PT  - JOURNAL ARTICLE
AU  - Tao Chen
AU  - Leslie M. Tompkins
AU  - Linhao Li
AU  - Haishan Li
AU  - Gregory Kim
AU  - Yuxin Zheng
AU  - Hongbing Wang
TI  - A Single Amino Acid Controls the Functional Switch of Human Constitutive Androstane Receptor (CAR) 1 to the Xenobiotic-Sensitive Splicing Variant CAR3
AID  - 10.1124/jpet.109.159210
DP  - 2010 Jan 01
TA  - Journal of Pharmacology and Experimental Therapeutics
PG  - 106--115
VI  - 332
IP  - 1
4099  - http://jpet.aspetjournals.org/content/332/1/106.short
4100  - http://jpet.aspetjournals.org/content/332/1/106.full
SO  - J Pharmacol Exp Ther2010 Jan 01; 332
AB  - The constitutive androstane receptor (CAR) is constitutively activated in immortalized cell lines independent of xenobiotic stimuli. This feature of CAR has limited its use as a sensor for xenobiotic-induced expression of drug-metabolizing enzymes. Recent reports, however, reveal that a splicing variant of human CAR (hCAR3), which contains an insertion of five amino acids (APYLT), exhibits low basal but xenobiotic-inducible activities in cell-based reporter assays. Nonetheless, the underlying mechanisms of this functional shift are not well understood. We have now generated chimeric constructs containing various residues of the five amino acids of hCAR3 and examined their response to typical hCAR activators. Our results showed that the retention of alanine (hCAR1+A) alone is sufficient to confer the constitutively activated hCAR1 to the xenobiotic-sensitive hCAR3. It is noteworthy that hCAR1+A was significantly activated by a series of known hCAR activators, and displayed activation superior to that of hCAR3. Moreover, intracellular localization assays revealed that hCAR1+A exhibits nuclear accumulation upon 6-(4-chlorophenyl) imidazo[2,1-b][1,3]thiazole-5-carbaldehyde-O-(3,4-dichlorobenzyl) oxime (CITCO) treatment in COS1 cells, which differs from the spontaneous nuclear distribution of hCAR1 and the nontranslocatable hCAR3. Mammalian two-hybrid and glutathione S-transferase pull-down assays further demonstrated that hCAR1+A interacts with the coactivator SRC-1 and GRIP-1 at low level before activation, while at significantly enhanced level in the presence of CITCO. Thus, the alanine residue in the insertion of hCAR3 seems in charge of the xenobiotic response of hCAR3 through direct and indirect mechanisms. Activation of hCAR1+A may represent a sensitive avenue for the identification of hCAR activators.© 2010 by The American Society for Pharmacology and Experimental Therapeutics