THE BIOCHEMICAL CONSEQUENCES OF ALPHA-2,6(N) SIALYLTRANSFERASE INDUCTION BY DEXAMETHASONE ON SIALOGLYCOPROTEIN EXPRESSION IN THE RAT H411E HEPATOMA-CELL LINE

Previous studies have demonstrated sialyltransferase (ST) enzyme activ ity 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 p attern of sialoglycoprotein (SGP) expression, The induction of total S T activity by dexamethasone was concentration dependent with maximum i nduction occurring 12 h subsequent to drug addition, Western blot anal ysis demonstrated that the induction was associated with an increase i n the expression of the alpha 2,6(N) ST enzyme with no change in the e xpression levels of the alpha 2,3(N) enzyme, While the induction resul ted in an increase in the reaction velocity(V-max) of the enzyme for b oth 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 eff iciency of the existing enzyme(s) rather than an induction of novel ST enzymes, Lectin blot analysis of cellular glycoprotein expression dem onstrated no change in the expression patterns of either alpha 2,3 or alpha 2,6-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 followi ng induction of ST enzyme activity, This may be due to the high basal enzyme levels in the control cells already exhausting endogenous accep tor sites, (C) 1997 Federation of European Biochemical Societies.