Human carboxylesterase 1A2 expressed from carboxylesterase 1A1 and 1A2 genes is a potent predictor of CPT-11 cytotoxicity in vitro

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Abstract
The conversion of CPT-11 to its active form, SN-38, by carboxylesterases (CESs) is a critical event in CPT-11-induced cytotoxicity. Among the CESs, CES1 and CES2 probably play a major role in the metabolism, but the functional significance and molecular basis of CES1 on CPT-11 response remain unclear. We investigated CES1A1 (AB119997) and CES1A2 (AB187225), whose coding sequences were recently registered in GenBank, for CPT-11-induced cytotoxicity, anticipating novel biomarkers of CPT-11 response. Their coding sequences showed high homology, with only four amino acid differences in the N-terminal region, but our sequencing study of the 5' regions revealed that CES1A1 and CES1A2 had distinctive consensus sequences for transcription factors in the regions, implying differences in transcriptional regulation of the genes. We also identified three isoforms of CES1A1 gene--CES1A1a, CES1A1b and CES1A1c--and developed a detection method for CES1A1 and CES1A2 types of mRNA expression. Interestingly, CES1A2 type of mRNA was found to be expressed from both CES1A1b and CES1A1c isoforms and CES1A2, the promoter activity of the former was higher than that of the original CES1A2 gene. Finally, CES1A2 type of mRNA expression correlated with CPT-11 sensitivities of cancer cells. We demonstrated novel sequence structures and a functional role of CES1A genes in CPT-11 responses. We believe that our novel findings will be of key importance in developing a really useful prediction method for CPT-11 chemosensitivity.