INSULIN-LIKE GROWTH FACTOR-I IS AN AUTOCRINE REGULATOR OF HUMAN COLON CANCER CELL-DIFFERENTIATION AND GROWTH

The polyanionic compound suramin triggers enterocyte-like differentiation of the human colic adenocarcinoma cell clone HT29-D4. We now demonstrate that suramin interferes with the binding of IGF-I to its receptor at the surface of HT29-D4 cells. Half-maximum inhibition of I-125-IGF-I binding was obtained in the presence of 25-mu-g/ml suramin. Moreover, the drug was able to dissociate I-125-IGF previously bound to its cell surface receptor. Affinity labeling experiments demonstrated that I-125-IGF-I bound to a polypeptide of 140 kDa. When HT29-D4 cells were cultured in the presence of 10-mu-g/ml of alpha-IR3, a monoclonal antibody directed against the binding site of IGF-I, an inhibition of cell proliferation and a stimulation of cell differentiation was observed. After 10 days of treatment with alpha-IR3, HT29-D4 cells formed a regular monolayer of enterocyte-like cells exhibiting an apical brush border and tight junctions delimiting two domains of the plasma membrane (apical and basolateral). Furthermore, we show that IGF-I significantly increased the initial rate of glucose uptake by HT29-D4 cells, while we have previously shown that suramin decreased glucose consumption. From these data we conclude that IGF-I secreted by the cells themselves, stimulates proliferation of HT29-D4 cells via an autocrine mechanism. Blockade of this stimulation by suramin or by a specific monoclonal antibody inhibits cell growth, glucose uptake and triggers the process of enterocytic differentiation.