Endocrine Abstracts

TLR4 activation and the induction of endoplasmic reticulum stress (ERS) are two of the most important mechanisms connecting excessive dietary fat with insulin resistance. TLR4 activation is a primary event in the induction of cellular stress that contributes to increased inflammatory gene expression in metabolic diseases. However, the mechanisms linking these molecular events are unknown. The chaperones GRP78 and GRP94 play an important role during the assembly of newly translated TLR4 molecules. In addition, GRP94 escorts TLR4 to the cell membrane. Under prolonged activation the demand for newly synthesized TLR4 molecules increase, and thus, the demand for new chaperones. Therefore, we hypothesized that under increased activation of TLR4 the synthesis of the protein would not be matched by the expression of chaperones, thus, triggering ERS. To test this hypothesis, the monocyte cell line THP-1 was incubated with LPS and the expression/activation of proteins involved in ERS was determined. Time-course experiments revealed that LPS led to a 2.5-fold increase of TLR4 expression starting as early as 8 h, peaking after 24 h and remaining significantly increased after 48 h. The expression of GRP78 underwent a three-fold increase with a sharp rise at 24 h, while GRP94 increased by only 1.5-fold with a peak at 2 h and an early return to base-line levels. None of the chaperones were increased after 48 h. LPS-induced ERS was detected as early as 4 h after stimulus as detected by the evaluation of PERK/eIF2α, IRE1 and ATF6 pathways. Strong signals of ERS were still present after 48 h. The pre-incubation of THP-1 in glucose-deprived medium produced 2.5- and 11-fold increases of GRP94 and GRP78, respectively. Upon glucose deprivation, LPS could no longer induce ERS. Inhibition of chaperone expression by siRNA completely abrogated the effect of glucose deprivation to protect cells from LPS-induced ERS. Thus, insufficient LPS-induced chaperone expression links TLR4 signaling to ERS.

Declaration of interest: The authors declare that there is no conflict of interest that could be perceived as prejudicing the impartiality of the research project.

Funding: This work was supported however, funding details unavailable.