- [Show abstract] [Hide abstract] ABSTRACT: Carrageenan (CGN), a widely used food additive, has been shown to injure the epithelial barrier in animal models. This type of damage is a clinical feature of inflammatory bowel disease (IBD) in humans. In the present study, the effects of CGN on pro-apoptotic responses associated with macrophage inhibitory cytokine 1 (MIC-1) regulation in human enterocytes were evaluated. CGN up-regulated the expression of MIC-1 that promoted epithelial cell apoptosis. Although MIC-1 induction was dependent on pro-apoptotic p53 protein, the pro-survival protein activating transcription factor 3 (ATF3) was negatively regulated by p53 expression. However, MIC-1 enhanced the expression of the pro-survival protein ATF3 in enterocytes exposed to CGN. Functionally, MIC-1-mediated epithelial cell apoptosis was counteracted by the pro-survival action of ATF3 in response to CGN exposure. These findings demonstrated that the counterbalance between MIC-1 and ATF3 is critical for deciding the fate of enterocytes under the food chemical stress.
- [Show abstract] [Hide abstract] ABSTRACT: The cell-protective features of the ER stress response are chronically activated in vigorously growing malignant tumor cells, which provide cellular growth advantages over the adverse microenvironment including chemotherapy. As an intervention with ER stress responses in the intestinal cancer cells, preventive exposure to flavone apigenin potentiated super-induction of a regulatory transcription factor, activating transcription factor 3 (ATF3), which is also known to be an integral player coordinating ER stress response-related gene expression. ATF3 super-induction was due to increased turnover of ATF3 transcript via stabilization with HuR protein in the cancer cells under ER stress. Moreover, enhanced ATF3 caused inhibitory action against ER stress-induced cancer chemokines that are potent mediators determining the survival and metastatic potential of epithelial cancer cells. Although enhanced ATF3 was a negative regulator of the well-known pro-inflammatory transcription factor NF-κB, blocking of NF-κB signaling did not affect ER stress-induced chemokine expression. Instead, immediately expressed transcription factor early growth response protein 1 protein (EGR-1) was positively involved in cancer chemokine induction by ER stressors. ER stress-induced EGR-1 and subsequent chemokine production were repressed by ATF3. Mechanistically, ATF3 directly interacted with and recruited HDAC1 protein, which led to epigenetic suppression of EGR-1 expression and subsequent chemokine production. Conclusively, super-induced ATF3 attenuated ER stress-induced cancer chemokine expression by epigenetically interfering with induction of EGR-1, a transcriptional modulator crucial to cancer chemokine production. Thus, these results suggest a potent therapeutic intervention of ER stress response-related cancer-favoring events by ATF3.
- [Show abstract] [Hide abstract] ABSTRACT: In response to excessive nucleotide-binding oligomerization domain-containing protein 2 (Nod2) stimulation caused by mucosal bacterial components, gut epithelia need to activate regulatory machinery to maintain epithelial homeostasis. Activating transcription factor 3 (ATF3) is a representative regulator in the negative feedback loop that modulates TLR-associated inflammatory responses. In the current study, the regulatory effects of ribosomal stress-induced ATF3 on Nod2-stimulated proinflammatory signals were assessed. Ribosomal inactivation caused persistent ATF3 expression that in turn suppressed proinflammatory chemokine production facilitated by Nod2. Decreased chemokine production was due to attenuation of Nod2-activated NF-κB and early growth response protein 1 (EGR-1) signals by ATF3. However, the underlying molecular mechanisms involve two convergent regulatory pathways. Although ATF3 induced by ribosomal inactivation regulated Nod2-induced EGR-1 expression epigenetically through the recruitment of histone deacetylase 1, NF-κB regulation was associated with posttranscriptional regulation by ATF3 rather than epigenetic modification. ATF3 induced by ribosomal inactivation led to the destabilization of p65 mRNA caused by nuclear entrapment of transcript-stabilizing human Ag R protein via direct interaction with ATF3. These findings demonstrate that ribosomal stress-induced ATF3 is a critical regulator in the convergent pathways between EGR-1 and NF-κB, which contributes to the suppression of Nod2-activated proinflammatory gene expression.
- [Show abstract] [Hide abstract] ABSTRACT: NF-κB expression and activity are strictly regulated in gut epithelia to prevent overstimulation of pro-inflammatory responses following exposure to commensal bacteria. The effects of epithelial EGR-1 on responses to bacterial NF-κB-activating lipopolysaccharide (LPS) in intestinal epithelial cells under ribosomal stress were assessed. This was done to determine the potential of EGR-1 as a modulator of epithelial NF-κB signaling. Nuclear translocation of phosphorylated p65 protein was observed in the cells exposed to LPS although chemokine expression was marginally affected. In contrast, simultaneous exposure to LPS and ribosomal insults prevented epithelial NF-κB activation while chemokine expression was enhanced. The effect of EGR-1, another pro-inflammatory signaling mediator, was monitored to determine the involvement of this factor on chemokine production in response to this co-treatment. Similar to the previously reported ribosomal stress response, EGR-1 expression was elevated by ribosomal insults alone and positively affected gene expression of pro-inflammatory chemokines in the intestinal epithelial cells. However, EGR-1 suppression led to super-induction of chemokines by simultaneous treatment with LPS and ribosomal insult, indicating that EGR-1 is a negative modulator of chemokine gene expression. Particularly, mucosal ribosomal insult-triggered EGR-1 mediated PPARγ induction, which counteracted NF-κB activation by LPS. It can be thus concluded that EGR-1 regulates pro-inflammatory NF-κB activation by LPS via PPARγ although EGR-1 is a positive mediator of chemokine expression following ribosomal insult in intestinal epithelial cells.
- [Show abstract] [Hide abstract] ABSTRACT: Excessive and persistent insults during endoplasmic reticulum (ER) stress lead to apoptotic cell death that is implicated in a range of chronic inflammatory diseases and cancers. Macrophage inhibitory cytokine 1 (MIC-1), a member of the transforming growth factor-β superfamily, is diversely linked to the pathogenesis of cancer. To investigate the precise molecular mechanisms of MIC-1 gene regulation, ER stress and its related signals were studied in human colon cancer cells. Functionally, MIC-1 played pivotal roles in ER stress-linked apoptotic death, which was also influenced by C/EBP homologous protein, a well known apoptotic mediator of ER stress. ER stress enhanced MIC-1 mRNA stability instead of transcriptional activation, and there were two mechanistic translocations critical for mRNA stabilization. First, C/EBP homologous protein triggered protein kinase C-linked cytosolic translocation of the HuR/ELAVL1 (Elav-like RNA-binding protein 1) RNA-binding protein, which bound to and stabilized MIC-1 transcript. As the second critical in-and-out regulation, ER stress-activated ERK1/2 signals contributed to enhanced stabilization of MIC-1 transcript by controlling the extended holding of the nucleated mRNA in the stress granules fusing with the mRNA-decaying processing body. We propose that these two sequential in-and-out modulations can account for stabilized transcription and subsequent translation of pro-apoptotic MIC-1 gene in human cancer cells under ER stress.