Kevin M. Claunch 

Introduction. The Unfolded Protein Response (UPR) is a critical homeostatic pathway in mammalian cells that is triggered by ER dysfunction and subsequent accumulation of misfolded and damaged proteins[1,7]. Activation of the UPR is achieved through three transmembrane proteins, one of which is IRE-1, which catalyzes the splicing of X-box binding protein-1 (Xbp-1), a transcription factor of UPR-specific genes[7]. Activation of the UPR is modulated in part by the activity of deubiquitinating enzymes, which function both as antagonists of the ubiquitin-proteosome pathway involved in protein turnover and as inhibitors of the UPR[7]. In addition to its role in cellular homeostasis, the UPR has been implicated in host defense to a number of pathogens, such as    Listeria monocytogenes [5], a leading cause of food-borne illness among the elderly, immunocompromised, and pregnant women3. The UPR has thus been investigated as a potential drug target for the treatment of L. monocytogenes infection[2,6]. Two derivates of the compound WP1130, compounds 9 and 6, were found to activate the UPR via inactivation of the deubiquitinating enzyme USP14[2,6]. Inactivation of USP14 by these compounds frees IRE-1, allowing it to splice Xbp-1 mRNA and induce the UPR[4,7]. Methods. RAW264.7 mouse macrophages were infected with L. monocytogenes and incubated with compounds 9 and 6 to determine if these compounds reduce bacterial replication in cell culture[2,6]. Inside-out immunofluorescence microscopy was also performed to determine if compound 6 limited bacterial internalization. In addition, both compounds were tested for direct antimicrobial activity by performing bacterial growth assays in the presence or absence of  each compound[6]. Results. Both compounds 9 and 6 were found to significantly reduce replication of L. monocytogenes in RAW264.7 macrophages with no detectable toxicity to the macrophages[2,6]. In addition, compound 6 was found to limit internalization of the bacteria into RAW264.7 cells with roughly one bacterial   cell internalized for every three that remained extracellular[6]. While these compounds act by inducing the UPR, they also may have some moderate direct antimicrobial effects as well, as evidenced by a modest decrease in bacterial growth rate when L. monocytogenes was grown in the presence of each compound in broth alone[2,6]. Conclusions. These findings indicate that both compounds 9 and 6 are promising candidates for further research and development as novel therapeutics for L. monocytogenes infection. Although these studies were performed in cell culture only, further work to verify or enhance efficacy of these drugs in animal models of infection is warranted.

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