Chaperone Hsp27, a Novel Subunit of AUF1 Protein Complexes, Functions in AU-Rich Element-Mediated mRNA Decay

Abstract

Controlled, transient cytokine production by monocytes depends heavily upon rapid mRNA degradation, conferred by 3 untranslated region-localized AU-rich elements (AREs) that associate with RNA-binding proteins. The ARE-binding protein AUF1 forms a complex with cap-dependent translation initiation factors and heat shock proteins to attract the mRNA degradation machinery. We refer to this protein assembly as the AUF1-and signal transduction-regulated complex, ASTRC. Rapid degradation of ARE-bearing mRNAs (AREmRNAs) requires ubiquitination of AUF1 and its destruction by proteasomes. Activation of monocytes by adhesion to capillary endothelium at sites of tissue damage and subsequent proinflammatory cytokine induction are prominent features of inflammation, and ARE-mRNA stabilization plays a critical role in the induction process. Here, we demonstrate activation-induced subunit rearrangements within ASTRC and identify chaperone Hsp27 as a novel subunit that is itself an ARE-binding protein essential for rapid ARE-mRNA degradation. As Hsp27 has well-characterized roles in protein ubiquitination as well as in adhesion-induced cytoskeletal remodeling and cell motility, its association with ASTRC may provide a sensing mechanism to couple proinflammatory cytokine induction with monocyte adhesion and motility.Many mRNAs encoding proteins transiently required for inflammatory responses, cell proliferation, and intracellular signaling are labile due to AU-rich elements (AREs) in their 3Ј untranslated regions (UTRs) (14,21,57). ARE association by ELAV-like (embryonic lethal, abnormal vision) proteins, such as HuR, blocks ARE-mediated mRNA decay (AMD) (31). By contrast, association of proteins such as AUF1, tristetraprolin (TTP), BRF1 (butyrate-responsive factor-1), K-homology splicing regulatory protein (KSRP), ring finger K-homology domain 1 (RKHD1), polymyositisscleroderma 75-kDa antigen (PM-Scl75), or microRNA miR16 or miR289 with an ARE promotes AMD (6,8,12,18,24,34,43). The phosphorylation state of TTP, BRF1, and AUF1 affects AMD efficiency (3,37,51,56), indicating that signal transduction networks regulate this pathway.AUF1 has four protein isoforms-p37, p40, p42, and p45-generated by alternative pre-mRNA splicing (50). Based upon extensive biochemical studies of AUF1, we proposed an integrated, three-step model for induction of AMD by AUF1 via assembly of a trans-acting complex that targets the mRNA for degradation (52). The first step is dynamic AUF1 dimer binding to an ARE and formation of an oligomeric AUF1 complex (7, 52). Stabilizing ARE-binding proteins (AUBPs) may compete with AUF1 for binding to the ARE during this step, thus preventing AUF1 oligomerization and subsequent factor recruitment (25). Binding of AUF1 to an ARE then permits the second step involving recruitment of additional trans-acting factors including eukaryotic translation initiation factor eIF4G, poly(A)-binding protein, dual-functional heat shock/AUBPs Hsp/Hsc70 (27), and additional unknown proteins, forming a multisubunit AUF1-and signal tra...

HSP27-Mediated Extracellular and Intracellular Signaling Pathways Synergistically Confer Chemoresistance in Squamous Cell Carcinoma of Tongue

“…Our observation is consistent with previous findings where HSP27 is able to prevent cells from apoptosis by interacting with Cyto C (23) or by interfering the apoptotic signaling upstream of the mitochondrial Cyto C release (24,25). In addition, it was reported that HSP27 as a subunit of AUF1 protein complexes can also act as a RNA-binding protein to mediate mRNA decay (26). Enhanced HSP27 expression in response to oxidative stress binds to the 3 0 -untranslated region of BIM mRNA to repress its translation and subsequently prevent neuronal death in cerebellar granule neurons (27).…”

Modification and Reorganization of the Cytoprotective Cellular Chaperone Hsp27 during Herpes Simplex Virus Type 1 Infection

“…AREs are instability motifs found within the 3Ј untranslated region of some transcripts and serve as markers for rapid degradation. Recently published research has shown that knockdown of Hsp27 (all forms) resulted in decreased mRNA decay (29). During HSV-1 infection, although most cellular mRNAs are degraded through the activity of the viral vhs (virion host shutoff) protein (27), some, like that of the cytokine regulator IEX-1, are stabilized (15).…”

Chaperone Hsp27 Modulates AUF1 Proteolysis and AU-Rich Element-Mediated mRNA Degradation

“…AUF1 consists of four isoforms of 37, 40, 42, and 45 kDa generated by alternative pre-mRNA splicing of a single pre-mRNA. They associate with heat shock proteins Hsc70-Hsp70 and Hsp27, translation initiation factor eIF4G, poly(A)-binding protein (PABP), and other unidentified proteins to form a multisubunit complex we refer to as AUF1-and signal transduction-regulated complex (ASTRC) (15,27). This complex recruits messenger ribonucleases to degrade ARE mRNAs.…”

“…This complex recruits messenger ribonucleases to degrade ARE mRNAs. RNA interference (RNAi)-mediated knockdown of either AUF1 or Hsp27 stabilizes TNF-␣ ARE mRNA, indicating that both proteins are essential for ARE-mediated mRNA degradation (AMD) (27).…”