Lipopolysaccharide differentially modulates expression of cytokines and cyclooxygenases in dorsal root ganglion cells via Toll-like receptor-4 dependent pathways

- Isolated dorsal root ganglion neurons express TLR4 on their cell surface.
- Cytokine production by LPS-stimulated DRG cells is NF-κB-dependent.
- LPS-stimulated COX-2 is dependent on both NF-κB and EGFR transactivation.
- LPS-stimulated PGE2 and PGI2 production in a COX-2-dependent manner.
- Activation of TLR4 on DRG neurons by endogenous ligands may underlie pain states.

We have examined the functional expression of Toll-like receptor 4 (TLR4) in adult male rat dorsal root ganglion (DRG) cells in culture by studying changes in pro-inflammatory cytokines and cyclooxygenase (COX)-dependent prostanoid production. In the mixed population of DRG neurons and glial cells, only DRG neurons expressed cell surface TLR4 along with MD-2 and CD14. This classical TLR4 signaling complex on DRG neurons responded to lipopolysaccharide (LPS) with a TLR4-dependent and time-dependent increase in interleukin-1β and tumor necrosis factor-α mRNA expression which was entirely dependent on NF-κB activity. In contrast, after 2-h incubation with DRG cells, LPS-stimulated COX-2 was regulated by both NF-κB and transactivation of epidermal growth factor receptor (EGFR) with potential downstream activation of ERK1/2 and p38 kinase. In contrast to this evidence for myeloid differentiation primary response gene-88 (MyD88)-dependent signaling, no evidence was obtained for TIR-domain-containing adaptor-inducing interferon-ß (TRIF)-dependent signaling from TLR4 in DRG neurons. LPS surprisingly produced a time-dependent decrease in COX-1 protein which likely facilitates the COX-2-dependent production of prostaglandin E2 and prostacyclin. Our study is the first to demonstrate the activation of TLR4-dependent production of prostaglandin E2 and prostacyclin in DRG cell cultures. Our findings support the concept that the activation of TLR4 on primary sensory neurons by endogenous ligands may underlie neuropathic and inflammatory pain states.

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