TRANSCRIPTIONAL AND POSTTRANSCRIPTIONAL REGULATION OF ALPHA-1,3-GALACTOSYLTRANSFERASE IN ACTIVATED ENDOTHELIAL-CELLS RESULTS IN DECREASED EXPRESSION OF GAL-ALPHA-1,3GAL

Gal alpha 1,3Gal carbohydrate residues are present in the glycoprotein s and glycolipids of lower mammals, and appear to be involved in the b inding specificity of several membrane receptors, We report here that endothelial cells stimulated with lipopolysaccharide or inflammatory c ytokines modulate their expression of UPD-Gal:beta-D-Gal alpha 1,3-gal actosyltransferase (alpha 1,3GT), the Golgi enzyme that attaches a gal actose in alpha 1,3 configuration to an N-acetyllactosamine acceptor. Upon activation, the steady state level of mRNA is transiently increas ed, the modifications being paralleled by a transcriptional regulation of the gene. Cell-associated enzyme activity, on the other hand, fall s rapidly after activation, before being up-and downregulated with kin etics that parallel those of the mRNA, and after 3 days reaches a leve l representing 40-60 % of the activity in cells before activation. Ove rall Gal alpha 1,3Gal expression at the cell surface follows enzyme ac tivity, except that it is insensitive to the rapid and transient reduc tion of activity occurring shortly after activation. This reduced alph a 1,3GT activity in stimulated EC is correlated with lower stability o f the protein, and with a switch in the expression of the isoform patt ern, isoform 1 being predominant in resting cells whereas after activa tion it is isoform 2 that predominates, The two isoforms, however, app ear to have similar intrinsic stability, so that the reduced stability of the enzyme in activated EC probably results from an induced proteo lytic degradation pathway.