Polyamines regulate the stability of activating transcription factor-2 mRNA through RNA-binding protein HuR in intestinal epithelial cells

Astrocytic spermidine insufficiency contributes to enhanced pain sensitivity associated with ApoE4

Neuropathic pain is a chronic condition with limited effective treatments, closely associated with astrocytes and their role in central sensitization. Apolipoprotein E (ApoE), predominantly expressed in astrocytes in central nervous system, exists in three ApoE isoforms (ApoE2, ApoE3, and ApoE4) in humans, with ApoE4 linked to increased susceptibility to neurological diseases. However, the relationship between ApoE4 and neuropathic pain, as well as underlying mechanisms, remains poorly understood. Here, we demonstrated that mice expressing human ApoE4 (ApoE4-TR) displayed increased pain sensitivity following spared nerve injury (SNI) compared to ApoE3-TR mice. This increased sensitivity was also observed in mice with astrocyte-specific expression of ApoE4, achieved through Cre-mediated recombination. Metabolomic profiling revealed reduced spermidine levels in the spinal dorsal horn of ApoE4-TR mice relative to ApoE3-TR mice. Daily gavage administration of spermidine alleviated mechanical pain to a comparable level in ApoE3-TR and ApoE4-TR mice, as assessed by von Frey test. However, lower dose of spermidine effectively alleviated neuropathic pain in ApoE3-TR mice but showed reduced efficacy in ApoE4-TR mice, likely due to limited spermidine retention in ApoE4 astrocytes, as demonstrated in vitro. Transcriptomic analysis identified Nos2 as a critical gene upregulated in ApoE4-TR mice. Mechanistically, spermidine suppressed Nos2 expression by inhibiting the NF-κB pathway in astrocytes, thereby alleviating neuropathic pain. These findings highlight an enhanced pain sensitivity associated with ApoE4 and suggest spermidine as a potential therapeutic strategy, emphasizing a tailored dosage approach for ApoE4 carriers.

Research progress on the microbial metabolism and transport of polyamines and their roles in animal gut homeostasis

Polyamines (putrescine, spermidine, and spermine) are aliphatic compounds ubiquitous in prokaryotes and eukaryotes. Positively charged polyamines bind to negatively charged macromolecules, such as nucleic acids and acidic phospholipids, and are involved in physiological activities including cell proliferation, differentiation, apoptosis and gene regulation. Intracellular polyamine levels are regulated by biosynthesis, catabolism and transport. Polyamines in the body originate from two primary sources: dietary intake and intestinal microbial metabolism. These polyamines are then transported into the bloodstream, through which they are distributed to various tissues and organs to exert their biological functions. Polyamines synthesized by intestinal microorganisms serve dual critical roles. First, they are essential for maintaining polyamine concentrations within the digestive tract. Second, through transcriptional and post-transcriptional mechanisms, these microbial-derived polyamines modulate the expression of genes governing key processes in intestinal epithelial cells-including proliferation, migration, apoptosis, and cell-cell interactions. Collectively, these regulatory effects help maintain intestinal epithelial homeostasis and ensure the integrity of the gut barrier. In addition, polyamines interact with the gut microbiota to maintain intestinal homeostasis by promoting microbial growth, biofilm formation, swarming, and endocytosis vesicle production, etc. Supplementation with polyamines has been demonstrated to be important in regulating host intestinal microbial composition, enhancing nutrient absorption, and improving metabolism and immunity. In this review, we will focus on recent advances in the study of polyamine metabolism and transport in intestinal microbes and intestinal epithelial cells. We then summarize the scientific understanding of their roles in intestinal homeostasis, exploring the advances in cellular and molecular mechanisms of polyamines and their potential clinical applications, and providing a rationale for polyamine metabolism as an important target for the treatment of intestinal-based diseases.

ONC212 enhances YM155 cytotoxicity by triggering SLC35F2 expression and NOXA-dependent MCL1 degradation in acute myeloid leukemia cells

Although the anticancer activity of ONC212 has been reported, the precise mechanism underlying its apoptotic effects remains unclear. In this study, we investigated the apoptotic mechanism of ONC212 in acute myeloid leukemia (AML) cells. ONC212 induces apoptosis, MCL1 downregulation, and mitochondrial depolarization in AML U937 cells. Ectopic MCL1 expression alleviates mitochondria-mediated apoptosis in ONC212-treated U937 cells. ONC212 triggers AKT phosphorylation, inducing NOX4-dependent ROS production and promoting HuR transcription. HuR-mediated ATF4 mRNA stabilization stimulates NOXA and SLC35F2 expression; ONC212-induced upregulation of NOXA leads to MCL1 degradation. The synergistic effect of ONC212 on YM155 cytotoxicity was dependent on increased SLC35F2 expression. In addition, YM155 feedback facilitated the activation of the ONC212-induced signaling pathway. A similar mechanism explains ONC212- and ONC212/YM155-induced AML HL-60 cell death. The continuous treatment of U937 cells with the benzene metabolite hydroquinone (HQ) generated U937/HQ cells, exhibiting enhanced responsiveness to the cytotoxic effects of ONC212. In U937/HQ cells, ONC212 triggered apoptosis through NOXA-mediated MCL1 downregulation, enhancing YM155 cytotoxicity. Collectively, our data suggested that ONC212 upregulated SLC35F2 expression and triggered NOXA-mediated MCL1 degradation in U937, U937/HQ, and HL-60 cells by activating the AKT/NOX4/HuR/ATF4 pathway. The ONC212-induced signaling pathway showed anti-AML activity and enhanced YM155 cytotoxicity.

miR-542-5p targets c-myc and negates the cell proliferation effect of SphK1 in intestinal epithelial cells

Intestinal epithelial barrier defects occur commonly during a variety of pathological conditions, though their underlying mechanisms are not completely understood. Sphingosine-1-phosphate (S1P) has been shown to be a critical regulator of proliferation and of maintenance of an intact intestinal epithelial barrier, as is also sphingosine kinase 1 (SphK1), the rate-limiting enzyme for S1P synthesis. SphK1 has been shown to modulate its effect on intestinal epithelial proliferation through increased levels of c-myc. We conducted genome-wide profile analysis to search for differential microRNA expression related to overexpressed SphK1 demonstrating adjusted expression of microRNA 542-5p (miR-542-5p). Here, we show that miR-542-5p is regulated by SphK1 activity and is an effector of c-myc translation that ultimately serves as a critical regulator of the intestinal epithelial barrier. miR-542-5p directly regulates c-myc translation through direct binding to the c-myc mRNA. Exogenous S1P analogs administered in vivo protect murine intestinal barrier from damage due to mesenteric ischemia reperfusion, and damaged intestinal tissue had increased levels of miR-542-5p. These results indicate that miR-542-5p plays a critical role in the regulation of S1P-mediated intestinal barrier function, and may highlight a novel role in potential therapies.

Polyamines and Their Metabolism: From the Maintenance of Physiological Homeostasis to the Mediation of Disease

The polyamines spermidine and spermine are positively charged aliphatic molecules. They are critical in the regulation of nucleic acid and protein structures, protein synthesis, protein and nucleic acid interactions, oxidative balance, and cell proliferation. Cellular polyamine levels are tightly controlled through their import, export, de novo synthesis, and catabolism. Enzymes and enzymatic cascades involved in polyamine metabolism have been well characterized. This knowledge has been used for the development of novel compounds for research and medical applications. Furthermore, studies have shown that disturbances in polyamine levels and their metabolic pathways, as a result of spontaneous mutations in patients, genetic engineering in mice or experimentally induced injuries in rodents, are associated with multiple maladaptive changes. The adverse effects of altered polyamine metabolism have also been demonstrated in in vitro models. These observations highlight the important role these molecules and their metabolism play in the maintenance of physiological normalcy and the mediation of injury. This review will attempt to cover the extensive and diverse knowledge of the biological role of polyamines and their metabolism in the maintenance of physiological homeostasis and the mediation of tissue injury.

Role of Polyamines and Hypusine in β Cells and Diabetes Pathogenesis

The polyamines-putrescine, spermidine, and spermine-are polycationic, low molecular weight amines with cellular functions primarily related to mRNA translation and cell proliferation. Polyamines partly exert their effects via the hypusine pathway, wherein the polyamine spermidine provides the aminobutyl moiety to allow posttranslational modification of the translation factor eIF5A with the rare amino acid hypusine (hydroxy putrescine lysine). The "hypusinated" eIF5A (eIF5Ahyp) is considered to be the active form of the translation factor necessary for the translation of mRNAs associated with stress and inflammation. Recently, it has been demonstrated that activity of the polyamines-hypusine circuit in insulin-producing islet β cells contributes to diabetes pathogenesis under conditions of inflammation. Elevated levels of polyamines are reported in both exocrine and endocrine cells of the pancreas, which may contribute to endoplasmic reticulum stress, oxidative stress, inflammatory response, and autophagy. In this review, we have summarized the existing research on polyamine-hypusine metabolism in the context of β-cell function and diabetes pathogenesis.

Polyamines in Gut Epithelial Renewal and Barrier Function

Polyamines regulate a variety of physiological functions and are involved in pathogenesis of diverse human diseases. The epithelium of the mammalian gut mucosa is a rapidly self-renewing tissue in the body, and its homeostasis is preserved through well-controlled mechanisms. Here, we highlight the roles of cellular polyamines in maintaining the integrity of the gut epithelium, focusing on the emerging evidence of polyamines in the regulation of gut epithelial renewal and barrier function. Gut mucosal growth depends on the available supply of polyamines to the dividing cells in the crypts, and polyamines are also essential for normal gut epithelial barrier function. Polyamines modulate expression of various genes encoding growth-associated proteins and intercellular junctions via distinct mechanisms involving RNA-binding proteins and noncoding RNAs. With the rapid advance of polyamine biology, polyamine metabolism and transport are promising therapeutic targets in our efforts to protect the gut epithelium and barrier function in patients with critical illnesses.

The RNA-binding protein HuR regulates intestinal epithelial restitution by modulating Caveolin-1 gene expression

The integrity of the intestinal mucosal barrier protects hosts against pathological conditions. Early mucosal restitution after wounding refers to epithelial cell migration into a defect. The RNA-binding protein HuR plays an important role in the posttranscriptional regulation of gene expression and is involved in many aspects of cellular physiology. In the present study, we investigated the role of HuR in the regulation of cell migration through the posttranscriptional regulation of Caveolin-1 (Cav-1). Online software was used to identify Cav-1 mRNA as a potential target of HuR. The interaction of HuR with Cav-1 mRNA was investigated via ribonucleoprotein immunoprecipitation (RNP IP) assays and biotin pulldown analysis. HuR was found to bind specifically to the Cav-1 3'-UTR rather than the coding region or 5'-UTR. Transfection of cells with siHuR decreased both HuR protein levels and Cav-1 protein levels; conversely, ectopic overexpression of HuR via infection of cells with an adenoviral vector containing HuR cDNA (AdHuR) increased Cav-1 protein levels without disturbing Cav-1 mRNA levels. Thus, HuR enhanced Cav-1 expression in vitro by stimulating Cav-1 translation. Intestinal epithelium-specific HuR knockout in mice decreased Cav-1 protein levels without changing Cav-1 mRNA levels, consistent with the in vitro results. Decreasing the levels of HuR via siHuR transfection inhibited early epithelial repair, but this effect was reversed by ectopic overexpression of GFP-tagged Cav-1. These results indicate that posttranscriptional regulation of Cav-1 gene expression by HuR plays a critical role in the regulation of rapid epithelial repair after wounding.

The activating transcription factor 2: an influencer of cancer progression

In contrast to the continuous increase in survival rates for many cancer entities, colorectal cancer (CRC) and pancreatic cancer are predicted to be ranked among the top 3 cancer-related deaths in the European Union by 2025. Especially, fighting metastasis still constitutes an obstacle to be overcome in CRC and pancreatic cancer. As described by Fearon and Vogelstein, the development of CRC is based on sequential mutations leading to the activation of proto-oncogenes and the inactivation of tumour suppressor genes. In pancreatic cancer, genetic alterations also attribute to tumour development and progression. Recent findings have identified new potentially important transcription factors in CRC, among those the activating transcription factor 2 (ATF2). ATF2 is a basic leucine zipper protein and is involved in physiological and developmental processes, as well as in tumorigenesis. The mutation burden of ATF2 in CRC and pancreatic cancer is rather negligible; however, previous studies in other tumours indicated that ATF2 expression level and subcellular localisation impact tumour progression and patient prognosis. In a tissue- and stimulus-dependent manner, ATF2 is activated by upstream kinases, dimerises and induces target gene expression. Dependent on its dimerisation partner, ATF2 homodimers or heterodimers bind to cAMP-response elements or activator protein 1 consensus motifs. Pioneering work has been performed in melanoma in which the dual role of ATF2 is best understood. Even though there is increasing interest in ATF2 recently, only little is known about its involvement in CRC and pancreatic cancer. In this review, we summarise the current understanding of the underestimated ‘cancer gene chameleon’ ATF2 in apoptosis, epithelial-to-mesenchymal transition and microRNA regulation and highlight its functions in CRC and pancreatic cancer. We further provide a novel ATF2 3D structure with key phosphorylation sites and an updated overview of all so-far available mouse models to study ATF2 in vivo.

RNA-binding protein HuR regulates translation of vitamin D receptor modulating rapid epithelial restitution after wounding

Homeostasis of the intestinal epithelium is tightly regulated by numerous extracellular and intracellular factors including vitamin D and the vitamin D receptor (VDR). VDR is highly expressed in the intestinal epithelium and is implicated in many aspects of gut mucosal pathophysiology, but the exact mechanism that controls VDR expression remains largely unknown. The RNA-binding protein human antigen R (HuR) regulates the stability and translation of target mRNAs and thus modulates various cellular processes and functions. Here we report a novel role of HuR in the posttranscriptional control of VDR expression in the intestinal epithelium. The levels of VDR in the intestinal mucosa decreased significantly in mice with ablated HuR, compared with control mice. HuR silencing in cultured intestinal epithelial cells (IECs) also reduced VDR levels, whereas HuR overexpression increased VDR abundance; neither intervention changed cellular Vdr mRNA content. Mechanistically, HuR bound to Vdr mRNA via its 3'-untranslated region (UTR) and enhanced VDR translation in IECs. Moreover, VDR silencing not only inhibited IEC migration over the wounded area in control cells but also prevented the increased migration in cells overexpressing HuR, although it did not alter IEC proliferation in vitro and growth of intestinal organoids ex vivo. The human intestinal mucosa from patients with inflammatory bowel diseases exhibited decreased levels of both HuR and VDR. These results indicate that HuR enhances VDR translation by directly interacting with its mRNA via 3'-UTR and that induced VDR by HuR is crucial for rapid intestinal epithelial restitution after wounding.

Interaction between HuR and circPABPN1 Modulates Autophagy in the Intestinal Epithelium by Altering ATG16L1 Translation

Intestinal epithelial autophagy is crucial for host defense against invasive pathogens, and defects in this process occur frequently in patients with inflammatory bowel disease (IBD) and other mucosal disorders, but the exact mechanism that activates autophagy is poorly defined. Here, we investigated the role of RNA-binding protein HuR (human antigen R) in the posttranscriptional control of autophagy-related genes (ATGs) in the intestinal epithelium. We found that targeted deletion of HuR in intestinal epithelial cells (IECs) specifically decreased the levels of ATG16L1 in the intestinal mucosa. Intestinal mucosa from patients with IBD exhibited reduced levels of both HuR and ATG16L1. HuR directly interacted with Atg16l1 mRNA via its 3' untranslated region and enhanced ATG16L1 translation, without affecting Atg16l1 mRNA stability. Circular RNA circPABPN1 blocked HuR binding to Atg16l1 mRNA and lowered ATG16L1 production. HuR silencing in cultured IECs also prevented rapamycin-induced autophagy, which was abolished by overexpressing ATG16L1. These findings indicate that HuR regulates autophagy by modulating ATG16L1 translation via interaction with circPABPN1 in the intestinal epithelium.

HuR Reduces Radiation-Induced DNA Damage by Enhancing Expression of ARID1A

Tumor suppressor ARID1A, a subunit of the chromatin remodeling complex SWI/SNF, regulates cell cycle progression, interacts with the tumor suppressor TP53, and prevents genomic instability. In addition, ARID1A has been shown to foster resistance to cancer therapy. By promoting non-homologous end joining (NHEJ), ARID1A enhances DNA repair. Consequently, ARID1A has been proposed as a promising therapeutic target to sensitize cancer cells to chemotherapy and radiation. Here, we report that ARID1A is regulated by human antigen R (HuR), an RNA-binding protein that is highly expressed in a wide range of cancers and enables resistance to chemotherapy and radiation. Our results indicate that HuR binds ARID1A mRNA, thereby increasing its stability in breast cancer cells. We further find that ARID1A expression suppresses the accumulation of DNA double-strand breaks (DSBs) caused by radiation and can rescue the loss of radioresistance triggered by HuR inhibition, suggesting that ARID1A plays an important role in HuR-driven resistance to radiation. Taken together, our work shows that HuR and ARID1A form an important regulatory axis in radiation resistance that can be targeted to improve radiotherapy in breast cancer patients.

RNA-Binding Protein HuR Regulates Rac1 Nucleocytoplasmic Shuttling Through Nucleophosmin in the Intestinal Epithelium

BACKGROUND & AIMS

The mammalian intestinal epithelium is a rapidly self-renewing tissue in the body, and its homeostasis is tightly regulated via well-controlled mechanisms. The RNA-binding protein HuR is essential for maintaining gut epithelial integrity, and targeted deletion of HuR in intestinal epithelial cells (IECs) disrupts mucosal regeneration and delays repair after injury. Here, we defined the role of HuR in regulating subcellular distribution of small guanosine triphosphatase Rac1 and investigated the implication of nucleophosmin (NPM) as a molecular chaperone in this process.

METHODS

Studies were conducted in intestinal epithelial tissue-specific HuR knockout (IE-HuR^-/-) mice and cultured IEC-6 cells, derived from rat small intestinal crypts. Functions of HuR and NPM in vitro were investigated via their gene silencing and overexpression.

RESULTS

The abundance of cytoplasmic Rac1 in the small intestinal mucosa increased significantly in IE-HuR^-/- mice, although HuR deletion did not alter total Rac1 levels. HuR silencing in cultured IECs also increased the cytoplasmic Rac1 levels, without an effect on whole-cell Rac1 content. In addition, HuR deficiency in the intestinal epithelium decreased the levels of NPM in IE-HuR^-/- mice and cultured IECs. NPM physically interacted with Rac1 and formed the NPM/Rac1 complex. NPM silencing decreased the NPM/Rac1 association and inhibited nuclear accumulation of Rac1, along with an increase in cytoplasmic abundances of Rac1. In contrast, ectopically expressed NPM enhanced Rac1 nuclear translocation and restored Rac1 subcellular localization to near normal in HuR-deficient cells.

CONCLUSIONS

These results indicate that HuR regulates Rac1 nucleocytoplasmic shuttling in the intestinal epithelium by altering NPM expression.

ATF2, a paradigm of the multifaceted regulation of transcription factors in biology and disease

Stringent transcriptional regulation is crucial for normal cellular biology and organismal development. Perturbations in the proper regulation of transcription factors can result in numerous pathologies, including cancer. Thus, understanding how transcription factors are regulated and how they are dysregulated in disease states is key to the therapeutic targeting of these factors and/or the pathways that they regulate. Activating transcription factor 2 (ATF2) has been studied in a number of developmental and pathological conditions. Recent findings have shed light on the transcriptional, post-transcriptional, and post-translational regulatory mechanisms that influence ATF2 function, and thus, the transcriptional programs coordinated by ATF2. Given our current knowledge of its multiple levels of regulation and function, ATF2 represents a paradigm for the mechanistic complexity that can regulate transcription factor function. Thus, increasing our understanding of the regulation and function of ATF2 will provide insights into fundamental regulatory mechanisms that influence how cells integrate extracellular and intracellular signals into a genomic response through transcription factors. Characterization of ATF2 dysfunction in the context of pathological conditions, particularly in cancer biology and response to therapy, will be important in understanding how pathways controlled by ATF2 or other transcription factors might be therapeutically exploited. In this review, we provide an overview of the currently known upstream regulators and downstream targets of ATF2.

HuR Enhances Early Restitution of the Intestinal Epithelium by Increasing Cdc42 Translation

The mammalian intestinal mucosa exhibits a spectrum of responses after acute injury and repairs itself rapidly to restore the epithelial integrity. The RNA-binding protein HuR regulates the stability and translation of target mRNAs and is involved in many aspects of gut epithelium homeostasis, but its exact role in the regulation of mucosal repair after injury remains unknown. We show here that HuR is essential for early intestinal epithelial restitution by increasing the expression of cell division control protein 42 (Cdc42) at the posttranscriptional level. HuR bound to the Cdc42 mRNA via its 3' untranslated region, and this association specifically enhanced Cdc42 translation without an effect on the Cdc42 mRNA level. Intestinal epithelium-specific HuR knockout not only decreased Cdc42 levels in mucosal tissues, but it also inhibited repair of damaged mucosa induced by mesenteric ischemia/reperfusion in the small intestine and by dextran sulfate sodium in the colon. Furthermore, Cdc42 silencing prevented HuR-mediated stimulation of cell migration over the wounded area by altering the subcellular distribution of F-actin. These results indicate that HuR promotes early intestinal mucosal repair after injury by increasing Cdc42 translation and demonstrate the importance of HuR deficiency in the pathogenesis of delayed mucosal healing in certain pathological conditions.

Transcriptional regulation of importin-α1 by JunD modulates subcellular localization of RNA-binding protein HuR in intestinal epithelial cells

The RNA-binding protein HuR is crucial for normal intestinal mucosal regeneration by modulating the stability and translation of target mRNAs, but the exact mechanism underlying HuR trafficking between the cytoplasm and nucleus remains largely unknown. Here we report a novel function of transcription factor JunD in the regulation of HuR subcellular localization through the control of importin-α₁ expression in intestinal epithelial cells (IECs). Ectopically expressed JunD specifically inhibited importin-α₁ at the transcription level, and this repression is mediated via interaction with CREB-binding site that was located at the proximal region of importin-α₁ promoter. Reduction in the levels of importin-α₁ by JunD increased cytoplasmic levels of HuR, although it failed to alter whole cell HuR levels. Increased levels of endogenous JunD by depleting cellular polyamines also inhibited importin-α₁ expression and increased cytoplasmic HuR levels, whereas JunD silencing rescued importin-α₁ expression and enhanced HuR nuclear translocation in polyamine-deficient cells. Moreover, importin-α₁ silencing protected IECs against apoptosis, which was prevented by HuR silencing. These results indicate that JunD regulates HuR subcellular distribution by downregulating importin-α₁, thus contributing to the maintenance of gut epithelium homeostasis.

Post-transcriptional regulation of Wnt co-receptor LRP6 and RNA-binding protein HuR by miR-29b in intestinal epithelial cells

MicroRNAs (miRNAs) control gene expression by binding to their target mRNAs for degradation and/or translation repression and are implicated in many aspects of cellular physiology. Our previous study shows that miR-29b acts as a biological repressor of intestinal mucosal growth, but its exact downstream targets remain largely unknown. In the present study, we found that mRNAs, encoding Wnt co-receptor LRP6 (low-density lipoprotein-receptor-related protein 6) and RNA-binding protein (RBP) HuR, are novel targets of miR-29b in intestinal epithelial cells (IECs) and that expression of LRP6 and HuR is tightly regulated by miR-29b at the post-transcriptional level. miR-29b interacted with both Lrp6 and HuR mRNAs via their 3′-UTRs and inhibited LRP6 and HuR expression by destabilizing Lrp6 and HuR mRNAs and repressing their translation. Studies using heterologous reporter constructs revealed a greater repressive effect of miR-29b through a single binding site in the Lrp6 or HuR 3′-UTR, whereas deletion mutation of this site prevented miR-29b-induced repression of LRP6 and HuR expression. Repression of HuR by miR-29b in turn also contributed to miR-29b-induced LRP6 inhibition, since ectopic overexpression of HuR in cells overexpressing miR-29b restored LRP6 expression to near normal levels. Taken together, our results suggest that miR-29b inhibits expression of LRP6 and HuR post-transcriptionally, thus playing a role in the regulation of IEC proliferation and intestinal epithelial homoeostasis.

Competition between RNA-binding proteins CELF1 and HuR modulates MYC translation and intestinal epithelium renewal

The mammalian intestinal epithelium is one of the most rapidly self-renewing tissues in the body, and its integrity is preserved through strict regulation. The RNA-binding protein (RBP) ELAV-like family member 1 (CELF1), also referred to as CUG-binding protein 1 (CUGBP1), regulates the stability and translation of target mRNAs and is implicated in many aspects of cellular physiology. We show that CELF1 competes with the RBP HuR to modulate MYC translation and regulates intestinal epithelial homeostasis. Growth inhibition of the small intestinal mucosa by fasting in mice was associated with increased CELF1/Myc mRNA association and decreased MYC expression. At the molecular level, CELF1 was found to bind the 3'-untranslated region (UTR) of Myc mRNA and repressed MYC translation without affecting total Myc mRNA levels. HuR interacted with the same Myc 3'-UTR element, and increasing the levels of HuR decreased CELF1 binding to Myc mRNA. In contrast, increasing the concentrations of CELF1 inhibited formation of the [HuR/Myc mRNA] complex. Depletion of cellular polyamines also increased CELF1 and enhanced CELF1 association with Myc mRNA, thus suppressing MYC translation. Moreover, ectopic CELF1 overexpression caused G1-phase growth arrest, whereas CELF1 silencing promoted cell proliferation. These results indicate that CELF1 represses MYC translation by decreasing Myc mRNA association with HuR and provide new insight into the molecular functions of RBPs in the regulation of intestinal mucosal growth.

Modulation by miR-29b of intestinal epithelium homoeostasis through the repression of menin translation

Menin regulates distinct cellular functions by regulating gene transcription through its interaction with partner transcription factors, but the exact mechanisms that control menin levels remain largely unknown. In the present study we report that Men1 mRNA, encoding menin, is a novel target of miR-29b and that miR-29b/Men1 mRNA association regulates menin expression post-transcriptionally in rat intestinal epithelial cells (IECs). Overexpression of a miR-29b precursor lowered the levels of Men1 mRNA modestly, but reduced new synthesis of menin robustly; conversely, antagonism of miR-29b enhanced menin protein synthesis and steady-state levels. The repressive effect of miR-29b on menin expression was mediated through a single binding site in the coding region of Men1 mRNA, because point mutation of this site prevented miR-29b-induced repression of menin translation. Increasing cellular polyamines due to overexpression of ornithine decarboxylase (ODC) enhanced menin translation by reducing miR-29b, whereas polyamine depletion by inhibiting ODC increased it, thus suppressing menin expression. Moreover, an increase in menin abundance in an miR-29b-silenced population of IECs led to increased sensitivity to apoptosis, which was prevented by silencing menin. These findings indicate that miR-29b represses translation of Men1 mRNA, in turn affecting intestinal epithelial homoeostasis by altering IEC apoptosis.

RNA-binding protein HuR promotes growth of small intestinal mucosa by activating the Wnt signaling pathway

Inhibition of growth of the intestinal epithelium, a rapidly self-renewing tissue, is commonly found in various critical disorders. The RNA-binding protein HuR is highly expressed in the gut mucosa and modulates the stability and translation of target mRNAs, but its exact biological function in the intestinal epithelium remains unclear. Here, we investigated the role of HuR in intestinal homeostasis using a genetic model and further defined its target mRNAs. Targeted deletion of HuR in intestinal epithelial cells caused significant mucosal atrophy in the small intestine, as indicated by decreased cell proliferation within the crypts and subsequent shrinkages of crypts and villi. In addition, the HuR-deficient intestinal epithelium also displayed decreased regenerative potential of crypt progenitors after exposure to irradiation. HuR deficiency decreased expression of the Wnt coreceptor LDL receptor-related protein 6 (LRP6) in the mucosal tissues. At the molecular level, HuR was found to bind the Lrp6 mRNA via its 3'-untranslated region and enhanced LRP6 expression by stabilizing Lrp6 mRNA and stimulating its translation. These results indicate that HuR is essential for normal mucosal growth in the small intestine by altering Wnt signals through up-regulation of LRP6 expression and highlight a novel role of HuR deficiency in the pathogenesis of intestinal mucosal atrophy under pathological conditions.

RNA-binding proteins and microRNAs in gastrointestinal epithelial homeostasis and diseases

The epithelium of gastrointestinal (GI) mucosa is a rapidly self-renewing tissue in the body, and its homeostasis is preserved through strict regulation of cell proliferation and apoptosis. Epithelial cells originate from a small number of pluripotent stem cells, which divide to either renew themselves or become committed crypt cells. RNA-binding proteins (RBPs) and microRNAs (miRNAs) regulate gene expression at the posttranscriptional level and are recently shown to modulate GI mucosal growth and repair after injury. Here we highlight the roles of RBPs HuR, CUG-binding protein 1, AU-binding factor 1, and several GI epithelial-specific miRNAs in gut mucosal homeostasis and diseases and also further analyze the mechanisms through which RBPs and miRNAs modulate the stability and translation of target mRNAs.

Jnk2 deletion disrupts intestinal mucosal homeostasis and maturation by differentially modulating RNA-binding proteins HuR and CUGBP1

Homeostasis and maturation of the mammalian intestinal epithelium are preserved through strict regulation of cell proliferation, apoptosis, and differentiation, but the exact mechanism underlying this process remains largely unknown. c-Jun NH2-terminal kinase 2 (JNK2) is highly expressed in the intestinal mucosa, and its activation plays an important role in proliferation and also mediates apoptosis in cultured intestinal epithelial cells (IECs). Here, we investigated the in vivo function of JNK2 in the regulation of intestinal epithelial homeostasis and maturation by using a targeted gene deletion approach. Targeted deletion of the jnk2 gene increased cell proliferation within the crypts in the small intestine and disrupted mucosal maturation as indicated by decreases in the height of villi and the villus-to-crypt ratio. JNK2 deletion also decreased susceptibility of the intestinal epithelium to apoptosis. JNK2-deficient intestinal epithelium was associated with an increase in the level of the RNA-binding protein HuR and with a decrease in the abundance of CUG-binding protein 1 (CUGBP1). In studies in vitro, JNK2 silencing protected intestinal epithelial cell-6 (IEC-6) cells against apoptosis and this protection was prevented by inhibiting HuR. Ectopic overexpression of CUGBP1 repressed IEC-6 cell proliferation, whereas CUGBP1 silencing enhanced cell growth. These results indicate that JNK2 is essential for maintenance of normal intestinal epithelial homeostasis and maturation under biological conditions by differentially modulating HuR and CUGBP1.

Posttranscriptional Regulation of Intestinal Epithelial Tight Junction Barrier by RNA-binding Proteins and microRNAs

Intestinal epithelial tight junctions (TJs) are a specialized structure that determines the cell polarity and prevents the diffusion of toxins, allergens, and pathogens from the lumen into the tissue. TJs are highly dynamic and its constituent protein complexes undergo continuously remodeling and turnover under tight regulation by numerous extracellular and intracellular factors. RNA-binding proteins (RBPs) and microRNAs (miRNAs) regulate gene expression at the posttranscriptional level and are involved in many aspects of cellular physiology. An increasing body of evidence indicates that RBPs including HuR and CUG-binding protein 1 and miRNAs such as miR-192 modulate the stability and translation of mRNAs encoding TJ proteins and play an important role in the control of intestinal epithelial TJ barrier function. In this mini-review article, we highlight the changes in TJ expression and intestinal epithelial TJ barrier function after activation or inactivation of RBPs and miRNAs and further analyze in some detail the mechanisms through which the stability and translation of TJ mRNAs are regulated by RBPs and miRNAs.

Inhibition of Smurf2 translation by miR-322/503 modulates TGF-β/Smad2 signaling and intestinal epithelial homeostasis

Smurf2 is an E3 ubiquitin ligase that regulates TGF-β/Smad signaling and is implicated in a wide variety of cellular responses. miR-322 and miR-503 repress Smurf2 translation and thus modulate TGF-β/Smad2 signaling and intestinal epithelial homeostasis.

Smad ubiquitin regulatory factor 2 (Smurf2) is an E3 ubiquitin ligase that regulates transforming growth factor β (TGF-β)/Smad signaling and is implicated in a wide variety of cellular responses, but the exact mechanisms that control Smurf2 abundance are largely unknown. Here we identify microRNA-322 (miR-322) and miR-503 as novel factors that regulate Smurf2 expression posttranscriptionally. Both miR-322 and miR-503 interact with Smurf2 mRNA via its 3′-untranslated region (UTR) and repress Smurf2 translation but do not affect total Smurf2 mRNA levels. Studies using heterologous reporter constructs reveal a greater repressive effect of miR-322/503 through a single binding site in the Smurf2 3′-UTR, whereas point mutation of this site prevents miR-322/503–induced repression of Smurf2 translation. Increased levels of endogenous Smurf2 via antagonism of miR-322/503 inhibits TGF-β–induced Smad2 activation by increasing degradation of phosphorylated Smad2. Furthermore, the increase in Smurf2 in intestinal epithelial cells (IECs) expressing lower levels of miR-322/503 is associated with increased resistance to apoptosis, which is abolished by Smurf2 silencing. These findings indicate that miR-322/503 represses Smurf2 translation, in turn affecting intestinal epithelial homeostasis by altering TGF-β/Smad2 signaling and IEC apoptosis.

Sphingosine kinase 1 overexpression stimulates intestinal epithelial cell proliferation through increased c-Myc translation

Sphingosine-1-phosphate (S1P), through mechanisms that are not completely understood, is shown to modulate cellular proliferation, which is critically important for maintaining the integrity of intestinal epithelium. Here, we show that increased S1P promotes proliferation in intestinal epithelial cells. We found that overexpression of sphingosine kinase 1 (SphK1), the rate-limiting enzyme for S1P synthesis, significantly increased cell proliferation and that this occurred through enhanced expression of c-Myc. Further, we found that the increased pattern of expression of c-Myc occurred predominantly due to its increased translation. The overexpressed SphK1 led to increased checkpoint kinase 2 and enhanced HuR phosphorylation which allowed for increased translation of c-Myc mRNA through HuR binding at the 3'-untranslated regions. Our findings demonstrate that S1P modulates intestinal cell proliferation and provides new insights as to the mechanistic actions of SphK1 and S1P in maintaining intestinal epithelial homeostasis.

A role for polyamine regulators in ESC self-renewal

Embryonic stem cells (ESCs) depend on extensive regulatory networks to coordinate their self-renewal and differentiation. The polyamine pathway regulator AMD1 was recently implicated in ESC self-renewal and directed differentiation of ESCs to neural precursor cells (NPCs). The polyamines spermine and spermidine are essential for a wide range of biological processes, and their levels are tightly regulated. Here, we review the polyamine pathway and discuss how it can impact polyamine levels, cellular methylation and hypusinated EIF5A levels. We discuss how it could feed into regulation of ESC self-renewal and directed differentiation. We show that in addition to AMD1, a second rate-limiting enzyme in the polyamine pathway, ODC1, can also promote ESC self-renewal, and that both Amd1 and Odc1 can partially substitute for Myc during cellular reprogramming. We propose that both Amd1 and Odc1 are essential regulators of ESCs and function to ensure high polyamine levels to promote ESC self-renewal.

Competitive binding of CUGBP1 and HuR to occludin mRNA controls its translation and modulates epithelial barrier function

The present study shows that RNA-binding proteins CUGBP1 and HuR jointly regulate the translation of occludin and play a crucial role in the maintenance of tight junction integrity.

RNA-binding proteins CUG-binding protein 1 (CUGBP1) and HuR are highly expressed in epithelial tissues and modulate the stability and translation of target mRNAs. Here we present evidence that CUGBP1 and HuR jointly regulate the translation of occludin and play a crucial role in the maintenance of tight junction (TJ) integrity in the intestinal epithelial cell monolayer. CUGBP1 and HuR competed for association with the same occludin 3′-untranslated region element and regulated occludin translation competitively and in opposite directions. CUGBP1 overexpression decreased HuR binding to occludin mRNA, repressed occludin translation, and compromised the TJ barrier function, whereas HuR overexpression inhibited CUGBP1 association with occludin mRNA and promoted occludin translation, thereby enhancing the barrier integrity. Repression of occludin translation by CUGBP1 was due to the colocalization of CUGBP1 and tagged occludin RNA in processing bodies (P-bodies), and this colocalization was prevented by HuR overexpression. These findings indicate that CUGBP1 represses occludin translation by increasing occludin mRNA recruitment to P-bodies, whereas HuR promotes occludin translation by blocking occludin mRNA translocation to P-bodies via the displacement of CUGBP1.

Spermine attenuates the action of the DNA intercalator, actinomycin D, on DNA binding and the inhibition of transcription and DNA replication

The anticancer activity of DNA intercalators is related to their ability to intercalate into the DNA duplex with high affinity, thereby interfering with DNA replication and transcription. Polyamines (spermine in particular) are almost exclusively bound to nucleic acids and are involved in many cellular processes that require nucleic acids. Until now, the effects of polyamines on DNA intercalator activities have remained unclear because intercalation is the most important mechanism employed by DNA-binding drugs. Herein, using actinomycin D (ACTD) as a model, we have attempted to elucidate the effects of spermine on the action of ACTD, including its DNA-binding ability, RNA and DNA polymerase interference, and its role in the transcription and replication inhibition of ACTD within cells. We found that spermine interfered with the binding and stabilization of ACTD to DNA. The presence of increasing concentrations of spermine enhanced the transcriptional and replication activities of RNA and DNA polymerases, respectively, in vitro treated with ActD. Moreover, a decrease in intracellular polyamine concentrations stimulated by methylglyoxal-bis(guanylhydrazone) (MGBG) enhanced the ACTD-induced inhibition of c-myc transcription and DNA replication in several cancer cell lines. The results indicated that spermine attenuates ACTD binding to DNA and its inhibition of transcription and DNA replication both in vitro and within cells. Finally, a synergistic antiproliferative effect of MGBG and ACTD was observed in a cell viability assay. Our findings will be of significant relevance to future developments in combination with cancer therapy by enhancing the anticancer activity of DNA interactors through polyamine depletion.

Insights from the HuR-interacting transcriptome: ncRNAs, ubiquitin pathways, and patterns of secondary structure dependent RNA interactions

The HuR protein regulates the expression of thousands of cellular transcripts by modulating mRNA splicing, trafficking, translation, and stability. Although it serves as a model of RNA-protein interactions, many features of HuR's interactions with RNAs remain unknown. In this report, we deployed the cryogenic RNA immunoprecipitation technique to analyze HuR-interacting RNAs with the Affymetrix all-exon microarray platform. We revealed several thousand novel HuR-interacting RNAs, including hundreds of non-coding RNAs such as natural antisense transcripts from stress responsive loci. To gain insight into the mechanisms of specificity and sensitivity of HuR's interaction with its target RNAs, we searched HuR-interacting RNAs for composite patterns of primary sequence and secondary structure. We provide evidence that secondary structures of 66-75 nucleotides enhance HuR's recognition of its specific RNA targets composed of short primary sequence patterns. We validated thousands of these RNAs by analysis of overlap with recently published findings, including HuR's interaction with RNAs in the pathways of RNA splicing and stability. Finally, we observed a striking enrichment for members of ubiquitin ligase pathways among the HuR-interacting mRNAs, suggesting a new role for HuR in the regulation of protein degradation to mirror its known function in protein translation.

ATF2 - at the crossroad of nuclear and cytosolic functions

An increasing number of transcription factors have been shown to elicit oncogenic and tumor suppressor activities, depending on the tissue and cell context. Activating transcription factor 2 (ATF2; also known as cAMP-dependent transcription factor ATF-2) has oncogenic activities in melanoma and tumor suppressor activities in non-malignant skin tumors and breast cancer. Recent work has shown that the opposing functions of ATF2 are associated with its subcellular localization. In the nucleus, ATF2 contributes to global transcription and the DNA damage response, in addition to specific transcriptional activities that are related to cell development, proliferation and death. ATF2 can also translocate to the cytosol, primarily following exposure to severe genotoxic stress, where it impairs mitochondrial membrane potential and promotes mitochondrial-based cell death. Notably, phosphorylation of ATF2 by the epsilon isoform of protein kinase C (PKCε) is the master switch that controls its subcellular localization and function. Here, we summarize our current understanding of the regulation and function of ATF2 in both subcellular compartments. This mechanism of control of a non-genetically modified transcription factor represents a novel paradigm for 'oncogene addiction'.

The RNA-binding protein human antigen R controls global changes in gene expression during Schwann cell development

An important prerequisite to myelination in peripheral nerves is the establishment of one-to-one relationships between axons and Schwann cells. This patterning event depends on immature Schwann cell proliferation, apoptosis, and morphogenesis, which are governed by coordinated changes in gene expression. Here, we found that the RNA-binding protein human antigen R (HuR) was highly expressed in immature Schwann cells, where genome-wide identification of its target mRNAs in vivo in mouse sciatic nerves using ribonomics showed an enrichment of functionally related genes regulating these processes. HuR coordinately regulated expression of several genes to promote proliferation, apoptosis, and morphogenesis in rat Schwann cells, in response to NRG1, TGFβ, and laminins, three major signals implicated in this patterning event. Strikingly, HuR also binds to several mRNAs encoding myelination-related proteins but, contrary to its typical function, negatively regulated their expression, likely to prevent ectopic myelination during development. These functions of HuR correlated with its abundance and subcellular localization, which were regulated by different signals in Schwann cells.

Bile salts increase epithelial cell proliferation through HuR-induced c-Myc expression

BACKGROUND

Bile salts increase intestinal mucosal proliferation through an increase in c-Myc, a transcription factor that controls the expression of numerous translation regulatory proteins. HuR is an RNA-binding protein that regulates translation of target mRNAs. RNA-binding proteins can control mRNA stability by binding to AU- and U-rich elements located in the 3'-untranslated regions (3'-UTRs) of target mRNAs.

AIM

To determine how bile salt-induced c-Myc stimulates enterocyte proliferation.

METHODS

Enterocyte proliferation was measured both in vivo using C57Bl6 mice and in vitro using IEC-6 cells after taurodeoxycholate (TDCA) supplementation. HuR and c-Myc protein expression was determined by immunoblot. c-Myc mRNA expression was determined by PCR. HuR expression was inhibited using specific small interfering RNA. HuR binding to c-Myc mRNA was determined by immunoprecipitation.

RESULTS

TDCA increased enterocyte proliferation in vivo and in vitro. TDCA stimulates translocation of HuR from the nucleus to the cytoplasm. Cytoplasmic HuR regulates c-Myc translation by HuR binding to the 3'-UTR of c-Myc mRNA. Increased TDCA-induced c-Myc increases enterocyte proliferation.

CONCLUSIONS

Bile salts have beneficial effects on the intestinal epithelial mucosa, which are important in maintaining intestinal mucosal integrity and function. These data further support an important beneficial role of bile salts in regulation of mucosal growth and repair. Decreased enterocyte exposure to luminal bile salts, as occurs during critical illness, liver failure, starvation, and intestinal injury, may have a detrimental effect on mucosal integrity.

Polyamines inhibit the assembly of stress granules in normal intestinal epithelial cells regulating apoptosis

Polyamines regulate multiple signaling pathways and are implicated in many aspects of cellular functions, but the exact molecular processes governed by polyamines remain largely unknown. In response to environmental stress, repression of translation is associated with the assembly of stress granules (SGs) that contain a fraction of arrested mRNAs and are thought to function as mRNA storage. Here we show that polyamines modulate the assembly of SGs in normal intestinal epithelial cells (IECs) and that induced SGs following polyamine depletion are implicated in the protection of IECs against apoptosis. Increasing the levels of cellular polyamines by ectopic overexpression of the ornithine decarboxylase gene decreased cytoplasmic levels of SG-signature constituent proteins eukaryotic initiation factor 3b and T-cell intracellular antigen-1 (TIA-1)-related protein and repressed the assembly of SGs induced by exposure to arsenite-induced oxidative stress. In contrast, depletion of cellular polyamines by inhibiting ornithine decarboxylase with α-difluoromethylornithine increased cytoplasmic eukaryotic initiation factor 3b and TIA-1 related protein abundance and enhanced arsenite-induced SG assembly. Polyamine-deficient cells also exhibited an increase in resistance to tumor necrosis factor-α/cycloheximide-induced apoptosis, which was prevented by inhibiting SG formation with silencing SG resident proteins Sort1 and TIA-1. These results indicate that the elevation of cellular polyamines represses the assembly of SGs in normal IECs and that increased SGs in polyamine-deficient cells are crucial for increased resistance to apoptosis.

Polyamines and Gut Mucosal Homeostasis

The epithelium of gastrointestinal (GI) mucosa has the most rapid turnover rate of any tissue in the body and its integrity is preserved through the dynamic balance between cell migration, proliferation, growth arrest and apoptosis. To maintain tissue homeostasis of the GI mucosa, the rates of epithelial cell division and apoptosis must be highly regulated by various extracellular and intracellular factors including cellular polyamines. Natural polyamines spermidine, spermine and their precursor putrescine, are organic cations in eukaryotic cells and are implicated in the control of multiple signaling pathways and distinct cellular functions. Normal intestinal epithelial growth depends on the available supply of polyamines to the dividing cells in the crypts, and polyamines also regulate intestinal epithelial cell (IEC) apoptosis. Although the specific molecular processes controlled by polyamines remains to be fully defined, increasing evidence indicates that polyamines regulate intestinal epithelial integrity by modulating the expression of various growth-related genes. In this review, we will extrapolate the current state of scientific knowledge regarding the roles of polyamines in gut mucosal homeostasis and highlight progress in cellular and molecular mechanisms of polyamines and their potential clinical applications.

miR-503 represses CUG-binding protein 1 translation by recruiting CUGBP1 mRNA to processing bodies

This study shows that microRNA-503 interacts with the CUG-binding protein 1 (CUGBP1) mRNA and represses its translation by recruiting the CUGBP1 mRNA to processing bodies.

microRNAs (miRNAs) and RNA-binding proteins (RBPs) jointly regulate gene expression at the posttranscriptional level and are involved in many aspects of cellular functions. The RBP CUG-binding protein 1 (CUGBP1) destabilizes and represses the translation of several target mRNAs, but the exact mechanism that regulates CUGBP1 abundance remains elusive. In this paper, we show that miR-503, computationally predicted to associate with three sites of the CUGBP1 mRNA, represses CUGBP1 expression. Overexpression of an miR-503 precursor (pre-miR-503) reduced the de novo synthesis of CUGBP1 protein, whereas inhibiting miR-503 by using an antisense RNA (antagomir) enhanced CUGBP1 biosynthesis and elevated its abundance; neither intervention changed total CUGBP1 mRNA levels. Studies using heterologous reporter constructs revealed a greater repressive effect of miR-503 through the CUGBP1 coding region sites than through the single CUGBP1 3′-untranslated region target site. CUGBP1 mRNA levels in processing bodies (P-bodies) increased in cells transfected with pre-miR-503, while silencing P-body resident proteins Ago2, RCK, or LSm4 decreased miR-503–mediated repression of CUGBP1 expression. Decreasing the levels of cellular polyamines reduced endogenous miR-503 levels and promoted CUGBP1 expression, an effect that was prevented by ectopic miR-503 overexpression. Repression of CUGBP1 by miR-503 in turn altered the expression of CUGBP1 target mRNAs and thus increased the sensitivity of intestinal epithelial cells to apoptosis. These findings identify miR-503 as both a novel regulator of CUGBP1 expression and a modulator of intestinal epithelial homoeostasis.

The RNA-binding protein HuR stabilizes survivin mRNA in human oesophageal epithelial cells

Overexpression of survivin, a member of the IAP (inhibitor of apoptosis) family, has been correlated with poorer outcomes in multiple malignancies, including oesophageal cancer. The regulatory mechanisms, particularly at the post-transcriptional level, involved in survivin overexpression are not well understood. Previous work from our group has shown that the RNA-binding protein HuR (Hu antigen R), which is also overexpressed in several malignancies, stabilizes the mRNA of XIAP (X-linked IAP), another IAP family member. In the present study, we demonstrate the binding of HuR to a 288 bp fragment in the 3'-UTR (untranslated region) of survivin mRNA in human oesophageal epithelial cells. Unexpectedly, overexpression of HuR led to a decrease in survivin expression. This was associated with decreased survivin mRNA and promoter activity, suggesting a decrease in transcription. Levels of p53, a negative transcriptional regulator of survivin, increased following HuR overexpression, in conjunction with enhanced p53 mRNA stability. Silencing p53 prior to HuR overexpression resulted in increased survivin protein and mRNA stability. These results demonstrate that, in the absence of p53, HuR overexpression results in increased survivin mRNA stability and protein expression. This provides an additional explanation for the increased survivin expression observed in oesophageal cancer cells that have lost p53.

Chk2-dependent HuR phosphorylation regulates occludin mRNA translation and epithelial barrier function

Occludin is a transmembrane tight junction (TJ) protein that plays an important role in TJ assembly and regulation of the epithelial barrier function, but the mechanisms underlying its post-transcriptional regulation are unknown. The RNA-binding protein HuR modulates the stability and translation of many target mRNAs. Here, we investigated the role of HuR in the regulation of occludin expression and therefore in the intestinal epithelial barrier function. HuR bound the 3′-untranslated region of the occludin mRNA and enhanced occludin translation. HuR association with the occludin mRNA depended on Chk2-dependent HuR phosphorylation. Reduced HuR phosphorylation by Chk2 silencing or by reduction of Chk2 through polyamine depletion decreased HuR-binding to the occludin mRNA and repressed occludin translation, whereas Chk2 overexpression enhanced (HuR/occludin mRNA) association and stimulated occludin expression. In mice exposed to septic stress induced by cecal ligation and puncture, Chk2 levels in the intestinal mucosa decreased, associated with an inhibition of occludin expression and gut barrier dysfunction. These results indicate that HuR regulates occludin mRNA translation through Chk2-dependent HuR phosphorylation and that this influence is crucial for maintenance of the epithelial barrier integrity in the intestinal tract.

Posttranscriptional regulation of ornithine decarboxylase

Activity of the polyamine biosynthetic enzyme ornithine decarboxylase (ODC) and intracellular levels of ODC protein are controlled very tightly. Numerous studies have described ODC regulation at the levels of transcription, translation, and protein degradation in normal cells and dysregulation of these processes in response to oncogenic stimuli. Although posttranscriptional regulation of ODC has been well documented, the RNA binding proteins (RBPs) that interact with ODC mRNA and control synthesis of the ODC protein have not been defined. Using Ras-transformed rat intestinal epithelial cells (Ras12V cells) as a model, we have begun identifying the RBPs that associate with the ODC transcript. Binding of RBPs could potentially regulate ODC synthesis by either changing mRNA stability or rate of mRNA translation. Techniques for measuring RBP binding and translation initiation are described here. Targeting control of ODC translation or mRNA decay could be a valuable method of limiting polyamine accumulation and subsequent tumor development in a variety of cancers.

Role of RNA binding protein HuR in ductal carcinoma in situ of the breast

HuR is a ubiquitously expressed RNA-binding protein that modulates gene expression at the post-transcriptional level. It is predominantly nuclear, but can shuttle between the nucleus and the cytoplasm. While in the cytoplasm HuR can stabilize its target transcripts, many of which encode proteins involved in carcinogenesis. While cytoplasmic HuR expression is a marker of reduced survival in breast cancer, its role in precursor lesions of malignant diseases is unclear. To address this we explored HuR expression in atypical ductal hyperplasia (ADH) and in ductal in situ carcinomas (DCIS). We show that cytoplasmic HuR expression is elevated in both ADH and DCIS when compared to normal controls, and that this expression associated with high grade, progesterone receptor negativity and microinvasion and/or tumour-positive sentinel nodes of the DCIS. To study the mechanisms of HuR in breast carcinogenesis, HuR expression was silenced in an immortalized breast epithelial cell line (184B5Me), which led to reduction in anchorage-independent growth, increased programmed cell death and inhibition of invasion. In addition, we identified two novel target transcripts (CTGF and RAB31) that are regulated by HuR and that bind HuR protein in this cell line. Our results show that HuR is aberrantly expressed at early stages of breast carcinogenesis and that its inhibition can lead to suppression of this process. ArrayExpress Accession No. E-MEXP-3035.

Role of RNA-Binding Proteins in MAPK Signal Transduction Pathway

Mitogen-activated protein kinases (MAPKs), which are found in all eukaryotes, are signal transducing enzymes playing a central role in diverse biological processes, such as cell proliferation, sexual differentiation, and apoptosis. The MAPK signaling pathway plays a key role in the regulation of gene expression through the phosphorylation of transcription factors. Recent studies have identified several RNA-binding proteins (RBPs) as regulators of MAPK signaling because these RBPs bind to the mRNAs encoding the components of the MAPK pathway and regulate the stability of their transcripts. Moreover, RBPs also serve as targets of MAPKs because MAPK phosphorylate and regulate the ability of RBPs to bind and stabilize target mRNAs, thus controlling various cellular functions. In this review, we present evidence for the significance of the MAPK signaling in the regulation of RBPs and their target mRNAs, which provides additional information about the regulatory mechanism underlying gene expression. We further present evidence for the clinical importance of the posttranscriptional regulation of mRNA stability and its implications for drug discovery.

Harmine activates intrinsic and extrinsic pathways of apoptosis in B16F-10 melanoma

Background

Harmine is a beta-carboline alkaloid from the plant Peganum harmala. Previous studies found that harmine inhibited metastasis of B16F-10 melanoma cells. This study aims to elucidate the role of harmine in apoptosis of B16F-10 cells.

Methods

B16F-10 melanoma cells were treated in the presence and absence of harmine in vitro. Morphological changes, cell cycle and expression of various pro and anti- apoptotic genes were analyzed for the study of apoptosis.

Results

Morphological observation and DNA laddering assay showed that harmine treated cells displayed marked apoptotic characteristics, such as nuclear fragmentation, appearance of apoptotic bodies and DNA laddering fragment. TUNEL assay and flow cytometric analysis also confirmed apoptosis. Furthermore, RT-PCR analysis showed that harmine induced apoptosis in B16F-10 melanoma cells by up-regulating Bax and activating Caspase-3, 9 and p53 and down-regulating Bcl-2. Harmine also up-regulated Caspase-8 and Bid, indicating that harmine affected both extrinsic and intrinsic pathways of apoptosis. This study also showed inhibitory effects of harmine on some transcription factors and pro- inflammatory cytokines that protect cell from apoptosis.

Conclusion

Harmine activates both intrinsic and extrinsic pathways of apoptosis and regulates some transcription factors and pro-inflammatory cytokines.

Posttranscriptional regulation of gene expression in epithelial cells by polyamines

In addition to regulating gene transcription, polyamines also potently modulate gene expression posttranscriptionally. Posttranscriptional gene regulation, which includes processes such as mRNA transport, turnover, and translation, involves specific mRNA sequences (cis-element) that interact with transacting factors such as RNA-binding proteins (RBPs) and microRNAs. U- or AU-rich elements (ARE) are the best characterized cis-acting sequences located in the 3'-untranslated regions of many labile mRNAs. Several RBPs, including AUF1, BRF1, TTP, and KSRP, promote ARE-mRNA decay through the recruitment of the ARE-bearing mRNA to sites of mRNA degradation, whereas RBPs such as HuR, HuB, HuC, and HuD stabilize target mRNAs and stimulate their translation. HuR is one of the best-studied RBPs and has emerged as a key regulator of posttranscriptional control of gene expression and its activity is tightly regulated by cellular polyamines. Ribonucleoprotein immunoprecipitation assays and biotin pull-down assays are two major methods used extensively in experiments investigating the roles and mechanisms of cellular polyamines in the posttranscriptional regulation and are described in detail in this chapter.

Association between expression of embryonic lethal abnormal vision-like protein HuR and cyclooxygenase-2 in oral squamous cell carcinoma

BACKGROUND

Increased cytoplasmic HuR expression has been noted in several cancer types, where it may contribute to the increased cyclooxygenase-2 (COX-2) expression observed during tumorigenesis and metastasis.

METHODS

To assess the correlation between COX-2 and HuR in oral squamous cell carcinoma (OSCC), the expression patterns of HuR and COX-2 were assessed via immunohistochemistry analyses of 103 OSCC samples.

RESULTS

Cytoplasmic HuR expression was significantly associated with COX-2 expression (p < .025) and lymph node metastasis (p < .050) and distant metastasis (p < .025). In multivariate analysis, cytoplasmic HuR expression was identified as an independent prognostic parameter for reduced overall survival. The inhibition of HuR expression by siRNA or leptomycin B (LMB) caused a reduction in the inducibility of COX-2 in oral cancer cells.

CONCLUSION

Our results indicate that the cytoplasmic expression of HuR is associated with COX-2 expression in OSCCs and HuR can regulate COX-2 expression in oral cancer cell lines.

Polyamines regulate the stability of JunD mRNA by modulating the competitive binding of its 3' untranslated region to HuR and AUF1

Polyamines critically regulate all mammalian cell growth and proliferation by mechanisms such as the repression of growth-inhibitory proteins, including JunD. Decreasing the levels of cellular polyamines stabilizes JunD mRNA without affecting its transcription, but the exact mechanism whereby polyamines regulate JunD mRNA degradation has not been elucidated. RNA-binding proteins HuR and AUF1 associate with labile mRNAs bearing AU-rich elements located in the 3' untranslated regions (3'-UTRs) and modulate their stability. Here, we show that JunD mRNA is a target of HuR and AUF1 and that polyamines modulate JunD mRNA degradation by altering the competitive binding of HuR and AUF1 to the JunD 3'-UTR. The depletion of cellular polyamines enhanced HuR binding to JunD mRNA and decreased the levels of JunD transcript associated with AUF1, thus stabilizing JunD mRNA. The silencing of HuR increased AUF1 binding to the JunD mRNA, decreased the abundance of HuR-JunD mRNA complexes, rendered the JunD mRNA unstable, and prevented increases in JunD mRNA and protein in polyamine-deficient cells. Conversely, increasing the cellular polyamines repressed JunD mRNA interaction with HuR and enhanced its association with AUF1, resulting in an inhibition of JunD expression. These results indicate that polyamines modulate the stability of JunD mRNA in intestinal epithelial cells through HuR and AUF1 and provide new insight into the molecular functions of cellular polyamines.

Cytoplasmic accumulation of the RNA-binding protein HuR stabilizes the ornithine decarboxylase transcript in a murine nonmelanoma skin cancer model

Ornithine decarboxylase (ODC) is the first and usually rate-limiting enzyme in the polyamine biosynthetic pathway. Under normal physiological conditions, polyamine content and ODC enzyme activity are highly regulated. However, the induction of ODC activity is an early step in neoplastic transformation. The studies described here use normal mouse keratinocytes (C5N cells), and spindle carcinoma cells (A5 cells) to explore the regulation of ODC in nonmelanoma skin cancer development. Previous results have shown that induction of ODC activity is both necessary and sufficient for the promotion of skin tumors. We see a marked increase in ODC enzyme activity in A5 cells compared with C5N keratinocytes, which correlates with a 4-fold stabilization of ODC mRNA. These data suggest that ODC is post-transcriptionally regulated in skin tumor development. Thus, we sought to investigate whether the ODC transcript interacts with the RNA-binding protein HuR, which is known to bind to and stabilize its target mRNAs. We show that HuR is able to bind to the ODC 3'-UTR in A5 cells but not in C5N cells. Immunofluorescence results reveal that HuR is present in both the nucleus and cytoplasm of A5 cells, whereas C5N cells exhibit strictly nuclear localization of HuR. Knockdown experiments in A5 cells showed that when HuR is depleted, ODC RNA becomes less stable, and ODC enzyme activity decreases. Together, these data support the hypothesis that HuR plays a causative role in ODC up-regulation during nonmelanoma skin cancer development by binding to and stabilizing the ODC transcript.

Inflammation: cytokines and RNA-based regulation

The outcome of an inflammatory response depends upon the coordinated regulation of a variety of both pro-inflammatory and anti-inflammatory cytokines and other proteins. Regulation of these inflammation mediators can occur at multiple levels, including transcription, mRNA translation, post-translational modifications, and mRNA degradation. Post-transcriptional regulation has been shown to play an important role in controlling the expression of these mediators, allowing for normal initiation and resolution of the inflammatory response. Many inflammatory mediators have unstable mRNAs due, in part, to the presence of AU-rich elements in their 3'-untranslated regions. Increasing numbers of RNA-binding proteins have been identified that can bind to these AU-rich elements and then regulate the stability and/or translation of the mRNA. This review summarizes current knowledge about the role of several RNA-binding proteins that act through AU-rich elements to post-transcriptionally regulate the biosynthesis of proteins involved in inflammation.

Post-transcriptional regulation of MEK-1 by polyamines through the RNA-binding protein HuR modulating intestinal epithelial apoptosis

MEK-1 [MAPK (mitogen-activated protein kinase) kinase-1] is an important signal transducing enzyme that is implicated in many aspects of cellular functions. In the present paper, we report that cellular polyamines regulate MEK-1 expression at the post-transcriptional level through the RNA-binding protein HuR (Hu-antigen R) in IECs (intestinal epithelial cells). Decreasing the levels of cellular polyamines by inhibiting ODC (ornithine decarboxylase) stabilized MEK-1 mRNA and promoted its translation through enhancement of the interaction between HuR and the 3'-untranslated region of MEK-1 mRNA, whereas increasing polyamine levels by ectopic ODC overexpression destabilized the MEK-1 transcript and repressed its translation by reducing the abundance of HuR-MEK-1 mRNA complex; neither intervention changed MEK-1 gene transcription via its promoter. HuR silencing rendered the MEK-1 mRNA unstable and inhibited its translation, thus preventing increases in MEK-1 mRNA and protein in polyamine-deficient cells. Conversely, HuR overexpression increased MEK-1 mRNA stability and promoted its translation. Inhibition of MEK-1 expression by MEK-1 silencing or HuR silencing prevented the increased resistance of polyamine-deficient cells to apoptosis. Moreover, HuR overexpression did not protect against apoptosis if MEK-1 expression was silenced. These results indicate that polyamines destabilize the MEK-1 mRNA and repress its translation by inhibiting the association between HuR and the MEK-1 transcript. Our findings indicate that MEK-1 is a key effector of the HuR-elicited anti-apoptotic programme in IECs.

Induced ATF-2 represses CDK4 transcription through dimerization with JunD inhibiting intestinal epithelial cell growth after polyamine depletion

Intestinal epithelium is a rapidly self-renewing tissue in the body, and its homeostasis is tightly regulated by numerous factors including polyamines. Decreased levels of cellular polyamines increase activating transcription factor (ATF)-2, but the exact role and mechanism of induced ATF-2 in the regulation of intestinal epithelial cell (IEC) growth remain elusive. Cyclin-dependent kinase (CDK) 4 is necessary for the G1-to-S phase transition during the cell cycle, and its expression is predominantly controlled at the transcription level. Here, we reported that induced ATF-2 following polyamine depletion repressed CDK4 gene transcription in IECs by increasing formation of the ATF-2/JunD heterodimers. ATF-2 formed complexes with JunD as measured by immunoprecipitation using the ATF-2 and JunD antibodies and by glutathione S-transferase (GST) pull-down assays using GST-ATF-2 fusion proteins. Studies using various mutants of GST-ATF-2 revealed that formation of the ATF-2/JunD dimers depended on the COOH-terminal basic region-leucine zipper domain of ATF-2. Polyamine depletion increased ATF-2/JunD complex and inhibited CDK4 transcription as indicated by a decrease in the levels of CDK4-promoter activity and its mRNA. ATF-2 silencing not only prevented inhibition of CDK4 transcription in polyamine-deficient cells but also abolished repression of CDK4 expression induced by ectopic JunD overexpression. ATF-2 silencing also promoted IEC growth in polyamine-depleted cells. These results indicate that induced ATF-2/JunD association following polyamine depletion represses CDK4 transcription, thus contributing to the inhibition of IEC growth.