p21(WAF1/CIP1) upregulation through the stress granule-associated protein CUGBP1 confers resistance to bortezomib-mediated apoptosis

Overexpression of CUGBP1 is associated with the progression of non-small cell lung cancer

The multifunctional RNA-binding protein CUGBP1 regulates multiple aspects of nuclear and cytoplasmic messenger RNA (mRNA) processing, including splicing, stabilization, and translation of mRNAs. Previous studies have shown that CUGBP1 is overexpressed in non-small-cell lung cancer (NSCLC) tissues, but the pathological functions of CUGBP1 in tumorigenesis and development are unknown. Here, we provide the first evidence demonstrating the clinicopathological significance of CUGBP1 in NSCLC. Using immunohistochemistry, the levels of CUGBP1 expression in NSCLC tissues and adjacent non-cancerous tissues were examined and determined to be associated with differentiation. Short hairpin RNA-induced downregulation of CUGBP1 promoted apoptosis and decreased proliferation in the A549 NSCLC cell line. Moreover, Western blot analysis indicated that the depletion of CUGBP1 increased the protein levels of cyclin D1, BAD, BAX, Jun D, and E-cadherin, while the cyclin B1 level decreased. Knockdown of CUGBP1 decreased β-catenin and vimentin levels and increased E-cadherin expression, suggesting that CUGBP1 may contribute significantly to epithelial to mesenchymal transition (EMT) progression. These results demonstrate the importance of CUGBP1 in the biological and pathological functions of NSCLC and indicate its potential as a therapeutic target for NSCLC.

Cellular uptake kinetics of bortezomib in relation to efficacy in myeloma cells and the influence of drug transporters

PURPOSE

Despite overall successful application to multiple myeloma patients, clinical efficacy of the proteasome inhibitor bortezomib is typically challenged by primary and secondary resistance of unknown origin. So far, the potential impact of intracellular concentrations on drug efficacy of bortezomib and the influence of drug transporters are unknown.

METHODS

We determined cellular bortezomib kinetics in nine myeloma cell lines using ultrahigh-performance liquid chromatography coupled to tandem mass spectrometry. The potential influence of drug transporters on the uptake kinetics observed in these cell lines was investigated by testing substrate characteristics of bortezomib for several transporters in over-expressing model cells. Additionally, transporter mRNA expression was quantified in myeloma cell lines by real-time polymerase chain reaction (RT-PCR).

RESULTS

All myeloma cells revealed an extensive intracellular bortezomib accumulation (47.5-183 ng/ml) exceeding extracellular concentrations (0.04-0.17 ng/ml) by more than factor 1,000. Only organic anion-transporting polypeptide 1B1 facilitated the uptake in over-expressing cells, however, to a negligible extent (factor 1.36). Bortezomib efflux via P-glycoprotein was confirmed by demonstrating reduced sensitivity (IC50 11.6 vs. 2.8 ng/ml) and intracellular concentrations (-56.1%) in over-expressing cells compared to controls. RT-PCR revealed a varying but overall weak transporter expression in the studied myeloma cells without any correlation to intracellular concentrations. Although principally valid as demonstrated in the P-glycoprotein over-expressing cell model, there was no significant correlation between intracellular concentrations and bortezomib efficacy in myeloma cell lines.

CONCLUSION

Differences in intracellular concentrations in myeloma cell lines neither result from variable transporter expression nor represent the main factor determining bortezomib efficacy in vitro.

Stress granules, P-bodies and cancer

Cancer cells are exposed to adverse conditions in the tumor microenvironment, and utilize post-transcriptional control mechanisms to re-program gene expression in ways that enhance cell survival. Stress granules and processing bodies are RNA-containing granules that contribute to this process by modulating cellular signaling pathways, metabolic machinery, and stress response programs. This review examines evidence implicating RNA granules in the pathogenesis of cancer and discusses their potential as targets for anticancer therapies. This article is part of a Special Issue entitled: Translation and Cancer.

Tudor-SN, a component of stress granules, regulates growth under salt stress by modulating GA20ox3 mRNA levels in Arabidopsis

The Tudor-SN protein (TSN) is universally expressed and highly conserved in eukaryotes. In Arabidopsis, TSN is reportedly involved in stress adaptation, but the mechanism involved in this adaptation is not understood. Here, we provide evidence that TSN regulates the mRNA levels of GA20ox3, a key enzyme for gibberellin (GA) biosynthesis. The levels of GA20ox3 transcripts decreased in TSN1/TSN2 RNA interference (RNAi) transgenic lines and increased in TSN1 over-expression (OE) transgenic lines. The TSN1 OE lines displayed phenotypes that may be attributed to the overproduction of GA. No obvious defects were observed in the RNAi transgenic lines under normal conditions, but under salt stress conditions these lines displayed slower growth than wild-type (WT) plants. Two mutants of GA20ox3, ga20ox3-1 and -2, also showed slower growth under stress than WT plants. Moreover, a higher accumulation of GA20ox3 transcripts was observed under salt stress. The results of a western blot analysis indicated that higher levels of TSN1 accumulated after salt treatment than under normal conditions. Subcellular localization studies showed that TSN1 was uniformly distributed in the cytoplasm under normal conditions but accumulated in small granules and co-localized with RBP47, a marker protein for stress granules (SGs), in response to salt stress. The results of RNA immunoprecipitation experiments indicated that TSN1 bound GA20ox3 mRNA in vivo. On the basis of these findings, we conclude that TSN is a novel component of plant SGs that regulates growth under salt stress by modulating levels of GA20ox3 mRNA.

[Role of HRI in apoptosis resistance]

When exposed to environmental stresses, cells activate defence mechanisms to adapt stress and inhibit apoptotic pathways leading to their survival. Stressed cells also reduce their general metabolism in part by inhibiting mRNA translation, thereby saving energy needed to repair stress-induced damages. Under stress conditions, the inhibition of mRNA translation occurs mainly at its initiation step through the phosphorylation of the translation initiation factor eIF2α. One of the four kinases known to phosphorylate eIF2α is heme-regulated inhibitor (HRI). The activation of HRI occurs under conditions of heme deficiency, oxidative stress and treatment with anti-cancer drugs such as proteasome inhibitors. In this article, we discuss the role of HRI in promoting cell resistance to stress-mediated apoptosis.

Wig-1 regulates cell cycle arrest and cell death through the p53 targets FAS and 14-3-3σ

Wig-1, also known as ZMAT3, is a p53 target gene that encodes an RNA-binding zinc-finger protein involved in the regulation of mRNA stability through binding to AU-rich elements (AREs). We have used microarray analysis to identify novel Wig-1 target mRNAs. We identified 2447 transcripts with >fourfold differential expression between Wig-1 and control small interfering (si)RNA-treated HCT116 cells. Several p53 target genes were among the deregulated transcripts. We found that Wig-1 regulates FAS and 14-3-3σ mRNA independently of p53. We show that Wig-1 binds to FAS mRNA 3'-UTR and decreases its stability through an ARE in the 3'-UTR. Depletion of Wig-1 was associated with increased cell death and reduced cell cycle arrest upon DNA damage. Our results suggest a role of Wig-1 as a survival factor that directs the p53 stress response toward cell cycle arrest rather than apoptosis through the regulation of FAS and 14-3-3σ mRNA levels.

Control of mRNA turnover: implication of cytoplasmic RNA granules

The control of mRNA turnover is essential for the cell to rationalize its mRNA content both under physiological conditions and upon stress. Several mechanisms involved in the control of mRNA turnover have been elucidated. These include surveillance mechanisms such as nonsense-mediated decay, non-stop mediated decay and non-go-mediated decay that eliminate aberrant mRNAs, and regulatory mechanisms including AU-mediated decay, GU-mediated decay, and CDE-mediated decay that ensure mRNA plasticity. In general, the mechanisms of RNA decay rely on interactions between specific cis-acting RNA elements and selected RNA-binding proteins that either prevent the degradation of mRNA targets or induce the recruitment of decaying effectors leading to mRNA degradation. Formation of cytoplasmic RNA granules including processing bodies, stress granules, UV granules, and exosome granules have recently emerged as an additional mechanism that control mRNA turnover of selected mRNAs. Here we will review briefly review the main mechanisms that control mRNA decay and highlight possible implication of RNA granules in such mechanisms.

5-Fluorouracil affects assembly of stress granules based on RNA incorporation

The antimetabolite 5-fluorouracil is a widely used chemotherapeutic for the treatment of several solid cancers. However, resistance to 5-fluorouracil remains a major drawback in its clinical use. In this study we report that treatment of HeLa cells with 5-fluorouracil resulted in de novo assembly of stress granules. Moreover, we revealed that stress granule assembly under stress conditions as well as disassembly is altered in cells treated with 5-fluorouracil. Notably, we discovered that RACK1, a protein mediating cell survival and apoptosis, is a component of 5-fluorouracil-induced stress granules. To explore the mode of action of 5-fluorouracil accountable for de novo stress granule assembly, we analyzed 5-fluorouracil metabolites and noticed that stress granule assembly is caused by RNA, not DNA incorporating 5-fluorouracil metabolites. Interestingly, we observed that other RNA incorporating drugs also cause assembly of stress granules. Thus, our results suggest that incorporation of chemotherapeutics into RNA may result in stress granule assembly with potential significance in chemoresistance.

The proteasome inhibitor bortezomib is a potent inducer of zinc finger AN1-type domain 2a gene expression: role of heat shock factor 1 (HSF1)-heat shock factor 2 (HSF2) heterocomplexes

The zinc finger AN1-type domain 2a gene, also known as arsenite-inducible RNA-associated protein (AIRAP), was recently identified as a novel human canonical heat shock gene strictly controlled by heat shock factor (HSF) 1. Little is known about AIRAP gene regulation in human cells. Here we report that bortezomib, a proteasome inhibitor with anticancer and antiangiogenic properties used in the clinic for treatment of multiple myeloma, is a potent inducer of AIRAP expression in human cells. Using endothelial cells as a model, we unraveled the molecular mechanism regulating AIRAP expression during proteasome inhibition. Bortezomib induces AIRAP expression at the transcriptional level early after treatment, concomitantly with polyubiquitinated protein accumulation and HSF activation. AIRAP protein is detected at high levels for at least 48 h after bortezomib exposure, together with the accumulation of HSF2, a factor implicated in differentiation and development regulation. Different from heat-mediated induction, in bortezomib-treated cells, HSF1 and HSF2 interact directly, forming HSF1-HSF2 heterotrimeric complexes recruited to a specific heat shock element in the AIRAP promoter. Interestingly, whereas HSF1 has been confirmed to be critical for AIRAP gene transcription, HSF2 was found to negatively regulate AIRAP expression after bortezomib treatment, further emphasizing an important modulatory role of this transcription factor under stress conditions. AIRAP function is still not defined. However, the fact that AIRAP is expressed abundantly in primary human cells at bortezomib concentrations comparable with plasma levels in treated patients suggests that AIRAP may participate in the regulatory network controlling proteotoxic stress during bortezomib treatment.

Lithium down-regulates histone deacetylase 1 (HDAC1) and induces degradation of mutant huntingtin

Lithium is an effective mood stabilizer that has been clinically used to treat bipolar disorder for several decades. Recent studies have suggested that lithium possesses robust neuroprotective and anti-tumor properties. Thus far, a large number of lithium targets have been discovered. Here, we report for the first time that HDAC1 is a target of lithium. Lithium significantly down-regulated HDAC1 at the translational level by targeting HDAC1 mRNA. We also showed that depletion of HDAC1 is essential for the neuroprotective effects of lithium and for the lithium-mediated degradation of mutant huntingtin through the autophagic pathway. Our studies explain the multiple functions of lithium and reveal a novel mechanism for the function of lithium in neurodegeneration.

Bortezomib for the treatment of previously untreated multiple myeloma

Management of multiple myeloma (MM) has been drastically changed in the last 10 years thanks to the introduction of novel agents, which, combined with the backbone of classical chemotherapy, have led to a significant improvement in disease control. Bortezomib is the first reversible proteasome inhibitor approved for the treatment of MM, with wide synergism in vitro and in vivo with a plethora of drugs active for MM. In patients eligible for autologous stem cell transplantation (ASCT), the achievement of complete response or very good partial response before ASCT is associated with prolonged progression-free and overall survival. Thus, the goal of induction regimens should include, at least for younger patients, a continued improvement of the quality and depth of the achieved response. This article is focused on reviewing the major efforts in frontline therapy for MM, including bortezomib-containing induction regimens in patients either eligible or ineligible for ASCT.

Naringin inhibits growth potential of human triple-negative breast cancer cells by targeting β-catenin signaling pathway

Triple-negative (ER-/PR-/HER2-) breast cancer (TNBC) is a severe clinical problem because of its relatively poorer prognosis, aggressive behavior and lack of targeted therapies. Naringin, a major flavonoid extracted from citrus fruits, has been reported to exert promising anticancer activities. However, the detailed antitumor mechanism of naringin still remains enigmatic. In this study, TNBC cell lines-based in vitro and in vivo models were used to explore the anticancer effect and mechanism of naringin. Our data demonstrated that naringin inhibited cell proliferation, and promoted cell apoptosis and G1 cycle arrest, accompanied by increased p21 and decreased survivin. Meanwhile, β-catenin signaling pathway was found to be suppressed by naringin. In contrast, over-expressing β-catenin by adenoviral vector system in TNBC cells reversed the antitumor activity of naringin, and regulated p21 and survivin. Correspondingly, the antitumor potential of naringin was also observed in naringin-treated MDA-MB-231 xenograft mice, while immunohistochemical analysis of tumors from naringin-treated mice showed higher expression of p21 and lower expression of survivin and active β-catenin. Taken together, these results indicate that naringin could inhibit growth potential of TNBC cells by modulating β-catenin pathway, which suggests naringin might be used as a potential supplement for the prevention and treatment of breast cancer.

Calreticulin-dimerization induced by post-translational arginylation is critical for stress granules scaffolding

Protein arginylation mediated by arginyl-tRNA protein transferase is a post-translational modification that occurs widely in biology, it has been shown to regulate protein and properties and functions. Post-translational arginylation is critical for embryogenesis, cardiovascular development and angiogenesis but the molecular effects of proteins arginylated in vivo are largely unknown. In the present study, we demonstrate that arginylation reduces CRT (calreticulin) thermostability and induces a greater degree of dimerization and oligomerization. R-CRT (arginylated calreticulin) forms disulfide-bridged dimers that are increased in low Ca(2+) conditions at physiological temperatures, a similar condition to the cellular environment that it required for arginylation of CRT. Moreover, R-CRT self-oligomerizes through non-covalent interactions that are enhanced at temperatures above 40 °C, condition that mimics the heat shock treatment where R-CRT is the only isoespecies of CRT that associates in cells to SGs (stress granules). We show that in cells lacking CRT the scaffolding of larger SGs is impaired; the transfection with CRT (hence R-CRT expression) restores SGs assembly whereas the transfection with CRT mutated in Cys146 does not. Thus, R-CRT disulfide-bridged dimers (through Cys146) are essential for the scaffolding of larger SGs under heat shock, although these dimers are not required for R-CRT association to SGs. The alteration in SGs assembly is critical for the normal cellular recover of cells after heat induced stress. We conclude that R-CRT is emerging as a novel protein that has an impact on the regulation of SGs scaffolding and cell survival.

Carfilzomib

This spotlight review focuses on the second-generation proteasome inhibitor carfilzomib, which was recently approved by the U.S. Food and Drug Administration for treatment of relapsed and refractory multiple myeloma patients who have received at least 2 prior therapies, including bortezomib and an immunomodulatory agent, and have demonstrated disease progression on or within 60 days of the completion of the last therapy. This review focuses on clinical trial data leading to drug approval and provides advice for treating physicians who are now accessing this drug for patients.

Inactivation of the mTORC1-eukaryotic translation initiation factor 4E pathway alters stress granule formation

Stress granules (SG) are cytoplasmic multimeric RNA bodies that form under stress conditions known to inhibit cap-dependent translation. SG contain translation initiation factors, RNA binding proteins, and signaling molecules. SG are known to inhibit apoptotic pathways, thus contributing to chemo- and radioresistance in tumor cells. However, whether stress granule formation involves oncogenic signaling pathways is currently unknown. Here, we report a novel role of the mTORC1-eukaryotic translation initiation factor 4E (eIF4E) pathway, a key regulator of cap-dependent translation initiation of oncogenic factors, in SG formation. mTORC1 specifically drives the eIF4E-mediated formation of SG through the phosphorylation of 4E-BP1, a key factor known to inhibit formation of the mTORC1-dependent eIF4E-eIF4GI interactions. Disrupting formation of SG by inactivation of mTOR with its specific inhibitor pp242 or by depletion of eIF4E or eIF4GI blocks the SG-associated antiapoptotic p21 pathway. Finally, pp242 sensitizes cancer cells to death in vitro and inhibits the growth of chemoresistant tumors in vivo. This work therefore highlights a novel role of the oncogenic mTORC1-eIF4E pathway, namely, the promotion of formation of antiapoptotic SG.

Characterization of fragile X mental retardation protein recruitment and dynamics in Drosophila stress granules

The RNA-binding protein Fragile X Mental Retardation (FMRP) is an evolutionarily conserved protein that is particularly abundant in the brain due to its high expression in neurons. FMRP deficiency causes fragile X mental retardation syndrome. In neurons, FMRP controls the translation of target mRNAs in part by promoting dynamic transport in and out neuronal RNA granules. We and others have previously shown that upon stress, mammalian FMRP dissociates from translating polysomes to localize into neuronal-like granules termed stress granules (SG). This localization of FMRP in SG is conserved in Drosophila. Whether FMRP plays a key role in SG formation, how FMRP is recruited into SG, and whether its association with SG is dynamic are currently unknown. In contrast with mammalian FMRP, which has two paralog proteins, Drosophila FMR1 (dFMRP) is encoded by a single gene that has no paralog. Using this genetically simple model, we assessed the role of dFMRP in SG formation and defined the determinants required for its recruitment in SG as well as its dynamics in SG. We show that dFMRP is dispensable for SG formation in vitro and ex vivo. FRAP experiments showed that dFMRP shuttles in and out SG. The shuttling activity of dFMRP is mediated by a protein-protein interaction domain located at the N-terminus of the protein. This domain is, however, dispensable for the localization of dFMRP in SG. This localization of dFMRP in SG requires the KH and RGG motifs which are known to mediate RNA binding, as well as the C-terminal glutamine/asparagine rich domain. Our studies thus suggest that the mechanisms controlling the recruitment of FMRP into SG and those that promote its shuttling between granules and the cytosol are uncoupled. To our knowledge, this is the first demonstration of the regulated shuttling activity of a SG component between RNA granules and the cytosol.

Overexpression of RNA-binding protein CELF1 prevents apoptosis and destabilizes pro-apoptotic mRNAs in oral cancer cells

CELF1 RNA-binding protein, otherwise called CUGBP1, associates and coordinates the degradation of GU-rich element (GRE) containing mRNA’s encoding factors important for cell growth, migration and apoptosis. Although many substrates of CELF1 have been identified, the biological significance of CELF1-mediated mRNA decay remains unclear. As the processes modulated by CELF1 are frequently disrupted in cancer, we investigated the expression and role of CELF1 in oral squamous cancer cells (OSCCs). We determined that CELF1 is reproducibly overexpressed in OSCC tissues and cell lines. Moreover, depletion of CELF1 reduced proliferation and increased apoptosis in OSCCs, but had negligible effect in non-transformed cells. We found that CELF1 associates directly with the 3′UTR of mRNAs encoding the pro-apoptotic factors BAD, BAX and JunD and mediates their rapid decay. Specifically, 3′UTR fragment analysis of JunD revealed that the GRE region is critical for binding with CELF1 and expression of JunD in oral cancer cells. In addition, silencing of CELF1 rendered BAD, BAX and JunD mRNAs stable and increased their protein expression in oral cancer cells. Taken together, these results support a critical role for CELF1 in modulating apoptosis and implicate this RNA-binding protein as a cancer marker and potential therapeutic target.

CELFish ways to modulate mRNA decay

The CELF family of RNA-binding proteins regulates many steps of mRNA metabolism. Although their best characterized function is in pre-mRNA splice site choice, CELF family members are also powerful modulators of mRNA decay. In this review we focus on the different modes of regulation that CELF proteins employ to mediate mRNA decay by binding to GU-rich elements. After starting with an overview of the importance of CELF proteins during development and disease pathogenesis, we then review the mRNA networks and cellular pathways these proteins regulate and the mechanisms by which they influence mRNA decay. Finally, we discuss how CELF protein activity is modulated during development and in response to cellular signals. We conclude by highlighting the priorities for new experiments in this field. This article is part of a Special Issue entitled: RNA Decay mechanisms.

HDAC inhibitor L-carnitine and proteasome inhibitor bortezomib synergistically exert anti-tumor activity in vitro and in vivo

Combinations of proteasome inhibitors and histone deacetylases (HDAC) inhibitors appear to be the most potent to produce synergistic cytotoxicity in preclinical trials. We have recently confirmed that L-carnitine (LC) is an endogenous HDAC inhibitor. In the current study, the anti-tumor effect of LC plus proteasome inhibitor bortezomib (velcade, Vel) was investigated both in cultured hepatoma cancer cells and in Balb/c mice bearing HepG2 tumor. Cell death and cell viability were assayed by flow cytometry and MTS, respectively. Gene, mRNA expression and protein levels were detected by gene microarray, quantitative real-time PCR and Western blot, respectively. The effect of Vel on the acetylation of histone H3 associated with the p21(cip1) gene promoter was examined by using ChIP assay and proteasome peptidase activity was detected by cell-based chymotrypsin-like (CT-like) activity assay. Here we report that (i) the combination of LC and Vel synergistically induces cytotoxicity in vitro; (ii) the combination also synergistically inhibits tumor growth in vivo; (iii) two major pathways are involved in the synergistical effects of the combinational treatment: increased p21(cip1) expression and histone acetylation in vitro and in vivo and enhanced Vel-induced proteasome inhibition by LC. The synergistic effect of LC and Vel in cancer therapy should have great potential in the future clinical trials.

Downregulation of extracellular signal-regulated kinase 1/2 activity by calmodulin KII modulates p21Cip1 levels and survival of immortalized lymphocytes from Alzheimer's disease patients

Previously, we reported a Ca(2+)/calmodulin (CaM)-dependent impairment of apoptosis induced by serum deprivation in Alzheimer's disease (AD) lymphoblasts. These cell lines showed downregulation of extracellular signal-regulated kinase (ERK)1/2 activity and elevated content of p21 compared with control cells. The aim of this study was to delineate the molecular mechanism underlying the distinct regulation of p21 content in AD cells. Quantitative reverse transcription polymerase chain reaction analysis demonstrated increased p21 messenger RNA (mRNA) levels in AD cells. The ERK1/2 inhibitor, PD98059, prevented death of control cells and enhanced p21 mRNA and protein levels. The CaM antagonist, calmidazolium, and the CaMKII inhibitor, KN-62, normalized the survival pattern of AD lymphoblasts by augmenting ERK1/2 activation and reducing p21 mRNA and protein levels. Upregulation of p21 transcription in AD cells appears to be the consequence of increased activity of forkhead box O3a (FOXO3a) as the result of diminished ERK1/2-mediated phosphorylation of this transcription factor, which in turn facilitates its nuclear accumulation. Murine double minute 2 (MDM2) protein levels were decreased in AD cells relative to control lymphoblasts, suggesting an impairment of FOXO3a degradation.

Characterization of Fragile X Mental Retardation Protein granules formation and dynamics in Drosophila

FMRP is an evolutionarily conserved protein that is highly expressed in neurons and its deficiency causes fragile X mental retardation syndrome. FMRP controls the translation of target mRNAs in part by promoting their dynamic transport in neuronal RNA granules. We have previously shown that high expression of mammalian FMRP induces formation of granules termed FMRP granules. These RNA granules are reminiscent of neuronal granules, of stress granules, as well as of the recently described in vitro-assembled granules. In contrast with mammalian FMRP, which has two paralog proteins, Drosophila FMRP (dFMRP) is encoded by a single gene that has no paralog. Using this genetically simple organism, we investigated formation and dynamics of FMRP granules. We found that increased expression of dFMRP in Drosophila cells induces the formation of dynamic dFMRP RNA granules. Mutagenesis studies identified the N-terminal protein–protein domain of dFMRP as a key determinant for FMRP granules formation. The RGG RNA binding motif of dFMRP is dispensable for dFMRP granules formation since its deletion does not prevent formation of those granules. Deletion of the RGG motif reduced, however, dFMRP trafficking between FMRP granules and the cytosol. Similarly, deletion of a large part of the KH RNA binding motif of dFMRP had no effect on formation of dFMRP-granules, but diminished the shuttling activity of dFMRP. Our results thus suggest that the mechanisms controlling formation of RNA granules and those promoting their dynamics are uncoupled. This study opens new avenues to further elucidate the molecular mechanisms controlling FMRP trafficking with its associated mRNAs in and out of RNA granules.

The RNA-binding protein CUG-BP1 increases survivin expression in oesophageal cancer cells through enhanced mRNA stability

Survivin, a member of the IAP (inhibitor of apoptosis protein) family, plays important roles in maintaining cellular homoeostasis and regulating cell-cycle progression. This IAP is overexpressed in oesophageal cancer cells, leading to uncontrolled cell growth and resistance to apoptosis. CUG-BP1 (CUG-binding protein 1) is an RNA-binding protein that regulates the stability and translational efficiency of target mRNAs. In the present paper, we report that CUG-BP1 is overexpressed in oesophageal cancer cell lines and human oesophageal cancer specimens. CUG-BP1 associates with the 3'-untranslated region of survivin mRNA, thereby stabilizing the transcript and elevating its expression in oesophageal cancer cells. Our results show that overexpression of CUG-BP1 in oesophageal epithelial cells results in increased survivin mRNA stability and consequently survivin protein expression. Conversely, silencing CUG-BP1 in oesophageal cancer cells destabilizes survivin mRNA, lowering the level of survivin protein. In addition, we have found that altering CUG-BP1 expression modulates susceptibility to chemotherapy-induced apoptosis. Overexpression of CUG-BP1 in oesophageal epithelial cells increases resistance to apoptosis, whereas silencing CUG-BP1 makes oesophageal cancer cells more susceptible to chemotherapy-induced apoptosis. Co-transfection experiments with small interfering RNA directed against survivin suggest that the anti-apoptotic role for CUG-BP1 is not entirely dependent on its effect on survivin expression.

Development of peptide-based reversing agents for p-glycoprotein-mediated resistance to carfilzomib

Carfilzomib is a novel class of peptidyl epoxyketone proteasome inhibitor and has demonstrated promising activity in multiple clinical trials to treat patients with multiple myeloma and other types of cancers. Here, we investigated molecular mechanisms underlying acquired resistance to carfilzomib and a potential strategy to restore cellular sensitivity to carfilzomib. H23 and DLD-1 cells (human lung and colon adenocarcinoma cell lines) with acquired resistance to carfilzomib displayed marked cross-resistance to YU-101, a closely related proteasome inhibitor, and paclitaxel, a known substrate of Pgp. However, carfilzomib-resistant cells remained sensitive to bortezomib, a clinically used dipeptide with boronic acid pharmacophore. In accordance with these observations, carfilzomib-resistant H23 and DLD-1 cells showed marked upregulation of P-glycoprotein (Pgp) as compared to their parental controls, and coincubation with verapamil, a Pgp inhibitor, led to an almost complete restoration of cellular sensitivity to carfilzomib. These results indicate that Pgp upregulation plays a major role in the development of carfilzomib resistance in these cell lines. In developing a potential strategy to overcome carfilzomib resistance, we as a proof of concept prepared a small library of peptide analogues derived from the peptide backbone of carfilzomib and screened these molecules for their activity to restore carfilzomib sensitivity when cotreated with carfilzomib. We found that compounds as small as dipeptides are sufficient in restoring carfilzomib sensitivity. Taken together, we found that Pgp upregulation plays a major role in the development of resistance to carfilzomib in lung and colon adenocarcinoma cell lines and that small peptide analogues lacking the pharmacophore can be used as agents to reverse acquired carfilzomib resistance. Our findings may provide important information in developing a potential strategy to overcome drug resistance.

The translation inhibitor pateamine A prevents cachexia-induced muscle wasting in mice

Cachexia, or muscle-wasting syndrome, is one of the major causes of death in patients affected by diseases such as cancer, AIDS and sepsis. However, no effective anti-cachectic treatment is currently available. Here we show that a low dose of pateamine A, an inhibitor of translation initiation, prevents muscle wasting caused by the cytokines interferon γ and tumour necrosis factor α or by C26-adenocarcinoma tumours. Surprisingly, although high doses of pateamine A abrogate general translation, low doses selectively inhibit the expression of pro-cachectic factors such as inducible nitric oxide synthase. This selectivity depends on the 5'UTR of inducible nitric oxide synthase messenger RNA (mRNA) that, unlike the 5'UTR of MyoD mRNA, promotes the recruitment of inducible nitric oxide synthase mRNA to stress granules, where its translation is repressed. Collectively, our data provide a proof of principle that nontoxic doses of compounds such as pateamine A could be used as novel drugs to combat cachexia-induced muscle wasting.

Rad knockdown induces mitochondrial apoptosis in bortezomib resistant leukemia and lymphoma cells

To understand the molecular mechanism(s) underlying bortezomib resistance, we sought to identify potential target genes that were differentially expressed in bortezomib-resistant leukemia cells versus parental controls. Microarray analysis revealed that the mRNA levels of Rad (Ras associated with diabetes) were higher in the bortezomib-resistant Jurkat (Jurkat-R) cells than in the parental control cells. The importance of Rad for bortezomib resistance was supported by three observations. First, Rad knockdown overcame bortezomib resistance and induced mitochondrial apoptosis via Noxa/Bcl-2 modulation. Second, Rad decreased cell death in response to bortezomib. Third, leukemia and lymphoma cell lines (K-562, Raji, IM-9 and Jurkat-R) with elevated Rad expression levels showed higher degrees of bortezomib resistance versus those (Sup-B15, JVM-2, U266 and Jurkat) with low Rad expression levels (r=0.48, P=0.0004). Thus, Rad over expression could be a molecular target to improve bortezomib sensitivity in human leukemia and lymphoma.

Arginylated calreticulin at plasma membrane increases susceptibility of cells to apoptosis

Post-translational modifications of proteins are important for the regulation of cell fate and functions; one of these post-translational modifications is arginylation. We have previously established that calreticulin (CRT), an endoplasmic reticulum resident, is also one of the arginylated substrates found in the cytoplasm. In the present study, we describe that arginylated CRT (R-CRT) binds to the cell membrane and identified its role as a preapoptotic signal. We also show that cells lacking arginyl-tRNA protein transferase are less susceptible to apoptosis than wild type cells. Under these conditions R-CRT is present on the cell membrane but at early stages is differently localized in stress granules. Moreover, cells induced to undergo apoptosis by arsenite show increased R-CRT on their cell surface. Exogenously applied R-CRT binds to the cell membrane and is able to both increase the number of cells undergoing apoptosis in wild type cells and overcome apoptosis resistance in cells lacking arginyl-tRNA protein transferase that express R-CRT on the cell surface. Thus, these results demonstrate the importance of surface R-CRT in the apoptotic response of cells, implying that post-translational arginylation of CRT can regulate its intracellular localization, cell function, and survival.

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.

An emerging role of RNA-binding proteins as multifunctional regulators of lymphocyte development and function

Sequence-specific RNA-binding proteins (RBP) and the regulation of RNA decay have long been recognized as important regulators of the inflammatory response. RBP influence gene expression throughout the lifespan of the mRNA by regulating splicing, polyadenylation, cellular localization, translation, and decay. Increasing evidence now indicates that these proteins, together with the RNA decay machinery that they recruit, also regulate the development and activation of lymphocytes. The activity of RBP is regulated by the same signal transduction pathways that govern lymphocyte development and differentiation in response to antigen and cytokine receptor engagement. Roles for these proteins in regulating the diverse functions of lymphocytes are becoming increasingly apparent.