ALBERT

Description: Background Palmitic acid, the most common saturated free fatty acid, has been implicated in ER (endoplasmic reticulum) stress-mediated apoptosis. This lipoapotosis is dependent, in part, on the upregulation of the activating transcription factor-4 (ATF4). To better understand the mechanisms by which palmitate upregulates the expression level of ATF4, we integrated literature information on palmitate-induced ER stress signaling into a discrete dynamic model. The model provides an in silico framework that enables simulations and predictions. The model predictions were confirmed through further experiments in human hepatocellular carcinoma (HepG2) cells and the results were used to update the model and our current understanding of the signaling induced by palmitate. Results The three key things from the in silico simulation and experimental results are: 1) palmitate induces different signaling pathways (PKR (double-stranded RNA-activated protein kinase), PERK (PKR-like ER kinase), PKA (cyclic AMP (cAMP)-dependent protein kinase A) in a time dependent-manner, 2) both ATF4 and CREB1 (cAMP-responsive element-binding protein 1) interact with the Atf4 promoter to contribute to a prolonged accumulation of ATF4, and 3) CREB1 is involved in ER-stress induced apoptosis upon palmitate treatment, by regulating ATF4 expression and possibly Ca2+ dependent-CaM (calmodulin) signaling pathway. Conclusion The in silico model helped to delineate the essential signaling pathways in palmitate-mediated apoptosis.

Description: Background MicroRNA-155 (miR-155) is the diced product of the MIR155HG gene. miR-155 regulates the expression of many immune-specific transcripts, is overexpressed in many human lymphomas, and has oncogenic activity in mouse transgenic models. MIR155HG has been proposed to be a target gene for transcription factor NF-κB largely due to the positive correlation between high nuclear NF-κB activity and increased miR-155 expression following treatment with NF-κB inducers or in subsets of hematopoietic cancers. Nevertheless, direct regulation of the human MIR155HG promoter by NF-κB has not been convincingly demonstrated previously. Results This report shows that induction of NF-κB activity rapidly leads to increased levels of both primary MIR155HG mRNA and mature miR-155 transcripts. We have mapped an NF-κB-responsive element to a position approximately 178 nt upstream of the MIR155HG transcription start site. The -178 site is specifically bound by the NF-κB p50/p65 heterodimer and is required for p65-induced reporter gene activation. Moreover, the levels of miR-155 in nine human B-lymphoma cell lines generally correlate with increased nuclear NF-κB proteins. Conclusion Overall, the identification of an NF-κB-responsive site in the MIR155HG proximal promoter suggests that MIR155HG is a direct NF-κB target gene in vivo. Understanding NF-κB-mediated regulation of miR-155 could lead to improved immune cell-related diagnostic tools and targeted therapies.