GENETIC DEFICIENCY IN LOW-DENSITY-LIPOPROTEIN RECEPTOR-RELATED PROTEIN CONFERS CELLULAR-RESISTANCE TO PSEUDOMONAS EXOTOXIN-A - EVIDENCE THAT THIS PROTEIN IS REQUIRED FOR UPTAKE AND DEGRADATION OF MULTIPLE LIGANDS

The low density lipoprotein receptor-related protein (LRP) is a large multifunctional receptor implicated in the cellular uptake of function ally diverse ligands. Biochemical evidence suggests that LRP is a clea rance receptor for apoE-rich remnant lipoproteins, lipoprotein lipase, alpha2-macroglobulin/protease complexes, plasminogen activator/inhibi tor complexes, the active protease tissue=type plasminogen activator a nd exotoxin A from Pseudomonas aeruginosa. Mice genetically deficient in LRP die early during gestation, underscoring the essential physiolo gical role of this gene in vivo. To study the effect of LRP deficiency at the cellular level, we have used Pseudomonas exotoxin A (PEA) to s elect murine embryonic fibroblasts that are genetically deficient in L RP. Our results demonstrate that this single gene defect is sufficient to confer resistance to PEA on cultured cells. In addition, embryonic fibroblasts lacking LRP are unable to bind, internalize and degrade m ethylamine-activated alpha2-macroglobulin and complexes of urokinase w ith plasminogen activator inhibitor-1. Furthermore, cellular uptake an d degradation of receptor-associated protein, a 39 kDa accessory prote in of LRP, is reduced by 90% in the absence of LRP. These results prov ide genetic evidence for the multifunctional nature of LRP and its cru cial role in protease/inhibitor complex metabolism.