EXPRESSION OF INSULIN-LIKE GROWTH FACTOR-II AND INSULIN-LIKE GROWTH-FACTOR BINDING-PROTEINS DURING CACO-2 CELL-PROLIFERATION AND DIFFERENTIATION

The components of the insulin-like growth factor (IGF) axis and their roles in regulating proliferation and differentiation of the human col on adenocarcinoma cell line, Caco-2, have been investigated. Caco-2 ce lls proliferated in serum-free medium at 75% the rate observed in medi um containing 10% fetal bovine serum. IGF-I (10 nM) increased Caco-2 c ell growth in serum-free medium, but not to the rate seen with serum. Multiple IGF-II mRNA species were produced by Caco-2 cells, but IGF-I mRNA was undetectable. Secretion of radioimmunoassayable IGF-II corres ponded with steady-state levels of IGF-II mRNA, neither of which was o bserved to change markedly over the course of 16 days of Caco-2 cell d ifferentiation. Levels of sucrase-isomaltase mRNA, a marker for entero cytic differentiation, increased 12-fold between days 5 and 16 of cult ure. Northern blotting of total RNA and ligand blot and immunoblot ana lyses of serum-free conditioned medium revealed that Caco-2 cells prod uce several IGF binding proteins (IGFBPs), including IGFBP-2, -3, and -4, as well as a 31,000 M(r) species that was not identified. The patt ern of IGFBP secretion changed dramatically during Caco-2 cell differe ntiation: IGFBP-3 and IGFBP-2 increased 8.5-fold and 5-fold, respectiv ely, whereas IGFBP-4 and the 31,000 M(r) species decreased 43% and 90% . Caco-2 cell clones stably transfected with a human IGFBP-4 cDNA cons truct exhibited a 60% increase in steady-state level of IGFBP-4 mRNA, and secreted twice as much IGFBP-4 protein as controls. Moreover, IGFB P-4-overexpressing cells proliferated at only 25% the rate of control cells in serum-free medium, in conjunction with a 70% increase in expr ession of sucrase-isomaltase. In summary, these studies indicate that a complex IGF axis is involved in autocrine regulation of Caco-2 cell proliferation and differentiation. (C) 1996 Wiley-Liss, Inc.