LEVEL OF RECEPTOR-ASSOCIATED PROTEIN MODERATES CELLULAR-SUSCEPTIBILITY TO PSEUDOMONAS EXOTOXIN-A

Pseudomonas exotoxin A (PE) enters mammalian cells via a receptor-medi ated endocytic pathway. The initial step in this pathway is binding to the multiligand receptor termed the alpha(2)-macroglobulin receptor/l ow-density lipoprotein receptor-related protein (LRP). Binding of toxi n, and of the many other ligands that bind to LRP, is blocked by the a ddition of a 39-kDa receptor-associated protein (RAP), sere we show th at approximately 40% of the cell-associated LRP is on the surface of t oxin-sensitive mouse LM fibroblasts and thus accessible for toxin inte rnalization. The remainder is located intracellularly, primarily in th e Golgi region. Mammalian cells exhibit a wide range of sensitivity to PE. To investigate possible reasons for this, we examined the express ion levels of both LRP and RAP. Results from a variety of cell lines i ndicated that there was a positive correlation between LRP expression and toxin sensitivity. In the absence of LRP, cells were as much as 20 0-fold more resistant to PE compared with sensitive cells. A second gr oup of resistant cells expressed LRP but had a high level of RAP. Thus , a toxin-resistant phenotype would be expected when cells expressed e ither low levels of LRP or high levels of LRP in the presence of high levels of RAP. We hypothesize that RAP has a pivotal role in moderatin g cellular susceptibility to PE.