SEROTONIN INCREASES MACROPHAGE UPTAKE OF OXIDIZED LOW-DENSITY-LIPOPROTEIN

Macrophage uptake of oxidized low density lipoprotein leads to cellula r cholesterol accumulation and foam cell formation. Atherogenesis invo lves platelet activation with subsequent serotonin release. Incubation of J-774 A.1 murine macrophages as well as mouse peritoneal macrophag es with serotonin for 2-18 hours at 37-degrees-C, followed by cell was h and a further incubation of the cells for 18 hours in the presence o f oxidized LDL (protein concentration 10-75 mg/l), resulted in up to 8 5% elevation in cellular uptake of the lipoprotein. Maximal effect was found after preincubation of the cells for 18 hours in the presence o f 25-mu-mol/l serotonin. At lower or higher serotonin concentrations a nd with longer or shorter times of preincubation, cellular degradation of oxidized LDL was reduced. Under similar incubation conditions (wit h oxidized LDL), macrophage cholesterol accumulation was also increase d by serotonin from 65 to 75-mu-g cholesterol per mg cell protein. Thi s effect of serotonin was the result of a serotonin-mediated increase in the affinity of oxidized LDL towards its receptor without a signifi cant change in the number of the receptors. Specific binding of seroto nin to J-774 A.1 macrophages was demonstrated. Ten-mu-mol/l of ketanse rine (serotonin antagonist) completely blocked the stimulatory effect of serotonin on the cellular uptake of oxidized LDL. After injection o f serotonin into the peritoneal cavity of thioglycolate-stimulated mic e (250 nmol/mouse), the collected macrophages showed a 34% enhanced de gradation of oxidized [I-125]LDL compared with control cells collected from mice that were not injected with serotonin. Platelets from patie nts with carcinoid syndrome contained about twice as much serotonin as platelets from healthy subjects. Platelets were activated with collag en (1 mg/l) and platelet-conditioned medium was collected. Platelet-co nditioned medium from healthy subjects enhanced the cellular uptake of oxidized LDL by 33%, whereas a similar concentration (protein concent ration 15 mg/l) of platelet-conditioned medium from the patients incre ased macrophage uptake of oxidized LDL by 75%. Incubation of macrophag es with platelet conditioned medium in the presence of ketanserine com pletely abolished the stimulation of oxidized LDL degradation, implyin g serotonin involvement in this process. We conclude that serotonin en hancement of oxidized LDL uptake by macrophages may be relevant in ath erogenesis where both platelet activation and foam cell formation occu r.