Apoptosis, accompanied with cytoplasmic shrinkage, nuclear condensation and caspase activation induced by LPS, is thought to proceed via activation of LPS receptor, which is comprised of the toll-like receptor (TLR) 4 and MD-2. We report that LPS induced apoptosis in the human monoblastic cell line U937 that was pretreated with interferon γ(U937IFN), but not in cells that were untreated or treated with retinoic acid (U937RA). To reveal the reason why interferon LipoPolySaccharide (LPS); Apoptosis; LPS receptor; Differentiation; Monocyte LipoPolySaccharide (LPS); Apoptosis; LPS receptor; Differentiation; Monocyte γ(IFN) confers apoptotic susceptibility to U937 cells, we focused on the LPS receptor. IFN stimulated the expression of cell surface TLR4 and the transcription level of MD-2 mRNA. By transfecting a designed antisense oligonucleotide (ASO) into U937IFN cells before exposing to LPS, the expression level of TLR4 was lowered to levels comparable to those in U937 cells; ASO transfected-cells showed no cleavage of poly (ADP-ribose) polymerase (PARP) and exhibited no morphological changes. Addition of an anti-TLR4 monoclonal antibody to the cultures of U937IFN cells along with LPS resulted in reduction of the Ac-DEVD-MCA hydrolysis and inhibition of the PARP cleavage. Neither a loss of function form of FADD, an adaptor molecule for death receptor signaling, nor a neutralization antibody against TNF-α or Fas prevented LPS-triggered apoptosis, indicating that the death ligand and death receptor were not activated. These results suggest that IFN induces expression of the LPS receptor, resulting in increased susceptibility to LPS-induced apoptosis.
Author(s): Taku Kuwabara and Shinobu Imajoh-Ohmi