Oxidized low density lipoprotein inhibits macrophage apoptosis by blocking ceramide generation, thereby maintaining protein kinase B activation and Bcl-XL levels.
Macrophages play a central role in the development and progression of atherosclerotic lesions. It is well known that oxidized low density lipoprotein (ox-LDL) promotes the recruitment of monocytes (which differentiate to macrophages) into the intima. We reported recently that ox-LDL blocks apoptosis in bone marrow-derived macrophages deprived of macrophage colony-stimulating factor (M-CSF) by a mechanism involving protein kinase B (PKB) (Hundal, R., Salh, B., Schrader, J., Gómez-Muñoz, A., Duronio, V., and Steinbrecher, U. (2001) J. Lipid Res. 42, 1483-1491). The aims of the present study were 1) to define the apoptotic pathway involved in the pro-survival effect of ox-LDL; 2) to determine which PKB target mediated this effect; and 3) to identify mechanisms responsible for PKB activation by ox-LDL. Apoptosis following M-CSF withdrawal was accompanied by activation of the caspase 9-caspase 3 cascade and cytochrome c release from mitochondria, but the caspase 8 pathway was unaffected. M-CSF withdrawal resulted in a marked and selective reduction in Bcl-XL protein and mRNA levels, and this decrease was prevented by ox-LDL. The ability of ox-LDL to preserve Bcl-XL levels was blocked by NFkappaB antagonists, thereby implicating IkappaB kinase as a key PKB target. M-CSF deprivation resulted in activation of acid sphingomyelinase and an increase in ceramide levels. Desipramine (a sphingomyelinase inhibitor) prevented the increase in ceramide and inhibited apoptosis after M-CSF deprivation. Ox-LDL completely blocked the increase in acid sphingomyelinase activity as well as the increase in ceramide after M-CSF deprivation. Pretreatment of macrophages with C2-ceramide reversed the effect of ox-LDL on PKB and macrophage survival. These results indicate that ox-LDL prevents apoptosis in M-CSF-deprived macrophages at least in part by inhibiting acid sphingomyelinase. This in turn prevents ceramide-induced down-regulation of PKB, the activity of which is required to maintain production of Bcl-XL.
Department of Medicine, University of British Columbia, Vancouver V5Z 3P1, Canada.
Rajinder S
RS
Hundal
Antonio
A
Gómez-Muñoz
Jennifer Y
JY
Kong
Baljinder S
BS
Salh
Anthony
A
Marotta
Vincent
V
Duronio
Urs P
UP
Steinbrecher
D000818:Animals; D017209:Apoptosis / Q000187:drug effects* / Q000502:physiology; D002478:Cells, Cultured; D002518:Ceramides / Q000037:antagonists & inhibitors*; D015536:Down-Regulation / Q000187:drug effects; D004789:Enzyme Activation / Q000187:drug effects; D005260:Female; D008077:Lipoproteins, LDL / Q000494:pharmacology*; D008264:Macrophages / Q000473:pathology* / Q000502:physiology; D051379:Mice; D017346:Protein Serine-Threonine Kinases; D011518:Proto-Oncogene Proteins / Q000378:metabolism*; D051057:Proto-Oncogene Proteins c-akt; D019253:Proto-Oncogene Proteins c-bcl-2 / Q000378:metabolism*; D015398:Signal Transduction / Q000187:drug effects; D051020:bcl-X Protein
C494812:Bcl2l1 protein, mouse; D002518:Ceramides; D008077:Lipoproteins, LDL; D011518:Proto-Oncogene Proteins; D019253:Proto-Oncogene Proteins c-bcl-2; D051020:bcl-X Protein; C099252:oxidized low density lipoprotein; D017346:Protein Serine-Threonine Kinases; D051057:Proto-Oncogene Proteins c-akt