Human carboxylesterase 2 splice variants: expression, activity, and role in the metabolism of irinotecan and capecitabine

Abstract

Carboxylesterases (CES) are enzymes that metabolize a wide variety of compounds including esters, thioesters, carbamates, and amides. In humans there are three known carboxylesterase genes CES1, CES2, and CES3. Irinotecan (CPT-11) and capecitabine are important chemotherapeutic prodrugs that are used for the treatment of colorectal cancer. Of the three CES isoenzymes, CES2 has the highest catalytic efficiency for irinotecan activation. There is large inter-individual variation in response to treatment with irinotecan. Life-threatening late-onset diarrhea has been reported in approximately 13% of patients receiving irinotecan. Several studies have reported single nucleotide polymorphisms (SNPs) for the CES2 gene. However, there has been no consensus on the effect of different CES2 SNPs and their relationship to CES2 RNA expression or irinotecan hydrolase activity. Three CES2 mRNA transcripts of approximately 2kb,3kb, and 4kb have been identified by multi-tissue northern analysis. The expressed sequence tag (EST) database indicates that CES2 undergoes several splicing events that could generate up to six potential proteins. Four of the proteins CES2, CES2458-473, CES2+64, CES21-93 were studied to characterize their expression and activity. Multi-tissue northern analysis revealed that CES2+64 corresponds to the 4kb and 3kb transcripts while CES21-93 is located only in the 4 kb transcript. CES2458-473 is an inactive splice variant that accounts for approximately 6% of the CES2 transcripts in normal and tumor colon tissue. There is large inter-individual variation in CES2 expression in both tumor and normal colon samples. Characterization of CES2+64 identified the protein as normal CES2 indicating that the signal peptide is recognized in spite of the additional 64 amino acids at the N-terminus. Sub-cellular localization studies revealed that CES2 and CES2+64 localize to the ER, and CES21-93 localizes to the cytoplasm. To date CES2 SNP data has not provided any explanation for the high inter-individual variability in response to irinotecan treatment. Multi-tissue northern blots indicate that CES2 is expressed in a tissue specific manner. We have identified the CES2 variants which correspond to each mRNA transcript. This information will be critical to defining the role of CES2 variants in the different tissues.