Surface association of pregnancy-associated plasma protein-A accounts for its colocalization with activated macrophages.
Intense immunostaining for pregnancy-associated plasma protein-A (PAPP-A), a newly characterized metalloproteinase in the insulin-like growth factor system, colocalizes with activated macrophages in human atherosclerotic plaque. To determine macrophage regulation of PAPP-A expression, we developed two models of human macrophages with basal and activated phenotypes. THP-1 cells and peripheral blood monocytes could be differentiated into macrophages and activated upon specific treatment regimens with phorbol myristate acetate, macrophage colony-stimulating factor, and interleukin-1beta. Activation was assessed by cell secretion of tumor necrosis factor-alpha, which increased 30- to 100-fold with activation. Activated macrophages also secreted matrix metalloproteinase-9. However, no PAPP-A mRNA or PAPP-A antigen could be detected in these cells under any condition. Upon incubation with recombinant PAPP-A, we found that activated macrophages bound and internalized more PAPP-A than unactivated macrophages or monocytes. Internalization accounted for at least 50% of macrophage-associated PAPP-A, as assessed in studies with cytochalasin B. Membrane-bound PAPP-A retained protease activity, whereas internalized PAPP-A had little or no activity. Similar experiments carried out with a mutated variant of PAPP-A, which retains functionality as a protease but is unable to bind surface-associated glycosaminoglycan, showed no macrophage association or internalization. Absence of PAPP-A expression was confirmed in activated macrophages isolated from a hypercholesterolemic rabbit model of atherosclerosis. We therefore conclude that PAPP-A is not synthesized in, but rather is bound and internalized by, macrophages. Our findings likely account for the observed intense immunostaining for PAPP-A colocalizing with activated macrophages and may have physiological significance in the development of vulnerable plaque.
Conover
Cheryl A
CA
Division of Endocrinology, Metabolism, and Nutrition, Department of Internal Medicine, Mayo Clinic College of Medicine, Rochester, Minnesota, USA. Conover.Cheryl@mayo.edu
Harrington
Sean C
SC
Bale
Laurie K
LK
Oxvig
Claus
C
D000818:Animals; D050197:Atherosclerosis / Q000209:etiology / Q000378:metabolism*; D002454:Cell Differentiation / Q000187:drug effects; D002460:Cell Line; D004195:Disease Models, Animal; D004705:Endocytosis / Q000187:drug effects; D006801:Humans; D006937:Hypercholesterolemia / Q000150:complications; D018974:Insulin-Like Growth Factor Binding Protein 4 / Q000378:metabolism; D053583:Interleukin-1beta / Q000494:pharmacology; D008262:Macrophage Activation / Q000187:drug effects; D016173:Macrophage Colony-Stimulating Factor / Q000378:metabolism; D008264:Macrophages / Q000166:cytology / Q000187:drug effects / Q000378:metabolism*; D020780:Matrix Metalloproteinase 9 / Q000378:metabolism; D009154:Mutation; D011266:Pregnancy-Associated Plasma Protein-A / Q000235:genetics / Q000378:metabolism*; D011817:Rabbits; D011994:Recombinant Proteins / Q000494:pharmacology; D013755:Tetradecanoylphorbol Acetate / Q000494:pharmacology; D014409:Tumor Necrosis Factor-alpha / Q000378:metabolism
D018974:Insulin-Like Growth Factor Binding Protein 4; D053583:Interleukin-1beta; D011994:Recombinant Proteins; D014409:Tumor Necrosis Factor-alpha; D016173:Macrophage Colony-Stimulating Factor; D011266:Pregnancy-Associated Plasma Protein-A; D020780:Matrix Metalloproteinase 9; D013755:Tetradecanoylphorbol Acetate