Cellular events mediated by lipopolysaccharide-stimulated toll-like receptor 4 - MD-2 is required for activation of mitogen-activated protein kinasesand Elk-1
H. Yang et al., Cellular events mediated by lipopolysaccharide-stimulated toll-like receptor 4 - MD-2 is required for activation of mitogen-activated protein kinasesand Elk-1, J BIOL CHEM, 275(27), 2000, pp. 20861-20866
Lipopolysaccharide (LPS) stimulates multiple signaling events, including nu
clear factor-kappa B (NF-kappa B) activity and the mitogen-activated protei
n (MAP) kinases, ERK, JNK, and p38 in LPS-responsive cells, resulting in tr
anscriptional activation and cytokine generation. LPS-induced signaling via
toll-like receptor 4 (TLR4) results in the activation of the transcription
factor NF-kappa B. Since LPS activates other signaling cascades in respons
ive cells, the objective of this study was to determine whether such events
are mediated by TLR4 in response to LPS, me generated human embryonic kidn
ey cells (HEK293) that stably express TLR4 (HEK-TLR4) and examined their re
sponsiveness to LPS by measuring NF-kappa B activity and production of inte
rleukin-8 (IL-8). A trans-reporting system was used to measure the activity
of Elk-1, an ETS-domain transcription factor targeted by MAP kinase pathwa
ys. LPS stimulated NF-kappa B reporter activity and IL-8 production but not
Elk-1 activity in HEK-TLR4 cells. When MD-2, a protein associated with the
extracellular domain of TLR4, was expressed in these cells, there was a ma
rked increase in Elk-1 activity as well as ERK, JNK, and p38 MAP kinase pho
sphorylation in response to LPS. TLR4-mediated NF-kappa B reporter activity
and IL-8 production was enhanced by the expression of MD-2. This study dem
onstrates that expression of both TLR4 and MD-2 is required for LPS to acti
vate or augment the MAP kinase pathways, Elk-1 stimulation, and IL-8 genera
tion.