Universal mechanisms of exposure response: calmodulin, a regulator of the macrophage response to radiation and target of enzymatic repair

Heather S. Smallwood

doi:10.7273/000005869

Exposure to environmental toxicants requires a well orchestrated immune response by macrophages for optimal health of the organism during the adaptation to exposure and beyond. Previously we demonstrated a rapid upregulation of calmodulin (CaM) protein levels in activated macrophages following exposure to the bacterial cell wall component lipopolysaccharide (LPS). Increasing CaM protein abundance facilitates binding and stabilization of inducible nitric oxide synthase (iNOS), forming an active enzymatic complex. This complex is functionally important, as is apparent from the increased generation of NO and enhanced bactericidal activity of macrophages overexpressing CaM against Salmonella typhimurium. In contrast, we find that radiation exposure induces a slower dose-dependent increase in CaM abundance in macrophages that contributes to a decrease in the hypersensitivity region (HSR) of radiation exposure. Further, the overexpression of CaM results in increased radioresistance in macrophages, significantly increasing the LD50 of macrophages and preventing radiation induced cell death at 1 Gy. This maintenance of macrophage viability in response to radiation exposure occurs due to an increase in DNA repair pathways mediated by phosphorylated H2AX (i.e. a histone protein), which we find to be significantly enhanced by increases in CaM abundance. These results demonstrate an important role for CaM in mediating divergent adaptive responses to environmental toxicants. Critical to cellular adaptation to oxidative stress are antioxidant defense mechanisms, which we have demonstrated to include a cellular denitrase activity that is upregulated following macrophage exposure to LPS. We postulate this denitrase activity to be critical to the maintenance of tyrosine kinase-dependent signaling pathways involving select proteins, such as CaM, under conditions of oxidative stress. In addition, we find reduction of methionine sulfoxides in oxidized CaM, involving endogenous methionine sulfoxide reductase enzymes, and an endogenous peptidase that selectively cleaves the C-terminus Lys148 of oxidized CaM that is suggested to alter CaM function or targeting it for degradation. Taken together, these findings demonstrate a significant role for CaM in the differential response of macrophages to environmental stimuli and identify CaM as a target of selective enzymatic repair which maintains CaM function through conditions of oxidative stress that occur as a result of exposure to specific substances that activate macrophages.

Universal mechanisms of exposure response: calmodulin, a regulator of the macrophage response to radiation and target of enzymatic repair

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