Alveolar macrophages (AMs) represent the first line of innate immune defense in the lung. AMs use
pattern recognition receptors (
PRRs) to sense pathogens. The best studied PRR is
Toll-like receptor (TLR)4, which detects LPS from gram-negative bacteria. The
lipid mediator
prostaglandin (PG)E2 dampens AM immune responses by inhibiting the signaling events downstream of
PRRs. We examined the effect of
PGE2 on TLR4 expression in rat AMs. Although
PGE2 did not reduce the
mRNA levels of TLR4, it decreased TLR4
protein levels. The translation inhibitor
cycloheximide reduced TLR4
protein levels with similar kinetics as
PGE2, and its effects were not additive with those of the
prostanoid, suggesting that
PGE2 inhibits TLR at the translational level. The action of
PGE2 could be mimicked by the direct stimulator of cAMP formation,
forskolin, and involved E
prostanoid receptor 2
ligation and cAMP-dependent activation of unanchored type I
protein kinase A. Cells pretreated with
PGE2 for 24 hours exhibited decreased TNF-α
mRNA and
protein levels in response to LPS stimulation. Knockdown of TLR4
protein by
small interfering RNA to the levels achieved by
PGE2 treatment likewise decreased TNF-α
mRNA and
protein in response to LPS, establishing the functional significance of this
PGE2 effect. We provide the first evidence of a
lipid mediator acting through its cognate
G protein-coupled receptor to affect PRR translation. Because
PGE2 is produced in abundance at sites of
infection, its inhibitory effects on AM TLR4 expression have important implications for host defense in the lung.