RACK1 is a functional target of the E1A oncoprotein

The receptor for activated C kinase 1 (RACK1) mediating immune response in thick shell mussel Mytilus coruscus

The receptor for activated C kinase 1 (RACK1) is a intracellular receptor for the protein kinase C family which mediates various biological processes. Here, a novel RACK1 gene termed Mc-RACK1 was identified from thick shell mussel, Mytilus coruscus. Mc-RACK1 shared typical RACK1 domains containing WD repeats, PKC phosphorylation sites, N-myristoylation sites, PKC activation sites, TK phosphorylation site and WD motifs. Mc-RACK1 was constitutively expressed in all examined tissues, and its expression in gills, haemocytes and digestive glands were significantly up-regulated upon LPS challenge. Mc-RACK1 showed a significantly down-regulated expression in gills and haemocytes at the early phase upon copper exposure. Mc-RACK1 in haemocytes was silenced after receiving its dsRNA, meanwhile, the increases of SOD and CAT activity were investigated. Further, Mc-RACK1 could activate the NF-κB and ISRE reporter in HEK-293T cells. These suggested that Mc-RACK1 had a deeper involvement in mollusc immunity, and played an important role in antioxidant system.

The receptor for activated C kinase 1 (RACK1) functions in hematopoiesis through JNK activation in Chinese mitten crab Eriocheir sinensis

Receptor for activated C kinase 1 (RACK1) is a WD-domain repeating protein which involves in the mediation of various biological processes, including innate immune response. In the present study, a RACK1 (designed as EsRACK1) gene from Chinese mitten crab E. sinensis was cloned by rapid amplification of cDNA ends (RACE) technique. The full-length cDNA sequence of EsRACK1 was of 1117 bp with an open reading frame (ORF) of 957 bp encoding a polypeptide of 318 amino acids containing seven WD repeats. EsRACK1 shared 62%-99% similarities with previously identified RACK1s in amino acid sequence, and it was clustered with the RACK1 from Pacifastacus leniusculus in the phylogenetic tree. The mRNA transcripts of EsRACK1 were constitutively expressed in various tissues with the highest expression level in hepatopancreas. The expression of EsRACK1 mRNA in hemocytes were significantly up-regulated post the stimulations with Vibrio anguillarum and Pichia pastoris. After exposure to CdCl2 and pentachlorophenol, the transcripts of EsRACK1 in hemocytes were up-regulated at the late phase from 12 h. When EsRACK1 was knocked down by dsRNA based RNAi, the total hemocyte counts, new-born hemocytes and phosphorylation of JNK were all significantly decreased. In addition, EsRACK1 transcription and phosphorylation of JNK were both decreased in hematopoietic tissue post Aeromonas hydrophila challenge. All the results suggested that EsRACK1 was involved in the innate immune response of the crab and participated in the production of new-born hemocytes through activation of JNK.

Novel mechanism of JNK pathway activation by adenoviral E1A

The adenoviral oncoprotein E1A influences cellular regulation by interacting with a number of cellular proteins. In collaboration with complementary oncogenes, E1A fully transforms primary cells. As part of this action, E1A inhibits transcription of c-Jun:Fos target genes while promoting that of c-Jun:ATF2-dependent genes including jun. Both c-Jun and ATF2 are hyperphosphorylated in response to E1A. In the current study, E1A was fused with the ligand binding domain of the estrogen receptor (E1A-ER) to monitor the immediate effect of E1A activation. With this approach we now show that E1A activates c-Jun N-terminal kinase (JNK), the upstream kinases MKK4 and MKK7, as well as the small GTPase Rac1. Activation of the JNK pathway requires the N-terminal domain of E1A, and, importantly, is independent of transcription. In addition, it requires the presence of ERM proteins. Downregulation of signaling components upstream of JNK inhibits E1A-dependent JNK/c-Jun activation. Taking these findings together, we show that E1A activates the JNK/c-Jun signaling pathway upstream of Rac1 in a transcription-independent manner, demonstrating a novel mechanism of E1A action.

RACK1, a versatile hub in cancer

RACK1 is a highly conserved intracellular adaptor protein with significant homology to Gβ and was originally identified as the anchoring protein for activated protein kinase C. In the past 20 years, the number of binding partners and validated cellular functions for RACK1 has increased, which facilitates clarification of its involvement in different biological events. In this review, we will focus on its role in cancer, summarizing its aberrant expression, pro- or anti-oncogenic effects and the underlying mechanisms in various cancers.

The human papillomavirus-16 E7 oncoprotein exerts antiapoptotic effects via its physical interaction with the actin-binding protein gelsolin

The oncoprotein E7 from human papillomavirus-16 (HPV-16 E7) plays a pivotal role in HPV postinfective carcinogenesis, and its physical interaction with host cell targets is essential to its activity. We identified a novel cellular partner for the viral oncoprotein: the actin-binding protein gelsolin (GSN), a key regulator of actin filament assembly and disassembly. In fact, biochemical analyses, generation of a 3D molecular interaction model and the use of specific HPV-16 E7 mutants provided clear cut evidence supporting the crucial role of HPV-16 E7 in affecting GSN integrity and function in human immortalized keratinocytes. Accordingly, functional analyses clearly suggested that stable HPV-16 E7 expression induced an imbalance between polymeric and monomeric actin in favor of the former. These events also lead to changes of cell cycle (increased S phase), to the inhibition of apoptosis and to the increase of cell survival. These results provide support to the hypotheses generated from the 3D molecular interaction model and encourage the design of small molecules hindering HPV-induced host cell reprogramming by specifically targeting HPV-16 E7-expressing cells.

Sprouty is a cytoplasmic target of adenoviral E1A oncoproteins to regulate the receptor tyrosine kinase signalling pathway

Background

Oncoproteins encoded by the early region of adenoviruses have been shown to be powerful tools to study gene regulatory mechanisms, which affect major cellular events such as proliferation, differentiation, apoptosis and oncogenic transformation. They are possesing a key role to favor viral replication via their interaction with multiple cellular proteins. In a yeast two-hybrid screen we have identified Sprouty1 (Spry1) as a target of adenoviral E1A Oncoproteins. Spry proteins are central and complex regulators of the receptor tyrosine kinase (RTK) signalling pathway. The deregulation of Spry family members is often associated with alterations of the RTK signalling and its downstream effectors, leading to the ERK pathway.

Results

Here, we confirm our yeast two-hybrid data, showing the interaction between Spry1 and E1A in GST pull-down and immunoprecipitation assays. We also demonstrated the interaction of E1A with two further Spry isoforms. Using deletion mutants we identified the N-terminus and the CR conserved region (CR) 3 of E1A- and the C-terminal half of Spry1, which contains the highly conserved Spry domain, as the essential sites for direct interaction between Spry and E1A. Immunofluorescent microscopy data revealed a co-localization of E1A_13S with Spry1 in the cytoplasm. SRE and TRE reporter assays demonstrated that co-expression of Spry1 with E1A_13S abolishes the inhibitory function of Spry1 in RTK signalling, which is consequently accompanied with a decrease of E1A_13S-induced gene expression.

Conclusions

These results establish Spry1 as a cytoplasmic localized cellular target for E1A oncoproteins to regulate the RTK signalling pathway, and consequently cellular events downstream of RTK that are essential for viral replication and transformation.

Receptor for Activated C Kinase-1 protein from Penaeus monodon (Pm-RACK1) participates in the shrimp antioxidant response

Cellular oxidative stress responses are caused in many ways, but especially by disease and environmental stress. After the initial burst of reactive oxygen species (ROS), the effective elimination of ROS is crucial for the survival of organisms and is mediated by antioxidant defense mechanisms. In this paper, we investigate the possible antioxidant function of Penaeus monodon Receptor for Activated C Kinase-1 (Pm-RACK1). When Pm-RACK1 was over-expressed in Escherichia coli cells or Spodoptera frugiperda (Sf9) insect cells exposed to H(2)O(2), it significantly protected the cells from oxidative damage induced by H(2)O(2). When recombinant Pm-RACK1 protein was expressed as a histidine fusion protein in E. coli and purified with a Ni(2+)-column it possessed antioxidant functions that protected DNA from metal-catalyzed oxidation. Shrimp (Penaeus vannamei) held at an alkaline pH had a much higher hepatopancreatic expression of Pm-RACK1 than in those held at pH 7.4. The exposure of shrimp to alkaline pH is also known to increase ROS production. These results provide strong evidence that Pm-RACK1 can participate in the shrimp antioxidant response induced by the formation of ROS.

RACK1 has the nerve to act: structure meets function in the nervous system

The receptor for activated protein kinase C 1 (RACK1) is an intracellular adaptor protein. Accumulating evidence attributes to this member of the tryptophan-aspartate (WD) repeat family the role of regulating several major nervous system pathways. Structurally, RACK1 is a seven-bladed-beta-propeller, interacting with diverse proteins having distinct structural folds. When bound to the IP3 receptor, RACK1 regulates intracellular Ca2+ levels, potentially contributing to processes such as learning, memory and synaptic plasticity. By binding to the NMDA receptor, it dictates neuronal excitation and sensitivity to ethanol. When bound to the stress-induced acetylcholinesterase variant AChE-R, RACK1 is implicated in stress responses and behavior, compatible with reports of RACK1 modulations in brain ageing and in various neurodegenerative diseases. This review sheds new light on both the virtues and the variety of neuronal RACK1 interactions and their physiological consequences.

Functional expression cloning reveals a central role for the receptor for activated protein kinase C 1 (RACK1) in T cell apoptosis

Mammalian cDNA expression cloning was used to identify novel genes that regulate apoptosis. Using a functional screen, we identified a partial cDNA for the receptor for activated protein kinase C 1 (RACK1) through selection for resistance to phytohemagglutinin and gamma-irradiation. Expression of this partial cDNA in T cell lines using a mammalian expression vector produced an increase in RACK1 expression and resulted in resistance to dexamethasone- and ultraviolet-induced apoptosis. Down-regulation of RACK1 using RNA interference abolished the resistance of the transfected cells to apoptosis. Overexpression of full-length RACK1 also resulted in the suppression of apoptosis mediated by several apoptotic stimuli, and this effect was quantitatively consistent with the effects of the original cDNA isolated on endogenous RACK1 levels. Together, these findings suggest that RACK1 plays an important role in the intracellular signaling pathways that lead to apoptosis in T cells.

Use of adenoviral E1A protein to analyze K18 promoter deregulation in colon carcinoma cells discloses a role for CtBP1 and BRCA1

Background

The promoter of the keratin 18 (K18) gene is 5- to 10-fold more active in tumorigenic (T-type) cell clones derived from the SW613-S human colon carcinoma cell line than in non-tumorigenic (NT-type) clones. We have reported previously that the mechanism responsible for this differential activity is acting on the minimal K18 promoter (TATA box and initiation site). This mechanism does not require the binding of a factor to a specific site on the DNA but involves the acetylation of a non-histone substrate. To get further insight into this mechanism, we investigated the effect of the adenovirus E1A protein on the activity of the K18 promoter, both in T and NT cells.

Results

Wild type adenovirus E1A protein and C-terminal deletion mutants inhibit the K18 promoter, specifically in T-type cells. The domain responsible for this inhibitory effect is located in the 12–25 region of the viral protein. E1A mutants that have lost this region but retain the PLDLS motif (the C-terminal binding site for CtBP1) stimulate the K18 promoter, specifically in NT cells. The inhibitory or stimulatory effects of the different E1A mutants are not dependent on a particular sequence of the promoter. An E1A N-terminal deletion mutant carrying point mutations in the PLDLS motif cannot stimulate the K18 promoter. CtBP1 interacts with CtIP, which is a known partner of BRCA1, itself a component of the RNA polymerase II holoenzyme. The stimulatory effect of two BRCA1 mutants, specifically in NT cells, implicates a tripartite BRCA1-CtIP-CtBP1 complex in the regulation of the K18 promoter.

Conclusion

Since we have shown previously that the K18 promoter is stimulated by deacetylase inhibitors, specifically in NT cells, we conclude that the activity of the promoter is repressed in NT cells by a mechanism involving the recruitment, by a BRCA1/CtIP complex, of CtBP1 and associated deacetylases to the preinitiation complex. We propose a model depicting the mechanism responsible for the differential activity of the K18 promoter between T and NT cells of the SW613-S cell line.

Regulation of expression of the Epstein-Barr virus BamHI-A rightward transcripts

The Epstein-Barr virus (EBV) BamHI-A rightward transcripts, or BARTs, are a family of mRNAs expressed in all EBV latency programs, including EBV-infected B cells in healthy carriers. Despite their ubiquitous expression, the regulation and biological function of BARTs are still unclear. In this study, the BART 5' termini were characterized by using a procedure that selects capped, full-length mRNAs. Two TATA-less promoter regions, designated P1 and P2, were mapped. P1 had relatively high basal activity in both epithelial and B cells, whereas P2 exhibited higher activity in epithelial cells. Upon EBV infection of B cells, transcription from P1 was detected soon after infection, while expression from P2 was delayed. Promoter-reporter assays in transiently transfected cells revealed that P1 and P2 were differentially regulated. Interferon regulatory factor 7 (IRF7) and IRF5 negatively regulated P1 activity. c-Myc and C/EBP family members positively regulated P2. Regulation of P2 by C/EBPs was characterized by electrophoretic mobility shift assay, chromatin immunoprecipitation, and reporter assays. More-abundant BART expression in epithelial cells correlated with the relative expression of positive and negative regulators in these cells.