LXRα is uniquely required for maximal reverse cholesterol transport and atheroprotection in ApoE-deficient mice

The liver X receptor (LXR) signaling pathway is an important modulator of atherosclerosis, but the relative importance of the two LXRs in atheroprotection is incompletely understood. We show here that LXR alpha, the dominant LXR isotype expressed in liver, plays a particularly important role in whole-body sterol homeostasis. In the context of the ApoE(-/-) background, deletion of LXR alpha, but not LXR beta, led to prominent increases in atherosclerosis and peripheral cholesterol accumulation. However, combined loss of LXR alpha and LXR beta on the ApoE(-/-) background led to an even more severe cholesterol accumulation phenotype compared to LXR alpha(-/-)ApoE(-/-) mice, indicating that LXR beta does contribute to reverse cholesterol transport (RCT) but that this contribution is quantitatively less important than that of LXR alpha. Unexpectedly, macrophages did not appear to underlie the differential phenotype of LXR alpha(-/-)ApoE(-/-) and LXR beta(-/-)ApoE(-/-) mice, as in vitro assays revealed no difference in the efficiency of cholesterol efflux from isolated macrophages. By contrast, in vivo assays of RCT using exogenously labeled macrophages revealed a marked defect in fecal sterol efflux in LXR alpha(-/-)ApoE(-/-) mice. Mechanistically, this defect was linked to a specific requirement for LXR alpha(-/-) in the expression of hepatic LXR target genes involved in sterol transport and metabolism. These studies reveal a previously unrecognized requirement for hepatic LXR alpha for optimal reverse cholesterol transport in mice. Hong, C., M. N. Bradley, X. Rong, X. Wang, A. Wagner, V. Grijalva, L. W. Castellani, J. Salazar, S. Realegeno, R. Boyadjian, A. M. Fogelman, B. J. Van Lenten, S. T. Reddy, A. J. Lusis, R. K. Tangirala, and P. Tontonoz. LXR alpha is uniquely required for maximal reverse cholesterol transport and atheroprotection in ApoE-deficient mice. J. Lipid Res. 2012. 53: 1126-1133.