Inflammatory signals called prostaglandins (PGs) facilitate induction of the febrile response to infection. Fever-alleviating drugs are used by millions of people every day and act by inhibiting cyclooxygenase (COX), which catalyses the first step in the formation of PGs from arachidonic acid. But the use of COX inhibitors is associated with severe side effects that are probably related to their non-specific effects on the synthesis of several PGs. Targeting PGs that are specifically involved in eliciting fever might permit the development of anti-pyretic drugs with an improved safety profile. A study published recently in Nature Neuroscience brings us one step closer to realizing this goal.
A team led by Anders Blomqvist investigated the role of microsomal prostaglandin E synthase-1 (mPges1) — which catalyses the second step in Pge2 production — in the febrile response, by knocking out its expression in mice. Mutant mice were indistinguishable from their wild-type littermates under normal physiological conditions. However, differences emerged following immune challenge with bacterial cell-wall lipopolysaccharide (LPS). Shortly after injection of LPS, the core body temperature of wild-type mice increased significantly and remained elevated for about six hours. By contrast, the temperature of immune-challenged mPges1-deficient mice did not differ from that of saline-injected controls. Direct injection of Pge2 into brains of mutants elicited a robust febrile response, confirming that these mPges1-deficient mice retained the capacity to respond to the product of mPges1 activity.
Levels of Pge2 in cerebrospinal fluid after administration of LPS mimicked the temperature pattern, increasing in wild-type subjects and remaining static in their mutant counterparts. Incubation of brain homogenates from immune-challenged mice with the substrate of mPges1, Pgh2, showed that these responses were correlated with enzymatic activity, or lack thereof, in the microsomal fraction. Reverse transcription-polymerase chain reaction was used to confirm that the physiological effects of LPS injection were due to differential expression of mPges1 in brain homogenates of wild-type and mutant mice.
Interestingly, expression of another PG-synthesizing enzyme — mPges2 — was not upregulated by immune challenge of wild-type mice, indicting that mPges1 is specifically involved in facilitating fever. Similarly, the response of Cox2 to LPS injection was not affected by the mPges1 mutation, being upregulated in both mutant and wild-type mice. These results show that, unlike present-generation COX inhibitors, compounds that target mPges1 should specifically inhibit the synthesis of fever-inducing Pge2, without affecting the production of other PGs.
In a related study in The Journal of Physiology, Andrej Romanovsky and colleagues identified another new mediator of the febrile response. They showed that platelet-activating factor (PAF) has pyrogenic activity and mediates LPS-induced fever. Peripheral injection with PAF caused the body temperature of rats to increase. This increase was more pronounced following administration of a physiologically relevant form of PAF (that is, PAF complexed with bovine serum albumin) compared with free PAF. The PAF receptor antagonist BN 52021 attenuated these febrile responses, as well as fever resulting from challenge with LPS, but it remains to be established whether PAF and its receptor are valid targets for the development of new anti-pyretics.
ORIGINAL RESEARCH PAPERS
Engblom, D. et al. Microsomal prostaglandin E synthase-1 is the central switch during immune-induced pyresis. Nature Neurosci. 6, 1137–1138 (2003)
Ivanov, A. I. et al. Platelet-activating factor: a previously unrecognized mediator of fever. J. Physiol. (Lond.) 17 October 2003 (doi: 10.1113/jphysiol.2003.055616)
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Farley, S. Heat relief. Nat Rev Neurosci 4, 934 (2003). https://doi.org/10.1038/nrn1292