The Constitutive Androstane Receptor (CAR): A Nuclear Receptor in Health and Disease

Constitutive Androstane Receptor (CAR, NR1I3), a member of the nuclear receptor superfamily of transcription factors, has emerged as one of the key regulators of the drug and xenobiotic metabolism. The unique feature that separates CAR from other members of the superfamily is that it remains active in the absence of ligand and is further regulated by activators. From its first isolation in 1994, a number of studies related to its distribution, characteristics, functions, and relation to other members of the superfamily have been conducted that place it centrally, governing many key events of the body. Human CAR is expressed relatively higher in liver and epithelial cells of the small intestine villi and less in heart, muscle, kidney, brain and lung. Though there are some controversies regarding its subcellular localization in different cell lines, in general, the subcellular localization of CAR is reported to be predominantly cytoplasmic, in complex with co-chaperone partners HSP90 and CCRP (cytoplasmic CAR retention protein). To execute transcription functions, nuclear translocation is a prerequisite event for a NR, including CAR. In this context, existence of two pathways is suggested, i) direct mechanism of action; and ii) indirect mechanism of action that is governed via nuclear translocation of CAR. Additionally, existence of species-specific differences in its modulation with ligands acting either as an agonist, antagonist or inverse agonist is also apparent. Like the other xenobiotic receptor PXR, CAR also functions as an alternative 'xenosensor' to defend the body against persistent chemical insults. It responds to diverse array of chemically distinct compounds, including endobiotics and xenobiotics, to regulate the clearance of noxious chemicals and toxic metabolites in liver and intestine via induction of genes involved in their metabolism. The usefulness of targeting CAR in metabolic diseases including bilirubinemia, obesity, type 2 diabetes mellitus, atherosclerosis, preeclampsia, hypertension, cholestasis and also in liver cancer is being extensively studied in animal models. However, to determine the human relevance it requires further investigation. Though a large number of natural and synthetic compounds act as modulators of CAR, designing new derivatives with defined therapeutic benefit need to be investigated. The purpose of this review is to highlight the general aspects of nuclear receptor CAR, its mechanism of action and importance in human health and disease.