Regulation of alternative splicing of liver scavenger receptor class B gene by estrogen and the involved regulatory splicing factors

LIMP-2 enhances cancer stem-like cell properties by promoting autophagy-induced GSK3β degradation in head and neck squamous cell carcinoma

Cancer stem cell-like cells (CSCs) play an integral role in the heterogeneity, metastasis, and treatment resistance of head and neck squamous cell carcinoma (HNSCC) due to their high tumor initiation capacity and plasticity. Here, we identified a candidate gene named LIMP-2 as a novel therapeutic target regulating HNSCC progression and CSC properties. The high expression of LIMP-2 in HNSCC patients suggested a poor prognosis and potential immunotherapy resistance. Functionally, LIMP-2 can facilitate autolysosome formation to promote autophagic flux. LIMP-2 knockdown inhibits autophagic flux and reduces the tumorigenic ability of HNSCC. Further mechanistic studies suggest that enhanced autophagy helps HNSCC maintain stemness and promotes degradation of GSK3β, which in turn facilitates nuclear translocation of β-catenin and transcription of downstream target genes. In conclusion, this study reveals LIMP-2 as a novel prospective therapeutic target for HNSCC and provides evidence for a link between autophagy, CSC, and immunotherapy resistance.

TRA2: The dominant power of alternative splicing in tumors

The dysregulation of alternative splicing (AS) is frequently found in cancer and considered as key markers for cancer progression and therapy. Transformer 2 (TRA2), a nuclear RNA binding protein, consists of transformer 2 alpha homolog (TRA2A) and transformer 2 beta homolog (TRA2B), and plays a role in the regulation of pre-mRNA splicing. Growing evidence has been provided that TRA2A and TRA2B are dysregulated in several types of tumors, and participate in the regulation of proliferation, migration, invasion, and chemotherapy resistance in cancer cells through alteration of AS of cancer-related genes. In this review, we highlight the role of TRA2 in tumorigenesis and metastasis, and discuss potential molecular mechanisms how TRA2 influences tumorigenesis and metastasis via controlling AS of pre-mRNA. We propose that TRA2Ais a novel biomarker and therapeutic target for cancer progression and therapy.

RNA splicing: a dual-edged sword for hepatocellular carcinoma

RNA splicing is the fundamental process that brings diversity at the transcriptome and proteome levels. The spliceosome complex regulates minor and major processes of RNA splicing. Aberrant regulation is often associated with different diseases, including diabetes, stroke, hypertension, and cancer. In the majority of cancers, dysregulated alternative RNA splicing (ARS) events directly affect tumor progression, invasiveness, and often lead to poor survival of the patients. Alike the rest of the gastrointestinal malignancies, in hepatocellular carcinoma (HCC), which alone contributes to ~ 75% of the liver cancers, a large number of ARS events have been observed, including intron retention, exon skipping, presence of alternative 3'-splice site (3'SS), and alternative 5'-splice site (5'SS). These events are reported in spliceosome and non-spliceosome complexes genes. Molecules such as MCL1, Bcl-X, and BCL2 in different isoforms can behave as anti-apoptotic or pro-apoptotic, making the spliceosome complex a dual-edged sword. The anti-apoptotic isoforms of such molecules bring in resistance to chemotherapy or cornerstone drugs. However, in contrast, multiple malignant tumors, including HCC that target the pro-apoptotic favoring isoforms/variants favor apoptotic induction and make chemotherapy effective. Herein, we discuss different splicing events, aberrations, and antisense oligonucleotides (ASOs) in modulating RNA splicing in HCC tumorigenesis with a possible therapeutic outcome.

Integrative analysis of transcriptome complexity in pig granulosa cells by long-read isoform sequencing

Background

In intensive and large-scale farms, abnormal estradiol levels in sows can cause reproductive disorders. The high incidence rate of reproductive disturbance will induce the elimination of productive sows in large quantities, and the poor management will bring great losses to the pig farms. The change in estradiol level has an important effect on follicular development and estrus of sows. To solve this practical problem and improve the productive capacity of sows, it is significant to further clarify the regulatory mechanism of estradiol synthesis in porcine granulosa cells (GCs). The most important function of granulosa cells is to synthesize estradiol. Thus, the studies about the complex transcriptome in porcine GCs are significant. As for precursor-messenger RNAs (pre-mRNAs), their post-transcriptional modification, such as alternative polyadenylation (APA) and alternative splicing (AS), together with long non-coding RNAs (lncRNAs), may regulate the functions of granulosa cells. However, the above modification events and their function are unclear within pig granulosa cells.

Methods

Combined PacBio long-read isoform sequencing (Iso-Seq) was conducted in this work for generating porcine granulosa cells' transcriptomic data. We discovered new transcripts and possible gene loci via comparison against reference genome. Later, combined Iso-Seq data were adopted to uncover those post-transcriptional modifications such as APA or AS, together with lncRNA within porcine granulosa cells. For confirming that the Iso-Seq data were reliable, we chose four AS genes and analyzed them through RT-PCR.

Results

The present article illustrated that pig GCs had a complex transcriptome, which gave rise to 8,793 APA, 3,465 AS events, 703 candidate new gene loci, as well as 92 lncRNAs. The results of this study revealed the complex transcriptome in pig GCs. It provided a basis for the interpretation of the molecular mechanism in GCs.

Characterization of SR-B2a and SR-B2b genes and their ability to promote GCRV infection in grass carp (Ctenopharyngodon idellus)

Scavenger receptor class B type 2 (SR-B2) is a pattern recognition receptor involved in innate immunity in mammals; however, the immunological function of SR-Bs in fish remains unclear. In this study, the full-length cDNA sequences of SR-B2a and SR-B2b from grass carp (Ctenopharyngodon idellus) were cloned and designated as CiSR-B2a and CiSR-B2b. Multiple alignments and phylogenetic analyses deduced that CiSR-B2a and CiSR-B2b had the highest evolutionary conservation and were closely related to the zebrafish (Danio rerio) homologs, DrSR-B2a and DrSR-B2b, respectively. Both CiSR-B2a and CiSR-B2b were expressed in all the tested tissues, with the highest expression levels found in the hepatopancreas. In Ctenopharyngodon idellus kidney cells (CIK), CiSR-B2a and CiSR-B2b were mainly located in the cytoplasm, and a small amount located on the plasma membrane. After challenge with Grass Carp Reovirus (GCRV), the expression of CiSR-B2a and CiSR-B2b were significantly upregulated in the spleen (about 10.27 and 27.19 times higher than that at 0 day, p < 0.01). With CiSR-B2a or CiSR-B2b overexpressed in CIK, the relative copy number of GCRV in the cells was both significantly increased compared to that in the control group, indicating that CiSR-B2a and CiSR-B2b may be important proteins during the infection processes of GCRV.

Hepatic cholesterol transport and its role in non-alcoholic fatty liver disease and atherosclerosis

Non-alcoholic fatty liver disease (NAFLD) is a quickly emerging global health problem representing the most common chronic liver disease in the world. Atherosclerotic cardiovascular disease represents the leading cause of mortality in NAFLD patients. Cholesterol metabolism has a crucial role in the pathogenesis of both NAFLD and atherosclerosis. The liver is the major organ for cholesterol metabolism. Abnormal hepatic cholesterol metabolism not only leads to NAFLD but also drives the development of atherosclerotic dyslipidemia. The cholesterol level in hepatocytes reflects the dynamic balance between endogenous synthesis, uptake, esterification, and export, a process in which cholesterol is converted to neutral cholesteryl esters either for storage in cytosolic lipid droplets or for secretion as a major constituent of plasma lipoproteins, including very-low-density lipoproteins, chylomicrons, high-density lipoproteins, and low-density lipoproteins. In this review, we describe decades of research aimed at identifying key molecules and cellular players involved in each main aspect of hepatic cholesterol metabolism. Furthermore, we summarize the recent advances regarding the biological processes of hepatic cholesterol transport and its role in NAFLD and atherosclerosis.

Targeting Foam Cell Formation in Atherosclerosis: Therapeutic Potential of Natural Products

Foam cell formation and further accumulation in the subendothelial space of the vascular wall is a hallmark of atherosclerotic lesions. Targeting foam cell formation in the atherosclerotic lesions can be a promising approach to treat and prevent atherosclerosis. The formation of foam cells is determined by the balanced effects of three major interrelated biologic processes, including lipid uptake, cholesterol esterification, and cholesterol efflux. Natural products are a promising source for new lead structures. Multiple natural products and pharmaceutical agents can inhibit foam cell formation and thus exhibit antiatherosclerotic capacity by suppressing lipid uptake, cholesterol esterification, and/or promoting cholesterol ester hydrolysis and cholesterol efflux. This review summarizes recent findings on these three biologic processes and natural products with demonstrated potential to target such processes. Discussed also are potential future directions for studying the mechanisms of foam cell formation and the development of foam cell-targeted therapeutic strategies.

Dysregulation of the Splicing Machinery Is Associated to the Development of Nonalcoholic Fatty Liver Disease

CONTEXT

Nonalcoholic fatty liver disease (NAFLD) is a common obesity-associated pathology characterized by hepatic fat accumulation, which can progress to fibrosis, cirrhosis, and hepatocellular carcinoma. Obesity is associated with profound changes in gene-expression patterns of the liver, which could contribute to the onset of comorbidities.

OBJECTIVE

As these alterations might be linked to a dysregulation of the splicing process, we aimed to determine whether the dysregulation in the expression of splicing machinery components could be associated with NAFLD.

PARTICIPANTS

We collected 41 liver biopsies from nonalcoholic individuals with obesity, with or without hepatic steatosis, who underwent bariatric surgery.

INTERVENTIONS

The expression pattern of splicing machinery components was determined using a microfluidic quantitative PCR-based array. An in vitro approximation to determine lipid accumulation using HepG2 cells was also implemented.

RESULTS

The liver of patients with obesity and steatosis exhibited a severe dysregulation of certain splicing machinery components compared with patients with obesity without steatosis. Nonsupervised clustering analysis allowed the identification of three molecular phenotypes of NAFLD with a unique fingerprint of alterations in splicing machinery components, which also presented distinctive hepatic and clinical-metabolic alterations and a differential response to bariatric surgery after 1 year. In addition, in vitro silencing of certain splicing machinery components (i.e., PTBP1, RBM45, SND1) reduced fat accumulation and modulated the expression of key de novo lipogenesis enzymes, whereas conversely, fat accumulation did not alter spliceosome components expression.

CONCLUSION

There is a close relationship between splicing machinery dysregulation and NAFLD development, which should be further investigated to identify alternative therapeutic targets.

ERβ agonist alters RNA splicing factor expression and has a longer window of antidepressant effectiveness than estradiol after long-term ovariectomy

BACKGROUND

Estrogen therapy (ET), an effective treatment for perimenopausal depression, often fails to ameliorate symptoms when initiated late after the onset of menopause. Our previous work has suggested that alternative splicing of RNA might mediate these differential effects of ET.

METHODS

Female Sprague–Dawley rats were treated with estradiol (E2) or vehicle 6 days (early ET) or 180 days (late ET) after ovariectomy (OVX). We investigated the differential expression of RNA splicing factors and tryptophan hydroxylase 2 (TPH2) protein using a customized RT2 Profiler PCR Array, reverse-transcription polymerase chain reaction, immunoprecipitation and behaviour changes in clinically relevant early and late ET.

RESULTS

Early ET, but not late ET, prolonged swimming time in the forced swim test and reduced anxiety-like behaviours in the elevated plus maze. It reversed OVX-increased (SFRS7 and SFRS16) or OVX-decreased (ZRSR2 and CTNNB1) mRNA levels of splicing factors and ERβ splicing changes in the brains of OVX rats. Early ET, but not late ET, also increased the expression of TPH2 and decreased monoamine oxidase A levels in the dorsal raphe in the brains of OVX rats. In late ET, only diarylpropionitrile (an ERβ-specific agonist) achieved similar results — not E2 (an ERα and ERβ agonist) or propylpyrazoletriol (an ERα-specific agonist).

LIMITATIONS

Our experimental paradigm mimicked early and late ET in the clinical setting, but the contribution of age and OVX might be difficult to distinguish.

CONCLUSION

These findings suggest that ERβ alternative splicing and altered responses in the regulatory system for serotonin may mediate the antidepressant efficacy of ET associated with the timing of therapy initiation. It is likely that ERβ-specific ligands would be effective estrogen-based antidepressants late after the onset of menopause.

Alternative RNA Splicing in the Pathogenesis of Liver Disease

Non-alcoholic fatty liver disease (NAFLD) is becoming increasingly prevalent due to the worldwide obesity epidemic and currently affects one-third of adults or about one billion people worldwide. NAFLD is predicted to affect over 50% of the world's population by the end of the next decade. It is the most common form of liver disease and is associated with increased risk for progression to a more severe form non-alcoholic steatohepatitis, as well as insulin resistance, type 2 diabetes mellitus, cirrhosis, and eventually hepatocellular carcinoma. This review article will focus on the role of alternative splicing in normal liver physiology and dysregulation in liver disease.

Low-density lipoprotein upregulate SR-BI through Sp1 Ser702 phosphorylation in hepatic cells

Scavenger receptor class B type I (SR-BI) is one of the key proteins in the process of reverse cholesterol transport (RCT), and its major function is to uptake high density lipoprotein (HDL) cholesterol from plasma into liver cells. The regulation of SR-BI expression is important for controlling serum lipid content and reducing the risks of cardiovascular diseases. Here we found that SR-BI expression was significantly increased by LDL in vivo and in vitro, and the transcription factor specific protein 1 (Sp1) plays a critical role in this process. Results from co-immunoprecipitation experiments indicate that the activation of SR-BI was associated with Sp1-recruited protein complexes in the promoter region of SR-BI, where histone acetyltransferase p300 was recruited and histone deacetylase HDAC1 was dismissed. As a result, histone acetylation increased, leading to activation of SR-BI transcription. With further investigation, we found that LDL phosphorylated Sp1 through ERK1/2 pathway, which affected Sp1 protein complexes formation in SR-BI promoter. Using mass spectrometry and site directed mutagenesis, a new Sp1 phosphorylation site Ser702 was defined to be associated with Sp1-HDAC1 interaction and may be important in SR-BI activation, shedding light on the knowledge of delicate mechanism of hepatic HDL receptor SR-BI gene modulation by LDL.

The Expanding Complexity of Estrogen Receptor Signaling in the Cardiovascular System

Estrogen has important effects on cardiovascular function including regulation of vascular function, blood pressure, endothelial relaxation, and the development of hypertrophy and cardioprotection. However, the mechanisms by which estrogen mediates these effects are still poorly understood. As detailed in this review, estrogen can regulate transcription by binding to 2 nuclear receptors, ERα and ERβ, which differentially regulate gene transcription. ERα and ERβ regulation of gene transcription is further modulated by tissue-specific coactivators and corepressors. Estrogen can bind to ERα and ERβ localized at the plasma membrane as well as G-protein-coupled estrogen receptor to initiate membrane delimited signaling, which enhances kinase signaling pathways that can have acute and long-term effects. The kinase signaling pathways can also mediate transcriptional changes and can synergize with the ER to regulate cell function. This review will summarize the beneficial effects of estrogen in protecting the cardiovascular system through ER-dependent mechanisms with an emphasis on the role of the recently described ER membrane signaling mechanisms.

20-hydroxyecdysone mediates non-canonical regulation of mosquito vitellogenins through alternative splicing

Vitellogenesis is one of the most well-studied physiological processes in mosquitoes. Expression of mosquito vitellogenin genes is classically described as being restricted to female adult reproduction. We report premature vitellogenin transcript expression in three vector mosquitoes: Culex tarsalis, Aedes aegypti and Anopheles gambiae. Vitellogenins expressed during non-reproductive stages are alternatively spliced to retain their first intron and encode premature termination codons. We show that intron retention results in transcript degradation by translation-dependent nonsense-mediated mRNA decay. This is probably an example of regulated unproductive splicing and translation (RUST), a mechanism known to regulate gene expression in numerous organisms but which has never been described in mosquitoes. We demonstrate that the hormone 20-hydroxyecdysone (20E) is responsible for regulating post-transcriptional splicing of vitellogenin. After exposure of previtellogenic fat bodies to 20E, vitellogenin expression switches from a non-productive intron-retaining transcript to a spliced protein-coding transcript. This effect is independent of factors classically known to influence transcription, such as juvenile hormone-mediated competence and amino acid signalling through the target of rapamycin pathway. Non-canonical regulation of vitellogenesis through RUST is a novel role for the multifunctional hormone 20E, and may have important implications for general patterns of gene regulation in mosquitoes.

Atorvastatin and hormone therapy influence expression of ABCA1, APOA1 and SCARB1 in mononuclear cells from hypercholesterolemic postmenopausal women

BACKGROUND

Reverse cholesterol transport (RCT) has been inversely related to atherosclerosis and cardiovascular risk. The influence of menopause in the RCT process is poorly understood and the effects of cholesterol-lowering interventions, including statins and hormone therapy (HT), on genes controlling the RCT in postmenopausal women are also unknown.

METHODS

The effects on serum lipids and expression profile of genes involved in RCT - APOA1, ABCA1, ABCG1, SCARB1 and LXRA - were evaluated by TaqMan(®) quantitative PCR in peripheral blood mononuclear cells (PBMC) from 87 postmenopausal hypercholesterolemic women treated with atorvastatin (AT, n=17), estrogen or estrogen plus progestin (HT, n=34) and estrogen or estrogen plus progestin associated with atorvastatin (HT+AT, n=36).

RESULTS

Atorvastatin and HT treatments reduced the mRNA levels of APOA1 and SCARB1, respectively, whereas ABCA1 expression was reduced after all treatments. Although the expression of LXRA, an important transcription factor controlling the expression of genes involved in RCT, was not modified after any treatment, it was correlated with ABCA1, APOA1 and SCARB1 RNAm values before and after treatments, however no correlation with ABCG1 was observed. In a linear regression analysis, HT was related to an increase in apoAI levels after treatment when compared to atorvastatin and, moreover, higher SCARB1 and ABCA1 basal expression were also associated with decreased apoAI levels after treatments.

CONCLUSION

ABCA1 mRNA levels are decreased by atorvastatin and HT, however these treatments have a differential effect on APOA1 and SCARB1 expression in PBMC from postmenopausal women. Basal ABCA1 and SCARB1 expression profile could be helpful markers in predicting the effect of atorvastatin and HT on RCT, according to the changes in apoAI levels in this sample population.

Expression of Tra2 β in Cancer Cells as a Potential Contributory Factor to Neoplasia and Metastasis

The splicing regulator proteins SRSF1 (also known as ASF/SF2) and SRSF3 (also known as SRP20) belong to the SR family of proteins and can be upregulated in cancer. The SRSF1 gene itself is amplified in some cancer cells, and cancer-associated changes in the expression of MYC also increase SRSF1 gene expression. Increased concentrations of SRSF1 protein promote prooncogenic splicing patterns of a number of key regulators of cell growth. Here, we review the evidence that upregulation of the SR-related Tra2β protein might have a similar role in cancer cells. The TRA2B gene encoding Tra2β is amplified in particular tumours including those of the lung, ovary, cervix, stomach, head, and neck. Both TRA2B RNA and Tra2β protein levels are upregulated in breast, cervical, ovarian, and colon cancer, and Tra2β expression is associated with cancer cell survival. The TRA2B gene is a transcriptional target of the protooncogene ETS-1 which might cause higher levels of expression in some cancer cells which express this transcription factor. Known Tra2β splicing targets have important roles in cancer cells, where they affect metastasis, proliferation, and cell survival. Tra2β protein is also known to interact directly with the RBMY protein which is implicated in liver cancer.

MicroRNAs 185, 96, and 223 repress selective high-density lipoprotein cholesterol uptake through posttranscriptional inhibition

Hepatic scavenger receptor class B type I (SR-BI) plays an important role in selective high-density lipoprotein cholesterol (HDL-C) uptake, which is a pivotal step of reverse cholesterol transport. In this study, the potential involvement of microRNAs (miRNAs) in posttranscriptional regulation of hepatic SR-BI and selective HDL-C uptake was investigated. The level of SR-BI expression was repressed by miRNA 185 (miR-185), miR-96, and miR-223, while the uptake of 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI)-HDL was decreased by 31.9% (P < 0.001), 23.9% (P < 0.05), and 15.4% (P < 0.05), respectively, in HepG2 cells. The inhibition of these miRNAs by their anti-miRNAs had opposite effects in these hepatic cells. The critical effect of miR-185 was further validated by the loss of regulation in constructs with mutated miR-185 target sites. In addition, these miRNAs directly targeted the 3' untranslated region (UTR) of SR-BI with a coordinated effect. Interestingly, the decrease of miR-96 and miR-185 coincided with the increase of SR-BI in the livers of ApoE KO mice on a high-fat diet. These data suggest that miR-185, miR-96, and miR-223 may repress selective HDL-C uptake through the inhibition of SR-BI in human hepatic cells, implying a novel mode of regulation of hepatic SR-BI and an important role of miRNAs in modulating cholesterol metabolism.

Alternative splicing in the regulation of cholesterol homeostasis

PURPOSE OF REVIEW

With the advent of whole-transcriptome sequencing, or RNA-seq, we now know that alternative splicing is a generalized phenomenon, with nearly all multiexonic genes subject to alternative splicing. In this review, we highlight recent studies examining alternative splicing as a modulator of cellular cholesterol homeostasis and as an underlying mechanism of dyslipidemia.

RECENT FINDINGS

A number of key genes involved in cholesterol metabolism are known to undergo functionally relevant alternative splicing. Recently, we have identified coordinated changes in alternative splicing in multiple genes in response to alterations in cellular sterol content. We and others have implicated several splicing factors as regulators of lipid metabolism. Furthermore, a number of cis-acting human gene variants that modulate alternative splicing have been implicated in a variety of human metabolic diseases.

SUMMARY

Alternative splicing is of importance in various types of genetically influenced dyslipidemias and in the regulation of cellular cholesterol metabolism.

Functional genomics of the human high-density lipoprotein receptor scavenger receptor BI: an old dog with new tricks

PURPOSE OF REVIEW

The athero-protective role of scavenger receptor BI (SR-BI) is primarily attributed to its ability to selectively transfer cholesteryl esters from high-density lipoproteins (HDLs) to the liver during reverse cholesterol transport (RCT). In this review, we highlight recent findings that reveal the impact of SR-BI on lipid levels and cardiovascular disease in humans. Moreover, additional responsibilities of SR-BI in modulating adrenal and platelet function, as well as female fertility in humans, are discussed.

RECENT FINDINGS

Heterozygote carriers of P297S, S112F and T175A-mutant SR-BI receptors were identified in patients with high HDL-cholesterol levels. HDL from P297S-SR-BI carriers was unable to mediate macrophage cholesterol efflux, whereas hepatocytes expressing P297S-SR-BI were unable to mediate the selective uptake of HDL-cholesteryl esters. S112F and T175A-mutant receptors exhibited similar impaired cholesterol transport functions in vitro. Reduced SR-BI function in P297S carriers was also associated with decreased steroidogenesis and altered platelet function. Further, human population studies identified SCARB1 variants associated with female infertility.

SUMMARY

Identification of SR-BI variants confirms the key role of this receptor in influencing lipid levels and RCT in humans. A deeper understanding of the contributions of SR-BI to steroidogenesis, platelet function and fertility is required in light of exploration of HDL-raising therapies aimed at reducing cardiovascular risk.

Elevated CETP Activity Improves Plasma Cholesterol Efflux Capacity From Human Macrophages in Women

Objective—

We aim to identify the impact of endogenous cholesteryl ester transfer protein (CETP) activity on plasma capacity to mediate free cholesterol efflux from human macrophages.

Methods and Results—

Endogenous plasma CETP activity was measured in a population of 348 women. We defined a low CETP group corresponding to subjects displaying an endogenous plasma CETP activity within the first tertile and a high CETP group corresponding to subjects with an endogenous plasma CETP activity within the third tertile. Subjects from the high CETP activity group displayed a significant increase in the capacity of their plasma (+8.2%; P =0.001) to mediate cholesterol efflux from human acute monocytic leukemia cell line human macrophages and from ATP-binding cassette transporter A1-dependent pathway (+23.4%; P =0.0001) as compared with those from the low CETP activity group. Multivariate analyses revealed that the impact of CETP activity was independent of plasma lipids levels. Pre–β1-high-density lipoprotein concentrations were significantly elevated (+29.6%; P =0.01) in the high CETP activity group as compared with the low CETP activity group. A positive correlation between pre–β1-high-density lipoprotein levels and plasma efflux efficiency from human acute monocytic leukemia cell line human macrophages was observed ( r =0.29, P =0.02).

Conclusion—

CETP leading to the improvement of plasma efflux capacity, as a result of efficient pre–β-high-density lipoprotein formation and ATP-binding cassette transporter A1 efflux, should be preserved to prevent lipid accumulation in human macrophages.

Progesterone receptor (PR) variants exist in breast cancer cells characterised as PR negative

Progesterone receptor (PR) expression is measured in breast cancer by immunohistochemistry using N-terminally targeted antibodies and serves as a biomarker for endocrine therapeutic decisions. Extensive PR alternative splicing has been reported which may generate truncated PR variant proteins which are not detected by current breast cancer screening or may alter the function of proteins detected in screening. However, the existence of such truncated PR variants remains controversial. We have characterised PR protein expression in breast cancer cell lines using commercial PR antibodies targeting different epitopes. Truncated PR proteins are detected in reportedly PR negative MDA-MB-231 cells using a C-terminally targeted antibody. Antibody specificity was confirmed by immunoblotting following siRNA knockdown of PR expression. We have further demonstrated that alternatively spliced PR mRNA is present in MDA-MB-231 cells and in reportedly PR-negative breast tumour tissue which could encode the truncated PR proteins detected by the C-terminal antibody. The potential function of PR variant proteins present in MDA-MB-231 cells was also assessed, indicating the ability of these PR variants to bind progesterone, interact with a nuclear PR co-factor and bind DNA. These findings suggest that alternative splicing may generate functional truncated PR variant proteins which are not detected by breast cancer screening using N-terminally targeted antibodies leading to misclassification as PR negative.

Estradiol up-regulates expression of the A + U-rich binding factor 1 (AUF1) gene in the sheep uterus

The A+U-rich binding factor 1 (AUF1 or HNRPD) gene produces predominant RNA binding proteins. The AUF1 transcript is alternatively spliced to produce four protein isoforms that stabilize or destabilize hundreds of mRNAs. Previously, we discovered that estradiol (E2) treatment of ovariectomized sheep increased concentrations of AUF1p45 protein which stabilized estrogen receptor alpha (ER) mRNA in the uterus. This study examined E2 regulation of AUF1 mRNAs in the sheep uterus. Northern analysis determined that E2 treatment increased concentrations of total AUF1 mRNAs twofold in endometrial and myometrial tissue compartments. In situ hybridization indicated that the increase was most intense in the glandular epithelium of endometrium. In a well characterized in vitro RNA stability assay, AUF1 3'UTR sequences were much more stable in uterine extracts from E2-treated ewes compared to extracts from control ewes. AUF1 mRNAs with alternative splicing of exons 2 and 7 (in the coding sequence) and exon 9 (in the 3'UTR) were identified. The only effect of E2 treatment on alternative splicing was that it reduced the percentage of AUF1 mRNAs containing exon 9-derived sequences. These data indicate that E2 up-regulates AUF1 and ER genes coordinately by a post-transcriptional mechanism.

Impact of gene variants on sex-specific regulation of human Scavenger receptor class B type 1 (SR-BI) expression in liver and association with lipid levels in a population-based study

BACKGROUND

Several studies have noted that genetic variants of SCARB1, a lipoprotein receptor involved in reverse cholesterol transport, are associated with serum lipid levels in a sex-dependent fashion. However, the mechanism underlying this gene by sex interaction has not been explored.

METHODS

We utilized both epidemiological and molecular methods to study how estrogen and gene variants interact to influence SCARB1 expression and lipid levels. Interaction between 35 SCARB1 haplotype-tagged polymorphisms and endogenous estradiol levels was assessed in 498 postmenopausal Caucasian women from the population-based Rancho Bernardo Study. We further examined associated variants with overall and SCARB1 splice variant (SR-BI and SR-BII) expression in 91 human liver tissues using quantitative real-time PCR.

RESULTS

Several variants on a haplotype block spanning intron 11 to intron 12 of SCARB1 showed significant gene by estradiol interaction affecting serum lipid levels, the strongest for rs838895 with HDL-cholesterol (p=9.2x10(-4)) and triglycerides (p=1.3x10(-3)) and the triglyceride:HDL cholesterol ratio (p=2.7x10(-4)). These same variants were associated with expression of the SR-BI isoform in a sex-specific fashion, with the strongest association found among liver tissue from 52 young women<45 years old (p=0.002).

CONCLUSIONS

Estrogen and SCARB1 genotype may act synergistically to regulate expression of SCARB1 isoforms and impact serum levels of HDL cholesterol and triglycerides. This work highlights the importance of considering sex-dependent effects of gene variants on serum lipid levels.

Oxidative stress-induced alternative splicing of transformer 2beta (SFRS10) and CD44 pre-mRNAs in gastric epithelial cells

The tra2beta gene encoding an alternative splicing regulator, transformer 2-beta (Tra2beta), generates five alternative splice variant transcripts (tra2beta1-5). Functionally active, full-length Tra2beta is encoded by tra2beta1 isoform. Expression and physiological significance of the other isoforms, particularly tra2beta4, are not fully understood. Rat gastric mucosa constitutively expressed tra2beta1 isoform and specifically generated tra2beta4 isoform that includes premature termination codon-containing exon 2, when exposed to restraint and water immersion stress. Treatment of a gastric cancer cell line (AGS) with arsenite (100 microM) preferentially generated tra2beta4 isoform and caused translocation of Tra2beta from the nucleus to the cytoplasm in association with enhanced phosphorylation during the initial 4-6 h (acute phase). Following the acute phase, AGS cells continued upregulated tra2beta1 mRNA expression, and higher amounts of Tra2beta were reaccumulated in their nuclei. Treatment with small interference RNAs targeting up-frameshift-1 or transfection of a plasmid containing tra2beta1 cDNA did not induce tra2beta4 isoform expression and did not modify the arsenite-induced expression of this isoform, suggesting that neither the nonsense-mediated mRNA decay nor the autoregulatory control by excess amounts of Tra2beta participated in the tra2beta4 isoform generation. Knockdown of Tra2beta facilitated skipping of the central variable region of the CD44 gene and suppressed cell growth. In contrast, overexpression of Tra2beta stimulated combinatorial inclusion of multiple variable exons in the region and cell growth. The similar skipping and inclusion of the variable region were observed in arsenite-treated cells. Our results suggest that Tra2beta may regulate cellular oxidative response by changing alternative splicing of distinct genes including CD44.

Normal hepatic cell surface localization of the high density lipoprotein receptor, scavenger receptor class B, type I, depends on all four PDZ domains of PDZK1

PDZK1 is a four PDZ domain-containing scaffold protein that binds to scavenger receptor class B, type I (SR-BI), the high density lipoprotein receptor, by its first PDZ domain (PDZ1). PDZK1 knock-out mice exhibit a >95% decrease in hepatic SR-BI protein and consequently an approximately 70% increase in plasma cholesterol in abnormally large high density lipoprotein particles. These defects are corrected by hepatic overexpression of full-length PDZK1 but not the PDZ1 domain alone, which partially restores SR-BI protein abundance but not cell surface expression or function. We have generated PDZK1 knock-out mice with hepatic expression of four PDZK1 transgenes encoding proteins with nested C-terminal truncations: pTEM, which lacks the three C-terminal residues (putative PDZ-binding motif), and PDZ1.2, PDZ1.2.3, or PDZ1.2.3.4, which contain only the first two, three, or four N-terminal PDZ domains, respectively, but not the remaining C-terminal sequences. Hepatic overexpression of pTEM restored normal hepatic SR-BI abundance, localization, and function. Hepatic overexpression of PDZ1.2 or PDZ1.2.3 partially restored SR-BI abundance ( approximately 12 or approximately 30% of wild type, respectively) but did not (PDZ1.2) or only slightly (PDZ1.2.3) restored hepatic SR-BI cell surface localization and function. Hepatic overexpression of PDZ1.2.3.4 completely restored SR-BI protein abundance, cell surface expression, and function (normalization of plasma cholesterol levels). Thus, all four PDZ domains in PDZK1, but not PDZ1-3 alone, are sufficient for its normal control of the abundance, localization, and therefore function of hepatic SR-BI, whereas the residues C-terminal to the PDZ4 domain, including the C-terminal putative PDZ-binding domain, are not required.

Overexpression of the PDZ1 domain of PDZK1 blocks the activity of hepatic scavenger receptor, class B, type I by altering its abundance and cellular localization

PDZK1 is a four-PDZ domain-containing scaffold protein that, via its first PDZ domain (PDZ1), binds to the C terminus of the high density lipoprotein (HDL) receptor scavenger receptor, class B, type I (SR-BI). Abolishing PDZK1 expression in PDZK1 knock-out (KO) mice leads to a post-transcriptional, tissue-specific decrease in SR-BI protein level and an increase in total plasma cholesterol carried in abnormally large HDL particles. Here we show that, although hepatic overexpression of PDZK1 restored normal SR-BI protein abundance and function in PDZK1 KO mice, hepatic overexpression of only the PDZ1 domain was not sufficient to restore normal SR-BI function. In wild-type mice, overexpression of the PDZ1 domain overcame the activity of the endogenous hepatic PDZK1, resulting in a 75% reduction in hepatic SR-BI protein levels and intracellular mislocalization of the remaining SR-BI. As a consequence, the plasma lipoproteins in PDZ1 transgenic mice resembled those in PDZK1 KO mice (hypercholesterolemia due to large HDL). These results indicate that the PDZ1 domain can control the abundance and localization, and therefore the function, of hepatic SR-BI and that structural features of PDZK1 in addition to its SR-BI-binding PDZ1 domain are required for normal hepatic SR-BI regulation.