Insulin Signaling and the Heat Shock Response Modulate Protein Homeostasis in the Caenorhabditis elegans Intestine during Infection

During infection, damage can occur to the host as an outcome of both pathogen virulence mechanisms and host defense strategies. Using aggregation of a model polyglutamine-containing protein as an indicator in Caenorhabditis elegans, we show that protein damage occurs specifically at the site of the host-pathogen interaction, the intestine, in response to various bacterial pathogens. We demonstrate that the insulin signaling pathway and the heat shock transcription factor (HSF-1) influence the amount of aggregation that occurs, in addition to heat shock proteins and oxidative stress enzymes. We also show that addition of the antioxidants epigallocatechin gallate and α-lipoic acid reduces polyglutamine aggregation. The influence of oxidative stress enzymes and exogenous antioxidants on protein aggregation suggests that reactive oxygen species produced by the host are a source of protein damage during infection. We propose a model in which heat shock proteins and oxidative stress enzymes regulated by insulin signaling and HSF-1 are required for tissue protection during infection, to minimize the effects of protein damage occurring as a result of host-pathogen interactions. During infection, damage can occur to the host as an outcome of both pathogen virulence mechanisms and host defense strategies. Using aggregation of a model polyglutamine-containing protein as an indicator in Caenorhabditis elegans, we show that protein damage occurs specifically at the site of the host-pathogen interaction, the intestine, in response to various bacterial pathogens. We demonstrate that the insulin signaling pathway and the heat shock transcription factor (HSF-1) influence the amount of aggregation that occurs, in addition to heat shock proteins and oxidative stress enzymes. We also show that addition of the antioxidants epigallocatechin gallate and α-lipoic acid reduces polyglutamine aggregation. The influence of oxidative stress enzymes and exogenous antioxidants on protein aggregation suggests that reactive oxygen species produced by the host are a source of protein damage during infection. We propose a model in which heat shock proteins and oxidative stress enzymes regulated by insulin signaling and HSF-1 are required for tissue protection during infection, to minimize the effects of protein damage occurring as a result of host-pathogen interactions. Innate immunity is comprised of strategies that allow an organism to immediately defend itself against an invading pathogen. In addition to mechanisms that actively destroy the infecting agent, the organism must protect itself from damage. Damage to the host can occur from virulence mechanisms of the pathogen, but also as a side effect of the immune response of the host. One pathway that we have shown contributes greatly to pathogen resistance in the model host Caenorhabditis elegans is insulin signaling (1Garsin D.A. Villanueva J.M. Begun J. Kim D.H. Sifri C.D. Calderwood S.B. Ruvkun G. Ausubel F.M. Science. 2003; 300: 1921Crossref PubMed Scopus (443) Google Scholar). In addition to increased pathogen resistance, loss of insulin signaling in C. elegans results in several cytoprotective phenotypes such as stress resistance (oxidative stress, heat stress) and long lifespan. The insulin signaling pathway consists of a receptor, DAF-2, that when stimulated activates a phosphatidylinositol 3-kinase signaling cascade that culminates in the phosphorylation and down-regulation of the transcription factor DAF-16. A mutation in DAF-2, or any of the other upstream signaling components prevents inhibition of DAF-16, causing greater transcriptional activity (reviewed in Refs. 2Finch C.E. Ruvkun G. Annu. Rev. Genomics Hum. Genet. 2001; 2: 435-462Crossref PubMed Scopus (295) Google Scholar, 3Guarente L. Kenyon C. Nature. 2000; 408: 255-262Crossref PubMed Scopus (1108) Google Scholar, 4Nelson D.W. Padgett R.W. Genes Dev. 2003; 17: 813-818Crossref PubMed Scopus (33) Google Scholar, 5Tatar M. Bartke A. Antebi A. Science. 2003; 299: 1346-1351Crossref PubMed Scopus (1044) Google Scholar). It was previously shown that pathogen resistance mediated by increased DAF-16 activity is dependent, at least in part, on the heat shock transcription factor HSF-1. 2The abbreviations used are:HSF-1heat shock transcription factorROSreactive oxygen speciesEGCGepigallocatechin gallateLAα-lipoic acidHSPsheat shock proteinsYFPyellow fluorescent proteinRNAiRNA interferingDAFabnormal DAuer Formation The loss of hsf-1 in a daf-2 mutant, or a mutant that overexpresses daf-16, causes a reduction in pathogen resistance and the overexpression of hsf-1 increases pathogen resistance (6Singh V. Aballay A. Proc. Natl. Acad. Sci. U. S. A. 2006; 103: 13092-13097Crossref PubMed Scopus (161) Google Scholar). HSF-1 likely mediates these effects by controlling the expression of genes encoding heat shock proteins (HSPs). HSPs are protein chaperones that bind to unfolded or damaged proteins and prevent aggregation until they can be refolded or recycled (7Buchner J. FASEB J. 1996; 10: 10-19Crossref PubMed Scopus (382) Google Scholar, 8Frydman J. Annu. Rev. Biochem. 2001; 70: 603-647Crossref PubMed Scopus (936) Google Scholar, 9Sakahira H. Breuer P. Hayer-Hartl M.K. Hartl F.U. Proc. Natl. Acad. Sci. U. S. A. 2002; 99: 16412-16418Crossref PubMed Scopus (199) Google Scholar, 10Horwitz J. Proc. Natl. Acad. Sci. U. S. A. 1992; 89: 10449-10453Crossref PubMed Scopus (1745) Google Scholar, 11Van Montfort R. Slingsby C. Vierling E. Adv. Protein Chem. 2001; 59: 105-156Crossref PubMed Scopus (384) Google Scholar). HSPs regulated by HSF-1 were demonstrated to be protective against bacterial pathogens (6Singh V. Aballay A. Proc. Natl. Acad. Sci. U. S. A. 2006; 103: 13092-13097Crossref PubMed Scopus (161) Google Scholar) and we showed that ROS, a possible source of cellular damage, is generated by the pathogen-exposed worm (12Chavez V. Mohri-Shiomi A. Maadani A. Vega L.A. Garsin D.A. Genetics. 2007; 176: 1567-1577Crossref PubMe