Mice deficient in
scavenger receptor class B type I (SR-BI) and
apolipoprotein E (apoE) [double knockout (DKO)
mice] develop
dyslipidemia, accelerated
atherosclerosis, and
myocardial infarction, and die prematurely. We examined effects of apoE and SR-BI deficiency on
macrophage cholesterol homeostasis. DKO
macrophages had increased total
cholesterol (TC) stores (220-380 microg/mg protein) compared with apoE-/- cells (40 microg/mg), showed significant
lysosomal lipid engorgement, and increased their TC by 34% after exposure to HDL. DKO
macrophages from apoE-/-
mice reconstituted with DKO
bone marrow showed less
cholesterol accumulation (89 microg/mg), suggesting that the
dyslipidemia of DKO
mice explains part of the cellular
cholesterol defect. However, analyses of DKO and apoE-/-
macrophages from transplanted apoE-/-
mice revealed a role for
macrophage SR-BI, inasmuch as the TC in DKO
macrophages increased by 10% in the presence of HDL, whereas apoE-/-
macrophage TC decreased by 33%. After incubation with HDL, the free
cholesterol (FC) increased by 29% in DKO
macrophages, and decreased by 8% in apoE-/- cells, and only DKO cells had FC in large peri-nuclear pools. Similar trends were observed with apoA-I as an acceptor. Thus, the abnormal
cholesterol homeostasis of DKO
macrophages is due to the plasma
lipid environment of DKO
mice and to altered trafficking of
macrophage cholesterol. Both factors are likely to contribute to the accelerated
atherosclerosis in DKO
mice.
