Cell polarity of the insulin-like growth factor system in human intestinal epithelial cells. Unique apical sorting of insulin-like growth factor binding protein-6 in differentiated human colon cancer cells.

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Abstract
In this study, we have used enterocyte-like differentiated HT29-D4 human colonic carcinoma cells cultured in a glucose-free medium (HT29-D4-GAL cells) on semi-permeable supports in order to investigate the polarity of the insulin-like growth factor (IGF) system. We report that these cells secrete endogenous IGF-II predominantly (66%) from the basolateral cell surface where type I IGF receptors are almost all (> 96%) localized. HT29-D4-GAL cells also secrete IGF-binding protein (IGFBP) -2, -4, and -6 as evidenced by Western ligand and immunoblot analyses of conditioned medium. IGFBP-2 and IGFBP-4 are secreted primarily into the basolateral side (71 and 87%, respectively), whereas IGFBP-6 is targeted to the apical surface (76%) as a possible consequence of an active sorting. Finally, HT29-D4-GAL cells are found to display responses to IGF-II added to the basolateral but not the apical membrane side in terms of intracellular tyrosine phosphorylation and long-term stimulation of amino acid uptake. This study indicates (a) that IGF-II is potentially capable of autocrine regulation on the basolateral side of HT29-D4-GAL cell, and (b) that IGFBP-6 has a unique pattern of secretory polarity. It supports the concept that a differential sorting of the various forms of IGFBPs might play a modulatory role in the maintenance of a functional polarity in the differentiated HT29-D4-GAL cells.

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