Quinolinate (Quin) is a classic example of a biochemical double-edged sword, acting as both essential metabolite and potent
neurotoxin. Quin is an important metabolite in the
kynurenine pathway of
tryptophan catabolism leading to the de novo synthesis of
nicotinamide adenine dinucleotide (
NAD+). As a precursor for
NAD+, Quin can direct a portion of
tryptophan catabolism toward replenishing cellular NAD+ levels in response to
inflammation and
infection. Intracellular Quin levels increase dramatically in response to immune stimulation [e.g.,
lipopolysaccharide (LPS) or
pokeweed mitogen (PWM)] in macrophages, microglia, dendritic cells, and other cells of the immune system. NAD+ serves numerous functions including energy production, the
poly ADP ribose polymerization (PARP) reaction involved in DNA repair, and the activity of various
enzymes such as the
NAD+-dependent deacetylases known as
sirtuins. We used highly specific
antibodies to
protein-coupled Quin to delineate cells that accumulate Quin as a key aspect of the response to immune stimulation and
infection. Here, we describe Quin staining in the brain, spleen, and liver after LPS administration to the brain or systemic PWM administration. Quin expression was strong in immune cells in the periphery after both treatments, whereas very limited Quin expression was observed in the brain even after direct LPS injection. Immunoreactive cells exhibited diverse morphology ranging from foam cells to cells with membrane extensions related to cell motility. We also examined
protein expression changes in the spleen after
kynurenine administration. Acute (8 h) and prolonged (48 h)
kynurenine administration led to significant changes in
protein expression in the spleen, including multiple changes involved with cytoskeletal rearrangements associated with cell motility.
Kynurenine administration resulted in several expression level changes in
proteins associated with
heat shock protein 90 (HSP90), a chaperone for the
aryl-hydrocarbon receptor (AHR), which is the primary
kynurenine metabolite receptor. We propose that cells with high levels of Quin are those that are currently releasing
kynurenine pathway metabolites as well as accumulating Quin for sustained NAD+ synthesis from
tryptophan. Further, we propose that the
kynurenine pathway may be linked to the regulation of cell motility in immune and
cancer cells.