Apolipoprotein AI and HDL3 inhibit spreading of primary human monocytes through a mechanism that involves cholesterol depletion and regulation of CDC42

The objective of the current study was to characterize the influence of high density lipoproteins (HDL) on processes related to the vascular recruitment of human monocytes, which may contribute to the anti-atherogenic properties of these lipoproteins. We show that HDL3 and apo AI inhibit the following processes in primary human monocytes: (1) M-CSF induced cell spreading; (2) M-CSF stimulated expression of surface molecules involved in adhesion, migration, and scavenging; (3) fMLP induced chemotaxis. These processes are obviously modulated by the regulation of cellular cholesterol pools as indicated by the following findings. In Tangier monocytes with defective apo AI induced cholesterol efflux, apo AI had no influence on the spreading response. In control cells, stimulation of cholesterol efflux by p-cyclodextrin mimicked the effect of apo AI and HDL3 on spreading and chemotaxis, whereas cholesterol loading with enzymatically modified LDL (E-LDL) showed the opposite effect. Finally, a similar inverse regulation by E-LDL and apo AI/HDL3 was also observed in regard to the surface expression of β1- and β2-integrins as well as the hemoglobin/haptoglobin scavenger receptor CD163 and the Fcγ-IIIaR CD16. CDC42 was identified as a potential downstream target linking changes in cellular cholesterol content to monocyte spreading and chemotaxis. Thus, CDC42 antisense markedly reduced spreading and, in parallel with their influence on monocyte spreading, HDL3, apo AI and p-cyclodextrin down-regulated CDC42 expression while E-LDL had the inverse effect. The apo AI induced decrease of CDC42 protein expression was paralleled by the reduction of active GTP-bound CDC42. In summary, we provide evidence that HDL3 and apo AI are able to inhibit processes in primary human monocytes, which are related to the recruitment of monocytes into the vessel wall and probably involve regulation of cellular cholesterol pools and CDC42 function.