Sodium methyldithiocarbamate (SMD, also called metam sodium) is the third most abundantly used conventional agricultural pesticide in the U.S. It spontaneously breaks down to form methylisothiocyanate MTC), which has been classified a toxic air contaminant by the California EPA and which shares some immunomodulatory actions, with SMD. Both SMD and MITC alter toll-like receptor signaling in mouse peritoneal macrophages and cause substantial increases in the concentration of IL-10 and substantial decreases in IL-12 (and other pro-inflammatory cytokines and chemokines) in mice treated with bacterial lipopolysaccharide (IPS). SMD also substantially reduces innate resistance to Escherichia coli peritonitis in mice. A number of other chemicals and drugs cause increased expression of IL-10 and decreased expression of IL-12. Because these changes would tend to decrease innate immunity, understanding the mechanisms by which SMD and MITC affect these parameters has broad implications for human health, even for persons not exposed to SMD or MITC. This project will test the following hypothesis: SMD alters signaling through TLR4 to decrease the production of IL-12 and to increase IL-10 production by altering cellular redox balance, altering copper availability, and inducing stress mediators. These effects play key roles in decreasing innate resistance to infection with E. coli.
Three aims will be used to test the hypothesis of this project:
Specific Aim 1 - Determine the mechanism(s) by which SMD and MITC increase LPS-induced IL-10 production and decrease LPS-induced IL-12 production by peritoneal macrophages.
Specific Aim 2 - Determine the mechanism(s) by which SMD alters activation of upstream kinases involved in IL-10 and IL-12 expression and determine the role of increased IL-10 expression in the SMD and MITC-induced decreases in IL-12 expression.
Specific Aim 3 - Determine the role of each of the mechanisms evaluated in Aims 1 and 2 in the decreased resistance to E. coli peritonitis in SMD or MITC treated mice. In each Aim the mechanisms to be investigated are those noted in the hypothesis (altered redox balance, altered copper availability, and induction of stress mediators).
In Aim 3, mathematical models will be developed to predict or describe the relationships between changes in signaling, gene expression, and host resistance. These results will represent a new (systems biology) approach to immunotoxicology. These results will allow mechanism based risk assessment for SMD and MITC for the first time. It is also expected that the detailed study of three important mechanisms of action, which will be examined in each aim, can be generalized to other situations or toxicants that alter these parameters.
