UGT1A10 drives tissue‐specific differences in the clearance of the chiral tobacco carcinogen, NNAL

Abstract

Tobacco‐specific nitrosamines (TSNAs) are among the most potent carcinogens found in tobacco, with 4‐(methylnitrosamino)‐1‐(3‐pyridyl)‐1‐butanone (NNK) the most potent and one of the most abundant. NNK is extensively metabolized to the equally carcinogenic chiral 4‐ (methylnitrosamino)‐1‐(3‐pyridyl)‐1‐butanol (NNAL). (S)‐NNAL exhibits higher stereoselective tissue retention in mice and humans and has been shown to be more carcinogenic in mice than its enantiomeric counterpart, (R)‐NNAL. This differential carcinogenic potential makes it increasingly important to know which UGT enzymes target specific NNAL enantiomers for glucuronidation. NNAL is detoxified primarily by four hepatic UDP‐glucuronyltransferase (UGT) enzymes: UGTs 1A9, 2B7, 2B10 and 2B17; each of these UGTs have been shown to exhibit different stereo‐specificity for the formation of glucuronides of the different NNAL enantiomers. The goal of the present study was to identify the UGTs involved in extra‐hepatic NNAL metabolism and examine differences in the stereospecific glucuronidation of NNAL between tissue types with different UGT expression. Large differences in stereo‐specificity for (S)‐ versus (R)‐NNAL were identified upon comparison of NNAL‐glucuronides (Gluc) formed in reactions of extra‐hepatic UGT1A10‐over‐expressing cell microsomes upon incubation with racemic NNAL, with an enantiomeric preference for (R)‐NNAL as a substrate when compared to (S)‐NNAL. When UGT1A10‐over‐expressing cell microsomes were incubated with pure NNAL enantiomers, large difference in kinetics were observed, with a Km=1.5 mM and Vmax=4.6 pmol/min/mg for (R)‐NNAL, vs a Km>16 mM and Vmax=1.3 pmol/min/mg for (S)‐NNAL. Human liver microsomes preferentially formed (S)‐NNAL‐Gluc while microsomes from human intestine, larynx, esophagus, tonsil, floor of mouth, and tongue all preferentially formed (R)‐NNAL‐Gluc upon incubation with racemic NNAL. Liver and intestine were shown to express UGTs 1A9, 2B7, and 2B17 while intestine, unlike liver, has also been shown to express UGT1A10. All other tissue types analyzed were shown to express only UGTs 1A10 and 2B17, with the exception of esophagus that also expressed UGT2B7. These data indicate that UGT1A10 may drive the difference in differential tissue‐specific clearance of the specific NNAL enantiomers known to induce tobacco‐related cancers.This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.