A Specialized Mechanism of Translation Mediated by FXR1a-Associated MicroRNP in Cellular Quiescence.

MicroRNAs predominantly decrease gene expression; however, specific mRNAs are translationally upregulated in quiescent (G0) mammalian cells and immature Xenopus laevis oocytes by an FXR1a-associated microRNA-protein complex (microRNP) that lacks the microRNP repressor, GW182. Their mechanism in these conditions of decreased mTOR signaling, and therefore reduced canonical (cap-and-poly(A)-tail-mediated) translation, remains undiscovered. Our data reveal that mTOR inhibition in human THP1 cells enables microRNA-mediated activation. Activation requires shortened/no poly(A)-tail targets; polyadenylated mRNAs are partially activated upon PAIP2 overexpression, which interferes with poly(A)-bound PABP, precluding PABP-enhanced microRNA-mediated inhibition and canonical translation. Consistently, inhibition of PARN deadenylase prevents activation. P97/DAP5, a homolog of canonical translation factor, eIF4G, which lacks PABP- and cap binding complex-interacting domains, is required for activation, and thereby for the oocyte immature state. P97 interacts with 3' UTR-binding FXR1a-associated microRNPs and with PARN, which binds mRNA 5' caps, forming a specialized complex to translate recruited mRNAs in these altered canonical translation conditions.

An Exportin-1-dependent microRNA biogenesis pathway during human cell quiescence.

The reversible state of proliferative arrest known as "cellular quiescence" plays an important role in tissue homeostasis and stem cell biology. By analyzing the expression of miRNAs and miRNA-processing factors during quiescence in primary human fibroblasts, we identified a group of miRNAs that are induced during quiescence despite markedly reduced expression of Exportin-5, a protein required for canonical miRNA biogenesis. The biogenesis of these quiescence-induced miRNAs is independent of Exportin-5 and depends instead on Exportin-1. Moreover, these quiescence-induced primary miRNAs (pri-miRNAs) are modified with a 2,2,7-trimethylguanosine (TMG)-cap, which is known to bind Exportin-1, and knockdown of Exportin-1 or trimethylguanosine synthase 1, responsible for (TMG)-capping, inhibits their biogenesis. Surprisingly, in quiescent cells Exportin-1-dependent pri-miR-34a is present in the cytoplasm together with a small isoform of Drosha, implying the existence of a different miRNA processing pathway in these cells. Our findings suggest that during quiescence the canonical miRNA biogenesis pathway is down-regulated and specific miRNAs are generated by an alternative pathway to regulate genes involved in cellular growth arrest.

FXR1a-associated microRNP: A driver of specialized non-canonical translation in quiescent conditions.

Eukaryotic protein synthesis is a multifaceted process that requires coordination of a set of translation factors in a particular cellular state. During normal growth and proliferation, cells generally make their proteome via conventional translation that utilizes canonical translation factors. When faced with environmental stress such as growth factor deprivation, or in response to biological cues such as developmental signals, cells can reduce canonical translation. In this situation, cells adapt alternative modes of translation to make specific proteins necessary for required biological functions under these distinct conditions. To date, a number of alternative translation mechanisms have been reported, which include non-canonical, cap dependent translation and cap independent translation such as IRES mediated translation. Here, we discuss one of the alternative modes of translation mediated by a specialized microRNA complex, FXR1a-microRNP that promotes non-canonical, cap dependent translation in quiescent conditions, where canonical translation is reduced due to low mTOR activity.

Upregulation of eIF5B controls cell-cycle arrest and specific developmental stages.

Proliferation arrest and distinct developmental stages alter and decrease general translation yet maintain ongoing translation. The factors that support translation in these conditions remain to be characterized. We investigated an altered translation factor in three cell states considered to have reduced general translation: immature Xenopus laevis oocytes, mouse ES cells, and the transition state of proliferating mammalian cells to quiescence (G0) upon growth-factor deprivation. Our data reveal a transient increase of eukaryotic translation initiation factor 5B (eIF5B), the eukaryotic ortholog of bacterial initiation factor IF2, in these conditions. eIF5B promotes 60S ribosome subunit joining and pre-40S subunit proofreading. eIF5B has also been shown to promote the translation of viral and stress-related mRNAs and can contribute indirectly to supporting or stabilizing initiator methionyl tRNA (tRNA-Met(i)) association with the ribosome. We find that eIF5B is a limiting factor for translation in these three conditions. The increased eIF5B levels lead to increased eIF5B complexes with tRNA-Met(i) upon serum starvation of THP1 mammalian cells. In addition, increased phosphorylation of eukaryotic initiation factor 2α, the translation factor that recruits initiator tRNA-Meti for general translation, is observed in these conditions. Importantly, we find that eIF5B is an antagonist of G0 and G0-like states, as eIF5B depletion reduces maturation of G0-like, immature oocytes and hastens early G0 arrest in serum-starved THP1 cells. Consistently, eIF5B overexpression promotes maturation of G0-like immature oocytes and causes cell death, an alternative to G0, in serum-starved THP1 cells. These data reveal a critical role for a translation factor that regulates specific cell-cycle transition and developmental stages.

Ribosome changes reprogram translation for chemosurvival in G0 leukemic cells.

Quiescent leukemic cells survive chemotherapy, with translation changes. Our data reveal that FXR1, a protein amplified in several aggressive cancers, is elevated in quiescent and chemo-treated leukemic cells and promotes chemosurvival. This suggests undiscovered roles for this RNA- and ribosome-associated protein in chemosurvival. We find that FXR1 depletion reduces translation, with altered rRNAs, snoRNAs, and ribosomal proteins (RPs). FXR1 regulates factors that promote transcription and processing of ribosomal genes and snoRNAs. Ribosome changes in FXR1-overexpressing cells, including RPLP0/uL10 levels, activate eIF2α kinases. Accordingly, phospho-eIF2α increases, enabling selective translation of survival and immune regulators in FXR1-overexpressing cells. Overriding these genes or phospho-eIF2α with inhibitors reduces chemosurvival. Thus, elevated FXR1 in quiescent or chemo-treated leukemic cells alters ribosomes that trigger stress signals to redirect translation for chemosurvival.

A post-transcriptional program of chemoresistance by AU-rich elements and TTP in quiescent leukemic cells.

BACKGROUND: Quiescence (G0) is a transient, cell cycle-arrested state. By entering G0, cancer cells survive unfavorable conditions such as chemotherapy and cause relapse. While G0 cells have been studied at the transcriptome level, how post-transcriptional regulation contributes to their chemoresistance remains unknown. RESULTS: We induce chemoresistant and G0 leukemic cells by serum starvation or chemotherapy treatment. To study post-transcriptional regulation in G0 leukemic cells, we systematically analyzed their transcriptome, translatome, and proteome. We find that our resistant G0 cells recapitulate gene expression profiles of in vivo chemoresistant leukemic and G0 models. In G0 cells, canonical translation initiation is inhibited; yet we find that inflammatory genes are highly translated, indicating alternative post-transcriptional regulation. Importantly, AU-rich elements (AREs) are significantly enriched in the upregulated G0 translatome and transcriptome. Mechanistically, we find the stress-responsive p38 MAPK-MK2 signaling pathway stabilizes ARE mRNAs by phosphorylation and inactivation of mRNA decay factor, Tristetraprolin (TTP) in G0. This permits expression of ARE mRNAs that promote chemoresistance. Conversely, inhibition of TTP phosphorylation by p38 MAPK inhibitors and non-phosphorylatable TTP mutant decreases ARE-bearing TNFα and DUSP1 mRNAs and sensitizes leukemic cells to chemotherapy. Furthermore, co-inhibiting p38 MAPK and TNFα prior to or along with chemotherapy substantially reduces chemoresistance in primary leukemic cells ex vivo and in vivo. CONCLUSIONS: These studies uncover post-transcriptional regulation underlying chemoresistance in leukemia. Our data reveal the p38 MAPK-MK2-TTP axis as a key regulator of expression of ARE-bearing mRNAs that promote chemoresistance. By disrupting this pathway, we develop an effective combination therapy against chemosurvival.

Analysis of MicroRNA-Mediated Translation Activation of In Vitro Transcribed Reporters in Quiescent Cells.

Quiescence (G0) is defined as an assortment of cell cycle arrested states that exhibit distinct properties. Leukemias harbor a subpopulation of G0 cells that can be enriched by growth factor deprivation or serum starvation. Target site reporters with shortened poly(A) tails show translation activation by microRNAs, via a noncanonical mechanism, when introduced into the nucleus of G0 cells. This is because recruitment by the activation causing FXR1a-microRNA-protein complex (FXR1a-microRNP) is nuclear and requires shortened poly(A) tails to avoid repressive factors and canonical translation. When introduced into the cytoplasm, target mRNAs and microRNAs are directed toward repression rather than translation activation. Leukemic cell lines are difficult to transfect but can be routinely nucleofected-where in vitro transcribed mRNA reporters and microRNAs are introduced into the nucleus of G0 leukemic cells. Nucleofection of a microRNA target reporter and either cognate, targeting microRNA, or control microRNA, into the nucleus of G0 cells, enables analysis of translation activation by microRNAs in G0. We discuss a modified protocol that we developed for transfection of mRNAs along with microRNAs to test translation regulation by microRNAs in G0 leukemic cells.

Serotonin transamidates Rab4 and facilitates its binding to the C terminus of serotonin transporter.

The serotonin transporter (SERT) on the plasma membrane is the major mechanism for the clearance of plasma serotonin (5-hydroxytryptamine (5HT)). The uptake rates of cells depend on the density of SERT molecules on the plasma membrane. Interestingly, the number of SERT molecules on the platelet surface is down-regulated when plasma 5HT ([5HT](ex)) is elevated. It is well reported that stimulation of cells with high [5HT](ex) induces transamidation of a small GTPase, Rab4. Modification with 5HT stabilizes Rab4 in its active, GTP-bound form, Rab4-GTP. Although investigating the mechanism by which elevated plasma 5HT level down-regulates the density of SERT molecules on the plasma membrane, we studied Rab4 and SERT in heterologous and platelet expression systems. Our data demonstrate that, in response to elevated [5HT](ex), Rab4-GTP co-localizes with and binds to SERT. The association of SERT with Rab4-GTP depends on: (i) 5HT modification and (ii) the GTP-binding ability of Rab4. Their association retains transporter molecules intracellularly. Furthermore, we mapped the Rab4-SERT association domain to amino acids 616-624 in the cytoplasmic tail of SERT. This finding provides an explanation for the role of the C terminus in the localization and trafficking of SERT via Rab4 in a plasma 5HT-dependent manner. Therefore, we propose that elevated [5HT](ex)"paralyzes" the translocation of SERT from intracellular locations to the plasma membrane by controlling transamidation and Rab4-GTP formation.