Effect of Bone Marrow Transplantation on Lipoprotein Metabolism and Atherosclerosis in LDL Receptor–Knockout Mice

Abstract

The LDL receptor (LDLR) plays an important role in the removal of LDL and its precursors, the intermediate and very low density lipoproteins, from the blood circulation. The receptor is expressed on various cell types. In this study the relative importance of the LDLR on macrophages for lipoprotein metabolism and atherogenesis was assessed. For this purpose, irradiated LDLR-knockout (-/-) mice were transplanted with bone marrow of normal C57BL/6J mice. DNA analysis showed that the transplanted mice were chimeric. The transplantation resulted in a slight decrease of total serum cholesterol when compared with LDLR-/- mice that were transplanted with LDLR-/- bone marrow. This modest decrease, however, did not reach statistical significance at all time points examined. This decrease can be almost completely attributed to a decrease in LDL cholesterol. The specific lowering of LDL cholesterol could clearly be observed at 4 weeks after transplantation, but the decrease was less at 12 weeks after transplantation. Quantification of atherosclerotic lesions of mice fed a 1% cholesterol diet for 6 months revealed that there were no differences in mean lesion area between mice transplanted with wild-type bone marrow or LDLR-/- bone marrow. We anticipate that in LDLR-/- mice transplanted with wild-type bone marrow, the LDLR is downregulated by the relatively high concentrations of circulating cholesterol. In vitro incubations of peritoneal macrophages with 125I-LDL indicated that the LDLR of these cells could be downregulated by 25-hydroxycholesterol. Peritoneal macrophages isolated from LDLR-/- mice transplanted with wild-type bone marrow, in contrast to those transplanted with LDLR-/- bone marrow, were able to degrade 125I-LDL, indicating that the capacity to express functional LDLR was achieved. In conclusion, introduction of the LDLR into LDLR -/- mice via bone marrow transplantation resulted in only a relatively modest decrease of LDL cholesterol that became less pronounced at later time points, possibly due to downregulation of the LDLR. To utilize the LDLR in macrophages for effective cholesterol lowering, either the sterol-regulatory elements have to be "silenced" or a high-expression LDLR construct has to be introduced into macrophages, eg, via transplantation of in vitro transfected hematopoietic stem cells.

The relative atherogenicity of VLDL and LDL is dependent on the topographic site

“…Second, LDLr expression in the vascular wall does not affect atherogenesis ( 27 ). Therefore, our data strongly suggest that whether LDL is more proatherogenic than VLDL is dependent on the topographic site.…”

Low Density Lipoprotein Receptor of Macrophages Facilitates Atherosclerotic Lesion Formation in C57Bl/6 Mice

“…We previously demonstrated that transplantation of C57Bl/6 bone marrow into LDLr(Ϫ/Ϫ) mice, which had serum cholesterol levels 9-fold higher than those of the mice in the present study even without cholate added to the diet, did not result in an increase of the atherosclerotic lesion area. 13 Although different animal models were used to analyze diet-induced (with and without the addition of cholate) atherosclerosis in these studies, which could make direct comparisons difficult, the present data strongly suggest that the LDLr can facilitate foam cell formation only under conditions of moderate cholesterol levels. Analysis of LDLr expression in more advanced foam cell-rich lesions of control transplanted C57Bl/63 C57Bl/6 mice revealed that in these mice, the macrophage LDLr is downregulated during the progression of lesion development.…”

“…Previously, we showed that the method of BMT in mice results in an almost complete replacement of recipient hemopoietic cells by those of donor origin. 13 During the weeks after BMT, the total serum cholesterol level was repeatedly determined. No significant differences between LDLr(Ϫ/Ϫ)3 C57Bl/6 and C57Bl/63 C57Bl/6 could be observed at 4 and 8 weeks after BMT.…”

“…Expression of the LDLr on macrophages may influence atherosclerotic lesion development when these cells migrate into the intima of the vessel wall. Recently, we 13 and others 14,15 transplanted C57Bl/6 bone marrow into LDLr(Ϫ/Ϫ) mice to study specifically the role of macrophage-expressed LDLr. The recipient hemopoietic cells were almost completely replaced by donor cells.…”

“…11,12 We and others have recently transferred wild-type LDLr(ϩ/ϩ) cells into LDLr(Ϫ/Ϫ) mice via bone marrow transplantation (BMT) in vivo and investigated the effects on hypercholesterolemia and atherosclerosis. [13][14][15] Transplantation of wildtype bone marrow into LDLr(Ϫ/Ϫ) mice resulted in a temporary decrease of total serum cholesterol. The mean atherosclerotic lesion area of these transplanted mice fed a cholesterol-rich diet was not different from that of control LDLr(Ϫ/Ϫ) mice that were transplanted with LDLr(Ϫ/Ϫ) bone marrow.…”