Received for publication May 13, 2008.
Revised August 6, 2008.
Accepted for publication August 7, 2008.

The expression of most UDP-glucuronosyltransferases (UGTs) is increased significantly during Caco-2 cell differentiation, while UGT1A6 is highly expressed also in undifferentiated cells

Abstract

The human colon carcinoma cell line Caco-2 is often employed as a model for intestinal drug absorption. To better understand xenobiotics glucuronidation in Caco-2 cells, we have examined the expression levels of different UDP-glucuronosyltransferases (UGTs) in them. The effects of two main factors were investigated, namely passage number and cell differentiation. Hence, the mRNA levels of 15 human UGTs of subfamilies 1A and 2B were assessed in both undifferentiated and fully differentiated cells, at four passage levels, P31, P37, P43 and P49. Quantitative RT-PCR was employed to determine the mRNA levels of individual UGTs and the values were normalized using beta-actin as a reference gene. The results indicate that although passage number in the tested range exerts a mild effect on the expression level of several UGTs, the contribution of cell differentiation is much larger. The expression of nearly all the UGTs that were examined in this study was significantly, sometimes greatly, increased during cell differentiation. UGT1A6 was a distinct exception to this rule, however, since it was already highly expressed in the undifferentiated cells. The mRNA findings were confirmed at the enzyme activity level by measuring the glucuronidation of 1-naphthol, a very good substrate for UGT1A6, as well as estradiol that is not glucuronidated by this enzyme. The results revealed that 1-naphthol glucuronidation activity was high in both the differentiated and undifferentiated cells, whereas estradiol glucuronidation was only detected in the differentiated cells. Caco-2 cell differentiation, thus, plays a major role in UGTs expression and ensuing metabolic reactions.

Key words: enzyme induction, glucuronidation, intestinal transport, phase II drug metabolism, UDP glucuronyltransferases