Uptake and catabolism of modified LDL in scavenger-receptor class A type I/II knock-out mice.
The liver is the major organ responsible for the uptake of modified low-density lipoprotein (LDL) from the blood circulation, with endothelial and Kupffer cells as major cellular uptake sites. Scavenger-receptors, which include various classes, are held responsible for this uptake. Mice deficient in scavenger-receptor class A types I and II were created and the fate of acetylated LDL (Ac-LDL) in vivo and its interaction with liver endothelial, Kupffer and peritoneal macrophages was characterized. Surprisingly, the decay in vivo (t12 < 2 min), tissue distribution and liver uptake (at 5 min it was 77.4 +/- 4.6% of the injected dose) of Ac-LDL in the knock-out mice were not significantly different from control mice (t12 < 2 min and liver uptake 79.1 +/- 4.6% of the injected dose). A separation of mice liver cells into parenchymal, endothelial and Kupffer cells 10 min after injection of Ac-LDL indicated that in both control and knock-out mice the liver endothelial cells were responsible for more than 70% of the liver uptake. Both in control and knock-out mice, preinjection of polyinosinic acid (poly I, 200 microg) completely blocked the liver uptake, indicating that both in control and knock-out mice the scavenger-receptors are sensitive to poly I. Preinjection of suboptimal poly I concentrations (20 and 50 microg) provided evidence that the serum decay and liver uptake of Ac-LDL is more readily inhibited in the knock-out mice as compared with the control mice, indicating less efficient removal of Ac-LDL in vivo in the knock-out mice under these conditions. Studies in vitro with isolated liver endothelial and Kupffer cells from knock-out mice indicate that the cell association of Ac-LDL during 2 h at 37 degrees C is 50 and 53% of the control, respectively, whereas the degradation reaches values of 58 and 63%. For peritoneal macrophages from knock-out mice the cell association of Ac-LDL was identical to the control mice whereas the Ac-LDL degradation in cells from the knock-out mice was 17% of the control. The low degradation capacity of peritoneal macrophages from knock-out mice for Ac-LDL indicates that scavenger-receptor class A types I and II play a quantitative important role in the degradation of Ac-LDL by macrophages. In liver, the contribution of scavenger-receptor class A types I and II to the maximal uptake and degradation of Ac-LDL by endothelial and Kupffer cells was 40-50%. Binding studies performed at 4 degrees C indicate that the lower rates of degradation are due to a lower number of surface receptors on the cells from the knock-out mice. From the in vitro and in vivo data it can be concluded that in addition to the classic scavenger-receptors class A types I and II liver does contain additional novel poly I-sensitive scavenger-receptors that facilitate efficient removal of Ac-LDL from the blood circulation. The availability of the scavenger-receptor class A types I and II knock-out mice will stimulate further molecular identification of these receptors.
Van Berkel
T J
TJ
Division of Biopharmaceutics, Leiden/Amsterdam Center for Drug Research, Sylvius Laboratory, University of Leiden, P.O. Box 9503, 2300 RA Leiden, The Netherlands.
Van Velzen
A
A
Kruijt
J K
JK
Suzuki
H
H
Kodama
T
T
D000107:Acetylation; D000818:Animals; D004727:Endothelium / Q000378:metabolism; D007728:Kupffer Cells / Q000378:metabolism; D008077:Lipoproteins, LDL / Q000378:metabolism*; D008099:Liver / Q000166:cytology / Q000378:metabolism*; D017737:Macrophages, Peritoneal / Q000378:metabolism*; D051379:Mice; D018345:Mice, Knockout; D011971:Receptors, Immunologic / Q000172:deficiency* / Q000235:genetics; D018110:Receptors, Lipoprotein / Q000378:metabolism; D051116:Receptors, Scavenger; D051117:Scavenger Receptors, Class A
D008077:Lipoproteins, LDL; C495140:Msr1 protein, mouse; D011971:Receptors, Immunologic; D018110:Receptors, Lipoprotein; D051116:Receptors, Scavenger; D051117:Scavenger Receptors, Class A