Enhanced macrophage resistance to Pseudomonas exotoxin A is correlated with decreased expression of the low-density lipoprotein receptor-related protein

Cellular intoxification by exotoxin A of Pseudomonas aeruginosa (PEA) begin s when PEA binds to its cellular receptor, the low-density lipoprotein rece ptor-related protein (LRP). This receptor is particularly abundant on macro phages. We hypothesize here that inducible changes in cellular expression l evels of the LRP represent an important mechanism by which macrophage susce ptibility to PEA is regulated by the host. We have examined the effect of l ipopolysaccharide (LPS) on LRP expression and PEA sensitivity in the macrop hage-like cell line HS-P. Using a [H-3]leucine incorporation assay to measu re inhibition of protein synthesis, we have demonstrated that HS-P macropha ges are highly sensitive to PEA and that PEA toxicity is decreased by the L RP antagonist receptor-associated protein. LPS pretreatment decreases HS-P PEA sensitivity in a time- and dose-dependent manner. The dose of toxin req uired to inhibit protein synthesis by 50% increased from 11.3 +/- 1.2 ng/ml in untreated cells to 25.7 +/- 2.0 ng/ml in cells treated with LPS. In pul se experiments, involving brief exposure to saturating concentrations of PE A, [H-3]leucine incorporation was more than threefold higher in cells pretr eated with LPS than in untreated macrophages. These changes in HS-P PEA sen sitivity following LPS treatment were consistently associated with a fivefo ld decrease in HS-P LRP mRNA expression as measured by Northern blot analys is and a three-and-a-half-fold decrease in HS-P LRP-specific ligand interna lization as determined by activated alpha(2)-macroglobulin internalization studies. These data demonstrate for the first time that modulation of LRP l evels by extracellular signaling molecules can alter cellular PEA sensitivi ty.