Objective: Mast cell chymase is a chymotryptic heparin proteoglycan-bound neutral protease that exerts its activity in extracellular fluids. We studied the effect of chymase on the apolipoprotein compositions and the abilities of plasmas from apolipoprotein (apo)A-I-knockout (A-I-KO) and wild-type (C57BL/6J) mice to stimulate efflux of cellular cholesterol from mouse macrophage foam cells.
Methods and results: The A-I-KO apolipoproteins compared with the wild-type (apoA-I, apoA-II, apoA-IV, and apoE) showed total lack of apoA-I, unaltered apoA-II, an absence of apoA-IV, and an increase of apoE. Despite these major differences, the 2 plasmas induced similar high-affinity efflux of cholesterol from the foam cells. Quantitative analysis of chymase-treated plasmas revealed (1) in A-I-KO plasma, complete loss of apoE and apoA-II, and (2) in wild-type plasma, slight reduction of apoA-I associated with complete depletion of the minor pre-beta-high density lipoprotein fraction, strong reduction of apoA-II, and complete depletion of apoA-IV and apoE. Both proteolyzed plasmas had lost the ability to induce cellular cholesterol efflux with high affinity. Addition of discoidal pre-beta-migrating reconstituted high density lipoprotein particles containing human apoA-I or apoA-II to the chymase-treated A-I-KO plasma fully restored its cholesterol efflux-inducing ability, indicating functional replacement of the proteolyzed apoE and apoA-II. Thus, chymase degraded all the nondeleted apolipoproteins of the A-I-KO plasma involved in the high-affinity efflux of cellular cholesterol.
Conclusions: This is the first indication that genetically engineered mice could be used as models for examining the hypothesis that extracellular proteases are involved in the development of atherosclerosis by inhibiting the apolipoprotein-mediated removal of macrophage cholesterol.