Abstract 762: Atherosclerosis Regression Promoted By An LXR Agonist Is Dependent On The Chemokine Receptor CCR7
We have developed a transplantation-based mouse model of atherosclerosis regression by allowing plaques to form in apoE−/ − mice, then changing the plaque’s plasma environment from hyperlipidemia to normolipidemia. In this model, we reported emigration of plaque foam cells to regional and systemic lymph nodes after 3 days in the regression environment (Llodrá et al. PNAS 2004). This emigration required the expression of the chemokine receptor CCR7 on the foam cells and was associated with up-regulation of their LXR mRNA (Trogan, Feig et al. PNAS 2006). Because the human and murine promoters have LXREs, we hypothesized that LXR was a regulator of CCR7. Using a murine model of immature DCs, D2.4 cells, we found that CCR7 expression is increased 8 –9X upon LXR activation by the agonist T0901317 (T09). Importantly, this increase is dependent on CCR7 gene transcription, because pre-treatment with actinomycin D abolished the observed response. When siRNA to LXR alpha and LXR beta was used in conjunction with T09, CCR7 gene expression was significantly blunted to only 2X that of the control (no T09). To extend the studies in vivo, we treated atherosclerotic apoE−/ − mice (n=9/group) with the LXR agonist (50 mg/kg/day) or saline for 2 weeks. Using laser capture microdissection, we isolated lesional macrophages and found expression of downstream targets of LXR such as ABCA1, ABCG1, and ABCG4 to be up-regulated by 3.7, 4.4, and 3.1X, respectively, as assessed by quantitative real-time PCR. Interestingly, CCR7 expression was found to be increased by 3.7X. This result was confirmed at the protein level using immunohistochemistry, which showed that CCR7 expression was increased in foam cells from the LXR-agonist treated but not the saline-treated group. Morphometric analyses revealed that foam cell content and lesion area decreased by 21% and 24%, respectively. The dependence of these relatively rapid (2 wks) effects on CCR7 was shown by treating apoE−/ − CCR7−/ − atherosclerotic mice (n=8) with T09. After 2 wks, foam cell content only decreased by approximately 9%. In conclusion, CCR7 is a required factor for atherosclerosis regression in a mouse model and LXR regulates its expression in vitro and in vivo.