INDUCTION OF LACTASE BIOSYNTHESIS IN THE HUMAN INTESTINAL EPITHELIAL-CELL LINE CACO-2

The human colonic adenocarcinoma cell line Caco-2 forms monolayers of differentiated enterocyte-like cells when cultured on permeable suppor ts. After confluency, Caco-2 cells express a number of brush-border en zymes including lactase-phlorizin hydrolase, sucrase-isomaltase and di peptidylpeptidase IV. We have studied, with particular emphasis on lac tase-phlorizin hydrolase, the modulation of biosynthesis of these enzy mes by stimulating second messenger systems. Forskolin induced lactase -phlorizin hydrolase synthesis approximately fourfold within 7 h, supp ressed sucrase-isomaltase synthesis, and had little effect on dipeptid ylpeptidase IV. Dibutyryl-cAMP, 8-bromo-cAMP and vasoactive intestinal peptide also increased lactase-phlorizin hydrolase biosynthesis, indi cating c-AMP dependent regulation. The induction of lactase-phlorizin hydrolase biosynthesis could be inhibited by actinomycin D and was pre ceded by a fourfold increase in lactase-phlorizin hydrolase mRNA level s, suggesting transcriptional control. Phorbol 12-myristate 13-acetate had an inhibitory effect on brush-border enzyme synthesis, in particu lar on sucrase-isomaltase, and blocked the forskolin-induced biosynthe sis of lactase-phlorizin hydrolase. Lactase-phlorizin hydrolase synthe sis was also inducible by hydrocortisone, but maximal induction requir ed at least 3 days during which time sucrase-isomaltase synthesis dimi nished. The results indicate opposite regulation of lactase-phlorizin hydrolase and sucrase-isomaltase via cAMP and corticosteroids, and sug gest that the Caco-2 cell line can serve as a model system to study as pects of the humoral regulation of human intestinal brush-border enzym es in cell culture.