated that T877A and W741L point mutations can reduce the binding affinity between BUD31 and AR. The RMSF and dynamic cross-correlation analysis indicated that amino acid point mutations can affect the motions of loop residues in the AR structure.

Results

These results indicated that AR co-regulator binding site AF2 can serve as a target for drug discovery to solve the resistance problem.

Complex Coordination of Cell Plasticity by a PGC-1α-controlled Transcriptional Network in Skeletal Muscle

Skeletal muscle cells exhibit an enormous plastic capacity in order to adapt to external stimuli. Even though our overall understanding of the molecular mechanisms that underlie phenotypic changes in skeletal muscle cells remains poor, several factors involved in the regulation and coordination of relevant transcriptional programs have been identified in recent years. For example, the peroxisome proliferator-activated receptor γ coactivator-1α (PGC-1α) is a central regulatory nexus in the adaptation of muscle to endurance training. Intriguingly, PGC-1α integrates numerous signaling pathways and translates their activity into various transcriptional programs. This selectivity is in part controlled by differential expression of PGC-1α variants and post-translational modifications of the PGC-1α protein. PGC-1α-controlled activation of transcriptional networks subsequently enables a spatio-temporal specification and hence allows a complex coordination of changes in metabolic and contractile properties, protein synthesis and degradation rates and other features of trained muscle. In this review, we discuss recent advances in our understanding of PGC-1α-regulated skeletal muscle cell plasticity in health and disease.

Hormone binding and co-regulator binding to the glucocorticoid receptor are allosterically coupled

The glucocorticoid receptor initiates the cellular response to glucocorticoid steroid hormones in vertebrates. Co-regulator proteins dock to the receptor in response to hormone binding and potentiate the transcriptional activity of the receptor by modifying DNA and recruiting essential transcription factors like RNA polymerase II. Hormones and co-regulators bind at distinct sites in the ligand binding domain yet function cooperatively to mediate transcriptional control. This study reveals and quantifies energetic coupling between two binding sites using purified components. Using a library of peptides taken from co-regulator proteins, we determine the pattern of co-regulator binding to the glucocorticoid receptor ligand binding domain. We show that peptides from co-regulators differ in their effects on hormone binding and kinetics. Peptides from DAX1 and SRC1 bind with similar affinity, but DAX1 binding is coupled to hormone binding, and SRC1 is not. Mechanistic details of co-regulator binding and coupling to the hormone binding pocket are uncovered by analysis of properties endowed by mutation of a key residue in the allosteric network connecting the sites.