The 3'-untranslated region of p21WAF1 mRNA is a composite cis-acting sequence bound by RNA-binding proteins from breast cancer cells, including HuR and poly(C)-binding protein

3'UTR Diversity: Expanding Repertoire of RNA Alterations in Human mRNAs

Genomic information stored in the DNA is transcribed to the mRNA and translated to proteins. The 3' untranslated regions (3'UTRs) of the mRNA serve pivotal roles in posttranscriptional gene expression, regulating mRNA stability, translation, and localization. Similar to DNA mutations producing aberrant proteins, RNA alterations expand the transcriptome landscape and change the cellular proteome. Recent global analyses reveal that many genes express various forms of altered RNAs, including 3'UTR length variants. Alternative polyadenylation and alternative splicing are involved in diversifying 3'UTRs, which could act as a hidden layer of eukaryotic gene expression control. In this review, we summarize the functions and regulations of 3'UTRs and elaborate on the generation and functional consequences of 3'UTR diversity. Given that dynamic 3'UTR length control contributes to phenotypic complexity, dysregulated 3'UTR diversity might be relevant to disease development, including cancers. Thus, 3'UTR diversity in cancer could open exciting new research areas and provide avenues for novel cancer theragnostics.

RNA binding proteins (RBPs) and their role in DNA damage and radiation response in cancer

Traditionally majority of eukaryotic gene expression is influenced by transcriptional and post-transcriptional events. Alterations in the expression of proteins that act post-transcriptionally can affect cellular signaling and homeostasis. RNA binding proteins (RBPs) are a family of proteins that specifically bind to RNAs and are involved in post-transcriptional regulation of gene expression and important cellular processes such as cell differentiation and metabolism. Deregulation of RNA-RBP interactions and any changes in RBP expression or function can lead to various diseases including cancer. In cancer cells, RBPs play an important role in regulating the expression of tumor suppressors and oncoproteins involved in various cell-signaling pathways. Several RBPs such as HuR, AUF1, RBM38, LIN28, RBM24, tristetrapolin family and Musashi play critical roles in various types of cancers and their aberrant expression in cancer cells makes them an attractive therapeutic target for cancer treatment. In this review we provide an overview of i). RBPs involved in cancer progression and their mechanism of action ii). the role of RBPs, including HuR, in breast cancer progression and DNA damage response and iii). explore RBPs with emphasis on HuR as therapeutic target for breast cancer therapy.

RNA-Binding Proteins in the Regulation of Adipogenesis and Adipose Function

The obesity epidemic represents a critical public health issue worldwide, as it is a vital risk factor for many diseases, including type 2 diabetes (T2D) and cardiovascular disease. Obesity is a complex disease involving excessive fat accumulation. Proper adipose tissue accumulation and function are highly transcriptional and regulated by many genes. Recent studies have discovered that post-transcriptional regulation, mainly mediated by RNA-binding proteins (RBPs), also plays a crucial role. In the lifetime of RNA, it is bound by various RBPs that determine every step of RNA metabolism, from RNA processing to alternative splicing, nucleus export, rate of translation, and finally decay. In humans, it is predicted that RBPs account for more than 10% of proteins based on the presence of RNA-binding domains. However, only very few RBPs have been studied in adipose tissue. The primary aim of this paper is to provide an overview of RBPs in adipogenesis and adipose function. Specifically, the following best-characterized RBPs will be discussed, including HuR, PSPC1, Sam68, RBM4, Ybx1, Ybx2, IGF2BP2, and KSRP. Characterization of these proteins will increase our understanding of the regulatory mechanisms of RBPs in adipogenesis and provide clues for the etiology and pathology of adipose-tissue-related diseases.

The RNA-binding protein PCBP1 represses lung adenocarcinoma progression by stabilizing DKK1 mRNA and subsequently downregulating β-catenin

Background

PolyC-RNA-binding protein 1 (PCBP1) functions as a tumour suppressor and RNA regulator that is downregulated in human cancers. Here, we aimed to reveal the biological function of PCBP1 in lung adenocarcinoma (LUAD).

Methods

First, PCBP1 was identified as an important biomarker that maintains LUAD through The Cancer Genome Atlas (TCGA) project screening and confirmed by immunohistochemistry and qPCR. Via colony formation, CCK8, IncuCyte cell proliferation, wound healing and Transwell assays, we confirmed that PCBP1 was closely related to the proliferation and migration of LUAD cells. The downstream gene DKK1 was discovered by RNA sequencing of PCBP1 knockdown cells. The underlying mechanisms were further investigated using western blot, qPCR, RIP, RNA pulldown and mRNA stability assays.

Results

We demonstrate that PCBP1 is downregulated in LUAD tumour tissues. The reduction in PCBP1 promotes the proliferation, migration and invasion of LUAD in vitro and in vivo. Mechanistically, the RNA-binding protein PCBP1 represses LUAD by stabilizing DKK1 mRNA. Subsequently, decreased expression of the DKK1 protein relieves the inhibitory effect on the Wnt/β-catenin signalling pathway. Taken together, these results show that PCBP1 acts as a tumour suppressor gene, inhibiting the tumorigenesis of LUAD.

Conclusions

We found that PCBP1 inhibits LUAD development by upregulating DKK1 to inactivate the Wnt/β-catenin pathway. Our findings highlight the potential of PCBP1 as a promising therapeutic target.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12967-022-03552-y.

Hu Antigen R (HuR) Protein Structure, Function and Regulation in Hepatobiliary Tumors

Simple Summary

Hepatobiliary tumors are a group of primary malignancies encompassing the liver, the intra- and extra-hepatic biliary tracts, and the gall bladder. Within the liver, hepatocellular carcinoma (HCC) is the most common type of primary cancer, which is, also, representing the third-most recurrent cause of cancer-associated death and the sixth-most prevalent type of tumor worldwide, nowadays. Although less frequent, cholangiocarcinoma (CCA) is, currently, a fatal cancer with limited therapeutic options. Here, we review the regulatory role of Hu antigen R (HuR), a ubiquitous member of the ELAV/Hu family of RNA-binding proteins (RBPs), in the pathogenesis, progression, and treatment of HCC and CCA. Overall, HuR is proposed as a valuable diagnostic and prognostic marker, as well as a therapeutic target in hepatobiliary cancers. Therefore, novel therapeutic approaches that can selectively modulate HuR function appear to be highly attractive for the clinical management of these types of tumors.

Abstract

Hu antigen R (HuR) is a 36-kDa ubiquitous member of the ELAV/Hu family of RNA-binding proteins (RBPs), which plays an important role as a post-transcriptional regulator of specific RNAs under physiological and pathological conditions, including cancer. Herein, we review HuR protein structure, function, and its regulation, as well as its implications in the pathogenesis, progression, and treatment of hepatobiliary cancers. In particular, we focus on hepatocellular carcinoma (HCC) and cholangiocarcinoma (CCA), tumors where the increased cytoplasmic localization of HuR and activity are proposed, as valuable diagnostic and prognostic markers. An overview of the main regulatory axes involving HuR, which are associated with cell proliferation, invasion, metastasis, apoptosis, and autophagy in HCC, is provided. These include the transcriptional, post-transcriptional, and post-translational modulators of HuR function, in addition to HuR target transcripts. Finally, whereas studies addressing the relevance of targeting HuR in CCA are limited, in the past few years, HuR has emerged as a potential therapeutic target in HCC. In fact, the therapeutic efficacy of some pharmacological inhibitors of HuR has been evaluated, in early experimental models of HCC. We, further, discuss the major findings and future perspectives of therapeutic approaches that specifically block HuR interactions, either with post-translational modifiers or cognate transcripts in hepatobiliary cancers.

Research progress on RNA‑binding proteins in breast cancer (Review)

Breast cancer is the most common malignancy among women, and the abnormal regulation of gene expression serves an important role in its occurrence and development. However, the molecular mechanisms underlying gene expression are highly complex and heterogeneous, and RNA-binding proteins (RBPs) are among the key regulatory factors. RBPs bind targets in an environment-dependent or environment-independent manner to influence mRNA stability and the translation of genes involved in the formation, progression, metastasis and treatment of breast cancer. Due to the growing interest in these regulators, the present review summarizes the most influential studies concerning RBPs associated with breast cancer to elucidate the role of RBPs in breast cancer and to assess how they interact with other key pathways to provide new molecular targets for the diagnosis and treatment of breast cancer.

The role of splicing factor PRPF8 in breast cancer

BACKGROUND:

Alternative splicing is a mechanism to produce different proteins with diverse functions from one gene. Many splicing factors play an important role in cancer progression. PRPF8 is a core protein component of the spliceosome complex, U4/U6-U5 tri-snRNP.

OBJECTIVE:

However, PRPF8 involved in mRNA alternative splicing are rarely included in the prognosis.

METHODS:

We found that PRPF8 was expressed in all examined cancer types. Further analyses found that PRPF8 expression was significantly different between the breast cancer and paracancerous tissues.

RESULTS:

Survival analyses showed that PRPF8-high patients had a poor prognosis, and the expression of PRPF8 is associated with distant metastasis-free survival (DMFS) and post progression survival (PPS). Gene Set Enrichment Analysis (GSEA) has revealed that PRPF8 expression is correlated with TGF-β, JAK-STAT, and cell cycle control pathways. Consistent with these results, upon PRPF8 silencing, the growth of MCF-7 cells was reduced, the ability of cell clone formation was weakened, and p⁢21 expression was increased.

CONCLUSIONS:

These results have revealed that PRPF8 is a significant factor for splicing in breast cancer progression.

LncRNA EIF1AX-AS1 promotes endometrial cancer cell apoptosis by affecting EIF1AX mRNA stabilization

Long non-coding RNAs (lncRNAs) have been found to play an important role in the occurrence and development of endometrial carcinoma (EC). Here, using RNA sequencing analysis, we systemically screened and identified the lncRNA EIF1AX-AS1, which is aberrantly down-regulated in clinical EC tissues and closely correlated with tumor type. EIF1AX-AS1 markedly inhibited EC cell proliferation and promoted apoptosis in vitro and in vivo. Mechanistically, EIF1AX-AS1 interacts with EIF1AX mRNA and poly C binding protein 1 (PCBP1), which promote eukaryotic translation initiation factor 1A, X-linked (EIF1AX) mRNA degradation. Intriguingly, interaction with IRES-related proteins Y-box binding protein 1 (YBX-1), EIF1AX promotes c-Myc translation through the internal ribosome enter site pathway. c-Myc promotes EIF1AX transcription and thus forms a feed-forward loop to regulate EC cell proliferation. Taken together, these data reveal new insights into the biology driving EC proliferation and highlights the potential of lncRNAs as biomarkers for prognosis and future therapeutic targets for cancer.

MBNL2 Regulates DNA Damage Response via Stabilizing p21

RNA-binding proteins are frequently dysregulated in human cancer and able to modulate tumor cell proliferation as well as tumor metastasis through post-transcriptional regulation on target genes. Abnormal DNA damage response and repair mechanism are closely related to genome instability and cell transformation. Here, we explore the function of the RNA-binding protein muscleblind-like splicing regulator 2 (MBNL2) on tumor cell proliferation and DNA damage response. Transcriptome and gene expression analysis show that the PI3K/AKT pathway is enriched in MBNL2-depleted cells, and the expression of cyclin-dependent kinase inhibitor 1A (p21^CDKN1A) is significantly affected after MBNL2 depletion. MBNL2 modulates the mRNA and protein levels of p21, which is independent of its canonical transcription factor p53. Moreover, depletion of MBNL2 increases the phosphorylation levels of checkpoint kinase 1 (Chk1) serine 345 (S345) and DNA damage response, and the effect of MBNL2 on DNA damage response is p21-dependent. MBNL2 would further alter tumor cell fate after DNA damage, MBNL2 knockdown inhibiting DNA damage repair and DNA damage-induced senescence, but promoting DNA damage-induced apoptosis.

Poly C Binding Protein 1 Regulates p62/SQSTM1 mRNA Stability and Autophagic Degradation to Repress Tumor Progression

Accumulating evidence show that Poly C Binding Protein 1 (PCBP1) is deleted in distinct types of tumors as a novel tumor suppressor, but its tumor suppression mechanism remains elusive. Here, we firstly describe that downregulation of PCBP1 is significantly associated with clinical ovarian tumor progression. Mechanistically, PCBP1 overexpression affects various autophagy-related genes expression at various expression levels to attenuate the intrinsic cell autophagy, including the autophagy-initiating ULK, ATG12, ATG7 as well as the bona fide marker of autophagosome, LC3B. Accordingly, knockdown of the endogenous PCBP1 in turn enhances autophagy and less cell death. Meanwhile, PCBP1 upregulates p62/SQSTM1 via inhibition p62/SQSTM1 autophagolysome and proteasome degradation as well as its mRNA stability, consequently accompanying with the caspase 3 or 8 activation for tumor cell apoptosis. Importantly, clinical ovary cancer sample analysis consistently validates the relevance of PCBP1 expression to both p62/SQSTM1 and caspase-8 to overall survival, and indicates PCBP1 may be a master player to repress tumor initiation. Taken together, our results uncover the tumorigenic mechanism of PCBP1 depletion and suggest that inhibition of tumor cell autophagy with autophagic inhibitors could be an effective therapeutical strategy for PCBP1-deficient tumor.

The RNA Binding Protein HuR: A Promising Drug Target for Anticancer Therapy

The stability of mRNA is one of the key factors governing the regulation of eukaryotic gene expression and function. Human antigen R (HuR) is an RNA-binding protein that regulates the stability, translation, and nucleus-to-cytoplasm shuttling of its target mRNAs. While HuR is normally localized within the nucleus, it has been shown that HuR binds mRNAs in the nucleus and then escorts the mRNAs to the cytoplasm where HuR protects them from degradation. It contains several RNA recognition motifs, which specifically bind to adenylate and uridylate-rich regions within the 3'-untranslated region of the target mRNA to mediate its effect. Many of the HuR target mRNAs encode proteins important for cell growth, tumorigenesis, angiogenesis, tumor inflammation, invasion and metastasis. HuR overexpression is known to correlate well with high-grade malignancy and poor prognosis in many tumor types. Thus, HuR has emerged as an attractive drug target for cancer therapy. Novel small molecule HuR inhibitors have been identified by high throughput screening and new formulations for targeted delivery of HuR siRNA to tumor cells have been developed with promising anticancer activity. This review summarizes the significant role of HuR in cancer development, progression, and poor treatment response. We will discuss the potential and challenges of targeting HuR therapeutically.

GRK2-Dependent HuR Phosphorylation Regulates HIF1α Activation under Hypoxia or Adrenergic Stress

Adaptation to hypoxia is a common feature in solid tumors orchestrated by oxygen-dependent and independent upregulation of the hypoxia-inducible factor-1α (HIF-1α). We unveiled that G protein-coupled receptor kinase (GRK2), known to be overexpressed in certain tumors, fosters this hypoxic pathway via phosphorylation of the mRNA-binding protein HuR, a central HIF-1α modulator. GRK2-mediated HuR phosphorylation increases the total levels and cytoplasmic shuttling of HuR in response to hypoxia, and GRK2-phosphodefective HuR mutants show defective cytosolic accumulation and lower binding to HIF-1α mRNA in hypoxic Hela cells. Interestingly, enhanced GRK2 and HuR expression correlate in luminal breast cancer patients. GRK2 also promotes the HuR/HIF-1α axis and VEGF-C accumulation in normoxic MCF7 breast luminal cancer cells and is required for the induction of HuR/HIF1-α in response to adrenergic stress. Our results point to a relevant role of the GRK2/HuR/HIF-1α module in the adaptation of malignant cells to tumor microenvironment-related stresses.

RNA-binding protein, human antigen R regulates hypoxia-induced autophagy by targeting ATG7/ATG16L1 expressions and autophagosome formation

Autophagy, a prosurvival mechanism offers a protective role during acute kidney injury. We show novel findings on the functional role of RNA binding protein, HuR during hypoxia-induced autophagy in renal proximal tubular cells-2 (HK-2). HK-2 cells showed upregulated expressions of HuR and autophagy-related proteins such as autophagy related 7 (ATG7), autophagy related 16 like 1 (ATG16L1), and LC3II under hypoxia. Increased autophagosome formation was visualized as LC3 puncta in hypoxic cells. Further, short hairpin-RNA-mediated loss of HuR function in HK-2 cells significantly decreased ATG7 and ATG16L1 protein expressions. Bioinformatics prediction revealed HuR motif binding on the coding region of ATG7 and AU-rich element at 3'UTR ATG16L1 messnger RNA (mRNA). The RNA immunoprecipitation study showed that HuR was predominantly associated with ATG7 and ATG16L1 mRNAs under hypoxia. In addition, HuR enhanced autophagosome formation by regulating LC3II expressions. These results show that HuR regulates ATG7 and ATG16L1 expressions and thereby mediate autophagy in HK-2 cells. Importantly, HuR knockdown cells underwent apoptosis during hypoxia as observed through the terminal deoxynucleotidyl transferase dUTP nick end labeling assay. Collectively, these findings show the crucial role of HuR under hypoxia by regulating autophagy and suppressing apoptosis in renal tubular cells.

RBMS2 inhibits the proliferation by stabilizing P21 mRNA in breast cancer

Background

RNA binding proteins (RBPs) play an important role in regulating the metabolism of target RNAs. Aberrant expression of RBPs plays a vital role in the initiation and development of many cancers. The RBM family, which has the conserved RNA binding motif RNP1 and RNP2, shares the similar function in RNA processing and RBMS2 is a member of them. P21, also named CDKN1A, promotes cell cycle arrest and plays an important role in halting cell proliferation. In our study, we identified RBMS2 as a tumor suppressor in breast cancer. It inhibited the proliferation of breast cancer by positively regulating the stability of P21 mRNA in posttranscriptional way.

Methods

TCGA was used to identify differentially expressed RBPs in breast cancer.

The effect of RBMS2 on breast cancer proliferation was evaluated in vitro using CCK-8 assays, colony formation assays and cell-cycle assays and the in vivo effect was investigated using a mouse tumorigenicity model.

The main pathway and genes regulated by RBMS2 was detected by RNA sequencing. The RNA immunoprecipitation combined with dual-luciferase reporter assay were conducted to testify the direct binding between RBMS2 and P21. Rescue assay was used to detect P21 as the main target of RBMS2.

Results

The expression of RBMS2 was lower in breast cancer compared with normal tissues and was a favorable biomarker in breast cancer. RBMS2 inhibited the proliferation of breast cancer and P21 was the main target of RBMS2. RBMS2 stabilized the mRNA of P21 by directly binding to the AU-rich element of 3′-UTR region. Anti-proliferation activity induced by overexpression of RBMS2 was rescued by interfering with the expression of P21.

Conclusion

In conclusion, RBMS2 acted as a tumor suppressor in breast cancer and positively regulated the expression of P21 by stabilizing its mRNA.

Electronic supplementary material

The online version of this article (10.1186/s13046-018-0968-z) contains supplementary material, which is available to authorized users.

PCBP1 depletion promotes tumorigenesis through attenuation of p27Kip1 mRNA stability and translation

Background

Poly C Binding Protein 1 (PCBP1) is an RNA-binding protein that binds and regulates translational activity of subsets of cellular mRNAs. Depletion of PCBP1 is implicated in various carcinomas, but the underlying mechanism in tumorigenesis remains elusive.

Methods

We performed a transcriptome-wide screen to identify novel bounding mRNA of PCBP1. The bind regions between PCBP1 with target mRNA were investigated by using point mutation and luciferase assay. Cell proliferation, cell cycle, tumorigenesis and cell apoptosis were also evaluated in ovary and colon cancer cell lines. The mechanism that PCBP1 affects p27 was analyzed by mRNA stability and ribosome profiling assays. We analyzed PCBP1 and p27 expression in ovary, colon and renal tumor samples and adjacent non-tumor tissues using RT-PCR, Western Blotting and immunohistochemistry. The prognostic significance of PCBP1 and p27 also analyzed using online databases.

Results

We identified cell cycle inhibitor p27^Kip1 (p27) as a novel PCBP1-bound transcript. We then demonstrated that binding of PCBP1 to p27 3’UTR via its KH1 domain mainly stabilizes p27 mRNA, while enhances its translation to fuel p27 expression, prior to p27 protein degradation. The upregulated p27 consequently inhibits cell proliferation, cell cycle progression and tumorigenesis, whereas promotes cell apoptosis under paclitaxel treatment. Conversely, knockdown of PCBP1 in turn compromises p27 mRNA stability, leading to lower p27 level and tumorigenesis in vivo. Moreover, forced depletion of p27 counteracts the tumor suppressive ability of PCBP1 in the same PCBP1 over-expressing cells. Physiologically, we showed that decreases of both p27 mRNA and its protein expressions are well correlated to PCBP1 depletion in ovary, colon and renal tumor samples, independent of the p27 ubiquitin ligase Skp2 level. Correlation of PCBP1 with p27 is also found in the tamoxifen, doxorubincin and lapatinib resistant breast cancer cells of GEO database.

Conclusion

Our results thereby indicate that loss of PCBP1 expression firstly attenuates p27 expression at post-transcriptional level, and subsequently promotes carcinogenesis. PCBP1 could be used as a diagnostic marker to cancer patients.

Electronic supplementary material

The online version of this article (10.1186/s13046-018-0840-1) contains supplementary material, which is available to authorized users.

HuR silencing elicits oxidative stress and DNA damage and sensitizes human triple-negative breast cancer cells to radiotherapy

HuR is an mRNA-binding protein whose overexpression in cancer cells has been associated with poor prognosis and resistance to therapy. While reports on HuR overexpression contributing to chemoresistance exist, limited information is available on HuR and radioresistance especially in triple-negative breast cancer (TNBC).

In this study we investigated the role of HuR in radiation resistance in three TNBC (MDA-MB-231, MDA-MB-468 and Hs578t) cell lines. Endogenous HuR expression was higher in TNBC cells compared to normal cells. siRNA mediated knockdown of HuR (siHuR) markedly reduced HuR mRNA and protein levels compared to scrambled siRNA (siScr) treatment. Further, siHuR treatment sensitized TNBC cells to ionizing radiation at 2 Gy compared to siScr treatment as evidenced by the significant reduction in clonogenic cell survival from 59%, 49%, and 65% in siScr-treated cells to 40%, 33%, and 46% in siHuR-treated MDA-MB-231, MDA-MB-468 and Hs578t cells, respectively. Molecular studies showed increased ROS production and inhibition of thioredoxin reductase (TrxR) in HuR knockdown cells contributed to radiosensitization. Associated with increased ROS production was evidence of increased DNA damage, demonstrated by a significant increase (p < 0.05) in γ-H2AX foci that persisted for up to 24 h in siHuR plus radiation treated cells compared to control cells. Further, comet assay revealed that HuR-silenced cells had larger and longer-lasting tails than control cells, indicating higher levels of DNA damage. In conclusion, our studies demonstrate that HuR knockdown in TNBC cells elicits oxidative stress and DNA damage resulting in radiosensitization.

Emerging role of HuR in inflammatory response in kidney diseases

Human antigen R (HuR) is a member of the embryonic lethal abnormal vision (ELAV) family which can bind to the A/U rich elements in 3' un-translated region of mRNA and regulate mRNA splicing, transportation, and stability. Unlike other members of the ELAV family, HuR is ubiquitously expressed. Early studies mainly focused on HuR function in malignant diseases. As researches proceed, more and more proofs demonstrate its relationship with inflammation. Since most kidney diseases involve pathological changes of inflammation, HuR is now suggested to play a pivotal role in glomerular nephropathy, tubular ischemia-reperfusion damage, renal fibrosis and even renal tumors. By regulating the mRNAs of target genes, HuR is causally linked to the onset and progression of kidney diseases. Reports on this topic are steadily increasing, however, the detailed function and mechanism of action of HuR are still not well understood. The aim of this review article is to summarize the present understanding of the role of HuR in inflammation in kidney diseases, and we anticipate that future research will ultimately elucidate the therapeutic value of this novel target.

Poly(C)-binding protein 1 (Pcbp1) regulates skeletal muscle differentiation by modulating microRNA processing in myoblasts

Regulation of gene expression during muscle development and disease remains incompletely understood. microRNAs are a class of small non-coding RNAs that regulate gene expression and function post-transcriptionally. The poly(C)-binding protein1 (Pcbp1, hnRNP-E1, or αCP-1) is an RNA-binding protein that has been reported to bind the 3'-UTRs of target genes to regulate mRNA stability and protein translation. However, Pcbp1's biological function and the general mechanism of action remain largely undetermined. Here, we report that Pcbp1 is a component of the miRNA-processing pathway that regulates miRNA biogenesis. siRNA-based inhibition of Pcbp1 in mouse skeletal muscle myoblasts led to dysregulated cellular proliferation and differentiation. We also found that Pcbp1 null mutant mice exhibit early embryonic lethality, indicating that Pcbp1 is indispensable for embryonic development. Interestingly, hypomorphic Pcbp1 mutant mice displayed defects in muscle growth due to defects in the proliferation and differentiation of myoblasts and muscle satellite cells, in addition to a slow to fast myofibril switch. Moreover, Pcbp1 modulated the processing of muscle-enriched miR-1, miR-133, and miR-206 by physically interacting with argonaute 2 (AGO2) and other miRNA pathway components. Our study, therefore, uncovers the important function of Pcbp1 in skeletal muscle and the microRNA pathway, signifying its potential as a therapeutic target for muscle disease.

Evidence Favoring a Positive Feedback Loop for Physiologic Auto Upregulation of hnRNP-E1 during Prolonged Folate Deficiency in Human Placental Cells

Background: Previously, we determined that heterogeneous nuclear ribonucleoprotein E1 (hnRNP-E1) functions as an intracellular physiologic sensor of folate deficiency. In this model, l-homocysteine, which accumulates intracellularly in proportion to the extent of folate deficiency, covalently binds to and thereby activates homocysteinylated hnRNP-E1 to interact with folate receptor-α mRNA; this high-affinity interaction triggers the translational upregulation of cell surface folate receptors, which enables cells to optimize folate uptake from the external milieu. However, integral to this model is the need for ongoing generation of hnRNP-E1 to replenish homocysteinylated hnRNP-E1 that is degraded.Objective: We searched for an interrelated physiologic mechanism that could also maintain the steady-state concentration of hnRNP-E1 during prolonged folate deficiency.Methods: A novel RNA-protein interaction was functionally characterized by using molecular and biochemical approaches in vitro and in vivo.Results: l-homocysteine triggered a dose-dependent high-affinity interaction between hnRNP-E1 and a 25-nucleotide cis element within the 5'-untranslated region of hnRNP-E1 mRNA; this led to a proportionate increase in these RNA-protein complexes, and translation of hnRNP-E1 both in vitro and within placental cells. Targeted perturbation of this RNA-protein interaction either by specific 25-nucleotide antisense oligonucleotides or mutation within this cis element or by small interfering RNA to hnRNP-E1 mRNA significantly reduced cellular biosynthesis of hnRNP-E1. Conversely, transfection of hnRNP-E1 mutant proteins that mimicked homocysteinylated hnRNP-E1 stimulated both cellular hnRNP-E1 and folate receptor biosynthesis. In addition, ferrous sulfate heptahydrate [iron(II)], which also binds hnRNP-E1, significantly perturbed this l-homocysteine-triggered RNA-protein interaction in a dose-dependent manner. Finally, folate deficiency induced dual upregulation of hnRNP-E1 and folate receptors in cultured human cells and tumor xenografts, and more selectively in various fetal tissues of folate-deficient dams.Conclusions: This novel positive feedback loop amplifies hnRNP-E1 during prolonged folate deficiency and thereby maximizes upregulation of folate receptors in order to restore folate homeostasis toward normalcy in placental cells. It will also functionally impact several other mRNAs of the nutrition-sensitive, folate-responsive posttranscriptional RNA operon that is orchestrated by homocysteinylated hnRNP-E1.

Phosphorylation of poly(rC) binding protein 1 (PCBP1) contributes to stabilization of mu opioid receptor (MOR) mRNA via interaction with AU-rich element RNA-binding protein 1 (AUF1) and poly A binding protein (PABP)

Gene regulation at the post-transcriptional level is frequently based on cis- and trans-acting factors on target mRNAs. We found a C-rich element (CRE) in mu-opioid receptor (MOR) 3'-untranslated region (UTR) to which poly (rC) binding protein 1 (PCBP1) binds, resulting in MOR mRNA stabilization. RNA immunoprecipitation and RNA EMSA revealed the formation of PCBP1-RNA complexes at the element. Knockdown of PCBP1 decreased MOR mRNA half-life and protein expression. Stimulation by forskolin increased cytoplasmic localization of PCBP1 and PCBP1/MOR 3'-UTR interactions via increased serine phosphorylation that was blocked by protein kinase A (PKA) or (phosphatidyl inositol-3) PI3-kinase inhibitors. The forskolin treatment also enhanced serine- and tyrosine-phosphorylation of AU-rich element binding protein (AUF1), concurrent with its increased binding to the CRE, and led to an increased interaction of poly A binding protein (PABP) with the CRE and poly(A) sites. AUF1 phosphorylation also led to an increased interaction with PCBP1. These findings suggest that a single co-regulator, PCBP1, plays a crucial role in stabilizing MOR mRNA, and is induced by PKA signaling by conforming to AUF1 and PABP.

Current Evidence and Future Perspectives on HuR and Breast Cancer Development, Prognosis, and Treatment

Hu-antigen R (HuR) is an RNA-binding posttranscriptional regulator that belongs to the Hu/ELAV family. HuR expression levels are modulated by a variety of proteins, microRNAs, chemical compounds, or the microenvironment, and in turn, HuR affects mRNA stability and translation of various genes implicated in breast cancer formation, progression, metastasis, and treatment. The aim of the present review is to critically summarize the role of HuR in breast cancer development and its potential as a prognosticator and a therapeutic target. In this aspect, all the existing English literature concerning HuR expression and function in breast cancer cell lines, in vivo animal models, and clinical studies is critically presented and summarized. HuR modulates many genes implicated in biological processes crucial for breast cancer formation, growth, and metastasis, whereas the link between HuR and these processes has been demonstrated directly in vitro and in vivo. Additionally, clinical studies reveal that HuR is associated with more aggressive forms of breast cancer and is a putative prognosticator for patients' survival. All the above indicate HuR as a promising drug target for cancer therapy; nevertheless, additional studies are required to fully understand its potential and determine against which types of breast cancer and at which stage of the disease a therapeutic agent targeting HuR would be more effective.

Poly C binding protein 1 represses autophagy through downregulation of LC3B to promote tumor cell apoptosis in starvation

Accumulating evidences indicate that poly C binding protein (PCBP1) is downregulated in various carcinomas as a tumor suppressor, but the underlying mechanism in suppression of tumorigenesis still remains elusive. Here, we found that PCBP1 overexpression attenuates tumor cell growth upon serum-free starvation. Notably, the autophagic degradation inhibitor, chloroquine, could mimic this suppressive effect in tumor cell growth. Autophagy analyses demonstrated that PCBP1 overexpression blocked autophagic flux of tumor cells under starvation conditions, while PCBP1 downregulation in turn refueled this autophagic flux, protecting cells from death. Mechanistically, PCBP1 overexpression attenuated microtubule-associated protein Light chain 3 (LC3B) mRNA stability to repress LC3B expression, resulting in the autophagy inhibition. Consequently, PCBP1 overexpression strongly triggered the caspase 3 and 8-mediated apoptosis of tumor cells and downregulated anti-apoptotic Bcl-2 expression upon starvation, which could be further synergized by autophagic inhibitor, indicating that PCBP1 not only inhibits tumor cell autophagy, but also renders them to apoptosis. Taken together, our results uncovered a novel mechanism of PCBP1 in repressing autophagy-mediated cell survival and indicated that inhibition of tumor cell autophagy by PCBP1 upregulation or with autophagic inhibitors could be an effective therapeutical strategy to colon and ovary tumors with low PCBP1 expression.

Downregulated Poly-C binding protein-1 is a novel predictor associated with poor prognosis in Acute Myeloid Leukemia

Background

Depletion of Poly-C binding protein-1(PCBP1) is implicated in various human malignancies. However, the underlying biological effect of PCBP1 in cancers, including acute myeloid leukemia (AML), still remains elusive. The purpose of this study was to examine the expression and clinical outcome of PCBP1in acute myeloid leukemia.

Methods

Bone marrow fluids of 88 newly diagnosed AML patients were sampled, and the PCBP1 mRNA expression level was evaluated using quantitative RT-PCR. The association between PCBP1 expression and clinicopathological features or the survival status of the patients was assessed by Chi-square test and Kaplan-Meier method.

Results

Comparing newly diagnosed AML patients to normal healthy donors, PCBP1 expression was significantly decreased in AML patients (P < 0.001). Conversely, PCBP1 expression had accordingly recovered back to normal in patients with complete remission (P < 0.001). Clinical feature analyses showed that PCBP1 expression was negatively correlated with white blood cell count (P = 0.024). In addition, patients with low PCBP1 expression had poor disease-free survival (11.8 % vs. 45.3 %; P = 0.01) and overall survival (18.2 % vs. 42.4 %; P = 0.032), respectively.

Conclusions

Taken together, our results showed for the first time that expression of PCBP1 was down-regulated in newly diagnosed AML patients and might be an independent prognostic marker in AML and should to be further investigated.

Regulation of Human PAX6 Expression by miR-7

The paired box gene 6 (PAX6) is a powerful mediator of eye and brain organogenesis whose spatiotemporal expression is exquisitely controlled by multiple mechanisms, including post-transcriptional regulation by microRNAs (miRNAs). In the present study, we use bioinformatic predictions to identify three candidate microRNA-7 (miR-7) target sites in the human PAX6 3′ untranslated region (3′-UTR) and demonstrate that two of them are functionally active in a human cell line. Furthermore, transient transfection of cells with synthetic miR-7 inhibits PAX6 protein expression but does not alter levels of PAX6 mRNA, suggesting that miR-7 induces translational repression of PAX6. Finally, a comparison of PAX6 3′-UTRs across species reveals that one of the functional miR-7 target sites is conserved, whereas the second functional target site is found only in primates. Thus, the interaction between PAX6 and miR-7 appears to be highly conserved; however, the precise number of sites through which this interaction occurs may have expanded throughout evolution.

Transcript stabilization by the RNA-binding protein HuR is regulated by cellular retinoic acid-binding protein 2

The RNA-binding protein HuR binds at 3' untranslated regions (UTRs) of target transcripts, thereby protecting them against degradation. We show that HuR directly interacts with cellular retinoic acid-binding protein 2 (CRABP2), a protein known to transport RA from the cytosol to the nuclear retinoic acid receptor (RAR). Association with CRABP2 dramatically increases the affinity of HuR toward target mRNAs and enhances the stability of such transcripts, including that of Apaf-1, the major protein in the apoptosome. We show further that its cooperation with HuR contributes to the ability of CRABP2 to suppress carcinoma cell proliferation. The data show that CRABP2 displays antioncogenic activities both by cooperating with RAR and by stabilizing antiproliferative HuR target transcripts. The observation that CRABP2 controls mRNA stabilization by HuR reveals that in parallel to participating in transcriptional regulation, the protein is closely involved in posttranscriptional regulation of gene expression.

Post-transcriptional regulation of DNA damage-responsive gene expression

SIGNIFICANCE

Production of proteins requires the synthesis, maturation, and export of mRNAs before their translation in the cytoplasm. Endogenous and exogenous sources of DNA damage pose a challenge to the co-ordinated regulation of gene expression, because the integrity of the DNA template can be compromised by DNA lesions. Cells recognize and respond to this DNA damage through a variety of DNA damage responses (DDRs). Failure to deal with DNA damage appropriately can lead to genomic instability and cancer.

RECENT ADVANCES

The p53 tumor suppressor plays a dominant role in DDR-dependent changes in gene expression, but this transcription factor is not solely responsible for all changes. Recent evidence indicates that RNA metabolism is integral to DDRs as well. In particular, post-transcriptional processes are emerging as important contributors to these complex responses.

CRITICAL ISSUES

Transcriptional, post-transcriptional, and translational regulation of gene expression is subject to changes in response to DNA damage. How these processes are intertwined in the unfolding of DDR is not fully understood.

FUTURE DIRECTIONS

Many complex regulatory responses combine to determine cell fate after DNA damage. Understanding how transcriptional, post-transcriptional, and translational processes interdigitate to create a web of regulatory interactions will be one of the key challenges to fully understand DDRs.

Loss of the nuclear receptor corepressor SLIRP compromises male fertility

Nuclear receptors (NRs) and their coregulators play fundamental roles in initiating and directing gene expression influencing mammalian reproduction, development and metabolism. SRA stem Loop Interacting RNA-binding Protein (SLIRP) is a Steroid receptor RNA Activator (SRA) RNA-binding protein that is a potent repressor of NR activity. SLIRP is present in complexes associated with NR target genes in the nucleus; however, it is also abundant in mitochondria where it affects mitochondrial mRNA transcription and energy turnover. In further characterisation studies, we observed SLIRP protein in the testis where its localization pattern changes from mitochondrial in diploid cells to peri-acrosomal and the tail in mature sperm. To investigate the in vivo effects of SLIRP, we generated a SLIRP knockout (KO) mouse. This animal is viable, but sub-fertile. Specifically, when homozygous KO males are crossed with wild type (WT) females the resultant average litter size is reduced by approximately one third compared with those produced by WT males and females. Further, SLIRP KO mice produced significantly fewer progressively motile sperm than WT animals. Electron microscopy identified disruption of the mid-piece/annulus junction in homozygous KO sperm and altered mitochondrial morphology. In sum, our data implicates SLIRP in regulating male fertility, wherein its loss results in asthenozoospermia associated with compromised sperm structure and mitochondrial morphology.

Multiple functions of the RNA-binding protein HuR in cancer progression, treatment responses and prognosis

The human embryonic lethal abnormal vision-like protein, HuR, is a member of the Hu family of RNA-binding proteins. Over the past decade, this ubiquitously expressed protein has been extensively investigated in cancer research because it is involved in the regulation of mRNA stability and translation in many cell types. HuR activity and function is associated with its subcellular distribution, transcriptional regulation, translational and post-translational modifications. HuR regulation of target mRNAs is based on the interaction between the three specific domains of HuR protein and one or several U- or AU-rich elements (AREs) in the untranslated region of target mRNAs. A number of cancer-related transcripts containing AREs, including mRNAs for proto-oncogenes, cytokines, growth factors, and invasion factors, have been characterized as HuR targets. It has been proposed that HuR has a central tumorigenic activity by enabling multiple cancer phenotypes. In this review, we comprehensively survey the existing evidence with regard to the diverse functions of HuR in caner development and progression. The current data also suggest that HuR might be a novel and promising therapeutic target and a marker for treatment response and prognostic evaluation.

Ovarian regulation of neuromedin U and its local actions in the ovary, mediated through neuromedin U receptor 2

Neuromedin U (NMU) was originally identified as an anorexigenic peptide that modulates appetite as well as energy homeostasis through the brain-gut axis. Although growing evidence has linked NMU activity with the development of female reproductive organs, no direct expression of and function for NMU in these organs has been pinpointed. Using a superovulated rat model, we found that NMU is directly expressed in the ovary, where its transcript level is tightly regulated by gonadotropins. Ovarian microdissection and immunohistochemical staining showed clearly that NMU is expressed mainly in theca/interstitial cells and to a moderate extent in granulosa cells. Primary cell studies together with reporter assays indicated the Nmu mRNA level in these cells is strongly induced via cAMP signaling, whereas this increase in expression can be reversed by the degradation message residing within its 3'-untranslated region, which recruits cis-acting mRNA degradation mechanisms, such as the gonadotropin-induced zinc finger RNA-binding protein Zfp36l1. This study also demonstrated that NMUR2, but not NMUR1, is the dominant NMU receptor in the ovary, where its expression is restricted to theca/interstitial cells. Treatment with NMU led to induction of the early response c-Fos gene, phosphorylation of extracellular signal-regulated kinase 1/2, and promotion of progesterone production in both developing and mature theca/interstitial cells. Taken as a whole, this study demonstrates that NMU and NMU receptor 2 compose a novel autocrine system in theca/interstitial cells in which the intensity of signaling is tightly controlled by gonadotropins.

Nuclear pore component Nup98 is a potential tumor suppressor and regulates posttranscriptional expression of select p53 target genes

The p53 tumor suppressor utilizes multiple mechanisms to selectively regulate its myriad target genes, which in turn mediate diverse cellular processes. Here, using conventional and single-molecule mRNA analyses, we demonstrate that the nucleoporin Nup98 is required for full expression of p21, a key effector of the p53 pathway, but not several other p53 target genes. Nup98 regulates p21 mRNA levels by a posttranscriptional mechanism in which a complex containing Nup98 and the p21 mRNA 3'UTR protects p21 mRNA from degradation by the exosome. An in silico approach revealed another p53 target (14-3-3σ) to be similarly regulated by Nup98. The expression of Nup98 is reduced in murine and human hepatocellular carcinomas (HCCs) and correlates with p21 expression in HCC patients. Our study elucidates a previously unrecognized function of wild-type Nup98 in regulating select p53 target genes that is distinct from the well-characterized oncogenic properties of Nup98 fusion proteins.

MDM2 expression is repressed by the RNA-binding protein RNPC1 via mRNA stability

The RNA-binding protein RNPC1 is a target of the p53 family and forms a feedback regulatory loop with the p53 family proteins. The MDM2 oncogene, a key negative regulator of p53, plays a critical role in a variety of fundamental cellular processes. MDM2 expression is found to be regulated via gene amplification, transcription, protein translation, and protein stability. In the current study, we reported a novel regulation of MDM2 by RNPC1 via mRNA stability. Specifically, we found that over-expression of RNPC1 decreases, whereas knockdown or knockout of RNPC1 increases, the level of MDM2 transcript and protein independent of p53. To uncover the underlying mechanism, we found that RNPC1 is able to destabilize the MDM2 transcript via binding to multiple AU-/U-rich elements in MDM2 3′UTR. Consistent with this, we showed that RNPC1 inhibits expression of exogenous MDM2 from a expression vector as long as the vector contains an AU-/U-rich element from MDM2 3′UTR. Finally, we showed that the RNA-binding activity of RNPC1 is required for binding to MDM2 transcript and consequently, for inhibiting MDM2 expression. Together, we uncover a novel regulation of MDM2 by the RNA-binding protein RNPC1 via mRNA stability.

The role of the 3' untranslated region in post-transcriptional regulation of protein expression in mammalian cells

The untranslated regions (UTRs) at the 3'end of mRNA transcripts contain important sequences that influence the fate of mRNA and thus proteosynthesis. In this review, we summarize the information known to date about 3'end processing, sequence characteristics including related binding proteins and the role of 3'UTRs in several selected signaling pathways to delineate their importance in the regulatory processes in mammalian cells. In addition to reviewing recent advances in the more well known aspects, such as cleavage and polyadenylation processes that influence mRNA stability and location, we concentrate on some newly emerging concepts of the role of the 3'UTR, including alternative polyadenylation sites in relation to proliferation and differentiation and the recognition of the multi-functional properties of non-coding RNAs, including miRNAs that commonly target the 3'UTR. The emerging picture is of a highly complex set of regulatory systems that include autoregulation, cooperativity and competition to fine tune proteosynthesis in context-dependent manners.

RNA processing and modification protein, carbon catabolite repression 4 (Ccr4), arrests the cell cycle through p21-dependent and p53-independent pathway

Ccr4d is a new member of the Ccr4 (carbon catabolite repression 4) family of proteins that are implicated in the regulation of mRNA stability and translation through mRNA deadenylation. However, Ccr4d is not believed to be involved in mRNA deadenylation. Thus, its biological function and mechanistic activity remain to be determined. Here, we report that Ccr4d is broadly expressed in various normal tissues, and the expression of Ccr4d is markedly down-regulated during cell cycle progression. We showed that Ccr4d inhibits cell proliferation and induces cell cycle arrest at G(1) phase. Our experiments further revealed that Ccr4d regulates the expression of p21 in a p53-independent manner. Mechanistic studies indicated that Ccr4d strongly bound to the 3'-UTR of p21 mRNA, leading to the stabilization of p21 mRNA. Interestingly, we found that the expression of Ccr4d is down-regulated in various tumor tissues. Collectively, our data indicate that Ccr4d functions as an anti-proliferating protein through the induction of cell cycle arrest via a p21-dependent and p53-independent pathway and suggest that Ccr4d might have an important role in carcinogenesis.

Manganese induces p21 expression in PC12 cells at the transcriptional level

Manganese is a common environmental and occupational pollutant. Excessive intake of manganese can cause toxicity known as manganism. Recently it has been demonstrated that unusual expression of cell cycle proteins and aberrant cell cycle progression in the central nervous system are involved in the pathogenesis of neurodegenerative diseases. The present studies were initiated to investigate whether p21 are induced after manganese exposure and its potential effects in vitro, with particular attention being given to understand the underlying regulatory mechanism of p21 induction by manganese in this process. We found that manganese induced DAergic cells injury and upregulation of p21 levels in nigrostriatal regions. Treatment of the PC12 cells with manganese resulted in a time- and concentration-dependent loss of cell viability. Analysis of cell cycle profile indicated that manganese blocked cell cycle progression by arresting the cell cycle at G2/M phase. Moreover, manganese treatment resulted in an increase in the mRNA and protein levels of p21, but did not have the same effect on other related factors. Silencing p21 by RNA interference showed a marked reversal of both G2/M arrest and the decrease in cell viability induced by manganese. Manganese did not stabilize the p21 protein and mRNA, and caused a marked increase in p21 mRNA levels together with an increase in its promoter activity, indicating a transcriptional mechanism. Overall, the in vivo and in vitro data suggest that exposure to manganese can increase p21 levels. An altered cell cycle status of PC12 cells can be induced by manganese through p21 up-regulation, and the induction of p21 occurs at the transcriptional level via promoter activation and mRNA induction.

Sequence requirements for RNA binding by HuR and AUF1

The stability of RNAs bearing AU-rich elements in their 3'-UTRs, and thus the level of expression of their protein products, is regulated by interactions with cytoplasmic RNA-binding proteins. Binding by HuR generally leads to mRNA stabilization and increased protein production, whereas binding by AUF1 isoforms generally lead to rapid degradation of the mRNA and reduced protein production. The exact nature of the interplay between these and other RNA-binding proteins remains unclear, although recent studies have shown close interactions between them and even suggested competition between the two for binding to their cognate recognition sequences. Other recent reports have suggested that the sequences recognized by the two proteins are different. We therefore performed a detailed in vitro analysis of the binding site(s) for HuR and AUF1 present in androgen receptor mRNA to define their exact target sequences, and show that the same sequence is contacted by both proteins. Furthermore, we analysed a proposed HuR target within the 3'-UTR of MTA1 mRNA, and show that the contacted bases lie outside of the postulated motif and are a better match to a classical ARE than the postulated motif. The defining features of these HuR binding sites are their U-richness and single strandedness.

Influence of physiologic folate deficiency on human papillomavirus type 16 (HPV16)-harboring human keratinocytes in vitro and in vivo

Although HPV16 transforms infected epithelial tissues to cancer in the presence of several co-factors, there is insufficient molecular evidence that poor nutrition has any such role. Because physiological folate deficiency led to the intracellular homocysteinylation of heterogeneous nuclear ribonucleoprotein E1 (hnRNP-E1) and activated a nutrition-sensitive (homocysteine-responsive) posttranscriptional RNA operon that included interaction with HPV16 L2 mRNA, we investigated the functional consequences of folate deficiency on HPV16 in immortalized HPV16-harboring human (BC-1-Ep/SL) keratinocytes and HPV16-organotypic rafts. Although homocysteinylated hnRNP-E1 interacted with HPV16 L2 mRNA cis-element, it also specifically bound another HPV16 57-nucleotide poly(U)-rich cis-element in the early polyadenylation element (upstream of L2L1 genes) with greater affinity. Together, these interactions led to a profound reduction of both L1 and L2 mRNA and proteins without effects on HPV16 E6 and E7 in vitro, and in cultured keratinocyte monolayers and HPV16-low folate-organotypic rafts developed in physiological low folate medium. In addition, HPV16-low folate-organotypic rafts contained fewer HPV16 viral particles, a similar HPV16 DNA viral load, and a much greater extent of integration of HPV16 DNA into genomic DNA when compared with HPV16-high folate-organotypic rafts. Subcutaneous implantation of 18-day old HPV16-low folate-organotypic rafts into folate-replete immunodeficient mice transformed this benign keratinocyte-derived raft tissue into an aggressive HPV16-induced cancer within 12 weeks. Collectively, these studies establish a likely molecular linkage between poor folate nutrition and HPV16 and predict that nutritional folate and/or vitamin-B(12) deficiency, which are both common worldwide, will alter the natural history of HPV16 infections and also warrant serious consideration as reversible co-factors in oncogenic transformation of HPV16-infected tissues to cancer.

HuR function in disease

The cytoplasmic events that control mammalian gene expression, primarily mRNA stability and translation, potently influence the cellular response to internal and external signals. The ubiquitous RNA-binding protein (RBP) HuR is one of the best-studied regulators of cytoplasmic mRNA fate. Through its post-transcriptional influence on specific target mRNAs, HuR can alter the cellular response to proliferative, stress, apoptotic, differentiation, senescence, inflammatory and immune stimuli. In light of its central role in important cellular functions, HuR's role in diseases in which these responses are aberrant is increasingly appreciated. Here, we review the mechanisms that control HuR function, its influence on target mRNAs, and how impairment in HuR-governed gene expression programs impact upon different disease processes. We focus on HuR's well-recognized implication in cancer and chronic inflammation, and discuss emerging studies linking HuR to cardiovascular, neurological, and muscular pathologies. We also discuss the progress, potential, and challenges of targeting HuR therapeutically.

A cis-acting region in the N-methyl-d-aspartate R1 3'-untranslated region interacts with the novel RNA-binding proteins beta subunit of alpha glucosidase II and annexin A2--effect of chronic ethanol exposure in vivo

A cis-acting region, Δ4, located in the 3'-untranslated region of N-methyl-d-aspartate R1(NR1) mRNA interacts with several trans-acting proteins present in polysomes purified from fetal cortical neurons. Chronic ethanol exposure of fetal cortical neurons increases Δ4 RNA-protein interactions. This increased interaction is due to an increase in one of the Δ4-binding trans-acting proteins identified as beta subunit of alpha glucosidase II (GIIβ). In this study, we examined whether ethanol-mediated regulation of NR1 mRNA in vivo is similar to that in vitro and whether Δ4-trans interactions are important for ethanol-mediated NR1 mRNA stability. Our data show that polysomal proteins from adult mouse cerebral cortex (CC) formed a complex with Δ4 RNA, suggesting the presence of NR1 mRNA-binding trans-acting proteins in CC polysomes. The intensity of the Δ4 RNA-protein complex was increased with polysomes from chronic ethanol-exposed CC. The Δ4 RNA-protein complex harbored GIIβ and a second trans-acting protein identified as annexin A2 (AnxA2). Ethanol-sensitive GIIβ was upregulated by 70% in ethanol-exposed CC. Heparin, a known binding partner of AnxA2, inhibited Δ4 RNA-protein complex formation. Transient transfection studies using chimeric constructs with and without the Δ4 region revealed that cis-trans interactions are important for ethanol-mediated stability of NR1 mRNA. Furthermore, our data highlight, for the first time, the presence of a binding site on the 3'-untranslated region of NR1 mRNA for AnxA2 and demonstrate the regulation of NR1 mRNA by AnxA2, GIIβ and a third NR1 mRNA-binding protein, which is yet to be identified.

PAK4 is required for regulation of the cell-cycle regulatory protein p21, and for control of cell-cycle progression

The serine/threonine kinase PAK4 regulates cytoskeletal architecture, and controls cell proliferation and survival. In most adult tissues PAK4 is expressed at low levels, but overexpression of PAK4 is associated with uncontrolled proliferation, inappropriate cell survival, and oncogenic transformation. Here we have studied for the first time, the role for PAK4 in the cell cycle. We found that PAK4 levels peak dramatically but transiently in the early part of G1 phase. Deletion of Pak4 was also associated with an increase in p21 levels, and PAK4 was required for normal p21 degradation. In serum-starved cells, the absence of PAK4 led to a reduction in the amount of cells in G1, and an increase in the amount of cells in G2/M phase. We propose that the transient increase in PAK4 levels at early G1 reduces p21 levels, thereby abrogating the activity of CDK4/CDK6 kinases, and allowing cells to proceed with the cell cycle in a precisely coordinated way.

The RNA-binding protein HuR opposes the repression of ERBB-2 gene expression by microRNA miR-331-3p in prostate cancer cells

ERBB-2 overexpression is associated with the development and progression of cancer and mediates its resistance to therapy. It has been suggested that post-transcriptional mechanisms control the overexpression of ERBB-2 in prostate cancer (PCa). We recently demonstrated that the 3'-untranslated region (3'-UTR) of ERBB-2 mRNA contains two specific target sites for binding of the microRNA miR-331-3p and that miR-331-3p represses ERBB-2 expression and signaling in PCa cells. Here we investigate a U-rich element situated in close proximity to the distal miR-331-3p target site in the ERBB-2 3'-UTR. Specific binding of HuR to this U-rich element promotes ERBB-2 expression in PCa cells. We show that HuR antagonizes the repressive action of miR-331-3p on its distal ERBB-2 3'-UTR target site. These results support a model in which the interplay between RNA-binding proteins and microRNAs controls the post-transcriptional regulation of gene expression and suggest that both HuR and miR-331-3p participate in the overexpression of ERBB-2 observed in some PCas.

Incrimination of heterogeneous nuclear ribonucleoprotein E1 (hnRNP-E1) as a candidate sensor of physiological folate deficiency

The mechanism underlying the sensing of varying degrees of physiological folate deficiency, prior to adaptive optimization of cellular folate uptake through the translational up-regulation of folate receptors (FR) is unclear. Because homocysteine, which accumulates intracellularly during folate deficiency, stimulated interactions between heterogeneous nuclear ribonucleoprotein E1 (hnRNP-E1) and an 18-base FR-α mRNA cis-element that led to increased FR biosynthesis and net up-regulation of FR at cell surfaces, hnRNP-E1 was a plausible candidate sensor of folate deficiency. Accordingly, using purified components, we evaluated the physiological basis whereby L-homocysteine triggered these RNA-protein interactions to stimulate FR biosynthesis. L-homocysteine induced a concentration-dependent increase in RNA-protein binding affinity throughout the range of physiological folate deficiency, which correlated with a proportionate increase in translation of FR in vitro and in cultured human cells. Targeted reduction of newly synthesized hnRNP-E1 proteins by siRNA to hnRNP-E1 mRNA reduced both constitutive and L-homocysteine-induced rates of FR biosynthesis. Furthermore, L-homocysteine covalently bound hnRNP-E1 via multiple protein-cysteine-S-S-homocysteine mixed disulfide bonds within K-homology domains known to interact with mRNA. These data suggest that a concentration-dependent, sequential disruption of critical cysteine-S-S-cysteine bonds by covalently bound L-homocysteine progressively unmasks an underlying RNA-binding pocket in hnRNP-E1 to optimize interaction with FR-α mRNA cis-element preparatory to FR up-regulation. Collectively, such data incriminate hnRNP-E1 as a physiologically relevant, sensitive, cellular sensor of folate deficiency. Because diverse mammalian and viral mRNAs also interact with this RNA-binding domain with functional consequences to their protein expression, homocysteinylated hnRNP-E1 also appears well positioned to orchestrate a novel, nutrition-sensitive (homocysteine-responsive), posttranscriptional RNA operon in folate-deficient cells.

Grape seed extract upregulates p21 (Cip1) through redox-mediated activation of ERK1/2 and posttranscriptional regulation leading to cell cycle arrest in colon carcinoma HT29 cells

Abnormalities in cell cycle progression provide unlimited replicative potential to cancer cells, and therefore targeting of key cell cycle regulators could be a sound cancer chemopreventive strategy. Earlier, we found that grape seed extract (GSE) increases Cip/p21 protein level and inhibits growth and induces apoptosis in human colon carcinoma HT29 cells both in vitro and in vivo. However, the mechanism of GSE-induced p21 upregulation and its role in biological efficacy of GSE are not known, which were investigated here. GSE treatment of HT29 cells resulted in a strong dose- and time-dependent phosphorylation of extracellular signal regulated kinase 1/2 (ERK1/2), consistent with p21 induction. The inhibition of sustained ERK1/2 activation by GSE using pharmacological inhibitors abrogated GSE-induced p21 upregulation. Furthermore, pretreatment of cells with N-acetylcysteine inhibited GSE-induced ERK1/2 phosphorylation as well as p21 upregulation, suggesting the involvement of GSE-induced oxidative stress as an upstream event. Consistent with this, GSE also decreased intracellular level of reduced glutathione. Next, we determined whether GSE-induced signaling regulates p21 expression at transcriptional and/or translational levels. GSE was found to increase the stability of p21 message with resultant increase in p21 protein level, but it did not alter the protein stability to a great extent. Importantly, knock-down of p21 abrogated GSE-induced G(1) arrest suggesting that p21 induction by GSE is essential for its G(1) arrest effect. Together, our results for the first time identify a central role of p21 induction and associated mechanism in GSE-induced cell cycle arrest in HT29 cells.

Histone deacetylase inhibitors mediate post-transcriptional regulation of p21WAF1 through novel cis-acting elements in the 3' untranslated region

Histone deacetylase inhibitors (HDACIs) are promising anti-tumor agents that selectively induce cell cycle arrest, differentiation and/or apoptosis of tumor cells. Fundamentally, HDACIs are proposed to function by activating the transcription of genes, including the potent cyclin dependent kinase inhibitor p21(WAF1). However, HDACIs primarily increase p21(WAF1) expression at the post-transcriptional level in HepG2 cells, implying that these anti-tumor agents regulate genes at multiple levels. Here, two novel cis-acting elements in the 3' untranslated region (UTR) of p21(WAF1) are identified that control the ability of HDACIs to induce p21(WAF1) mRNA stabilization. Collectively, these studies highlight the complexity of HDACIs in gene regulation.

The cyclin-dependent kinase inhibitor p21 is regulated by RNA-binding protein PCBP4 via mRNA stability

RNA-binding proteins (RBPs) play a major role in many post-transcriptional processes, including mRNA stability, alternative splicing and translation. PCBP4, also called MCG10, is an RBP belonging to the poly(C)-binding protein family and a target of p53 tumor suppressor. Ectopic expression of PCBP4 induces cell-cycle arrest in G₂ and apoptosis. To identify RNA targets regulated by PCBP4 and further decipher its function, we generated multiple cell lines in which PCBP4 is either inducibly over-expressed or knocked down. We found that PCBP4 expression decreases cyclin-dependent kinase inhibitor p21 induction in response to DNA damage. We also provided evidence that PCBP4 regulates p21 expression independently of p53. In addition, we showed that a deficiency in PCBP4 enhances p21 induction upon DNA damage. To validate PCBP4 regulation of p21, we made PCBP4-deficient mice and showed that p21 expression is markedly increased in PCBP4-deficient primary mouse embryo fibroblasts compared to that in wild-type counterparts. Finally, we uncovered that PCBP4 binds to the 3′-UTR of p21 transcript in vitro and in vivo to regulate p21 mRNA stability. Taken together, we revealed that PCBP4 regulates both basal and stress-induced p21 expression through binding p21 3′-UTR and modulating p21 mRNA stability.

Examination of the expanding pathways for the regulation of p21 expression and activity

p21(Waf1/Cip1/Sdi1) was originally identified as an inhibitor of cyclin-dependent kinases, a mediator of p53 in growth suppression and a marker of cellular senescence. p21 is required for proper cell cycle progression and plays a role in cell death, DNA repair, senescence and aging, and induced pluripotent stem cell reprogramming. Although transcriptional regulation is considered to be the initial control point for p21 expression, there is growing evidence that post-transcriptional and post-translational regulations play a critical role in p21 expression and activity. This review will briefly discuss the activity of p21 and focus on current knowledge of the determinants that control p21 transcription, mRNA stability and translation, and protein stability and activity.

RNPC1 modulates the RNA-binding activity of, and cooperates with, HuR to regulate p21 mRNA stability

P21, a cyclin-dependent kinase inhibitor, plays a pivotal role in the cell-cycle regulation in response to stress stimuli. P21 expression is highly regulated through transcriptional, post-transcriptional and post-translational mechanisms. Previously, we and others showed that p21 expression is regulated through p21 mRNA stability by RNPC1, a target of the p53 family and HuR, a member of the ELAV family RNA-binding proteins. HuR carries three highly conserved RNA recognition motifs (RRMs) whereas RNPC1 carries one. Here we found that the ability of RNPC1 to regulate p21 mRNA stability is dependent on HuR. We also found that RNPC1 and HuR physically interact, and the RRM domain in RNPC1 and RRM3 in HuR are necessary for their interaction. Interestingly, we found that RNPC1 and HuR, both of which can bind AU-rich elements (AREs) in p21 3'-UTR, preferentially bind the upstream and downstream AREs, respectively. Finally, we showed that the RNA-binding activity of HuR to p21 transcript was enhanced by RNPC1 in vitro and in vivo. Together, we hypothesize that RNPC1 modulates the RNA-binding activity of, and cooperates with, HuR to regulate p21 mRNA stability.

RNA-binding proteins Rbm38 and Rbm24 regulate myogenic differentiation via p21-dependent and -independent regulatory pathways

Skeletal muscle differentiation entails organized sequential events, including cell cycle arrest of proliferating myoblast cells and cell fusion, which lead to the formation of multinucleated myotubes. This process involves both transcriptional and post-transcriptional regulation of the gene expression of myogenic proteins, as well as cell-cycle related proteins. RNA-binding proteins bind to specific sequences of target RNA and regulate gene expression in a post-transcriptional manner. However, few tissue-specific RNA binding proteins have been identified. Herein, we report that the RNA binding proteins Rbm24 and Rbm38 were found to be preferentially expressed in muscle during differentiation in vitro. Further, knockdown of either by RNA interference suppressed cell-cycle arrest and delayed myogenic differentiation in C2C12 cells. In contrast, over-expression of Rbm24 or Rbm38 induced cell cycle arrest, and then had a positive effect on myogenic differentiation. Immunoprecipitation-RT-PCR analysis using tagged Rbm proteins indicated that Rbm38 binds to the p21 transcript in vivo. Consistent with this, differentiation of Rbm38 knockdown cells was rescued by over-expression of p21. Together, our results suggest that Rbm38 plays a crucial role in cell cycle arrest and myogenic differentiation via its binding to p21.

miR-331-3p regulates ERBB-2 expression and androgen receptor signaling in prostate cancer

MicroRNAs (miRNAs) are short, non-coding RNAs that regulate gene expression and are aberrantly expressed in human cancer. The ERBB-2 tyrosine kinase receptor is frequently overexpressed in prostate cancer and is associated with disease progression and poor survival. We have identified two specific miR-331-3p target sites within the ERBB-2 mRNA 3'-untranslated region and show that miR-331-3p expression is decreased in prostate cancer tissue relative to normal adjacent prostate tissue. Transfection of multiple prostate cancer cell lines with miR-331-3p reduced ERBB-2 mRNA and protein expression and blocked downstream phosphatidylinositol 3-kinase/AKT signaling. Furthermore, miR-331-3p transfection blocked the androgen receptor signaling pathway in prostate cancer cells, reducing activity of an androgen-stimulated prostate-specific antigen promoter and blocking prostate-specific antigen expression. Our findings provide insight into the regulation of ERBB-2 expression in cancer and suggest that miR-331-3p has the capacity to regulate signaling pathways critical to the development and progression of prostate cancer cells.

Induction of p21(CIP1/WAF1) expression by human T-lymphotropic virus type 1 Tax requires transcriptional activation and mRNA stabilization

HTLV-1 Tax can induce senescence by up-regulating the levels of cyclin-dependent kinase inhibitors p21^CIP1/WAF1 and p27^KIP1. Tax increases p27^KIP1 protein stability by activating the anaphase promoting complex/cyclosome (APC/C) precociously, causing degradation of Skp2 and inactivation of SCF^Skp2, the E3 ligase that targets p27^KIP1. The rate of p21^CIP1/WAF1 protein turnover, however, is unaffected by Tax. Rather, the mRNA of p21^CIP1/WAF1 is greatly up-regulated. Here we show that Tax increases p21 mRNA expression by transcriptional activation and mRNA stabilization. Transcriptional activation of p21^CIP1/WAF1 by Tax occurs in a p53-independent manner and requires two tumor growth factor-β-inducible Sp1 binding sites in the -84 to -60 region of the p21^CIP1/WAF1 promoter. Tax binds Sp1 directly, and the CBP/p300-binding activity of Tax is required for p21^CIP1/WAF1 trans-activation. Tax also increases the stability of p21^CIP1/WAF1 transcript. Several Tax mutants trans-activated the p21 promoter, but were attenuated in stabilizing p21^CIP1/WAF1 mRNA, and were less proficient in increasing p21^CIP1/WAF1 expression. The possible involvement of Tax-mediated APC/C activation in p21^CIP1/WAF1 mRNA stabilization is discussed.