A Conserved Mechanism for Hormesis in Molecular Systems.

Hormesis refers to dose-response phenomena where low dose treatments elicit a response that is opposite the response observed at higher doses. Hormetic dose-response relationships have been observed throughout all of biology, but the underlying determinants of many reported hormetic dose-responses have not been identified. In this report, we describe a conserved mechanism for hormesis on the molecular level where low dose treatments enhance a response that becomes reduced at higher doses. The hormetic mechanism relies on the ability of protein homo-multimers to simultaneously interact with a substrate and a competitor on different subunits at low doses of competitor. In this case, hormesis can be observed if simultaneous binding of substrate and competitor enhances a response of the homo-multimer. We characterized this mechanism of hormesis in binding experiments that analyzed the interaction of homotrimeric proliferating cell nuclear antigen (PCNA) with uracil DNA glycosylase (UNG2) and a fluorescein-labeled peptide. Additionally, the basic features of this molecular mechanism appear to be conserved with at least two enzymes that are stimulated by low doses of inhibitor: dimeric BRAF and octameric glutamine synthetase 2 (GS2). Identifying such molecular mechanisms of hormesis may help explain specific hormetic responses of cells and organisms treated with exogenous compounds.