Cannabinoid (CB2) receptor deficiency reduces the susceptibility of macrophages to oxidized LDL/oxysterol-induced apoptosis

Macrophage apoptosis is an important process in the pathophysiology of atherosclerosis. Oxidized low-density lipoproteins (OxLDL) are a major component of lesions and potently induce macrophage apoptosis. Cannabinoid receptor 2 (CB2), the predominant macrophage cannabinoid receptor, modulates several macrophage processes associated with ongoing atherosclerosis; however, the role of CB2 in macrophage apoptosis is unknown. To determine if CB2 influences a macrophage apoptotic pathway relevant to atherosclerosis, we examined the effect of CB2 deficiency on OxLDL-induced macrophage apoptosis. In situ terminal transferase-mediated dUTP nick end labeling (TUNEL) analysis of resident peritoneal macrophages detected significantly fewer apoptotic CB2(-/-) macrophages than CB2(+/+) macrophages after incubation with OxLDL (27.9 +/- 4.7% vs. 61.9 +/- 8.5%, P < 0.001) or 7-ketocholesterol (7KC) (18.9 +/- 10.5% vs. 54.1 +/- 6.9%, P < 0.001), an oxysterol component of OxLDL. Caspase-3 activity; proteolytic conversion of procaspase-3; and cleavage of a caspase-3 substrate, PARP, were also diminished in 7KC-treated CB2(-/-) macrophages. Furthermore, the deactivation of the prosurvival kinase, Akt, in response to 7KC was impaired in CB2(-/-) macrophages. These results suggest that CB2 expression increases the susceptibility of macrophages to OxLDL-induced apoptosis, in part, by modulating the effect of oxysterols on the Akt survival pathway and that CB2 may influence atherosclerosis by modulating lesional macrophage apoptosis.