Multiple mechanisms of ontogenic regulation of nuclear receptors during rat liver development.

Nuclear receptors (NRs) play pivotal roles in the regulation of genes contributing to hepatobiliary cholesterol and bile acid homeostasis. We have previously shown that transporters involved in bile formation are developmentally regulated and are poorly developed during the fetal stage, but their expression reached gradual maturity during the postnatal period. To define the molecular mechanisms underlying this regulation and the role that class II NRs and associated members [liver receptor homolog-1 (LRH-1) and short heterodimer partner (SHP)] play, we have analyzed the ontogeny of NR expression during liver development. Real-time PCR analysis of hepatic NR expression from fetal day 17 through adult revealed that steady-state mRNA levels for all NRs were very low during the embryonic period. However, mRNA levels peaked close to that of adult rats (>6 wk-old rats) by 4 wk of age for farnesoid X receptor (FXR), pregnane X receptor (PXR), liver X receptor-alpha (LXRalpha), peroxisome proliferator-activated receptor-alpha (PPARalpha), retinoid acid receptor-alpha (RARalpha), LRH-1, and SHP, whereas RXRalpha mRNA lagged behind. FXR, PXR, LXRalpha, RARalpha, and PPARalpha functional activity in liver nuclear extracts assayed by gel EMSA demonstrated that the activity attained adult levels by 4 wk of age, exhibiting a strict correlation with mRNA levels. Surprisingly, PPARalpha activity was delayed as seen by EMSA assay. Protein levels for NRs also corresponded to the mRNA and functional activity except for RXRalpha. RXRalpha protein levels were higher than message levels, suggesting increased protein stability. We conclude that expression of NRs during rat liver development is primarily regulated by transcriptional mechanisms, which in turn, control the regulation of bile acid and cholesterol metabolic pathways.