Modulation of protein tyrosine phosphorylation in human cells by LPS and enzymatically deacylated LPS

Previous studies have shown that the antagonism of LPS-induced responses by lipid A analogs and the induction of tolerance (adaptation) to LPS can occur without decreasing LPS binding to CD14. To learn more about these inhibitory mechanisms, we studied the effects of LPS and dLPS on an early response in the LPS signal pathway, protein tyrosine phosphorylation. Using CD14-expressing THP-1 cells, we found that very low concentrations of LPS stimulated tyrosine phosphorylation of a 42 kDa protein (p42). dLPS did not stimulate detectable phosphorylation of a 42 or any other cellular protein, but it inhibited the ability of LPS to induce this response. dLPS was a potent inhibitor when added to the cells at the same time as LPS, but not when added one or two minutes after LPS. Exposing cells to LPS for 3 hours induced a state of tolerance (adaptation) in which the cells were refractory to restimulation of p42 phosphorylation by LPS, but not by other agonists. The duration of LPS-induced tolerance (8-16 hours) was much longer than the duration of the refractory state resulting from dLPS pre-treatment (under 2 hours). Cellular binding and uptake of LPS were not significantly reduced by preexposing the cells to LPS or dLPS. The data indicate that the mechanisms of dLPS antagonism and LPS-induced tolerance occur distal to CD14 binding and proximal to p42 tyrosine phosphorylation.