Skewing of UV-induced cell death pathways using TLR4 ligands

Abstract

Abstract Toll-like receptor 4 (TLR4) is a canonical innate immune receptor for the Gram-negative bacterial outer membrane component lipopolysaccharide (LPS). TLR4 deficiency has been shown to contribute to UV-induced carcinogenesis in epidermal cells. UV-induced cellular damage results in apoptosis which is characterized by a non-inflammatory cell death through the cleavage and activation of caspase-3. Utilizing bacterial enzyme combinational chemistry (BECC) we generated hypoacylated lipid A moieties from Escherichia coli, Pseudomonas aeruginosa, and Salmonella typhimurium which have moderately attenuated immune-stimulatory capacity. Continuously exposing TLR4 to these innate immune ligands leads to tolerization and decreased expression of the receptor in mammalian cells. We hypothesize that skewing cell death away from apoptosis using lipid A mimetics to lower TLR4 expression prior to UV exposure will reduce carcinogenic potential. To analyze the roll of TLR4 in UV-induced carcinogenesis, wildtype and TLR4-deficient bone marrow-derived macrophage (BMDM) cell lines were tolerized with hexa-acylated or penta-acylated lipid A from multiple Gram-negative species and exposed to either 10 or 25 mJ/cm 2doses of UVB radiation. UV-irradiated BMDM cells show evidence of apoptosis including cleavage of DNA creating a ladder, whereas UV-irradiated BMDM cells tolerized with lipid A mimetics show a lack of apoptosis. The data suggests that tolerization of TLR4 using canonical ligands may be a useful tool in promoting DNA repair and maintaining immune responses following UV-induced damage. The research was supported by a Fisher College of Science and Mathematics Undergraduate Research Grant.