Ubiquitination and proteasome-mediated degradation of BRCA1 and BARD1 during steroidogenesis in human ovarian granulosa cells

P62 promotes FSH-induced antral follicle formation by directing degradation of ubiquitinated WT1

In females, the pathophysiological mechanism of poor ovarian response (POR) is not fully understood. Considering the expression level of p62 was significantly reduced in the granulosa cells (GCs) of POR patients, this study focused on identifying the role of the selective autophagy receptor p62 in conducting the effect of follicle-stimulating hormone (FSH) on antral follicles (AFs) formation in female mice. The results showed that p62 in GCs was FSH responsive and that its level increased to a peak and then decreased time-dependently either in ovaries or in GCs after gonadotropin induction in vivo. GC-specific deletion of p62 resulted in subfertility, a significantly reduced number of AFs and irregular estrous cycles, which were same as pathophysiological symptom of POR. By conducting mass spectrum analysis, we found the ubiquitination of proteins was decreased, and autophagic flux was blocked in GCs. Specifically, the level of nonubiquitinated Wilms tumor 1 homolog (WT1), a transcription factor and negative controller of GC differentiation, increased steadily. Co-IP results showed that p62 deletion increased the level of ubiquitin-specific peptidase 5 (USP5), which blocked the ubiquitination of WT1. Furthermore, a joint analysis of RNA-seq and the spatial transcriptome sequencing data showed the expression of steroid metabolic genes and FSH receptors pivotal for GCs differentiation decreased unanimously. Accordingly, the accumulation of WT1 in GCs deficient of p62 decreased steroid hormone levels and reduced FSH responsiveness, while the availability of p62 in GCs simultaneously ensured the degradation of WT1 through the ubiquitin‒proteasome system and autophagolysosomal system. Therefore, p62 in GCs participates in GC differentiation and AF formation in FSH induction by dynamically controlling the degradation of WT1. The findings of the study contributes to further study the pathology of POR.

The Role of Genetic Mutations in Mitochondrial-Driven Cancer Growth in Selected Tumors: Breast and Gynecological Malignancies

There is an increasing understanding of the molecular and cytogenetic background of various tumors that helps us better conceptualize the pathogenesis of specific diseases. Additionally, in many cases, these molecular and cytogenetic alterations have diagnostic, prognostic, and/or therapeutic applications that are heavily used in clinical practice. Given that there is always room for improvement in cancer treatments and in cancer patient management, it is important to discover new therapeutic targets for affected individuals. In this review, we discuss mitochondrial changes in breast and gynecological (endometrial and ovarian) cancers. In addition, we review how the frequently altered genes in these diseases (BRCA1/2, HER2, PTEN, PIK3CA, CTNNB1, RAS, CTNNB1, FGFR, TP53, ARID1A, and TERT) affect the mitochondria, highlighting the possible associated individual therapeutic targets. With this approach, drugs targeting mitochondrial glucose or fatty acid metabolism, reactive oxygen species production, mitochondrial biogenesis, mtDNA transcription, mitophagy, or cell death pathways could provide further tailored treatment.

BRCA1 deficiency in triple-negative breast cancer: Protein stability as a basis for therapy

Mutations in breast cancer-associated 1 (BRCA1) increase the lifetime risk of developing breast cancer by up to 51% over the risk of the general population. Many aspects of this multifunctional protein have been revealed, including its essential role in homologous recombination repair, E3 ubiquitin ligase activity, transcriptional regulation, and apoptosis. Although most studies have focused on BRCA1 deficiency due to mutations, only a minority of patients carry BRCA1 mutations. A recent study has suggested an expanded definition of BRCA1 deficiency with reduced BRCA1 levels, which accounts for almost half of all triple-negative breast cancer (TNBC) patients. Reduced BRCA1 levels can result from epigenetic modifications or increased proteasomal degradation. In this review, we discuss how this knowledge of BRCA1 function and regulation of BRCA1 protein stability can help overcome the challenges encountered in the clinic and advance current treatment strategies for BRCA1-related breast cancer patients, especially focusing on TNBC.

Regulation of BRCA1 stability through the tandem UBX domains of isoleucyl-tRNA synthetase 1

Aminoacyl-tRNA synthetases (ARSs) have evolved to acquire various additional domains. These domains allow ARSs to communicate with other cellular proteins in order to promote non-translational functions. Vertebrate cytoplasmic isoleucyl-tRNA synthetases (IARS1s) have an uncharacterized unique domain, UNE-I. Here, we present the crystal structure of the chicken IARS1 UNE-I complexed with glutamyl-tRNA synthetase 1 (EARS1). UNE-I consists of tandem ubiquitin regulatory X (UBX) domains that interact with a distinct hairpin loop on EARS1 and protect its neighboring proteins in the multi-synthetase complex from degradation. Phosphomimetic mutation of the two serine residues in the hairpin loop releases IARS1 from the complex. IARS1 interacts with BRCA1 in the nucleus, regulates its stability by inhibiting ubiquitylation via the UBX domains, and controls DNA repair function.

CRL4-DCAF8L2 E3 ligase promotes ubiquitination and degradation of BARD1

BARD1 is a tumor suppressor that is necessary for the functioning and stability of BRCA1, with which it forms a heterodimer and participates in the repair of DNA double-strand breaks. The cellular level of BARD1 and its interaction with BRCA1 are crucial for BRCA1/BARD1 function in homologous recombination and tumor suppression. However, the regulatory mechanism underpinning the stability of BARD1 is largely unclear. In this study, we identified DCAF8L2, a DDB1-Cullin associated factor (DCAF) associated with CRL4 E3 ligase, as a negative regulator of BARD1. Mechanistically, DCAF8L2 interacts with and targets BARD1 for ubiquitination and degradation. In addition, the interaction of DCAF8L2 with BARD1 through the RING domain could compete with the dimerization of BRCA1 and BARD1, leading to increased cellular uncoupling of BARD1 and BRCA1, subjecting the latter to degradation. The overexpression of DCAF8L2 compromises the homologous recombination process and confers cells with increased sensitivity to DNA damage. Furthermore, DCAF8L2 was aberrantly expressed in breast cancer cell lines. Our findings suggest that DCAF8L2 may play an oncogenic role in the pathogenesis of breast cancer, possibly by negative regulation of BARD1.

The fellowship of the RING: BRCA1, its partner BARD1 and their liaison in DNA repair and cancer

The breast cancer type 1 susceptibility protein (BRCA1) and its partner - the BRCA1-associated RING domain protein 1 (BARD1) - are key players in a plethora of fundamental biological functions including, among others, DNA repair, replication fork protection, cell cycle progression, telomere maintenance, chromatin remodeling, apoptosis and tumor suppression. However, mutations in their encoding genes transform them into dangerous threats, and substantially increase the risk of developing cancer and other malignancies during the lifetime of the affected individuals. Understanding how BRCA1 and BARD1 perform their biological activities therefore not only provides a powerful mean to prevent such fatal occurrences but can also pave the way to the development of new targeted therapeutics. Thus, through this review work we aim at presenting the major efforts focused on the functional characterization and structural insights of BRCA1 and BARD1, per se and in combination with all their principal mediators and regulators, and on the multifaceted roles these proteins play in the maintenance of human genome integrity.

The BRCA1/BARD1 ubiquitin ligase and its substrates

Mutations in breast cancer type 1 susceptibility protein (BRCA1) and its heterodimeric binding partner BARD1 confer a high risk for the development of breast and ovarian cancers. The sole enzymatic function of the BRCA1/BARD1 complex is as a RING-type E3 ubiquitin (Ub) ligase, leading to the deposition of Ub signals onto a variety of substrate proteins. Distinct types of Ub signals deposited by BRCA1/BARD1 (i.e. degradative vs. non-degradative; mono-Ub vs. poly-Ub chains) on substrate proteins mediate aspects of its function in DNA double-stranded break repair, cell-cycle regulation, and transcriptional regulation. While cancer-predisposing mutations in both subunits lead to the inactivation of BRCA1/BARD1 ligase activity, controversy remains as to whether its Ub ligase activity directly inhibits tumorigenesis. Investigation of BRCA1/BARD1 substrates using rigorous, well-validated mutants and experimental systems will ultimately clarify the role of its ligase activity in cancer and possibly establish prognostic and diagnostic metrics for patients with mutations. In this review, we discuss the Ub ligase function of BRCA1/BARD1, highlighting experimental approaches, mechanistic considerations, and reagents that are useful in the study of substrate ubiquitylation. We also discuss the current understanding of two well-established BRCA1/BARD1 substrates (nucleosomal H2A and estrogen receptor α) and several recently discovered substrates (p50, NF2, Oct1, and LARP7). Lessons from the current body of work should provide a road map to researchers examining novel substrates and biological functions attributed to BRCA1/BARD1 Ub ligase activity.

Cavin3 released from caveolae interacts with BRCA1 to regulate the cellular stress response

Caveolae-associated protein 3 (cavin3) is inactivated in most cancers. We characterized how cavin3 affects the cellular proteome using genome-edited cells together with label-free quantitative proteomics. These studies revealed a prominent role for cavin3 in DNA repair, with BRCA1 and BRCA1 A-complex components being downregulated on cavin3 deletion. Cellular and cell-free expression assays revealed a direct interaction between BRCA1 and cavin3 that occurs when cavin3 is released from caveolae that are disassembled in response to UV and mechanical stress. Overexpression and RNAi-depletion revealed that cavin3 sensitized various cancer cells to UV-induced apoptosis. Supporting a role in DNA repair, cavin3-deficient cells were sensitive to PARP inhibition, where concomitant depletion of 53BP1 restored BRCA1-dependent sensitivity to PARP inhibition. We conclude that cavin3 functions together with BRCA1 in multiple cancer-related pathways. The loss of cavin3 function may provide tumor cell survival by attenuating apoptotic sensitivity and hindering DNA repair under chronic stress conditions.

BRCA1 degradation in response to mitochondrial damage in breast cancer cells

BRCA1 is a well-studied tumor suppressor involved in the homologous repair of DNA damage, whereas PINK1, a mitochondrial serine/threonine kinase, is known to be involved in mitochondrial quality control. Genetic mutations of PINK1 and Parkin cause autosomal recessive early-onset Parkinson's disease. We found that in breast cancer cells, the mitochondrial targeting reagents, which all induce mitochondrial depolarization along with PINK1 upregulation, induced proteasomal BRCA1 degradation. This BRCA1 degradation was dependent on PINK1, and BRCA1 downregulation upon mitochondrial damage caused DNA double-strand breaks. BRCA1 degradation was mediated through the direct interaction with the E3 ligase Parkin. Strikingly, BRCA1 and PINK1/Parkin expression were inversely correlated in cancerous mammary glands from breast cancer patients. BRCA1 knockdown repressed cancer cell growth, and high BRCA1 expression predicted poor relapse-free survival in breast cancer patients. These observations indicate a novel mechanism by which mitochondrial damage is transmitted to the nucleus, leading to BRCA1 degradation.

SIRT2 promotes BRCA1-BARD1 heterodimerization through deacetylation

The breast cancer type I susceptibility protein (BRCA1) and BRCA1-associated RING domain protein I (BARD1) heterodimer promote genome integrity through pleiotropic functions, including DNA double-strand break (DSB) repair by homologous recombination (HR). BRCA1-BARD1 heterodimerization is required for their mutual stability, HR function, and role in tumor suppression; however, the upstream signaling events governing BRCA1-BARD1 heterodimerization are unclear. Here, we show that SIRT2, a sirtuin deacetylase and breast tumor suppressor, promotes BRCA1-BARD1 heterodimerization through deacetylation. SIRT2 complexes with BRCA1-BARD1 and deacetylates conserved lysines in the BARD1 RING domain, interfacing BRCA1, which promotes BRCA1-BARD1 heterodimerization and consequently BRCA1-BARD1 stability, nuclear retention, and localization to DNA damage sites, thus contributing to efficient HR. Our findings define a mechanism for regulation of BRCA1-BARD1 heterodimerization through SIRT2 deacetylation, elucidating a critical upstream signaling event directing BRCA1-BARD1 heterodimerization, which facilitates HR and tumor suppression, and delineating a role for SIRT2 in directing DSB repair by HR.

Minten et al. show that SIRT2, a sirtuin deacetylase and tumor suppressor protein, promotes BRCA1-BARD1 heterodimerization through deacetylation of BARD1 at conserved lysines within its RING domain. These findings elucidate a critical upstream signaling event directing BRCA1-BARD1 heterodimerization, which facilitates HR and tumor suppression.

LINC00152 Promotes Tumor Progression and Predicts Poor Prognosis by Stabilizing BCL6 From Degradation in the Epithelial Ovarian Cancer

Long non-coding RNA 00152 (LINC00152) is tumorigenic in multiple somatic malignancies. However, its prognostic significance and molecular mechanisms in the epithelial ovarian cancer (EOC) remain elusive. Here our study reveals that dysregulation of LINC00152 is a predictor of poor prognosis in patients with EOC and facilitates ovarian tumor growth and metastasis both in vitro and in vivo; the expression of LINC00152 positively correlates with the protein levels of BCL6 in EOC tissues and ovarian tumor cells; LINC00152 binds to Ser333 and Ser343 of BCL6 protein and stabilizes BCL6 from poly-ubiquitination thus facilitating the oncogenic functions in EOC. Moreover, overexpression of the mutant BCL6^S333A/S343A fails to rescue the reduced proliferation and invasion caused by the knockdown of endogenous BCL6 in LINC00152-overexpressing cells. Our study might not only offer clues to the network of lncRNA-protein interactions but also provide potential therapeutic targets for the tumor pharmacology.

Estrogen receptor alpha, G-protein coupled estrogen receptor 1, and aromatase: Developmental, sex, and region-specific differences across the rat caudate-putamen, nucleus accumbens core and shell

Sex steroid hormones such as 17β-estradiol (estradiol) regulate neuronal function by binding to estrogen receptors (ERs), including ERα and GPER1, and through differential production via the enzyme aromatase. ERs and aromatase are expressed across the nervous system, including in the striatal brain regions. These regions, comprising the nucleus accumbens core, shell, and caudate-putamen, are instrumental for a wide-range of functions and disorders that show sex differences in phenotype and/or incidence. Sex-specific estrogen action is an integral component for generating these sex differences. A distinctive feature of the striatal regions is that in adulthood neurons exclusively express membrane but not nuclear ERs. This long-standing finding dominates models of estrogen action in striatal regions. However, the developmental etiology of ER and aromatase cellular expression in female and male striatum is unknown. This omission in knowledge is important to address, as developmental stage influences cellular estrogenic mechanisms. Thus, ERα, GPER1, and aromatase cellular immunoreactivity was assessed in perinatal, prepubertal, and adult female and male rats. We tested the hypothesis that ERα, GPER1, and aromatase exhibits sex, region, and age-specific differences, including nuclear expression. ERα exhibits nuclear expression in all three striatal regions before adulthood and disappears in a region- and sex-specific time-course. Cellular GPER1 expression decreases during development in a region- but not sex-specific time-course, resulting in extranuclear expression by adulthood. Somatic aromatase expression presents at prepuberty and increases by adulthood in a region- but not sex-specific time-course. These data indicate that developmental period exerts critical sex-specific influences on striatal cellular estrogenic mechanisms.

Possible Mechanisms for Maintenance and Regression of Corpus Luteum Through the Ubiquitin-Proteasome and Autophagy System Regulated by Transcriptional Factors

The corpus luteum (CL) is an important tissue of the female reproductive process which is established through ovulation of the mature follicle. Pulsatile release of prostaglandin F_2α from the uterus leads to the regression of luteal cells and restarts the estrous cycle in most non-primate species. The rapid functional regression of the CL, which coincides with decrease of progesterone production, is followed by its structural regression. Although we now have a better understanding of how the CL is triggered to undergo programmed cell death, the precise mechanisms governing CL protein degradation in a very short period of luteolysis remains unknown. In this context, activation of ubiquitin-proteasome pathway (UPP), unfolded protein response (UPR) and autophagy are potential subcellular mechanisms involved. The ubiquitin-proteasome pathway (UPP) maintains tissue homeostasis in the face of both internal and external stressors. The UPP also controls physiological processes in many gonadal cells. Emerging evidence suggests that UPP dysfunction is involved in male and female reproductive tract dysfunction. Autophagy is activated when cells are exposed to different types of stressors such as hypoxia, starvation, and oxidative stress. While emerging evidence points to an important role for the UPP and autophagy in the CL, the key underlying transcriptional mechanisms have not been well-documented. In this review, we propose how CL regression may be governed by the ubiquitin-proteasome and autophagy pathways. We will further consider potential transcription factors which may regulate these events in the CL.

Structural analysis of BRCA1 reveals modification hotspot

Cancer cells afflicted with mutations in the breast cancer susceptibility protein (BRCA1) often suffer from increased DNA damage and genomic instability. The precise manner in which physical changes to BRCA1 influence its role in DNA maintenance remains unclear. We used single-particle electron microscopy to study the three-dimensional properties of BRCA1 naturally produced in breast cancer cells. Structural studies revealed new information for full-length BRCA1, engaging its nuclear binding partner, the BRCA1-associated RING domain protein (BARD1). Equally important, we identified a region in mutated BRCA1 that was highly susceptible to ubiquitination. We refer to this site as a modification "hotspot." Ubiquitin adducts in the hotspot region proved to be biochemically reversible. Collectively, we show how key changes to BRCA1 affect its structure-function relationship, and present new insights to potentially modulate mutated BRCA1 in human cancer cells.

53BP1 ablation rescues genomic instability in mice expressing 'RING-less' BRCA1

BRCA1 mutations strongly predispose affected individuals to breast and ovarian cancer, but the mechanism by which BRCA1 acts as a tumor suppressor is not fully understood. Homozygous deletion of exon 2 of the mouse Brca1 gene normally causes embryonic lethality, but we show that exon 2-deleted alleles of Brca1 are expressed as a mutant isoform that lacks the N-terminal RING domain. This "RING-less" BRCA1 protein is stable and efficiently recruited to the sites of DNA damage. Surprisingly, robust RAD51 foci form in cells expressing RING-less BRCA1 in response to DNA damage, but the cells nonetheless display the substantial genomic instability. Genomic instability can be rescued by the deletion of Trp53bp1, which encodes the DNA damage response factor 53BP1, and mice expressing RING-less BRCA1 do not show an increased susceptibility to tumors in the absence of 53BP1. Genomic instability in cells expressing RING-less BRCA1 correlates with the loss of BARD1 and a defect in restart of replication forks after hydroxyurea treatment, suggesting a role of BRCA1-BARD1 in genomic integrity that is independent of RAD51 loading.

Breast cancer 1 (BRCA1)-deficient embryos develop normally but are more susceptible to ethanol-initiated DNA damage and embryopathies

The breast cancer 1 (brca1) gene is associated with breast and ovarian cancers, and heterozygous (+/−) brca1 knockout progeny develop normally, suggesting a negligible developmental impact. However, our results show BRCA1 plays a broader biological role in protecting the embryo from oxidative stress. Sox2-promoted Cre-expressing hemizygous males were mated with floxed brca1 females, and gestational day 8 +/− brca1 conditional knockout embryos with a 28% reduction in protein expression were exposed in culture to the reactive oxygen species (ROS)-initiating drug ethanol (EtOH). Untreated +/− brca1-deficient embryos developed normally, but when exposed to EtOH exhibited increased levels of oxidatively damaged DNA, measured as 8-oxo-2'-deoxyguanosine, γH2AX, which is a marker of DNA double strand breaks that can result from 8-oxo-2'-deoxyguanosine, formation, and embryopathies at EtOH concentrations that did not affect their brca1-normal littermates. These results reveal that even modest BRCA1 deficiencies render the embryo more susceptible to drug-enhanced ROS formation, and corroborate a role for DNA oxidation in the mechanism of EtOH teratogenesis.

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Heterozygous (+/−) brca1 conditional knockout (cKO) embryos develop normally.

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+/− brca1 cKO embryos have 28% less BRCA1 protein than wild-type (WT) littermates.

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Ethanol-exposed BRCA1-deficient mice have more oxidatively damaged DNA than WTs.

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Ethanol-exposed BRCA1 cKO embryos exhibit more embryopathies than WT littermates.

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BRCA1 protects the embryo from ethanol-enhanced oxidative stress—a novel role.

Genetics of androgen metabolism in women with infertility and hypoandrogenism

Hypoandrogenism in women with low functional ovarian reserve (LFOR, defined as an abnormally low number of small growing follicles) adversely affects fertility. The androgen precursor dehydroepiandrosterone (DHEA) is increasingly used to supplement treatment protocols in women with LFOR undergoing in vitro fertilization. Due to differences in androgen metabolism, however, responses to DHEA supplementation vary between patients. In addition to overall declines in steroidogenic capacity with advancing age, genetic factors, which result in altered expression or enzymatic function of key steroidogenic proteins or their upstream regulators, might further exacerbate variations in the conversion of DHEA to testosterone. In this Review, we discuss in vitro studies and animal models of polymorphisms and gene mutations that affect the conversion of DHEA to testosterone and attempt to elucidate how these variations affect female hormone profiles. We also discuss treatment options that modulate levels of testosterone by targeting the expression of steroidogenic genes. Common variants in genes encoding DHEA sulphotransferase, aromatase, steroid 5α-reductase, androgen receptor, sex-hormone binding globulin, fragile X mental retardation protein and breast cancer type 1 susceptibility protein have been implicated in androgen metabolism and, therefore, can affect levels of androgens in women. Short of screening for all potential genetic variants, hormonal assessments of patients with low testosterone levels after DHEA supplementation facilitate identification of underlying genetic defects. The genetic predisposition of patients can then be used to design individualized fertility treatments.

HUWE1 interacts with BRCA1 and promotes its degradation in the ubiquitin-proteasome pathway

The cellular BRCA1 protein level is essential for its tumor suppression activity and is tightly regulated through multiple mechanisms including ubiquitn-proteasome system. E3 ligases are involved to promote BRCA1 for ubiquitination and degradation. Here, we identified HUWE1/Mule/ARF-BP1 as a novel BRCA1-interacting protein involved in the control of BRCA1 protein level. HUWE1 binds BRCA1 through its N-terminus degron domain. Depletion of HUWE1 by siRNA-mediated interference significantly increases BRCA1 protein levels and prolongs the half-life of BRCA1. Moreover, exogenous expression of HUWE1 promotes BRCA1 degradation through the ubiquitin-proteasome pathway, which could explain an inverse correlation between HUWE1 and BRCA1 levels in MCF10F, MCF7 and MDA-MB-231 breast cancer cells. Consistent with a functional role for HUWE1 in regulating BRCA1-mediated cellular response to DNA damage, depletion of HUWE1 by siRNA confers increased resistance to ionizing radiation and mitomycin. These data indicate that HUWE1 is a critical negative regulator of BRCA1 and suggest a new molecular mechanism for breast cancer pathogenesis.

Stabilization of the transcription factor Foxp3 by the deubiquitinase USP7 increases Treg-cell-suppressive capacity

Stable Foxp3 expression is required for the development of functional regulatory T (Treg) cells. Here, we demonstrate that the expression of the transcription factor Foxp3 can be regulated through the polyubiquitination of multiple lysine residues, resulting in proteasome-mediated degradation. Expression of the deubiquitinase (DUB) USP7 was found to be upregulated and active in Treg cells, being associated with Foxp3 in the nucleus. Ectopic expression of USP7 decreased Foxp3 polyubiquitination and increased Foxp3 expression. Conversely, either treatment with DUB inhibitor or USP7 knockdown decreased endogenous Foxp3 protein expression and decreased Treg-cell-mediated suppression in vitro. Furthermore, in a murine adoptive-transfer-induced colitis model, either inhibition of DUB activity or USP7 knockdown in Treg cells abrogated their ability to resolve inflammation in vivo. Our data reveal a molecular mechanism in which rapid temporal control of Foxp3 expression in Treg cells can be regulated by USP7, thereby modulating Treg cell numbers and function.

The F-box protein FBXO44 mediates BRCA1 ubiquitination and degradation

BRCA1 mutations account for a significant proportion of familial breast and ovarian cancers. In addition, reduced BRCA1 protein is associated with sporadic cancer cases in these tissues. At the cellular level, BRCA1 plays a critical role in multiple cellular functions such as DNA repair and cell cycle checkpoint control. Its protein level is regulated in a cell cycle-dependent manner. However, regulation of BRCA1 protein stability is not fully understood. Our earlier study showed that the amino terminus of BRCA1 harbors a degron sequence that is sufficient and necessary for conferring BRCA1 degradation. In the current study, we used mass spectrometry to identify Skp1 that regulates BRCA1 protein stability. Small interfering RNA screening that targets all human F-box proteins uncovered FBXO44 as an important protein that influences BRCA1 protein level. The Skp1-Cul1-F-box-protein44 (SCF(FBXO44)) complex ubiquitinates full-length BRCA1 in vitro. Furthermore, the N terminus of BRCA1 mediates the interaction between BRCA1 and FBXO44. Overexpression of SCF(FBXO44) reduces BRCA1 protein level. Taken together, our work strongly suggests that SCF(FBXO44) is an E3 ubiquitin ligase responsible for BRCA1 degradation. In addition, FBXO44 expression pattern in breast carcinomas suggests that SCF(FBXO44)-mediated BRCA1 degradation might contribute to sporadic breast tumor development.

The C-terminal proteolytic fragment of the breast cancer susceptibility type 1 protein (BRCA1) is degraded by the N-end rule pathway

The breast cancer susceptibility type 1 gene product (BRCA1) is cleaved by caspases upon the activation of apoptotic pathways. After proteolysis the C-terminal fragment has been reported to translocate to the cytoplasm and promote cell death. Here we report that the C-terminal fragment is unstable in cells as it is targeted for degradation by the N-end rule pathway. The data reveals that mutating the wild type N-terminal aspartate, of the C-terminal fragment, to valine stabilizes the fragment. If the N terminus is mutated to another N-terminal destabilizing residue, like arginine, the C-terminal fragment remains unstable in cells. Last, the C-terminal fragment of BRCA1 is stable in cells lacking ATE1, a component of the N-end rule pathway.

The BRCA1 ubiquitin ligase and homologous recombination repair

Impairment of homologous recombination (HR), a vital process employed during repair of DNA double strand breaks and stalled DNA replication, provides a valuable opportunity for the cell to become transformed. Once transformed, the impairment turns to be a target for therapy as exemplified by the synthetic lethal strategy such as poly (ADP-ribose) polymerase (PARP) inhibitor for BRCA1/2-defective breast and ovarian cancer. Hence, improving mechanistic understanding of HR has emerged as an urgent issue to address due to the high clinical demand. Ubiquitin modification plays a central role in HR and more than a few E3 ubiquitin ligases have been implicated in the process. However, the significance of the activity of one such key E3 ligase, BRCA1, has not yet been clarified and remains as a major obstacle in the field. Here, we review recent advances in our understanding of BRCA1 function in HR and discuss possible roles of the activity.

The prostaglandin transporter regulates adipogenesis and aromatase transcription

Cytochrome P450 aromatase, encoded by the CYP19 gene, catalyzes estrogen synthesis. In obese postmenopausal women, increased estrogen synthesis in adipose tissue has been linked to hormone-dependent breast carcinogenesis. Hence, it is important to elucidate the mechanisms that regulate CYP19 gene expression. Prostaglandin E(2) (PGE(2)) stimulates the cyclic AMP (cAMP) → protein kinase A (PKA) → cAMP responsive element binding protein (CREB) pathway leading to increased CYP19 transcription. The prostaglandin transporter (PGT) removes PGE(2) from the extracellular milieu and delivers it to the cytosol, where it is inactivated. The main objective of this study was to determine whether PGT regulates CYP19 transcription. Silencing of PGT in preadipocytes increased PGE(2) levels in the extracellular medium, thereby stimulating the cAMP → PKA pathway resulting in enhanced interaction between pCREB, p300, and the CYP19 I.3/II promoter. A reciprocal decrease in the interaction between the CYP19 I.3/II promoter and BRCA1, a repressor of CYP19 transcription, was observed. Overexpressing PGT reduced extracellular PGE(2) levels, suppressed the cAMP → PKA pathway, enhanced the interaction between BRCA1 and p300, and inhibited aromatase expression. We also compared the PGT → aromatase axis in preadipocytes versus adipocytes. Aromatase levels were markedly increased in preadipocytes versus adipocytes. This increase in aromatase was explained, at least in part, by reduced PGT levels leading to enhanced PGE(2) → cAMP → PKA signaling. In addition to regulating aromatase expression, PGT-mediated changes in extracellular PGE(2) levels were a determinant of adipocyte differentiation. Collectively, these results suggest that PGT modulates adipogenesis and thereby PGE(2)-mediated activation of the cAMP → PKA → CREB pathway leading to altered CYP19 transcription and aromatase activity.

BRCA1/BARD1 complex interacts with steroidogenic factor 1--A potential mechanism for regulation of aromatase expression by BRCA1

Germline mutations in BRCA1 predispose women to early onset of breast and ovarian cancers. Findings from previous studies support the notion that the tissue- and gender-specific tumor suppression function of BRCA1 is associated with its role in negative regulation of aromatase expression, the rate-limiting step in estrogen biosynthesis. The molecular mechanism of BRCA1 in regulating aromatase promoter activity remains to be elucidated. In this study, we demonstrate that, in an ovarian granulosa cell line KGN, steroidogenic factor 1 (SF-1) is required for aromatase PII promoter basal activity as well as the elevated aromatase expression mediated by BRCA1 knockdown. Furthermore, BRCA1 in KGN cells exists mainly as a heterodimer with BARD1. We provide evidence that the BRCA1/BARD1 complex interacts with SF-1 both in vivo and in vitro. However, the intrinsic ubiquitin E3 ligase activity of BRCA1/BARD1 does not appear to contribute to ubiquitynation of SF-1. We propose that the interaction between SF-1 and BRCA1/BARD1 may recruit BRCA1/BARD1 complex to the aromatase PII promoter for BRCA1/BARD1-mediate transcriptional repression.

HERC2 is an E3 ligase that targets BRCA1 for degradation

The breast cancer suppressor BRCA1 forms a stable heterodimeric E3 ubiquitin ligase with BARD1. Each protein controls the abundance and stability of the other, and loss of the interaction leads to BRCA1 degradation. Here, we show that HERC2, a protein recently implicated in DNA damage repair, targets BARD1-uncoupled BRCA1 for degradation. HERC2 shuttles between the nucleus and the cytoplasm. Its COOH-terminal HECT-containing domain interacts with an NH(2)-terminal degron domain in BRCA1. HERC2 ubiquitinates BRCA1; this reaction depends on Cys(4762) of HERC2, the catalytic ubiquitin binding site, and the degron of BRCA1. The HERC2-BRCA1 interaction is maximal during the S phase of the cell cycle and rapidly diminishes as cells enter G(2)-M, inversely correlated with the steady-state level of BRCA1. Significantly, HERC2 depletion antagonizes the effects of BARD1 depletion by restoring BRCA1 expression and G(2)-M checkpoint activity. Conversely, BARD1 protects BRCA1 from HERC2-mediated ubiquitination. Collectively, our findings identify a function for HERC2 in regulating BRCA1 stability in opposition to BARD1. The HERC2 expression in breast epithelial cells and breast carcinomas suggests that this mechanism may play a role in breast carcinogenesis.

Gender differences and estrogen effects in parkin null mice

Estrogens are considered neurotrophic for dopamine neurons. Parkinson's disease is more frequent in males than in females, and more prevalent in females with short reproductive life. Estrogens are neuroprotective against neurotoxic agents for dopamine neurons in vivo and in vitro. Here, we have investigated the role of estrogens in wild-type (WT) and parkin null mice (PK-/-). WT mice present sexual dimorphisms in neuroprotective mechanisms (Bcl-2/Bax, chaperones, and GSH), but some of these inter-sex differences disappear in PK-/-. Tyrosine hydroxylase (TH) protein and TH+ cells decreased earlier and more severely in female than in male PK-/- mice. Neuronal cultures from midbrain of WT and PK-/- mice were treated with estradiol from 10 min to 48 h. Short-term treatments activated the mitogen-activated protein kinase pathway of WT and PK-/- neurons and the phosphatidylinositol 3'-kinase/AKT/glycogen synthase kinase-3 pathway of WT but not of PK-/- cultures. Long-term treatments with estradiol increased the number of TH+ neurons, the TH expression, and the extension of neurites, and decreased the level of apoptosis, the expression of glial fibrillary acidic protein, and the number of microglial cells in WT but not in PK-/- cultures. The levels of estrogen receptor-alpha were elevated in midbrain cultures and in the striatum of adult PK-/- male mice, suggesting that suppression of parkin changes the estrogen receptor-alpha turnover. From our data, it appears that parkin participates in the cellular estrogen response which could be of interest in the management of parkin-related Parkinson's disease patients.

Proteasomal degradation of TRIM5alpha during retrovirus restriction

The host protein TRIM5α inhibits retroviral infection at an early post-penetration stage by targeting the incoming viral capsid. While the detailed mechanism of restriction remains unclear, recent studies have implicated the activity of cellular proteasomes in the restriction of retroviral reverse transcription imposed by TRIM5α. Here, we show that TRIM5α is rapidly degraded upon encounter of a restriction-susceptible retroviral core. Inoculation of TRIM5α-expressing human 293T cells with a saturating level of HIV-1 particles resulted in accelerated degradation of the HIV-1-restrictive rhesus macaque TRIM5α protein but not the nonrestrictive human TRIM5α protein. Exposure of cells to HIV-1 also destabilized the owl monkey restriction factor TRIMCyp; this was prevented by addition of the inhibitor cyclosporin A and was not observed with an HIV-1 virus containing a mutation in the capsid protein that relieves restriction by TRIMCyp IVHIV. Likewise, human TRIM5α was rapidly degraded upon encounter of the restriction-sensitive N-tropic murine leukemia virus (N-MLV) but not the unrestricted B-MLV. Pretreatment of cells with proteasome inhibitors prevented the HIV-1-induced loss of both rhesus macaque TRIM5α and TRIMCyp proteins. We also detected degradation of endogenous TRIM5α in rhesus macaque cells following HIV-1 infection. We conclude that engagement of a restriction-sensitive retrovirus core results in TRIM5α degradation by a proteasome-dependent mechanism.

Recent studies have identified several cellular proteins that restrict infection by a variety of retroviruses. One of these restriction factors, TRIM5α, is partially responsible for the differences in susceptibility of monkeys and humans to SIV and HIV-1, respectively. TRIM5α inhibits retrovirus infection soon after penetration into the target cell by associating with the viral protein CA, which forms the polymeric capsid shell of the viral core. Although the detailed mechanism of restriction is unknown, TRIM5α is postulated to alter the stability of the viral core, resulting in a failure to complete reverse transcription. The activity of cellular proteasomes, which are responsible for intracellular protein degradation, has also been implicated in TRIM5α-dependent attenuation of retroviral reverse transcription. In this study, we show that cellular TRIM5α is rapidly degraded in cells exposed to a restriction-sensitive retrovirus but not in cells infected with an unrestricted virus. Virus-induced degradation of TRIM5α was dependent on cellular proteasome activity, as inhibition with drugs blocking proteasome function also inhibited degradation of TRIM5α. These results provide additional support for a role of proteasomal degradation in TRIM5α-dependent retrovirus restriction and suggest a novel mechanism by which binding of TRIM5α to the viral capsid prevents infection.

EP2 and EP4 receptors regulate aromatase expression in human adipocytes and breast cancer cells. Evidence of a BRCA1 and p300 exchange

Cytochrome P450 aromatase (aromatase), a product of the CYP19 gene, catalyzes the synthesis of estrogens from androgens. Because aromatase-dependent estrogen biosynthesis has been linked to hormone-dependent breast carcinogenesis, it is important to elucidate the mechanisms that regulate CYP19 gene expression. The main objective of this study was to identify the receptors (EP) for prostaglandin E(2) (PGE(2)) that mediate the induction of CYP19 transcription in human adipocytes and breast cancer cells. Treatment with PGE(2) induced aromatase, an effect that was mimicked by either EP(2) or EP(4) agonists. Antagonists of EP(2) or EP(4) or small interference RNA-mediated down-regulation of these receptors suppressed PGE(2)-mediated induction of aromatase. PGE(2) via EP(2) and EP(4) stimulated the cAMP-->protein kinase A pathway resulting in enhanced interaction between P-CREB, p300, and the aromatase promoter I.3/II. Overexpressing a mutant form of p300 that lacks histone acetyltransferase activity suppressed PGE(2)-mediated induction of aromatase promoter activity. PGE(2) via EP(2) and EP(4) also caused a reduction in both the amounts of BRCA1 and the interaction between BRCA1 and the aromatase promoter I.3/II. Activation of the aromatase promoter by PGE(2) was suppressed by overexpressing wild-type BRCA1. Silencing of EP(2) or EP(4) also blocked PGE(2)-mediated induction of the progesterone receptor, a prototypic estrogen-response gene. In a mouse model, overexpressing COX-2 in the mammary gland, a known inducer of PGE(2) synthesis, led to increased aromatase mRNA and activity and reduced amounts of BRCA1; these effects were reversed by knocking out EP(2). Taken together, these results suggest that PGE(2) via EP(2) and EP(4) activates the cAMP-->PKA-->CREB pathway leading to enhanced CYP19 transcription and increased aromatase activity. Reciprocal changes in the interaction between BRCA1, p300, and the aromatase promoter I.3/II contributed to the inductive effects of PGE(2).