Phase-separation: a possible new layer for transcriptional regulation by glucocorticoid receptor

Glucocorticoids (GCs) are hormones involved in circadian adaptation and stress response, and it is also noteworthy that these steroidal molecules present potent anti-inflammatory action through GC receptors (GR). Upon ligand-mediated activation, GR translocates to the nucleus, and regulates gene expression related to metabolism, acute-phase response and innate immune response. GR field of research has evolved considerably in the last decades, providing varied mechanisms that contributed to the understanding of transcriptional regulation and also impacted drug design for treating inflammatory diseases. Liquid-liquid phase separation (LLPS) in cellular processes represents a recent topic in biology that conceptualizes membraneless organelles and microenvironments that promote, or inhibit, chemical reactions and interactions of protein or nucleic acids. The formation of these molecular condensates has been implicated in gene expression control, and recent evidence shows that GR and other steroid receptors can nucleate phase separation (PS). Here we briefly review the varied mechanisms of transcriptional control by GR, which are largely studied in the context of inflammation, and further present how PS can be involved in the control of gene expression. Lastly, we consider how the reported advances on LLPS during transcription control, specially for steroid hormone receptors, could impact the different modalities of GR action on gene expression, adding a new plausible molecular event in glucocorticoid signal transduction.

Female Reproductive Systems: Hormone Dependence and Receptor Expression

The female reproductive system which consists of the ovaries, uterus (myometrium, endometrium), Fallopian tubes, cervix and vagina is exquisitely sensitive to the actions of steroid hormones. The ovaries play a key role in the synthesis of bioactive steroids (oestrogens, androgens, progestins) that act both within the tissue (intracrine/paracrine) as well as on other reproductive organs following release into the blood stream (endocrine action). Sex steroid receptors encoded by the oestrogen (ESR1, ESR2), progesterone (PR) and androgen (AR) receptor genes, which are members of the superfamily of ligand activated transcription factors are widely expressed within these tissues. These receptors play critical role(s) in regulation of cell proliferation, ovulation, endometrial receptivity, myometrial cell function and inflammatory cell infiltration. Our understanding of their importance has been informed by studies on human tissues and cells, which have employed immunohistochemistry as well as a wide range of molecular and genetic methods to identify which processes are dependent steroid ligand activation. The development of mice with targeted deletions of each of these receptors has provided complementary data that has extended our appreciation of cell-cell interactions in the fine tuning of reproductive tissue function. This large body of work has formed the basis of new and improved therapeutics to treat conditions such as infertility.

Progesterone receptors in normal breast development and breast cancer

Progesterone receptors (PR) play a pivotal role in many female reproductive tissues such as the uterus, the ovary, and the mammary gland (MG). Moreover, PR play a key role in breast cancer growth and progression. This has led to the development and study of different progestins and antiprogestins, many of which are currently being tested in clinical trials for cancer treatment. Recent reviews have addressed the role of PR in MG development, carcinogenesis, and breast cancer growth. Thus, in this review, in addition to making an overview on PR action in normal and tumor breast, the focus has been put on highlighting the still unresolved topics on hormone treatment involving PR isoforms and breast cancer prognosis.

Phasing the intranuclear organization of steroid hormone receptors

Steroid receptors (SRs) encompass a family of transcription factors that regulate the expression of thousands of genes upon binding to steroid hormones and include the glucocorticoid, androgen, progesterone, estrogen and mineralocorticoid receptors. SRs control key physiological and pathological processes, thus becoming relevant drug targets. As with many other nuclear proteins, hormone-activated SRs concentrate in multiple discrete foci within the cell nucleus. Even though these foci were first observed ∼25 years ago, their exact structure and function remained elusive. In the last years, new imaging methodologies and theoretical frameworks improved our understanding of the intranuclear organization. These studies led to a new paradigm stating that many membraneless nuclear compartments, including transcription-related foci, form through a liquid–liquid phase separation process. These exciting ideas impacted the SR field by raising the hypothesis of SR foci as liquid condensates involved in transcriptional regulation. In this work, we review the current knowledge about SR foci formation under the light of the condensate model, analyzing how these structures may impact SR function. These new ideas, combined with state-of-the-art techniques, may shed light on the biophysical mechanisms governing the formation of SR foci and the biological function of these structures in normal physiology and disease.

The vitamin D receptor (VDR) binds to the nuclear matrix via its hinge domain: A potential mechanism for the reduction in VDR mediated transcription in mitotic cells

Vitamin D is best known for its regulation of calcium homeostasis. Vitamin D exerts its genomic actions via the vitamin D receptor (VDR). As a member of the superfamily of nuclear receptors (NR), the VDR is primarily located within the nucleus of non-dividing cells. We show here that the VDR relocates from the nucleus into the cytoplasm across all stages of cell division in CHO cells. Furthermore, we show that the VDR is transcriptionally inert during cell division. In addition, 1α, 25 dihydroxyvitamin D (1,25(OH)₂D₃) promotes VDR binding to the nuclear matrix. Finally, we assessed the structural nature of VDR binding to the nuclear matrix. Mutation of the hinge domain reduced VDR's ability to bind to the nuclear matrix and to initiate transcription in response to 1,25(OH)₂D₃. Taken together, our data suggest that the association between the VDR and the nuclear matrix accounts for the apparent cytosolic distribution as the matrix disperses within the cytoplasm when cells divide. This may also explain the dramatic reduction in VDR mediated transcription during cell division. Our data also confirm that similar to other NRs, the hinge domain of the VDR is responsible for this association.

A single-dose PK study of onapristone including the effect of food on absorption

PURPOSE

Onapristone is an antiprogestin with activity in breast cancer and is under investigation for use in endometrial, ovarian and prostate cancers. Megestrol acetate and abiraterone generally show variability in absorption and, depending on the formulation, food effect. This study was conducted to determine the effect of food on 10 mg oral immediate-release (IR) onapristone and to help identify a formulation to minimize variability.

METHODS

This is an open-label, randomized, crossover study to determine the pharmacokinetic profile of onapristone and its main metabolite, N-mono-desmethyl onapristone. Twelve healthy female subjects received 10 mg of oral IR onapristone after an overnight fast, or within 30 min of a high-fat, high-calorie meal with a 2-week washout between dosing periods.

RESULTS

Onapristone plasma t1/2 (mean ± SD) was 4.36 ± 0.81 h for the fasted state and 3.76 ± 0.36 h for the fed state. Following food, onapristone tmax was delayed from 1 to 4 h. Food intake was also associated with a small increase in AUC0-∞ of approximately 13 % and a statistically significant decrease in Cmax of approximately 18 %. One subject experienced a 23-day delay in menses after one 10 mg onapristone dose, while another subject experienced transient grade 2 NCI-CTCAE liver enzyme elevation 3 weeks post dose.

CONCLUSION

The results are consistent with previous observations, indicating that there is a small increase in onapristone exposure and a significant decrease in Cmax when taken with food. These changes are within acceptable limits set out by the FDA. Thus, our findings indicate that onapristone could be administered without regard to food.

Scaffold attachment factor B (SAFB)1 and SAFB2 cooperatively inhibit the intranuclear mobility and function of ERα

Estrogen receptor alpha (ERα) plays a key role in physiological and pathophysiological processes as a ligand-activated transcriptional factor that is regulated by cofactors. ERα-mediated transcriptional regulation is closely correlated with the mobility of ERα in the nucleus in association with the nuclear matrix, the framework for nuclear events including transcription. However, the relationship between ERα mobility and the cofactors of ERα is unclear. Scaffold attachment factor B1 (SAFB1) and its paralog SAFB2 are nuclear matrix binding proteins that have been characterized as ERα corepressors. Here, using chimeric fluorescent proteins (FPs), we show that SAFB1 and SAFB2 colocalize with ERα in the nucleus of living cells after 17β-estradiol (E2) treatment. Co-immunoprecipitation (co-IP) experiments indicated that ERα interacts with both SAFB1 and SAFB2 in the presence of E2. Fluorescence recovery after photobleaching analysis revealed that SAFB1 and SAFB2 each decrease ERα mobility, and interestingly, coexpression of SAFB1 and SAFB2 causes a synergistic reduction in ERα dynamics under E2 treatment. In accordance with these mobility changes, ERα-mediated transcription and proliferation are cooperatively inhibited by SAFB1 and SAFB2. These results indicate that SAFB1 and SAFB2 are crucial repressors for ERα dynamics in association with the nuclear matrix and that their synergistic regulation of ERα mobility is sufficient for inhibiting ERα function.

Degradation of thyroid hormone receptor beta 1: existence of stable and unstable forms

BACKGROUND

The degradation of many nuclear receptors is controlled by ligand-binding and mediated by the ubiquitin-proteasome pathway. However, the mechanisms implicated in thyroid hormone receptor (TR) degradation remain unclear. Our objective was to define the kinetics, mechanisms, and sub-cellular fractions involved in TRs degradation.

METHODS

We used pulse-chase analyses, time-course experiments carried out in presence of cycloheximide (to inhibit new protein synthesis), and biochemical fractionation with Western blot analyses to determine the kinetics of the degradation of the TRβ isoform, TRβ1, in transiently transfected QBI-HEK 293A cells.

RESULTS

We observed that TRβ1 degradation is mediated by the proteasome pathway. Also, the kinetics of TRβ1 degradation is atypical due to the co-existence of more than one TRβ1 population, located in different cellular compartments and having different stability profiles. Moreover, TRβ1 degradation was unaffected by a mutation in its putative PEST motif, which confers turnover of other proteins.

CONCLUSION

Our findings introduce novel evidence suggesting that stable and unstable forms of TRβ1, which might have distinct functions, co-exist in cells.

Progesterone action in human tissues: regulation by progesterone receptor (PR) isoform expression, nuclear positioning and coregulator expression

Progesterone is a critical regulator of normal female reproductive function, with diverse tissue-specific effects in the human. The effects of progesterone are mediated by its nuclear receptor (PR) that is expressed as two isoforms, PRA and PRB, which are virtually identical except that PRA lacks 164 amino acids that are present at the N-terminus of PRB. Considerable in vitro evidence suggests that the two PRs are functionally distinct and in animals, tissue-specific distribution patterns of PRA and PRB may account for some of the diversity of progesterone effects. In the human, PRA and PRB are equivalently expressed in most target cells, suggesting that alternative mechanisms control the diversity of progesterone actions. PR mediates the effects of progesterone by association with a range of coregulatory proteins and binding to specific target sequences in progesterone-regulated gene promoters. Ligand activation of PR results in redistribution into discrete subnuclear foci that are detectable by immunofluorescence, probably representing aggregates of multiple transcriptionally active PR-coregulator complexes. PR foci are aberrant in cancers, suggesting that the coregulator composition and number of complexes is altered. A large family of coregulators is now described and the range of proteins known to bind PR exceeds the complement required for transcriptional activation, suggesting that in the human, tissue-specific coregulator expression may modulate progesterone response. In this review, we examine the role of nuclear localization of PR, coregulator association and tissue-specific expression in modulating progesterone action in the human.