The biochemical consequences of α2,6(N) sialyltransferase induction by dexamethasone on sialoglycoprotein expression in the rat H411e hepatoma cell line

Abstract

Previous studies have demonstrated sialyltransferase (ST) enzyme activity to be induced in hepatic cells by corticosteroids. In this study, we used the H411e rat hepatoma cell line to further characterise this induction with particular reference to the subsequent changes in the pattern of sialoglycoprotein (SGP) expression. The induction of total ST activity by dexamethasone was concentration dependent with maximum induction occurring 12 h subsequent to drug addition. Western blot analysis demonstrated that the induction was associated with an increase in the expression of the α2,6(N) ST enzyme with no change in the expression levels of the α2,3(N) enzyme. While the induction resulted in an increase in the reaction velocity ( V max) of the enzyme for both the sugar donor (CMP-Neu5Ac) and the asialofetuin acceptor protein, there was no significant change in the enzyme affinity ( K m) for the substrates, suggestive of either an increase in the expression or efficiency of the existing enzyme(s) rather than an induction of novel ST enzymes. Lectin blot analysis of cellular glycoprotein expression demonstrated no change in the expression patterns of either α2,3 or α2,6-linked SGP following enzyme induction. These results suggest that the available acceptor sites for the terminal sialic acid group(s) may be fully occupied in the control cells and therefore there are no further sites onto which the sialic acid can be transferred following induction of ST enzyme activity. This may be due to the high basal enzyme levels in the control cells already exhausting endogenous acceptor sites.