Similar but distinct effects of the tristetraprolin/TIS11 immediate-early proteins on cell survival

Turnover of PPP1R15A mRNA encoding GADD34 controls responsiveness and adaptation to cellular stress

The integrated stress response (ISR) is a key cellular signaling pathway activated by environmental alterations that represses protein synthesis to restore homeostasis. To prevent sustained damage, the ISR is counteracted by the upregulation of growth arrest and DNA damage-inducible 34 (GADD34), a stress-induced regulatory subunit of protein phosphatase 1 that mediates translation reactivation and stress recovery. Here, we uncover a novel ISR regulatory mechanism that post-transcriptionally controls the stability of PPP1R15A mRNA encoding GADD34. We establish that the 3' untranslated region of PPP1R15A mRNA contains an active AU-rich element (ARE) recognized by proteins of the ZFP36 family, promoting its rapid decay under normal conditions and stabilization for efficient expression of GADD34 in response to stress. We identify the tight temporal control of PPP1R15A mRNA turnover as a component of the transient ISR memory, which sets the threshold for cellular responsiveness and mediates adaptation to repeated stress conditions.

Post-transcriptional regulation of BIRC5/survivin expression and induction of apoptosis in breast cancer cells by tristetraprolin

Inhibition of apoptosis is one of the hallmarks of cancer and is a target of various therapeutic interventions. BIRC5 is an inhibitor of apoptosis that is aberrantly expressed in cancer leading to sustained growth of tumours. Post-transcriptional control mechanisms involving RNA-binding proteins and AU-rich elements (AREs) are fundamental to many cellular processes and changes in the expression or function of these proteins can promote an aberrant and pathological phenotype. BIRC5 mRNA has an ARE in its 3' UTR making it a candidate for regulation by the RNA binding proteins tristetraprolin (TTP) and HuR (ELAVL1). In this study, we investigated the binding of TTP and HuR by RNA-immunoprecipitation assays and found that these proteins were associated with BIRC5 mRNA to varying extents. Consequently, BIRC5 expression decreased when TTP was overexpressed and apoptosis was induced. In the absence of TTP, BIRC5 mRNA was stabilized, protein expression increased and the number of apoptotic cells declined. As an ARE-mRNA stabilizing protein, recombinant HuR led to upregulation of BIRC5 expression, whereas HuR silencing was concomitant with downregulation of BIRC5 mRNA and protein and increased cell death. Survival analyses demonstrated that increased TTP and low BIRC5 expression predicted an overall better prognosis compared to dysregulated TTP and high BIRC5. Thus, the results present a novel target of ARE-mediated post-transcriptional regulation.

Nicotine-mediated OTUD3 downregulation inhibits VEGF-C mRNA decay to promote lymphatic metastasis of human esophageal cancer

Nicotine addiction and the occurrence of lymph node spread are two major significant factors associated with esophageal cancer's poor prognosis; however, nicotine's role in inducing lymphatic metastasis of esophageal cancer remains unclear. Here we show that OTU domain-containing protein 3 (OTUD3) is downregulated by nicotine and correlates with poor prognosis in heavy-smoking esophageal cancer patients. OTUD3 directly interacts with ZFP36 ring finger protein (ZFP36) and stabilizes it by inhibiting FBXW7-mediated K48-linked polyubiquitination. ZFP36 binds with the VEGF-C 3-'UTR and recruits the RNA degrading complex to induce its rapid mRNA decay. Downregulation of OTUD3 and ZFP36 is essential for nicotine-induced VEGF-C production and lymphatic metastasis in esophageal cancer. This study establishes that the OTUD3/ZFP36/VEGF-C axis plays a vital role in nicotine addiction-induced lymphatic metastasis, suggesting that OTUD3 may serve as a prognostic marker, and induction of the VEGF-C mRNA decay might be a potential therapeutic strategy against human esophageal cancer.

Tristetraprolin: A cytosolic regulator of mRNA turnover moonlighting as transcriptional corepressor of gene expression

Tristetraprolin (TTP) is a nucleocytoplasmic 326 amino acid protein whose sequence is characterized by possessing two CCCH-type zinc finger domains. In the cytoplasm TTP function is to promote the degradation of mRNAs that contain adenylate/uridylate-rich elements (AREs). Mechanistically, TTP promotes the recruitment of poly(A)-specific deadenylases and exoribonucleases. By reducing the half-life of about 10% of all the transcripts in the cell TTP has been shown to participate in multiple cell processes that include regulation of gene expression, cell proliferation, metabolic homeostasis and control of inflammation and immune responses. However, beyond its role in mRNA decay, in the cell nucleus TTP acts as a transcriptional coregulator by interacting with chromatin modifying enzymes. TTP has been shown to repress the transactivation of NF-κB and estrogen receptor suggesting the possibility that it participates in the transcriptional regulation of hundreds of genes in human cells and its possible involvement in breast cancer progression. In this review, we discuss the cytoplasmic and nuclear functions of TTP and the effect of the dysregulation of its protein levels in the development of human diseases. We suggest that TTP be classified as a moonlighting tumor supressor protein that regulates gene expression through two different mechanims; the decay of ARE-mRNAs and a transcriptional coregulatory function.

Cataloguing the phosphorylation sites of tristetraprolin (TTP): Functional implications for inflammatory diseases

Tristetraprolin (TTP) is a destabilizing mRNA binding protein known to regulate gene expression of a wide variety of targets, including those that control inflammation. TTP expression, regulation and function is controlled by phosphorylation. While the importance of key serine (S) sites (S⁵² and S¹⁷⁸ in mice and S¹⁸⁶ in humans) has been recognized, other sites on the hyperphosphorylated TTP protein have more recently emerged as playing an important role in regulating cellular signalling and downstream functions of TTP. In order to propel investigation of TTP and fully exploit its potential as a drug target in inflammatory disease, this review will catalogue TTP phosphorylation sites in both the murine and human TTP protein, the known and unknown roles and functions of these sites, the kinases and phosphatases that act upon TTP and overview methodological approaches to increase our knowledge of this important protein regulated by phosphorylation.

The Tristetraprolin Family of RNA-Binding Proteins in Cancer: Progress and Future Prospects

Post-transcriptional regulation of gene expression plays a key role in cellular proliferation, differentiation, migration, and apoptosis. Increasing evidence suggests dysregulated post-transcriptional gene expression as an important mechanism in the pathogenesis of cancer. The tristetraprolin family of RNA-binding proteins (RBPs), which include Zinc Finger Protein 36 (ZFP36; commonly referred to as tristetraprolin (TTP)), Zinc Finger Protein 36 like 1 (ZFP36L1), and Zinc Finger Protein 36 like 2 (ZFP36L2), play key roles in the post-transcriptional regulation of gene expression. Mechanistically, these proteins function by binding to the AU-rich elements within the 3′-untranslated regions of their target mRNAs and, in turn, increasing mRNA turnover. The TTP family RBPs are emerging as key regulators of multiple biological processes relevant to cancer and are aberrantly expressed in numerous human cancers. The TTP family RBPs have tumor-suppressive properties and are also associated with cancer prognosis, metastasis, and resistance to chemotherapy. Herein, we summarize the various hallmark molecular traits of cancers that are reported to be regulated by the TTP family RBPs. We emphasize the role of the TTP family RBPs in the regulation of trait-associated mRNA targets in relevant cancer types/cell lines. Finally, we highlight the potential of the TTP family RBPs as prognostic indicators and discuss the possibility of targeting these TTP family RBPs for therapeutic benefits.

RNA-Binding Proteins in Acute Leukemias

Acute leukemias are genetic diseases caused by translocations or mutations, which dysregulate hematopoiesis towards malignant transformation. However, the molecular mode of action is highly versatile and ranges from direct transcriptional to post-transcriptional control, which includes RNA-binding proteins (RBPs) as crucial regulators of cell fate. RBPs coordinate RNA dynamics, including subcellular localization, translational efficiency and metabolism, by binding to their target messenger RNAs (mRNAs), thereby controlling the expression of the encoded proteins. In view of the growing interest in these regulators, this review summarizes recent research regarding the most influential RBPs relevant in acute leukemias in particular. The reported RBPs, either dysregulated or as components of fusion proteins, are described with respect to their functional domains, the pathways they affect, and clinical aspects associated with their dysregulation or altered functions.

Bi-phased regulation of the post-transcriptional inflammatory response by Tristetraprolin levels

AU-rich elements (AREs) are cis-acting instability and translation inhibition elements that are present in the 3ʹUTR of most inducible inflammatory mRNAs such as TNF and Cxcl2. mRNAs that contain AREs are, by default, repressed and only transiently expressed in response to stimuli. They are targeted by the inducible RNA-binding protein Tristetraprolin (TTP) which blocks their translation and facilitates their decay, thereby contributing to the quick termination of their expression. The exogenous over-expression of TTP in HEK293 cells can unexpectedly lead to the upregulation and extended expression of a nanoLuciferase reporter that contains the ARE of TNF. Here we show that, a moderate downregulation of the highly expressed endogenous TTP after LPS induction by siRNA in macrophages can lead to a reduction in the release of TNF and Cxcl2. We propose that, in contrast to their canonical function, very high levels of induced TTP at the onset of the inflammatory response can enhance the expression of ARE-mRNAs at the post-transcriptional level, independently of phosphorylation status. As the inflammatory response progresses, TTP levels diminish but they continuously regain their ability to reduce the expression of ARE-mRNAs to reach a turning point of ‘optimal TTP level’ with a maximum ability to repress ARE-mRNA expression. Below this level, a further reduction in TTP levels now leads to the loss of canonical-TTP function resulting in increased ARE-mRNA expression. These novel findings should contribute to the understanding of feedback loops that control the kinetics of the inflammatory response.

The RNA binding protein tristetraprolin down-regulates autophagy in lung adenocarcinoma cells

Tristetraprolin (TTP) is the most well-known member of RNA-binding zinc-finger protein that play a significant role in accelerating mRNA decay. Increasingly studies have reported that TTP was functioned as a tumor suppressor gene in several types of carcinomas, while its underlying mechanism is not clear yet. In the current study, we found that TTP overexpression decreased cell proliferation and increased cell death in lung adenocarcinoma cells, with the cell cycle arrest at the S phase. Remarkably, instead of inducing cell apoptosis directly, TTP overexpression alters cell autophagy. Our studies demonstrate that TTP overexpression has no effect on apoptosis related genes, but decreases the expression of autophagy-related genes, including Beclin 1 and LC3II. The level of autophagy flux assessed by infection with the mGFP-RFP-LC3 adenovirus construction has been blocked by TTP overexpression. Moreover, the autophagic vacuoles number detected by transmission electron microscopy decreased with TTP expression up-regulation. Our results indicate, for the first time, that TTP suppresses cell proliferation and increases cell death through cell autophagy pathway in lung cancer cells. Our study provides a new angle of view for TTP function as a tumor suppressor which could be targeted in tumor treatment.

ZFP36L1 and ZFP36L2 inhibit cell proliferation in a cyclin D-dependent and p53-independent manner

ZFP36 family members include ZFP36, ZFP36L1, and ZFP36L2, which belong to CCCH-type zinc finger proteins with two tandem zinc finger (TZF) regions. Whether ZFP36L1 and ZFP36L2 have antiproliferative activities similar to that of ZFP36 is unclear. In this study, when ZFP36L1 or ZFP36L2 was overexpressed in T-REx-293 cells, cell proliferation was dramatically inhibited and the cell cycle was arrested at the G1 phase. The levels of cell-cycle-related proteins, including cyclin B, cyclin D, cyclin A, and p21, decreased; however, p53 increased in ZFP36L1-or ZFP36L2-overexpressing T-REx-293 cells. Forced expression of ZFP36L1 or ZFP36L2 also inhibited cell proliferation and cyclin D gene expression in three human colorectal cancer cell lines: HCT116 p53^+/+, HCT116 p53^−/−, and SW620 (mutated p53) cells. However, it increased p53 and p21 expression only in HCT116 p53^+/+ cells. Knockdown of ZFP36L1 or ZFP36L2 increased cell proliferation and cyclin D expression; furthermore, the mutation of the TZF of ZFP36L1 or ZFP36L2 caused them to lose their antiproliferative ability, to the extent that they could not inhibit cyclin D expression in these three cell lines. The results indicated that ZFP36L1 and ZFP36L2 play a negative role in cell proliferation; the underlying mechanisms might be mediated through a cyclin D-dependent and p53-independent pathway.

The RNA-binding protein Tristetraprolin (TTP) is a critical negative regulator of the NLRP3 inflammasome

The NLRP3 inflammasome is a central regulator of inflammation in many common diseases, including atherosclerosis and type 2 diabetes, driving the production of pro-inflammatory mediators such as IL-1β and IL-18. Due to its function as an inflammatory gatekeeper, expression and activation of NLRP3 need to be tightly regulated. In this study, we highlight novel post-transcriptional mechanisms that can modulate NLRP3 expression. We have identified the RNA-binding protein Tristetraprolin (TTP) as a negative regulator of NLRP3 in human macrophages. TTP targets AU-rich elements in the NLRP3 3'-untranslated region (UTR) and represses NLRP3 expression. Knocking down TTP in primary macrophages leads to an increased induction of NLRP3 by LPS, which is also accompanied by increased Caspase-1 and IL-1β cleavage upon NLRP3, but not AIM2 or NLRC4 inflammasome activation. Furthermore, we found that human NLRP3 can be alternatively polyadenylated, producing a short 3'-UTR isoform that excludes regulatory elements, including the TTP- and miRNA-223-binding sites. Because TTP also represses IL-1β expression, it is a dual inhibitor of the IL-1β system, regulating expression of the cytokine and the upstream controller NLRP3.

Treatment with MAPKAP2 (MK2) inhibitor and DNA methylation inhibitor, 5-aza dC, synergistically triggers apoptosis in hepatocellular carcinoma (HCC) via tristetraprolin (TTP)

Over 100 putative driver genes that are associated with multiple recurrently altered pathways were detected in hepatocellular carcinoma (HCC), suggesting that multiple pathways will need to be inhibited for any therapeutic method to be effective. In this context, functional modification of the RNA regulating protein, tristetraprolin (TTP) that regulates approximately 2500 genes represents a promising strategy in HCC therapy. Since overexpression of TTP induces cell death in all cell types, it would be useful to target the regulator of TTP. In this study, we applied an inhibitor to MAPKAP2 (MK2) that suppresses TTP function. Importantly, cBIOportal for HCC genomics shows that expression level of the MK2 gene correlates with clinical outcome of HCC. We show that upon treatment with MK2 inhibitor, all 5 HCC cell lines, namely HepG2, Huh7, Hep3B, HLE and HLF, reduced cell growth, especially HepG2 and Hep3B cells underwent apoptosis. Simultaneously, TTP target genes such as c-Myc, IER3 or AKT-1 were downregulated. Depletion of the TTP gene rescued cells from apoptosis and restored the TTP-target mRNA expression in the presence of MK2 inhibitor. Furthermore, MK2 was activated in primary HCC that express TTP at high level. The TTP gene was induced upon treatment with DNA methylation inhibitor, 5-aza dC or interferon in three other cell lines, Huh7, HLE or HLF. Upon treatment with MK2 inhibitor and 5-aza dC or interferon these cells underwent apoptosis. The depletion of TTP in these cells partially rescued them from apoptosis, suggesting that the MK2/TTP pathway plays a role in proliferation and maintenance of HCCs. Notably, under the same conditions human hepatocyte cells (THLE-2) did not undergo apoptosis. These data also suggest that MK2 inhibitor with 5-aza dC or interferon may be a useful tool for therapy against HCC.

The RNA-binding protein TTP is a global post-transcriptional regulator of feedback control in inflammation

RNA-binding proteins (RBPs) facilitate post-transcriptional control of eukaryotic gene expression at multiple levels. The RBP tristetraprolin (TTP/Zfp36) is a signal-induced phosphorylated anti-inflammatory protein guiding unstable mRNAs of pro-inflammatory proteins for degradation and preventing translation. Using iCLIP, we have identified numerous mRNA targets bound by wild-type TTP and by a non-MK2-phosphorylatable TTP mutant (TTP-AA) in 1 h LPS-stimulated macrophages and correlated their interaction with TTP to changes at the level of mRNA abundance and translation in a transcriptome-wide manner. The close similarity of the transcriptomes of TTP-deficient and TTP-expressing macrophages upon short LPS stimulation suggested an effective inactivation of TTP by MK2, whereas retained RNA-binding capacity of TTP-AA to 3′UTRs caused profound changes in the transcriptome and translatome, altered NF-κB-activation and induced cell death. Increased TTP binding to the 3′UTR of feedback inhibitor mRNAs, such as Ier3, Dusp1 or Tnfaip3, in the absence of MK2-dependent TTP neutralization resulted in a strong reduction of their protein synthesis contributing to the deregulation of the NF-κB-signaling pathway. Taken together, our study uncovers a role of TTP as a suppressor of feedback inhibitors of inflammation and highlights the importance of fine-tuned TTP activity-regulation by MK2 in order to control the pro-inflammatory response.

Control of pro-angiogenic cytokine mRNA half-life in cancer: the role of AU-rich elements and associated proteins

Control of mRNA half-life plays a central role in normal development and disease. Several pathological conditions, such as inflammation and cancer, tightly correlate with deregulation in mRNA stability of pro-inflammatory genes. Among these, pro-angiogenesis cytokines, which play a crucial role in the formation of new blood vessels, normally show rapid mRNA decay patterns. The mRNA half-life of these genes appears to be regulated by mRNA-binding proteins that interact with AU-rich elements (AREs) in the 3'-untranslated region of mRNAs. Some of these RNA-binding proteins, such as tristetraprolin (TTP), ARE RNA-binding protein 1, and KH-type splicing regulatory protein, normally promote mRNA degradation. Conversely, other proteins, such as embryonic lethal abnormal vision-like protein 1 (HuR) and polyadenylate-binding protein-interacting protein 2, act as antagonists, stabilizing the mRNA. The steady state levels of mRNA-binding proteins and their relative ratio is often perturbed in human cancers and associated with invasion and aggressiveness. Compelling evidence also suggests that underexpression of TTP and overexpression of HuR may be a useful prognostic and predictive marker in breast, colon, prostate, and brain cancers, indicating a potential therapeutic approach for these tumors. In this review, we summarize the main mechanisms involved in the regulation of mRNA decay of pro-angiogenesis cytokines in different cancers and discuss the interactions between the AU-rich-binding proteins and their mRNA targets.

RNase L attenuates mitogen-stimulated gene expression via transcriptional and post-transcriptional mechanisms to limit the proliferative response

The cellular response to mitogens is tightly regulated via transcriptional and post-transcriptional mechanisms to rapidly induce genes that promote proliferation and efficiently attenuate their expression to prevent malignant growth. RNase L is an endoribonuclease that mediates diverse antiproliferative activities, and tristetraprolin (TTP) is a mitogen-induced RNA-binding protein that directs the decay of proliferation-stimulatory mRNAs. In light of their roles as endogenous proliferative constraints, we examined the mechanisms and functional interactions of RNase L and TTP to attenuate a mitogenic response. Mitogen stimulation of RNase L-deficient cells significantly increased TTP transcription and the induction of other mitogen-induced mRNAs. This regulation corresponded with elevated expression of serum-response factor (SRF), a master regulator of mitogen-induced transcription. RNase L destabilized the SRF transcript and formed a complex with SRF mRNA in cells providing a mechanism by which RNase L down-regulates SRF-induced genes. TTP and RNase L proteins interacted in cells suggesting that RNase L is directed to cleave TTP-bound RNAs as a mechanism of substrate specificity. Consistent with their concerted function in RNA turnover, the absence of either RNase L or TTP stabilized SRF mRNA, and a subset of established TTP targets was also regulated by RNase L. RNase L deficiency enhanced mitogen-induced proliferation demonstrating its functional role in limiting the mitogenic response. Our findings support a model of feedback regulation in which RNase L and TTP target SRF mRNA and SRF-induced transcripts. Accordingly, meta-analysis revealed an enrichment of RNase L and TTP targets among SRF-regulated genes suggesting that the RNase L/TTP axis represents a viable target to inhibit SRF-driven proliferation in neoplastic diseases.

Post-transcriptional regulation of BCL2 mRNA by the RNA-binding protein ZFP36L1 in malignant B cells

The human ZFP36 zinc finger protein family consists of ZFP36, ZFP36L1, and ZFP36L2. These proteins regulate various cellular processes, including cell apoptosis, by binding to adenine uridine rich elements in the 3' untranslated regions of sets of target mRNAs to promote their degradation. The pro-apoptotic and other functions of ZFP36 family members have been implicated in the pathogenesis of lymphoid malignancies. To identify candidate mRNAs that are targeted in the pro-apoptotic response by ZFP36L1, we reverse-engineered a gene regulatory network for all three ZFP36 family members using the 'maximum information coefficient' (MIC) for target gene inference on a large microarray gene expression dataset representing cells of diverse histological origin. Of the three inferred ZFP36L1 mRNA targets that were identified, we focussed on experimental validation of mRNA for the pro-survival protein, BCL2, as a target for ZFP36L1. RNA electrophoretic mobility shift assay experiments revealed that ZFP36L1 interacted with the BCL2 adenine uridine rich element. In murine BCL1 leukemia cells stably transduced with a ZFP36L1 ShRNA lentiviral construct, BCL2 mRNA degradation was significantly delayed compared to control lentiviral expressing cells and ZFP36L1 knockdown in different cell types (BCL1, ACHN, Ramos), resulted in increased levels of BCL2 mRNA levels compared to control cells. 3' untranslated region luciferase reporter assays in HEK293T cells showed that wild type but not zinc finger mutant ZFP36L1 protein was able to downregulate a BCL2 construct containing the BCL2 adenine uridine rich element and removal of the adenine uridine rich core from the BCL2 3' untranslated region in the reporter construct significantly reduced the ability of ZFP36L1 to mediate this effect. Taken together, our data are consistent with ZFP36L1 interacting with and mediating degradation of BCL2 mRNA as an important target through which ZFP36L1 mediates its pro-apoptotic effects in malignant B-cells.

Transforming growth factor β regulates P-body formation through induction of the mRNA decay factor tristetraprolin

Transforming growth factor β (TGF-β) is a potent growth regulator and tumor suppressor in normal intestinal epithelium. Likewise, epithelial cell growth is controlled by rapid decay of growth-related mRNAs mediated through 3' untranslated region (UTR) AU-rich element (ARE) motifs. We demonstrate that treatment of nontransformed intestinal epithelial cells with TGF-β inhibited ARE-mRNA expression. This effect of TGF-β was promoted through increased assembly of cytoplasmic RNA processing (P) bodies where ARE-mRNA localization was observed. P-body formation was dependent on TGF-β/Smad signaling, as Smad3 deletion abrogated P-body formation. In concert with increased P-body formation, TGF-β induced expression of the ARE-binding protein tristetraprolin (TTP), which colocalized to P bodies. TTP expression was necessary for TGF-β-dependent P-body formation and promoted growth inhibition by TGF-β. The significance of this was observed in vivo, where colonic epithelium deficient in TGF-β/Smad signaling or TTP expression showed attenuated P-body levels. These results provide new insight into TGF-β's antiproliferative properties and identify TGF-β as a novel mRNA stability regulator in intestinal epithelium through its ability to promote TTP expression and subsequent P-body formation.

Tristetraprolin expression and microRNA-mediated regulation during simian immunodeficiency virus infection of the central nervous system

BACKGROUND

The RNA-binding protein tristetraprolin (TTP) participates in normal post-transcriptional control of cytokine and chemokine gene expression, dysregulation of which contributes to the HIV-associated neurocognitive disorders. Transcriptional and post-transcriptional regulation of TTP has been described, including regulation by microRNA-29a. In the simian immunodeficiency virus (SIV) model of HIV CNS disease, control of cytokine/chemokine expression coincides with the end of acute phase infection. This control is lost during progression to disease. In this study, we assessed TTP regulation and association with cytokine regulation in the brain during SIV infection.

RESULTS

Quantitation of TTP expression over the course of SIV infection revealed downregulation of TTP during acute infection, maintenance of relatively low levels during asymptomatic phase, and increased expression only during late-stage CNS disease, particularly in association with severe disease. The ability of miR-29a to regulate TTP was confirmed, and evidence for additional miRNA targeters of TTP was found. However, increased miR-29a expression in brain was not found to be significantly negatively correlated with TTP. Similarly, increased TTP during late-stage disease was not associated with lower cytokine expression.

CONCLUSIONS

TTP expression is regulated during SIV infection of the CNS. The lack of significant negative correlation of miR-29a and TTP expression levels suggests that while miR-29a may contribute to TTP regulation, additional factors are involved. Reduced TTP expression during acute infection is consistent with increased cytokine production during this phase of infection, but the increases in TTP observed during late-stage infection were insufficient to halt runaway cytokine levels. While antisense inhibitors of the post-transcriptional targeters of TTP identified here could conceivably be used further to augment TTP regulation of cytokines, it is possible that high levels of TTP are undesirable. Additional research is needed to characterize members of the miRNA/TTP/cytokine regulatory network and identify nodes that may be best targeted therapeutically to ameliorate the effects of chronic inflammation in retrovirus-associated CNS disease.

ZFP36L1 negatively regulates plasmacytoid differentiation of BCL1 cells by targeting BLIMP1 mRNA

The ZFP36/Tis11 family of zinc-finger proteins regulate cellular processes by binding to adenine uridine rich elements in the 3′ untranslated regions of various mRNAs and promoting their degradation. We show here that ZFP36L1 expression is largely extinguished during the transition from B cells to plasma cells, in a reciprocal pattern to that of ZFP36 and the plasma cell transcription factor, BLIMP1. Enforced expression of ZFP36L1 in the mouse BCL1 cell line blocked cytokine-induced differentiation while shRNA-mediated knock-down enhanced differentiation. Reconstruction of regulatory networks from microarray gene expression data using the ARACNe algorithm identified candidate mRNA targets for ZFP36L1 including BLIMP1. Genes that displayed down-regulation in plasma cells were significantly over-represented (P = <0.0001) in a set of previously validated ZFP36 targets suggesting that ZFP36L1 and ZFP36 target distinct sets of mRNAs during plasmacytoid differentiation. ShRNA-mediated knock-down of ZFP36L1 in BCL1 cells led to an increase in levels of BLIMP1 mRNA and protein, but not for mRNAs of other transcription factors that regulate plasmacytoid differentiation (xbp1, irf4, bcl6). Finally, ZFP36L1 significantly reduced the activity of a BLIMP1 3′ untranslated region-driven luciferase reporter. Taken together, these findings suggest that ZFP36L1 negatively regulates plasmacytoid differentiation, at least in part, by targeting the expression of BLIMP1.

Tristetraprolin: roles in cancer and senescence

Cancer and senescence are both complex transformative processes that dramatically alter many features of cell physiology and their interactions with surrounding tissues. Developing the wide range of cellular features characteristic of these conditions requires profound alterations in global gene expression patterns, which can be achieved by suppressing, activating, or uncoupling cellular gene regulatory pathways. Many genes associated with the initiation and development of tumors are regulated at the level of mRNA decay, frequently through the activity of AU-rich mRNA-destabilizing elements (AREs) located in their 3'-untranslated regions. As such, cellular factors that recognize and control the decay of ARE-containing mRNAs can influence tumorigenic or senescent phenotypes mediated by products of these transcripts. In this review, we discuss evidence showing how suppressed expression and/or activity of the ARE-binding protein tristetraprolin (TTP) can contribute to these processes. Next, we outline current findings linking TTP suppression to exacerbation of individual tumorigenic phenotypes, and the roles of specific TTP substrate mRNAs in mediating these effects. Finally, we survey potential mechanisms that cells may employ to suppress TTP expression in cancer, and propose potential diagnostic and therapeutic strategies that may exploit the relationship between TTP expression and tumor progression or senescence.

ZFP36 expression impairs glioblastoma cell lines viability and invasiveness by targeting multiple signal transduction pathways

RNA binding proteins belonging to the TIS11/TTP gene family regulate the stability of multiple targets. Their inactivation or deregulated expression has recently been related to cancer, and it has been suggested that they are capable of displaying tumor suppressor activities. Here we describe three new targets of ZFP36 (PIM-1, PIM-3 and XIAP) and show by different approaches that its ectopic expression is capable of impairing glioblastoma cell lines viability and invasiveness by interfering with different transduction pathways. Moreover, we provide evidence that compounds capable of inducing the expression of TIS11/TTP genes determine a comparable biological effect on the same cell contexts.

Drosophila eyes absent is a novel mRNA target of the tristetraprolin (TTP) protein DTIS11

The Tristetraprolin (TTP) protein family includes four mammalian members (TTP, TIS11b, TIS11d, and ZFP36L3), but only one in Drosophila melanogaster (DTIS11). These proteins bind target mRNAs with AU-rich elements (AREs) via two C3H zinc finger domains and destabilize the mRNAs. We found that overexpression of mouse TIS11b or DTIS11 in the Drosophila retina dramatically reduced eye size, similar to the phenotype of eyes absent (eya) mutants. The eya transcript is one of many ARE-containing mRNAs in Drosophila. We showed that TIS11b reduced levels of eya mRNA in vivo. In addition, overexpression of Eya rescued the TIS11b overexpression phenotype. RNA pull-down and luciferase reporter analyses demonstrated that the DTIS11 RNA-binding domain is required for DTIS11 to bind the eya 3' UTR and reduce levels of eya mRNA. Moreover, ectopic expression of DTIS11 in Drosophila S2 cells decreased levels of eya mRNA and reduced cell viability. Consistent with these results, TTP proteins overexpressed in MCF7 human breast cancer cells were associated with eya homologue 2 (EYA2) mRNA, and caused a decrease in EYA2 mRNA stability and cell viability. Our results suggest that eya mRNA is a target of TTP proteins, and that downregulation of EYA by TTP may lead to reduced cell viability in Drosophila and human cells.

Cullin 4B is recruited to tristetraprolin-containing messenger ribonucleoproteins and regulates TNF-α mRNA polysome loading

TNF-α is a central mediator of inflammation and critical for host response to infection and injury. TNF-α biosynthesis is controlled by transcriptional and posttranscriptional mechanisms allowing for rapid, transient production. Tristetraprolin (TTP) is an AU-rich element binding protein that regulates the stability of the TNF-α mRNA. Using a screen to identify TTP-interacting proteins, we identified Cullin 4B (Cul4B), a scaffolding component of the Cullin ring finger ligase family of ubiquitin E3 ligases. Short hairpin RNA knockdown of Cul4B results in a significant reduction in TNF-α protein and mRNA in LPS-stimulated mouse macrophage RAW264.7 cells as well as a reduction in TTP protein. TNF-α message t(1/2) was reduced from 69 to 33 min in LPS-stimulated cells. TNF-3' untranslated region luciferase assays utilizing wild-type and mutant TTP-AA (S52A, S178A) indicate that TTP function is enhanced in Cul4B short hairpin RNA cells. Importantly, the fold induction of TNF-α mRNA polysome loading in response to LPS stimulation is reduced by Cul4B knockdown. Cul4B is present on the polysomes and colocalizes with TTP to exosomes and processing bodies, which are sites of mRNA decay. We conclude that Cul4B licenses the TTP-containing TNF-α messenger ribonucleoprotein for loading onto polysomes, and reduction of Cul4B expression shunts the messenger ribonucleoproteins into the degradative pathway.

The role of tristetraprolin in cancer and inflammation

Messenger RNA decay is a critical mechanism to control the expression of many inflammation- and cancer-associated genes. These transcripts are targeted for rapid degradation through AU-rich element (ARE) motifs present in the mRNA 3' untranslated region (3'UTR). Tristetraprolin (TTP) is an RNA-binding protein that plays a significant role in regulating the expression of ARE-containing mRNAs. Through its ability to bind AREs and target the bound mRNA for rapid degradation, TTP can limit the expression of a number of critical genes frequently overexpressed in inflammation and cancer. Regulation of TTP occurs on multiple levels through cellular signaling events to control transcription, mRNA turnover, phosphorylation status, cellular localization, association with other proteins, and proteosomal degradation, all of which impact TTP's ability to promote ARE-mediated mRNA decay along with decay-independent functions of TTP. This review summarizes the current understanding of post-transcriptional regulation of ARE-containing gene expression by TTP and discusses its role in maintaining homeostasis and the pathological consequences of losing TTP expression.

Novel phosphorylation-dependent ubiquitination of tristetraprolin by mitogen-activated protein kinase/extracellular signal-regulated kinase kinase kinase 1 (MEKK1) and tumor necrosis factor receptor-associated factor 2 (TRAF2)

Acute versus chronic inflammation is controlled by the accurate activation and regulation of interdependent signaling cascades. TNF-receptor 1 engagement concomitantly activates NF-κB and JNK signaling. The correctly timed activation of these pathways is the key to account for the balance between NF-κB-mediated cell survival and cell death, the latter fostered by prolonged JNK activation. Tristetraprolin (TTP), initially described as an mRNA destabilizing protein, acts as negative feedback regulator of the inflammatory response: it destabilizes cytokine-mRNAs but also acts as an NF-κB inhibitor by interfering with the p65/RelA nuclear import pathway. Our biochemical studies provide evidence that TTP contributes to the NF-κB/JNK balance. We find that the MAP 3-kinase MEKK1 acts as a novel TTP kinase that, together with the TNF receptor-associated factor 2 (TRAF2), constitutes not only a main determinate of the NF-κB-JNK cross-talk but also facilitates "TTP hypermodification": MEKK1 triggers TTP phosphorylation as prerequisite for its Lys-63-linked, TRAF2-mediated ubiquitination. Consequently, TTP no longer affects NF-κB activity but promotes the activation of JNK. Based on our data, we suggest a model where upon TNFα induction, TTP transits a hypo- to hypermodified state, thereby contributing to the molecular regulation of NF-κB versus JNK signaling cascades.

Constitutive ERK activity induces downregulation of tristetraprolin, a major protein controlling interleukin8/CXCL8 mRNA stability in melanoma cells

Most melanoma cells are characterized by the V600E mutation in B-Raf kinase. This mutation leads to increased expression of interleukin (CXCL8), which plays a key role in cell growth and angiogenesis. Thus CXCL8 appears to be an interesting therapeutic target. Hence, we performed vaccination of mice with GST-CXCL8, which results in a reduced incidence of syngenic B16 melanoma cell xenograft tumors. We next addressed the molecular mechanisms responsible for aberrant CXCL8 expression in melanoma. The CXCL8 mRNA contains multiples AU-rich sequences (AREs) that modulate mRNA stability through the binding of tristetraprolin (TTP). Melanoma cell lines express very low TTP levels. We therefore hypothesized that the very low endogenous levels of TTP present in different melanoma cell lines might be responsible for the relative stability of CXCL8 mRNAs. We show that TTP is actively degraded by the proteasome and that extracellular-regulated kinase inhibition results in TTP accumulation. Conditional expression of TTP in A375 melanoma cells leads to CXCL8 mRNA destabilization via its 3' untranslated regions (3'-UTR), and TTP overexpression reduces its production. In contrast, downregulation of TTP by short hairpin RNA results in upregulation of CXCL8 mRNA. Maintaining high TTP levels in melanoma cells decreases cell proliferation and autophagy and induces apoptosis. Sorafenib, a therapeutic agent targeting Raf kinases, decreases CXCL8 expression in melanoma cells through reexpression of TTP. We conclude that loss of TTP represents a key event in the establishment of melanomas through constitutive expression of CXCL8, which constitutes a potent therapeutic target.

Not1 mediates recruitment of the deadenylase Caf1 to mRNAs targeted for degradation by tristetraprolin

The carbon catabolite repressor protein 4 (Ccr4)–Negative on TATA (Not) complex controls gene expression at two levels. In the nucleus, it regulates the basal transcription machinery, nuclear receptor-mediated transcription and histone modifications. In the cytoplasm, the complex is required for messenger RNA (mRNA) turnover through its two associated deadenylases, Ccr4 and Caf1. Not1 is the largest protein of the Ccr4–Not complex and serves as a scaffold for other subunits of the complex. Here, we provide evidence that human Not1 in the cytoplasm associates with the C-terminal domain of tristetraprolin (TTP), an RNA binding protein that mediates rapid degradation of mRNAs containing AU-rich elements (AREs). Not1 shows extensive interaction through its central region with TTP, whereas binding of Caf1 is restricted to a smaller central domain within Not1. Importantly, Not1 is required for the rapid decay of ARE-mRNAs, and TTP can recruit the Caf1 deadenylase only in presence of Not1. Thus, cytoplasmic Not1 provides a platform that allows a specific RNA binding protein to recruit the Caf1 deadenylase and thereby trigger decay of its target mRNAs.

ZFP36L1 negatively regulates erythroid differentiation of CD34+ hematopoietic stem cells by interfering with the Stat5b pathway

ZFP36L1 is a member of a family of CCCH tandem zinc finger proteins (TTP family) able to bind to AU-rich elements in the 3'-untranslated region of mRNAs, thereby triggering their degradation. The present study suggests that such mechanism is used during hematopoiesis to regulate differentiation by posttranscriptionally modulating the expression of specific target genes. In particular, it demonstrates that ZFP36L1 negatively regulates erythroid differentiation by directly binding the 3' untranslated region of Stat5b encoding mRNA. Stat5b down-regulation obtained by ZFP36L1 overexpression results, in human hematopoietic progenitors, in a drastic decrease of erythroid colonies formation. These observations have been confirmed by silencing experiments targeting Stat5b and by treating hematopoietic stem/progenitor cells with drugs able to induce ZFP36L1 expression. Moreover, this study shows that different members of ZFP36L1 family act redundantly, because cooverexpression of ZFP36L1 and family member ZFP36 determines a cumulative effect on Stat5b down-regulation. This work describes a mechanism underlying ZFP36L1 capability to regulate hematopoietic differentiation and suggests a new target for the therapy of hematopoietic diseases involving Stat5b/JAK2 pathway, such as chronic myeloproliferative disorders.

ZFP36L1 is regulated by growth factors and cytokines in keratinocytes and influences their VEGF production

Keratinocyte-derived growth factors and cytokines play an important role in epidermal homeostasis and particularly in cutaneous wound repair. Thus, we analyzed a potential role of the ZFP36/tristetraprolin family of zinc finger proteins, which are targets of these factors, but also regulate their production, in keratinocytes. We show that expression of ZFP36, ZFP36L1, and ZFP36L2 is induced by a broad variety of growth factors and cytokines, and by scratch wounding. Since ZFP36L1 is a modulator of vascular endothelium growth factor (VEGF) mRNA stability, we subsequently used siRNA technology to inhibit ZFP36L1 gene expression. Notably, this treatment resulted in prolonged maintenance of elevated VEGF levels in HaCaT keratinocytes upon epidermal growth factor stimulation of these cells. Taken together, our results suggest an important role of ZFP36L1 in wound healing.

Functional switching of TGF-beta1 signaling in liver cancer via epigenetic modulation of a single CpG site in TTP promoter

BACKGROUND & AIMS

Acquisition of resistance to the antiproliferative effect of transforming growth factor (TGF)-beta1 is crucial for the malignant progression of cancers. In this study, we sought to determine whether deregulated expression of tristetrapolin (TTP), a negative posttranscriptional regulator of c-Myc, confers resistance to the antiproliferative effects of TGF-beta1 on liver cancer cells.

METHODS

The epigenetics of TTP promoter regulation and its effects on TGF-beta1 signaling were examined in hepatocellular carcinoma (HCC) cell lines and patient tissues.

RESULTS

TTP was down-regulated in HCC cell lines (10/11), compared with normal liver, as well as in tumor tissues (19/24) from paired HCC specimens. Methylation of a specific single CpG site located within the TGF-beta1-responsive region (TRR) of the TTP promoter was significantly associated with TTP down-regulation in both HCC cell lines and tumor tissues (r = -0.606383, P < .001). The singly methylated CpG site was specifically bound by a transcriptional repressor complex consisting of MECP2/c-Ski/DNMT3A and abolished the TGF-beta1-induced as well as basal-level expression of TTP. The epigenetic inactivation of TTP led to an increased half-life of c-Myc mRNA and blocked the cytostatic effect of TGF-beta1. Statistically significant correlations were observed between the single CpG site methylation and expression levels of TTP or c-Myc in clinical samples of HCC.

CONCLUSIONS

Abrogation of the post-transcriptional regulation of c-Myc via methylation of a specific single CpG site in the TTP promoter presents a novel mechanism for the gain of selective resistance to the antiproliferative signaling of TGF-beta1 in HCC.

The mRNA decay factor tristetraprolin (TTP) induces senescence in human papillomavirus-transformed cervical cancer cells by targeting E6-AP ubiquitin ligase

The RNA-binding protein tristetraprolin (TTP) regulates expression of many cancer-associated and proinflammatory factors through binding AU-rich elements (ARE) in the 3'-untranslated region (3'UTR) and facilitating rapid mRNA decay. Here we report on the ability of TTP to act in an anti-proliferative capacity in HPV18-positive HeLa cells by inducing senescence. HeLa cells maintain a dormant p53 pathway and elevated telomerase activity resulting from HPV-mediated transformation, whereas TTP expression counteracted this effect by stabilizing p53 protein and inhibiting hTERT expression. Presence of TTP did not alter E6 and E7 viral mRNA levels indicating that these are not TTP targets. It was found that TTP promoted rapid mRNA decay of the cellular ubiquitin ligase E6-associated protein (E6-AP). RNA-binding studies demonstrated TTP and E6-AP mRNA interaction and deletion of the E6-AP mRNA ARE-containing 3'UTR imparts resistance to TTP-mediated downregulation. Similar results were obtained with high-risk HPV16-positive cells that employ the E6-AP pathway to control p53 and hTERT levels. Furthermore, loss of TTP expression was consistently observed in cervical cancer tissue compared to normal tissue. These findings demonstrate the ability of TTP to act as a tumor suppressor by inhibiting the E6-AP pathway and indicate TTP loss to be a critical event during HPV-mediated carcinogenesis.

RNA-destabilizing factor tristetraprolin negatively regulates NF-kappaB signaling

Tristetraprolin (TTP) is a CCCH zinc finger-containing protein that destabilizes mRNA by binding to an AU-rich element. Mice deficient in TTP develop a severe inflammatory syndrome mainly because of overproduction of tumor necrosis factor alpha. We report here that TTP also negatively regulates NF-kappaB signaling at the transcriptional corepressor level, by which it may repress inflammatory gene transcription. TTP expression inhibited NF-kappaB-dependent transcription. However, overexpression of TTP did not affect reporter mRNA stability. Instead, TTP functioned as a corepressor of p65/NF-kappaB. In support of this concept, we found that TTP physically interacted with the p65 subunit of NF-kappaB and was also associated with HDAC1, -3, and -7 in vivo. Treatment with histone deacetylase inhibitors or small interfering RNA induced HDAC1 or HDAC3 knockdown completely or partly abolished the inhibitory activity of TTP on NF-kappaB reporter activation. Consistently, chromatin immunoprecipitation showed decreased recruitment of HDAC1 and increased recruitment of CREB-binding protein on the Mcp-1 promoter in TTP(-/-) cells compared with wild-type cells. Moreover, overexpression of TTP blocked CREB-binding protein-induced acetylation of p65/NF-kappaB. Taken together, these data suggest that TTP may also function in vivo as a modulator in suppressing the transcriptional activity of NF-kappaB.

TIS11 family proteins and their roles in posttranscriptional gene regulation

Posttranscriptional regulation of gene expression of mRNAs containing adenine-uridine rich elements (AREs) in their 3′ untranslated regions is mediated by a number of different proteins that interact with these elements to either stabilise or destabilise them. The present review concerns the TPA-inducible sequence 11 (TIS11) protein family, a small family of proteins, that appears to interact with ARE-containing mRNAs and promote their degradation. This family of proteins has been extensively studied in the past decade. Studies have focussed on determining their biochemical functions, identifying their target mRNAs, and determining their roles in cell functions and diseases.

The mRNA-destabilizing protein tristetraprolin is suppressed in many cancers, altering tumorigenic phenotypes and patient prognosis

AU-rich element-binding proteins (ARE-BP) regulate the stability and/or translational efficiency of mRNAs containing cognate binding sites. Many targeted transcripts encode factors that control processes such as cell division, apoptosis, and angiogenesis, suggesting that dysregulated ARE-BP expression could dramatically influence oncogenic phenotypes. Using several approaches, we evaluated the expression of four well-characterized ARE-BPs across a variety of human neoplastic syndromes. AUF1, TIA-1, and HuR mRNAs were not systematically dysregulated in cancers; however, tristetraprolin mRNA levels were significantly decreased across many tumor types, including advanced cancers of the breast and prostate. Restoring tristetraprolin expression in an aggressive tumor cell line suppressed three key tumorgenic phenotypes: cell proliferation, resistance to proapoptotic stimuli, and expression of vascular endothelial growth factor mRNA. However, the cellular consequences of tristetraprolin expression varied across different cell models. Analyses of gene array data sets revealed that suppression of tristetraprolin expression is a negative prognostic indicator in breast cancer, because patients with low tumor tristetraprolin mRNA levels were more likely to present increased pathologic tumor grade, vascular endothelial growth factor expression, and mortality from recurrent disease. Collectively, these data establish that tristetraprolin expression is frequently suppressed in human cancers, which in turn can alter tumorigenic phenotypes that influence patient outcomes.

Inhibition of tristetraprolin deadenylation by poly(A) binding protein

Tristetraprolin (TTP) is the prototype for a family of RNA binding proteins that bind the tumor necrosis factor (TNF) messenger RNA AU-rich element (ARE), causing deadenylation of the TNF poly(A) tail, RNA decay, and silencing of TNF protein production. Using mass spectrometry sequencing we identified poly(A) binding proteins-1 and -4 (PABP1 and PABP4) in high abundance and good protein coverage from TTP immunoprecipitates. PABP1 significantly enhanced TNF ARE binding by RNA EMSA and prevented TTP-initiated deadenylation in an in vitro macrophage assay of TNF poly(A) stability. Neomycin inhibited TTP-promoted deadenylation at concentrations shown to inhibit the deadenylases poly(A) ribonuclease and CCR4. Stably transfected RAW264.7 macrophages overexpressing PABP1 do not oversecrete TNF; instead they upregulate TTP protein without increasing TNF protein production. The PABP1 inhibition of deadenylation initiated by TTP does not require the poly(A) binding regions in RRM1 and RRM2, suggesting a more complicated interaction than simple masking of the poly(A) tail from a 3'-exonuclease. Like TTP, PABP1 is a substrate for p38 MAP kinase. Finally, PABP1 stabilizes cotransfected TTP in 293T cells and prevents the decrease in TTP levels seen with p38 MAP kinase inhibition. These findings suggest several levels of functional antagonism between TTP and PABP1 that have implications for regulation of unstable mRNAs like TNF.

Tristetraprolin down-regulates interleukin-8 and vascular endothelial growth factor in malignant glioma cells

Malignant gliomas are highly aggressive tumors of the central nervous system that rely on production of growth factors for tumor progression. Vascular endothelial growth factor (VEGF), interleukin-8 (IL-8), and tumor necrosis factor-alpha, for example, are up-regulated in these tumors to promote angiogenesis and proliferation. RNA stability, mediated through adenine and uridine-rich elements (ARE) in the 3' untranslated region, is a critical control point for regulating these growth factors. RNA half-life is predominantly governed by a balance between stabilizing and destabilizing factors that bind to ARE. We have previously shown that the stabilizing factor HuR is overexpressed in malignant gliomas and linked to RNA stabilization of angiogenic growth factors. Here, we report that the destabilizing factor tristetraprolin (TTP) is also ubiquitously expressed in primary malignant glioma tissues and cell lines. In contrast to benign astrogliotic tissues, however, the protein was hyperphosphorylated, with evidence implicating the p38/mitogen-activated protein kinase (MAPK) pathway. Conditional overexpression of TTP as a transgene in malignant glioma cells led to RNA destabilization of IL-8 and VEGF and down-regulation of protein production. Analysis of in vivo RNA binding indicated a shift of mRNA toward ectopic TTP and away from endogenous HuR. This biochemical phenotype was associated with a decrease in cell proliferation, loss of cell viability, and apoptosis. We postulate that hyperphosphorylation of TTP via p38/MAPK promotes progression of malignant gliomas by negatively regulating its RNA destabilizing function.

Strong induction of the Tis11B gene in myogenic differentiation

TIS11B is a zinc-finger protein of the tristetraprolin (TTP) family. Using cDNA microarray analysis, we could identify the Tis11B gene based on its differential expression in myogenesis. Here, we demonstrate that expression of the Tis11B gene is strongly induced during differentiation of the murine myoblast cell line C2C12. By contrast, expression of Ttp itself was not induced in myogenesis. Pretreatment of the cells with the translation inhibitor cycloheximide demonstrated that Tis11B was a primary response gene in this process. In addition, pretreatment with the transcription inhibitor actinomycin D demonstrated that gene expression was regulated at the transcriptional level. Since specific inhibitors of p38 MAP kinase completely blocked Tis11B induction, we conclude that expression of the Tis11B gene is regulated at least in part by this signaling pathway which plays a central role in myogenesis. Induction of Tis11B expression was also observed in primary myoblasts isolated from two different mouse strains, indicating physiological relevance of our results. In addition, TIS11B might also be an important player during myogenic differentiation and regeneration in vivo, as we detected a marked decrease in expression in several muscle tissues of the dystrophic mdx mouse, a model for continuous muscle degeneration and regeneration. These data suggest that TIS11B is an important regulator of myogenesis.

Tristetraprolin inhibits Ras-dependent tumor vascularization by inducing vascular endothelial growth factor mRNA degradation

Vascular endothelial growth factor (VEGF) is one of the most important regulators of physiological and pathological angiogenesis. Constitutive activation of the extracellular signal-regulated kinase (ERK) pathway and overexpression of VEGF are common denominators of tumors from different origins. We have established a new link between these two fundamental observations converging on VEGF mRNA stability. In this complex phenomenon, tristetraprolin (TTP), an adenylate and uridylate-rich element-associated protein that binds to VEGF mRNA 3'-untranslated region, plays a key role by inducing VEGF mRNA degradation, thus maintaining basal VEGF mRNA amounts in normal cells. ERKs activation results in the accumulation of TTP mRNA. However, ERKs reduce the VEGF mRNA-destabilizing effect of TTP, leading to an increase in VEGF expression that favors the angiogenic switch. Moreover, TTP decreases RasVal12-dependent VEGF expression and development of vascularized tumors in nude mice. As a consequence, TTP might represent a novel antiangiogenic and antitumor agent acting through its destabilizing activity on VEGF mRNA. Determination of TTP and ERKs status would provide useful information for the evaluation of the angiogenic potential in human tumors.

Comparative expression of tristetraprolin (TTP) family member transcripts in normal human tissues and cancer cell lines

The tristetraprolin (TTP) family of tandem zinc finger proteins comprises three members in man and most other mammals, with a fourth expressed in rodents. In mice, gene disruption of TTP itself leads to a systemic inflammatory syndrome that is mediated in large part by over-expression of tumor necrosis factor alpha (TNF). This increased expression is secondary to stabilization of the TNF mRNA in the TTP KO mice, a finding that led to the characterization of TTP as an mRNA binding protein that can promote the removal of the poly(A) tail from selected mRNAs and facilitate their nucleolytic destruction. The other human family members behave similarly to TTP in over-expression studies of transfected cells, but gene disruption experiments have implicated them in different physiological processes. In the present study, we developed a real-time PCR assay for all three human family members that allowed for comparative measurements of all three family members in the same tissues and cells. We used this assay to quantitate expression levels of all three transcripts in a variety of normal human tissues, as well as in the ;;NCI 60", a well characterized panel of human tumor cell lines. Although studies in fibroblasts and macrophages derived from knockout mice have failed to demonstrate compensatory expression of the family members in terms of transcript levels, it remains possible that the different family members can function as ;;TTP equivalents" in certain physiological or pathological circumstances.

Tristetraprolin (TTP)-14-3-3 complex formation protects TTP from dephosphorylation by protein phosphatase 2a and stabilizes tumor necrosis factor-alpha mRNA

Tumor necrosis factor (TNF)-alpha is a major cytokine produced by alveolar macrophages in response to pathogen-associated molecular patterns such as lipopolysaccharide. TNF-alpha secretion is regulated at both transcriptional and post-transcriptional levels. Post-transcriptional regulation occurs by modulation of TNF-alpha mRNA stability via the binding of tristetraprolin (TTP) to the adenosine/uridine-rich elements found in the 3'-untranslated region of the TNF-alpha transcript. Phosphorylation plays important roles in modulating mRNA stability, because activation of p38 MAPK by lipopolysaccharide stabilizes TNF-alpha mRNA. We hypothesized that the protein phosphatase 2A (PP2A) regulates this signaling pathway. Our results show that inhibition of PP2A by okadaic acid or small interference RNA significantly enhanced the stability of TNF-alpha mRNA. This result was associated with increased phosphorylation of p38 MAPK and MAPK-activated kinase 2 (MK-2). PP2A inhibition increased TTP phosphorylation and enhanced complex formation with chaperone protein 14-3-3. TTP physically interacted with PP2A in transfected mammalian cells. A functional consequence of TTP-14-3-3 complex formation appeared to be protection of TTP from dephosphorylation by inhibition of the binding of PP2A to phosphorylated TTP. Mutation of the MK-2 phosphorylation sites of TTP did not influence TNF-alpha adenosine/uridine-rich element binding and did not alter the increased TNF-alpha 3'-untranslated region-dependent luciferase activity induced by PP2A-small interference RNA silencing. Our data indicate that, although phosphorylation stabilizes TNF-alpha mRNA, PP2A regulates the mRNA stability by modulating the phosphorylation state of members of the p38/MK-2/TTP pathway.

Myocardial injury modulates the innate immune system and changes myocardial sensitivity

OBJECTIVE

Transverse aortic constriction (TAC) results in a transient increase of proinflammatory cytokines, which return to baseline levels within 3 d. In contrast to cytokine baseline levels, the myocardium remains capable to respond even stronger to a new stimulus. As the molecular mechanisms for this phenomenon are unknown, we tested whether TAC modulates the innate immune system in mice and changes the inflammatory reaction to a new stimulus.

METHODS

Following 3 d of TAC or sham-operation procedure (SOP), LPS (20 mg/kg) or PBS (control) were administered intraperitoneal for 10 min as well as for 6 h. Hemodynamic parameters were recorded to measure the effects of TAC and LPS. After TAC/SOP alone CD14 expression was monitored and after additional 6 h of LPS/PBS the expression of CD14, TLR4 and proinflammatory cytokines were determined by western-blot, ELISA and RNase protection assay, respectively. Following TAC/SOP and 10 min of LPS/PBS, NFkappaB activation was investigated by EMSA.

RESULTS

TAC induced cardiac hypertrophy and elevated blood pressure. LPS application led to hypotension and other symptoms of sepsis. CD14 expression increased after TAC alone and even further after additional LPS challenge. However, we did not detect changes of TLR4 expression. Also NFkappaB activation increased after LPS challenge higher in the TAC than in the SOP group. LPS-stimulation induced also higher cytokine expression in the TAC than in the SOP group.

CONCLUSION

TAC modulates innate immunity by regulating the expression of CD14 and changes the myocardial tissue to respond more powerful to LPS.

Posttranslational regulation of tristetraprolin subcellular localization and protein stability by p38 mitogen-activated protein kinase and extracellular signal-regulated kinase pathways

The p38 mitogen-activated protein kinase (MAPK) signaling pathway, acting through the downstream kinase MK2, regulates the stability of many proinflammatory mRNAs that contain adenosine/uridine-rich elements (AREs). It is thought to do this by modulating the expression or activity of ARE-binding proteins that regulate mRNA turnover. MK2 phosphorylates the ARE-binding and mRNA-destabilizing protein tristetraprolin (TTP) at serines 52 and 178. Here we show that the p38 MAPK pathway regulates the subcellular localization and stability of TTP protein. A p38 MAPK inhibitor causes rapid dephosphorylation of TTP, relocalization from the cytoplasm to the nucleus, and degradation by the 20S/26S proteasome. Hence, continuous activity of the p38 MAPK pathway is required to maintain the phosphorylation status, cytoplasmic localization, and stability of TTP protein. The regulation of both subcellular localization and protein stability is dependent on MK2 and on the integrity of serines 52 and 178. Furthermore, the extracellular signal-regulated kinase (ERK) pathway synergizes with the p38 MAPK pathway to regulate both stability and localization of TTP. This effect is independent of kinases that are known to be synergistically activated by ERK and p38 MAPK. We present a model for the actions of TTP and the p38 MAPK pathway during distinct phases of the inflammatory response.

Posttranscriptional mechanisms regulating the inflammatory response

The inflammatory response is a complex physiologic process that requires the coordinate induction of cytokines, chemokines, angiogenic factors, effector-enzymes, and proteases. Although transcriptional activation is required to turn on the inflammatory response, recent studies have revealed that posttranscriptional mechanisms play an important role by determining the rate at which mRNAs encoding inflammatory effector proteins are translated and degraded. Most posttranscriptional control mechanisms function to dampen the expression of pro-inflammatory proteins to ensure that potentially injurious proteins are not overexpressed during an inflammatory response. Here we discuss the factors that regulate the stability and translation of mRNAs encoding pro-inflammatory proteins.

Butyrate suppresses tumor necrosis factor alpha production by regulating specific messenger RNA degradation mediated through a cis-acting AU-rich element

OBJECTIVE

To study the capacity of butyrate to inhibit production of tumor necrosis factor alpha (TNFalpha) in macrophage-like synoviocytes (MLS) from patients with rheumatoid arthritis (RA), in human peripheral monocytes, and in murine RAW264.7 macrophages.

METHODS

The concentrations of TNFalpha in culture supernatants of these cells were measured using enzyme-linked immunosorbent assay. The expression levels of various messenger RNAs (mRNA), such as those for TNFalpha, the mRNA-binding protein TIS11B, and luciferase, were measured using real-time quantitative polymerase chain reaction. The in vitro effects of butyrate on transcriptional regulation were evaluated by transfection with various reporter plasmids in RAW264.7 macrophages. The effects of TIS11B on TNFalpha expression were examined using an overexpression model of TIS11B in RAW264.7 cells.

RESULTS

Butyrate suppressed TNFalpha protein and mRNA production in MLS and monocytes, but paradoxically enhanced transactivation of the TNFalpha promoter. Expression of the AU-rich element (ARE)-binding protein TIS11B was up-regulated by butyrate. Induction of TNFalpha mRNA by lipopolysaccharide was significantly inhibited when TIS11B was overexpressed. Butyrate facilitated the degradation of luciferase transcripts containing the 3'-untranslated region (3'-UTR) of TNFalpha, and this effect was dependent on the ARE in the 3'-UTR that is known to be involved in the regulation of mRNA degradation.

CONCLUSION

These results indicate that butyrate suppresses TNFalpha expression by facilitating mRNA degradation mediated through a cis-acting ARE. Butyrate has the ability to regulate TNFalpha at the mRNA level and is therefore a potential therapeutic drug for RA patients.

Butyrate response factor 1 enhances cisplatin sensitivity in human head and neck squamous cell carcinoma cell lines

Cisplatin is a widely used chemotherapeutic agent in head and neck squamous cell carcinoma (HNSCC). Resistance to cisplatin is a common feature of HNSCC. To identify genes that may regulate cisplatin sensitivity, we carried out a cDNA microarray analysis of gene expression in cisplatin-sensitive and cisplatin-resistant HNSCC-derived cell lines. Among genes differentially expressed by cisplatin treatment, we have confirmed the elevated expression of butyrate responsive factor 1 (BRF1) in cisplatin-sensitive HNSCC cells and have demonstrated that the expression level of BRF1 is associated with cisplatin-sensitivity. Specific inhibition of BRF1 expression using an antisense oligodeoxynucleotide (ODN) decreased the cisplatin-sensitivity and, on the contrary, overexpression of BRF1 increased cisplatin-sensitivity in HNSCC cells. Elevated expression of BRF1 decreased the level of the human inhibitor of apoptosis protein-2 (cIAP2) and increased the caspase-3 activity in HNSCC cells. In addition, elevated expression of BRF1 decreased the expression level of enhanced green fluorescent protein (EGFP) linked to a 3' terminal AU-rich element (ARE) of cIAP2 mRNA. These findings demonstrate that BRF1 expression enhanced cisplatin sensitivity in HNSCC cells by reducing the levels of cIAP2 mRNA.

Butyrate response factor 1 is regulated by parathyroid hormone and bone morphogenetic protein-2 in osteoblastic cells

Parathyroid hormone (PTH) exerts potent and diverse effects in bone and cartilage through activation of type 1 PTH receptors (PTH1R) capable of coupling to protein kinase A (PKA) and PKC. We have used macroarrays to identify zinc finger protein butyrate response factor-1 (BRF1) as a novel PTH regulated gene in clonal and normal osteoblasts of human and rodent origin. We further demonstrate that in human osteoblast-like OHS cells, biologically active hPTH(1-84) and hPTH(1-34) stimulate BRF1 mRNA expression in a dose- and time-dependent manner, while the amino-terminally truncated hPTH(3-84) which does not activate PTH1R has no effect. Moreover, using specific stimulators or inhibitors of PKA and PKC activity, the PTH-elicited BRF1 mRNA expression is mediated through the PKA signaling pathway. In mouse calvarial osteoblasts, BRF1 mRNA levels are upregulated by PTH(1-84) and reduced in response to bone morphogenetic protein 2 (BMP-2). Hence, our data showing that BRF1 is expressed in osteoblastic cells and regulated by PTH and BMP-2, suggest an important role for BRF1 in osteoblasts within the molecular network of PTH-dependent bone remodeling.

Expression, purification, and biochemical characterization of the antiinflammatory tristetraprolin: a zinc-dependent mRNA binding protein affected by posttranslational modifications

Tristetraprolin (TTP) is a hyperphosphorylated protein that destabilizes mRNA by binding to an AU-rich element (ARE). Mice deficient in TTP develop a severe inflammatory syndrome. The biochemical properties of TTP have not been adequately characterized, due to the difficulties in protein purification and lack of a high-titer antiserum. Full-length human TTP was expressed in human HEK293 cells and purified to at least 70% homogeneity. The purified protein was free of endogenous ARE binding activity, and was used for investigating its size, zinc dependency, and binding kinetics for tumor necrosis factor alpha mRNA ARE. A high-titer rabbit antiserum was raised against the MBP-hTTP fusion protein expressed in Escherichia coli. Cellular localization studies of the transfected cells indicated that approximately 80% of the expressed TTP was in the cytosol, with 20% in the nuclei. TTP from both locations bound to the ARE and formed similar complexes. The purified TTP was shown to be intact by N-terminal His-tag purification, C-terminal peptide sequencing, and mass spectrometry analysis. Results from size exclusion chromatography are consistent with the predominant form of active TTP being a tetramer. TTP's ARE binding activity was increased by 10 microM Zn(2+). The half-maximal binding of TTP from HEK293 cells was approximately 30 nM in assays containing 10 nM ARE. This value was about twice that of TTP from E. coli. TTP from HEK293 cells was highly phosphorylated, and its electrophoretic mobility was increased by alkaline phosphatase treatment and somewhat by T271A mutation, but not by PNGase F or S186A mutation. The gel mobility of TTP from E. coli was decreased by in vitro phosphorylation with p42/ERK2 and p38 mitogen-activated protein kinases. These results suggest that TTP's zinc-dependent ARE binding affinity is reduced by half by posttranslational modifications, mainly by phosphorylation but not by glycosylation, in mammalian cells. The results support a model in which each subunit of the TTP tetramer binds to one of the five overlapping UUAUUUAUU sequences of the ARE, resulting in a stable TTP-ARE complex.

Differential expression of proteins in rat plasma exposed to benzene

Benzene, a ubiquitous environmental contaminant, is an important solvent in the chemical industry and is also known as a constituent of petroleum. It has been reported that benzene is associated with hematotoxicity including leukemia in humans and cancer in laboratory animals. To study protein expression alterations in rat plasma exposed to benzene, rats were exposed to levels of 1, 10, 100 ppm benzine for 6 h/day and 5 d/week for 2 or 6 weeks. Two-dimensional gel electrophoresis of rat plasma was carried out, and approximately 1000 protein spots were detected on the gels. The 11 spots which showed significantly different expression were selected and identified with matrix-assisted laser desorption/ionization-time of flight-mass spectrometry. Analyzing the targeted 11 spots, there was no correlation between the 2 and 6 weeks benzene-inhaled groups on up-regulated proteins (zinc finger protein, and tristetraprolin) and on down-regulated proteins (cAMP-regulated guanine nucleotide exchange factor II, protein kinase and unknown protein). The overexpressed proteins (inhibitor of kappaB-like protein, GTP-binding protein rab14, T-cell receptor alpha chain, and somatostatin transactivating factor-1) were detected only in groups inhaling benzene for 6 weeks. Among them the expression level of T-cell receptor alpha chain was confirmed by Western blot.

The stability of tristetraprolin mRNA is regulated by mitogen-activated protein kinase p38 and by tristetraprolin itself

Tristetraprolin (TTP) is an mRNA-destabilizing protein that negatively regulates the expression of proinflammatory mediators such as tumor necrosis factor alpha, granulocyte/macrophage colony-stimulating factor, and cyclooxygenase 2. Here we investigate the regulation of TTP expression in the mouse macrophage cell line RAW264.7. We show that TTP mRNA is expressed in a biphasic manner following stimulation of cells with lipopolysaccharide and that the second phase of expression, like the first, is dependent on mitogen-activated protein kinase (MAPK) p38. MAPK p38 acts through a downstream kinase to stabilize TTP mRNA, and this stabilization is mediated by an adenosine/uridine-rich region at the 3'-end of the TTP 3'-untranslated region. Hence TTP is post-transcriptionally regulated in a similar manner to several proinflammatory genes. We also demonstrate that TTP is able to bind to its own 3'-untranslated region and negatively regulate its own expression, forming a feedback loop to limit expression levels.