Expression of liver X receptor alpha in rat fetal tissues at different developmental stages.

The liver X receptor (LXR) is a nuclear receptor that acts as a sterol sensor and metabolic regulator of cholesterol and lipid homeostasis. Using a novel LXRalpha-specific antibody for immunohistochemistry, we evaluated cellular expression of LXRalpha in fetal rat tissues. In the fetal liver, LXRalpha-positive macrophages appeared at 12 days and their number peaked at 18 days of gestation. In contrast, hepatocytes expressed LXRalpha during the later stage of gestation, suggesting the functional development of the liver during ontogeny. Later, macrophages in spleen and thymus expressed LXRalpha, and some mononuclear cells in the vascular lumen compatible to primitive/fetal macrophages in the fetal circulation were found to express LXRalpha. In vitro, rat monocytes did not express LXRalpha, but monocyte-derived macrophages cultured in the presence of macrophage-colony stimulating factor revealed the distinct expression of LXRalpha in nucleoli. These findings suggest that LXRalpha plays a role in the differentiation of fetal macrophages, particularly hepatic macrophages, in rat development.