Showing papers by "Kotb Abdelmohsen published in 2010"

Posttranscriptional regulation of cancer traits by HuR.

Abstract: Cancer-related gene expression programs are strongly influenced by posttranscriptional mechanisms. The RNA-binding protein HuR is highly abundant in many cancers. Numerous HuR-regulated mRNAs encode proteins implicated in carcinogenesis. Here, we review the collections of HuR target mRNAs that encode proteins responsible for implementing five major cancer traits. By interacting with specific mRNA subsets, HuR enhances the levels of proteins that (1) promote cell proliferation, (2) increase cell survival, (3) elevate local angiogenesis, (4) help the cancer cell evade immune recognition, and (5) facilitate cancer cell invasion and metastasis. We propose that HuR exerts a tumorigenic function by enabling these cancer phenotypes. We discuss evidence that links HuR to several specific cancers and suggests its potential usefulness in cancer diagnosis, prognosis, and therapy.

microRNA expression patterns reveal differential expression of target genes with age.

Abstract: Recent evidence supports a role for microRNAs (miRNAs) in regulating the life span of model organisms. However, little is known about how these small RNAs contribute to human aging. Here, we profiled the expression of over 800 miRNAs in peripheral blood mononuclear cells from young and old individuals by real-time RT-PCR analysis. This genome-wide assessment of miRNA expression revealed that the majority of miRNAs studied decreased in abundance with age. We identified nine miRNAs (miR-103, miR-107, miR-128, miR-130a, miR-155, miR-24, miR-221, miR-496, miR-1538) that were significantly lower in older individuals. Among them, five have been implicated in cancer pathogenesis. Predicted targets of several of these miRNAs, including PI3 kinase (PI3K), c-Kit and H2AX, were found to be elevated with advancing age, supporting a possible role for them in the aging process. Furthermore, we found that decreasing the levels of miR-221 was sufficient to cause a corresponding increase in the expression of the predicted target, PI3K. Taken together, these findings demonstrate that changes in miRNA expression occur with human aging and suggest that miRNAs and their predicted targets have the potential to be diagnostic indicators of age or age-related diseases.

hnRNP C promotes APP translation by competing with FMRP for APP mRNA recruitment to P bodies

Abstract: Amyloid precursor protein (APP) regulates neuronal synapse function, and its cleavage product Abeta is linked to Alzheimer's disease. Here, we present evidence that the RNA-binding proteins (RBPs) heterogeneous nuclear ribonucleoprotein (hnRNP) C and fragile X mental retardation protein (FMRP) associate with the same APP mRNA coding region element, and they influence APP translation competitively and in opposite directions. Silencing hnRNP C increased FMRP binding to APP mRNA and repressed APP translation, whereas silencing FMRP enhanced hnRNP C binding and promoted translation. Repression of APP translation was linked to colocalization of FMRP and tagged APP RNA within processing bodies; this colocalization was abrogated by hnRNP C overexpression or FMRP silencing. Our findings indicate that FMRP represses translation by recruiting APP mRNA to processing bodies, whereas hnRNP C promotes APP translation by displacing FMRP, thereby relieving the translational block.

MicroRNA profiling in human diploid fibroblasts uncovers miR-519 role in replicative senescence

Abstract: MicroRNAs (miRNAs) are short non-coding RNAs that regulate diverse biological processes by controlling the pattern of expressed proteins. In mammalian cells, miRNAs partially complement their target sequences leading to mRNA degradation and/or decreased mRNA translation. Here, we have analyzed transcriptome-wide changes in miRNAs in senescent relative to early-passage WI-38 human diploid fibroblasts (HDFs). Among the miRNAs downregulated with senescence were members of the let-7 family, while upregulated miRNAs included miR-1204, miR-663 and miR-519. miR-519 was recently found to reduce tumor growth at least in part by lowering the abundance of the RNA-binding protein HuR. Overexpression of miR-519a in either WI-38 or human cervical carcinoma HeLa cells triggered senescence, as measured by monitoring β-galactosidase activity and other senescence markers. These data suggest that miR-519 can suppress tumor growth by triggering senescence and that miR-519 elicits these actions by repressing HuR expression.

miR-375 Inhibits Differentiation of Neurites by Lowering HuD Levels

Abstract: Neuronal development and plasticity are maintained by tightly regulated gene expression programs. Here, we report that the developmentally regulated microRNA miR-375 affects dendrite formation and maintenance. miR-375 overexpression in mouse hippocampus potently reduced dendrite density. We identified the predominantly neuronal RNA-binding protein HuD as a key effector of miR-375 influence on dendrite maintenance. Heterologous reporter analysis verified that miR-375 repressed HuD expression through a specific, evolutionarily conserved site on the HuD 3′ untranslated region. miR-375 overexpression lowered both HuD mRNA stability and translation and recapitulated the effects of HuD silencing, which reduced the levels of target proteins with key functions in neuronal signaling and cytoskeleton organization (N-cadherin, PSD-95, RhoA, NCAM1, and integrin α1). Moreover, the increase in neurite outgrowth after brain-derived neurotrophic factor (BDNF) treatment was diminished by miR-375 overexpression; this effect was rescued by reexpression of miR-375-refractory HuD. Our findings indicate that miR-375 modulates neuronal HuD expression and function, in turn affecting dendrite abundance.

miR-519 suppresses tumor growth by reducing HuR levels.

Abstract: The RNA-binding protein HuR is highly abundant in many cancers. HuR expression was recently found to be repressed by microRNA miR-519, which potently lowered HuR translation without influencing HuR mRNA abundance. Here, we examined the levels of HuR and miR-519 in pairs of cancer and adjacent healthy tissues from ovary, lung, and kidney. In the three sample collections, the cancer specimens showed dramatically higher HuR levels, unchanged HuR mRNA concentrations, and markedly reduced miR-519 levels, when compared with healthy tissues. As tested using human cervical carcinoma cells, miR-519 reduced tumorigenesis in athymic mice. Compared with the tumors arising from control cells, cells overexpressing miR-519 formed significantly smaller tumors, while cells expressing reduced miR-519 levels gave rise to substantially larger tumors. Evidence that the miR-519-elicited reduction of HuR was critical for its tumor suppressor influence was obtained by reducing HuR, as HuR-silenced cells formed markedly smaller tumors and were unable to form large tumors even after lowering miR-519 abundance. Together, our data reveal that miR-519 inhibits tumorigenesis in large part by repressing HuR expression.

NF90 selectively represses the translation of target mRNAs bearing an AU-rich signature motif

Abstract: The RNA-binding protein nuclear factor 90 (NF90) has been implicated in the stabilization, transport and translational control of several target mRNAs. However, a systematic analysis of NF90 target mRNAs has not been performed. Here, we use ribonucleoprotein immunoprecipitation analysis to identify a large subset of NF90-associated mRNAs. Comparison of the 3′-untranslated regions (UTRs) of these mRNAs led to the elucidation of a 25- to 30-nucleotide, RNA signature motif rich in adenines and uracils. Insertion of the AU-rich NF90 motif (‘NF90m’) in the 3′UTR of an EGFP heterologous reporter did not affect the steady-state level of the chimeric EGFP-NF90m mRNA or its cytosolic abundance. Instead, the translation of EGFP-NF90m mRNA was specifically repressed in an NF90-dependent manner, as determined by analysing nascent EGFP translation, the distribution of chimeric mRNAs on polysome gradients and the steady-state levels of expressed EGFP protein. The interaction of endogenous NF90 with target mRNAs was validated after testing both endogenous mRNAs and recombinant biotinylated transcripts containing NF90 motif hits. Further analysis showed that the stability of endogenous NF90 target mRNAs was not significantly influenced by NF90 abundance, while their translation increased when NF90 levels were reduced. In summary, we have identified an AU-rich RNA motif present in NF90 target mRNAs and have obtained evidence that NF90 represses the translation of this subset of mRNAs.

Regulation of HuR by DNA Damage Response Kinases.

Abstract: As many DNA-damaging conditions repress transcription, posttranscriptional processes critically influence gene expression during the genotoxic stress response. The RNA-binding protein HuR robustly influences gene expression following DNA damage. HuR function is controlled in two principal ways: (1) by mobilizing HuR from the nucleus to the cytoplasm, where it modulates the stability and translation of target mRNAs and (2) by altering its association with target mRNAs. Here, we review evidence that two main effectors of ataxia-telangiectasia-mutated/ATM- and Rad3-related (ATM/ATR), the checkpoint kinases Chk1 and Chk2, jointly influence HuR function. Chk1 affects HuR localization by phosphorylating (hence inactivating) Cdk1, a kinase that phosphorylates HuR and thereby blocks HuR's cytoplasmic export. Chk2 modulates HuR binding to target mRNAs by phosphorylating HuR's RNA-recognition motifs (RRM1 and RRM2). We discuss how HuR phosphorylation by kinases including Chk1/Cdk1 and Chk2 impacts upon gene expression patterns, cell proliferation, and survival following genotoxic injury.

Response to Comment on “Increased MKK4 Abundance with Replicative Senescence Is Linked to the Joint Reduction of Multiple MicroRNAs”

Abstract: Moens and colleagues express concern that our report of phosphorylation of p38–regulated/activated protein kinase (PRAK) at Ser 93 in senescent cells may be incorrect as a result of their inability to detect phosphorylated PRAK with the same antibody. We provide additional characterization of the antibody that we used and show that there is a 42-kilodalton phosphoprotein detected by this antibody, but that this phosphoprotein may not be PRAK.