Serotonin increases macrophage uptake of oxidized low density lipoprotein.

Macrophage uptake of oxidized low density lipoprotein leads to cellular cholesterol accumulation and foam cell formation. Atherogenesis involves platelet activation with subsequent serotonin release. Incubation of J-774 A.1 murine macrophages as well as mouse peritoneal macrophages with serotonin for 2-18 hours at 37 degrees C, followed by cell wash and a further incubation of the cells for 18 hours in the presence of oxidized LDL (protein concentration 10-75 mg/l), resulted in up to 85% elevation in cellular uptake of the lipoprotein. Maximal effect was found after preincubation of the cells for 18 hours in the presence of 25 mumols/l serotonin. At lower or higher serotonin concentrations and with longer or shorter times of preincubation, cellular degradation of oxidized LDL was reduced. Under similar incubation conditions (with oxidized LDL), macrophage cholesterol accumulation was also increased by serotonin from 65 to 75 micrograms cholesterol per mg cell protein. This effect of serotonin was the result of a serotonin-mediated increase in the affinity of oxidized LDL towards its receptor without a significant change in the number of the receptors. Specific binding of serotonin to J-774 A.1 macrophages was demonstrated. Ten mumols/l of ketanserine (serotonin antagonist) completely blocked the stimulatory effect of serotonin on the cellular uptake of oxidized LDL. After injection of serotonin into the peritoneal cavity of thioglycolate-stimulated mice (250 nmol/mouse), the collected macrophages showed a 34% enhanced degradation of oxidized [125I]LDL compared with control cells collected from mice that were not injected with serotonin. Platelets from patients with carcinoid syndrome contained about twice as much serotonin as platelets from healthy subjects. Platelets were activated with collagen (1 mg/l) and platelet-conditioned medium was collected. Platelet-conditioned medium from healthy subjects enhanced the cellular uptake of oxidized LDL by 33%, whereas a similar concentration (protein concentration 15 mg/l) of platelet-conditioned medium from the patients increased macrophage uptake of oxidized LDL by 75%. Incubation of macrophages with platelet conditioned medium in the presence of ketanserine completely abolished the stimulation of oxidized LDL degradation, implying serotonin involvement in this process. We conclude that serotonin enhancement of oxidized LDL uptake by macrophages may be relevant in atherogenesis where both platelet activation and foam cell formation occur.