Abstract 4907: LDL-loaded Macrophages Convert Smooth Muscle Cells Into Foam Cell Like Cells by Direct Transport of LDL/cholesterol During Cell-cell-contact
Lipid-loaded macrophages [MP] and vascular smooth muscle cells [VSMC] are key players of atherosclerotic plaque development. However, little is known about their interaction. We hypothesized that MP modulate VSMC behavior directly by cell cell contact. Monocyte-derived MP were exposed to fluorescence-labeled acetylated LDL [FL-LDL] prior co-culture. Semi-confluent VSMC were co-cultured with the FL-LDL-tagged MP in a ratio of 1:3, respectively. Immune cytochemistry revealed that up to 20% of sm-actin positive cells (VSMC) gained Fl-LDL particles. Transfer of the particles was abolished by cell separation in a trans-well co-culture system. Time-lapse microscopy demonstrated that the VSMC acquired the FL-LDL from MP within hours. No phagocytosis of apoptotic bodies was involved in this process. 3D high magnification microscopy demonstrated that, within 24 h, the entire cytosol of single VSMC was filled with FL-LDL. FL-LDL/FL-cholesterol complexes were also transferred and resulted in cholesterol positive lipid droplet formation in VSMC. Usage of the acidotropic fluorescence dye LysoTracker™ demonstrated that some Fl-LDL was bioactive and reached the acidic lysosomes of VSMC. Marking all lysosomes with a fusion protein of LAMP1 and RFP demonstrated that the remaining FL-LDL was located in non acidic lysosomes. VSMC were then infected with a virus coding for a Rab5a-GFP fusion protein to mark all early endosomes. In contrast to endothelial cells (positive control) no co-localization of Rab5a-GFP and transported Fl-LDL in VSMC could be found, supporting a mechanism independent of phagocytosis. Rescue of the VSMC from co-culture and subsequent analysis demonstrated an increase in phagocytotic activity compared to VSMC co-cultured in a trans-well with MP. Xenogenic cell composure (rat VSMC, human MP) and subsequent quantitative RT-PCR with rat specific primers demonstrated induction of genes typical for MP (CD68 and Mac2) and a concomitant downregulation of cholersterol sensitve genes (HMG-CoA reductase) in VSMC. These results demonstrate an active and phagocytosis independent exchange of LDL/cholesterol from MP into VSMC and imply that MP are capable of transforming a plaque stabilizing VSMC into a plaque destabilizing foam cell like cell.