Regulation of monocyte induced cell migration by the RNA binding protein, FXR1.

FXR1 belongs to a family of RNA-binding proteins that play critical roles in post-transcriptional regulation of gene expression in immunity, development and cancer. FXR1 is associated with regulation of specific mRNAs in myocytes and macrophages. In quiescent cells (> 24 h of extended serum-starvation, ∼30-48 h or more), a spliced isoform of FXR1, FXR1a, promotes translation of the cytokine TNFα, independent of the effects of RNA levels. Here we examined the role of FXR1 in THP1 human monocytic leukemic cells that were grown in serum, as well as in early (24 h) serum-starvation conditions that demonstrates differences in gene expression mechanisms and is distinct from quiescent (> 24 h extended serum-starvation) cells. Global RNA profiling, conducted to investigate the role of FXR1 on mRNA levels, revealed that FXR1 affects levels of specific mRNAs in serum-grown and in early 24 h serum-starvation conditions. FXR1 decreases levels of several mRNAs, including as previously identified, CDKN1A (p21CIP1 or p21) mRNA in serum-grown cells. Interestingly, we find that FXR1 positively regulates mRNA levels of specific cytokines and chemokines in serum-grown and in early 24 h serum-starvation conditions. These include IL1ß and CCL2 that control cell migration. Accordingly, depletion and overexpression of FXR1 decreased and increased levels of CCL2 mRNA. Consistent with the reduced levels of IL1ß, CCL2 and other chemokines upon FXR1 depletion, our data reveal that depletion of FXR1 decreases the ability of these cells to induce cell migration of neighboring monocytic cells. These data reveal a new role of FXR1 in controlling induction of monocyte migration.

MicroRNA-mediated mRNA translation activation in quiescent cells and oocytes involves recruitment of a nuclear microRNP.

MicroRNAs can promote translation of specific mRNAs in quiescent (G0) mammalian cells and immature Xenopus laevis oocytes. We report that microRNA-mediated upregulation of target mRNAs in oocytes is dependent on nuclear entry of the microRNA; cytoplasmically-injected microRNA repress target mRNAs. Components of the activation microRNP, AGO, FXR1 (FXR1-iso-a) and miR16 are present in the nucleus and cytoplasm. Importantly, microRNA target mRNAs for upregulation, Myt1, TNFα and a reporter bearing the TNFα AU-rich, microRNA target sequence, are associated with AGO in immature oocyte nuclei and AGO2 in G0 human nuclei, respectively. mRNAs that are repressed or lack target sites are not associated with nuclear AGO. Crosslinking-coupled immunopurification revealed greater association of AGO2 with FXR1 in the nucleus compared to cytoplasm. Consistently, overexpression of FXR1-iso-a rescues activation of cytoplasmically-injected RNAs and in low density, proliferating cells. These data indicate the importance of a compartmentalized AGO2-FXR1-iso-a complex for selective recruitment for microRNA-mediated upregulation.