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  • Kasper Morton posted an update 4 months ago

    The extracellular pathogen Yersinia, the vacuole-bound pathogen Chlamydia, and the cytosolic pathogen Listeria, all require HRI to efficiently complete their respective cellular infection cycles. A common denominator among these three pathogens is that they all require access to the infected cell cytosol: either for their virulence factors to manipulate host cell processes or for the bacterium itself to reach the compartment in which it proliferates. How could a HRI-mediated process promote pathogen access to the host cytosol? A commonality among the three pathogens used in our studies is that their respective infection cycles are dependent on forming pores in infected host cellular membranes. In Gram-negative pathogens such as Yersinia and Chlamydia, the T3S secrete translocators that assemble poreforming structures in the host plasma membrane that mediate the transfer of effectors into the cytosol. In some respects this process resembles that which occurs in Listeriainfected cells. Following its invasion of the host cell, Listeria secretes monomeric LLO that, following its activation by the host-encoded gamma-interferon-inducible lysosomal thiol reductase, binds to and oligomerizes into poreforming structures within the endosomal membrane. In addition to allowing leakage of antimicrobial factors, the resulting pores are also thought to allow the access of co-expressed and secreted phospholipases to the inner leaflet of the endosomal membrane. It is possible that one or more of these events occur with reduced efficiency in HRI null cells. We believe that the activities of HRI described here neither involve it acting as an eIF2α kinase nor otherwise affecting protein synthesis. That T3S secretion is not coupled to host cell protein translation was indicated by our finding that cycloheximide treatment did not affect YopE-mediated disruption of the host cell cytoskeleton. The most compelling data, in our opinion, supporting the model that the infectionspecific activities of HRI are independent of its role as a regulator of protein synthesis is the fact that cells lacking the Ser51 residue of eIF2α are just as competent as wild-type cells in supporting both the T3S activity of Yersinia as well as the T3Sdependent intracellular proliferation of Chlamydia. However, eIF2α -expressing cells are more permissive for bacterial invasion indicating that eIF2 signaling does impact the initial events of the pathogen-host cell interaction. For example, Chlamydia is much more efficient at forming inclusions in eIF2α cells; however, the number of infectious EBs per inclusion is comparable between wild-type and eIF2α cells. This latter phenotype resembles that observed for PKR null cells indicating that PKR-mediated eIF2 signaling acts to oppose bacterial invasion but does not affect the subsequent maturation of the Chlamydial inclusion. Our findings are consistent with the observation first noted by Alexander of increased Chlamydial proliferation in cycloheximide-treated cells due to enhanced pathogen invasion. Owing to its broad activity in promoting the intracellular proliferation of pathogens, HRI may be an excellent target for the development of anti-microbial compounds. HRI is an especially attractive target since it is not required for responses to non-pathogens but interferes with specific virulence mechanisms. Recently, it has been shown that HRI activity can be reduced by either direct targeting with small molecules or indirectly by using natural products that inhibit the HRI-cofactor Hsp90. The targeting of host factors may make it less likely that a pathogen would evolve drug-resistance since the pathogen would not have genetic control over the interaction between the compound and its target. We recently demonstrated that interactions between Glycer-AGEs and the receptor for AGEs affect intracellular signaling, gene expression, and the release of pro-inflammatory molecules and also induce oxidative stress in numerous types of cells, all of which can contribute to the pathological changes observed in various chronic diseases. Furthermore, we detected increased hepatic RAGE expression and the enhanced production/accumulation of Glycer-AGEs in normal rats administered Glu-AGE-rich beverages. These findings indicate that Glu-AGEs, which are frequently found in beverages and foods, and hence, are taken into the body orally, enhance the production/accumulation of Glycer-AGEs, leading to Glycer- AGE-RAGE interactions. Our recent studies indicated that the serum levels of Glycer-AGEs, but not hemoglobin A1c, Glu-AGEs, or CML, could be used as biomarkers for predicting the progression of lifestyle-related diseases.We previously demonstrated that the enhanced production/accumulation of Glycer-AGEs after the oral consumption of Glu- AGEs plays an important role in the pathogenesis of vascular damage. A previous study indicated that a significant proportion of pro-inflammatory AGEs are derived from dietary components ; however, it is disputed whether such AGEs are a health risk. ELISA or liquid chromatography-mass spectrometry are usually used to assess the levels of AGEs in bodily secretions, foods, and beverages.