Affinity purification of ARE-binding proteins identifies polyA-binding protein 1 as a potential substrate in MK2-induced mRNA stabilization

MAPKAP kinase 2 phosphorylates tristetraprolin on in vivo sites including Ser178, a site required for 14-3-3 binding

MAPKAP kinase 2 (MK2) is required for tumor necrosis factor synthesis. Tristetraprolin (TTP) binds to the 3'-untranslated region of tumor necrosis factor mRNA and regulates its fate. We identified in vitro and in vivo phosphorylation sites in TTP using nanoflow high pressure liquid chromatography microelectrospray ionization tandem mass spectrometry and novel methods for direct digestion of TTP bound to affinity matrices (GSH-beads or anti-Myc linked to magnetic beads). MK2Delta3B, activated in Escherichia coli by p38alpha, phosphorylates TTP in vitro at major sites Ser(52) and Ser(178) (>10-fold in abundance) as well as at several minor sites that were detected after enriching for phosphopeptides with immobilized metal affinity chromatography. MK2 phosphorylation of TTP creates a functional 14-3-3 binding site. In cells, TTP was phosphorylated at Ser(52), Ser(178), Thr(250), and Ser(316) and at SP sites in a cluster (Ser(80)/Ser(82)/Ser(85)). Anisomycin treatment of NIH 3T3 cells increased phosphorylation of Ser(52) and Ser(178). Overexpression of MK2 sufficed to increase phosphorylation of Ser(52) and Ser(178) but not Ser(80)/Ser(82)/Ser(85) or Thr(250). Thus, Ser(52) and Ser(178) are putative MK2 sites in vivo. Identified phosphosite(s) may be biologic switches controlling mRNA stability and translation.

Translational control of tumor necrosis factor-related apoptosis-inducing ligand death receptor expression in melanoma cells

In the present study, it was found that tumor necrosis factor-related apoptosis-inducing ligand (TRAIL)-R2 protein expression did not correlate with mRNA expression in melanoma cell lines. In particular, early passage primary cultures from patients had low TRAIL-R2 protein expression compared with later passage cultures although TRAIL-R2 mRNA expression was similar in early and late passages. Similarly, cell lines made resistant to TRAIL by cultures in TRAIL had low TRAIL-R2 protein expression but normal levels of mRNA for TRAIL-R2. Expression from a luciferase reporter gene construct with the 3'-untranslated region (UTR) (but not the 5'-UTR) of TRAIL-R2 was suppressed when transfected into the TRAIL-selected (resistant) melanoma lines compared with that seen in the parental (sensitive) lines. Similar results were seen in early passage (resistant) cultures compared with late passage (sensitive) primary melanoma cultures. RNA gel shift assays demonstrated protein(s) binding to the 3'-UTR of TRAIL-R2 mRNA that were more evident in TRAIL-resistant cultures with low TRAIL-R2 protein expression. A 23-base fragment of the 3'-UTR inhibited binding of the proteins to the 3'-UTR, and a probe using this fragment bound to proteins in TRAIL-selected melanoma lines and early passage isolates of melanoma. Binding of the 3'-UTR probe to the cytosolic protein(s) was induced by exposure to TRAIL and was lost from the TRAIL selected lines 2-3 days after withdrawal of TRAIL from the cultures. These results are consistent with post-transcriptional regulation of TRAIL-R2 expression by cytosolic proteins induced by TRAIL that bind to the 3'-UTR region of TRAIL-R2 mRNA.

p38 Mitogen-activated protein kinase stabilizes mRNAs that contain cyclooxygenase-2 and tumor necrosis factor AU-rich elements by inhibiting deadenylation

AU-rich elements (AREs) in 3'-untranslated regions of mRNAs confer instability. They target mRNAs for rapid deadenylation and degradation and may enhance decapping. The p38 MAPK pathway stabilizes many otherwise unstable ARE-containing mRNAs encoding proteins involved in inflammation; however, the mRNA decay step(s) regulated by the signaling pathway are unknown. To investigate whether it regulates deadenylation or the decay of the mRNA body, we used a tetracycline-regulated beta-globin mRNA reporter system to transcribe pulses of mRNA of uniform length. We measured on Northern gels the migration of reporter mRNAs isolated from cells transfected only with reporter plasmid or co-transfected with an active mutant of MAPK kinase-6, and treated either with or without the p38 MAPK inhibitor SB 203580. Differences in migration were shown by RNase H mapping with oligo(dT) to be due to poly(A) shortening. Insertion of an ARE into the beta-globin reporter mRNA promoted rapid deadenylation and decay of hypo-adenylated reporter mRNA. p38 MAPK activation inhibited the deadenylation of reporter mRNAs containing either the cyclooxygenase-2 or tumor necrosis factor AREs. The regulation of deadenylation by p38 MAPK was found to be specific because deadenylation of the beta-globin reporter mRNA either lacking an ARE or containing the c-Myc 3'-untranslated region (which is not p38 MAPK-responsive) was unaffected by p38 MAPK. It was concluded that the p38 MAPK pathway predominantly regulates deadenylation, rather than decay of the mRNA body, and this provides an explanation for why p38 MAPK regulates mRNA stability in some situations and translation in others.

The GW182 protein colocalizes with mRNA degradation associated proteins hDcp1 and hLSm4 in cytoplasmic GW bodies

A novel cytoplasmic compartment referred to as GW bodies (GWBs) was initially identified using antibodies specific to a 182-kD protein termed GW182. GW182 was characterized by multiple glycine(G)-tryptophan(W) repeats and an RNA recognition motif (RRM) that bound a subset of HeLa cell messenger RNAs (mRNAs). The function of GWBs was not known; however, more recent evidence suggested similarities between GWBs and cytoplasmic structures that contain hLSm proteins and hDcp1, the human homolog to a yeast decapping enzyme subunit. In this study, we used antibodies to hLSm4 and hDcp1 to show that both of these markers of an mRNA degradation pathway colocalize to the same structures as GW182. Our studies demonstrate that GW182, hLSm4, and hDcp1 are found in the same cytoplasmic structures and suggest that GW182 is involved in the same mRNA processing pathway as hLSm4 and hDcp1.

Post-transcriptional regulation of gene expression by mitogen-activated protein kinase p38

The mitogen-activated protein kinase p38 pathway was originally identified as a signalling cascade activated by pro-inflammatory stimuli and cellular stresses, and playing a critical role in the translational regulation of pro-inflammatory cytokine synthesis. In almost a decade since this discovery, a great deal has been learned about the role of the p38 pathway in the post-transcriptional regulation of pro-inflammatory gene expression. However, important questions remain to be answered concerning the specificity and mechanism or mechanisms of action of p38. This review describes recent progress and remaining puzzles in the field of post-transcriptional regulation by p38.