Transcriptional Regulation of Carboxylesterase 1 in Human Liver: Role of the Nuclear Receptor Subfamily 1 Group H Member 3 and Its Splice Isoforms.

Carboxylesterase 1 () is the predominant carboxylesterase in the human liver, involved in metabolism of both xenobiotics and endogenous substrates. Genetic or epigenetic factors that alter activity or expression are associated with changes in drug response, lipid, and glucose homeostasis. However, the transcriptional regulation of in the human liver remains uncertain. By applying both the random forest and Sobol's Sensitivity Indices (SSI) to analyze existing liver RNA expression microarray data (GSE9588), we identified nuclear receptor subfamily 1 group H member 3 () liver X receptor (LXR) as a key factor regulating constitutive expression. This model prediction was validated using small interfering RNA (siRNA) knockdown and CRISPR-mediated transcriptional activation of in Huh7 and HepG2 cells. We found that 's activation of is splice isoform-specific, namely that increased expression of the isoform increased expression whereas did not. Also, in human liver samples, expression of -211 and are correlated, whereas and are not. This trend also occurs during differentiation of induced pluripotent stem cells (iPSCs) to hepatocytes, where only expression of the isoform parallels expression of Moreover, we found that treatment with the agonist T0901317 in HepG2 cells had no effect on expression. Overall, our results demonstrate a key role of in maintaining the constitutive expression of in the human liver. Furthermore, our results support that the effect of is splice isoform-specific and appears to be ligand independent. SIGNIFICANCE STATEMENT: Despite the central role of carboxylesterase 1 (CES1) in metabolism of numerous medications, little is known about its transcriptional regulation. This study identifies nuclear receptor subfamily 1 group H member 3 as a key regulator of constitutive CES1 expression and therefore is a potential target for future studies to understand interperson variabilities in CES1 activity and drug metabolism.