Inhibition of CACO-2 cell proliferation by (n-3) fatty acids: possible mediation by increased secretion of insulin-like growth factor binding protein-6

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The present study was performed to examine the effect of various polyunsaturated fatty acids (PUFAs) on the proliferation of the human colon adenocarcinoma cell line, Caco-2 cells. The ability of individual PUFAs to stimulate cell proliferation was examined by culturing cells in serum-free medium. Eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) inhibited cell proliferation, and therefore, cells cultured with 100 μM EPA or DHA reached much lower final densities compared to cells cultured with 2 μM linoleic acid (control) or 100 μM linoleic acid (LA). Insulin-like growth factors (IGFs) are autocrine and paracrine growth promoters of a variety of cells, and a family of IGF binding proteins (IGFBPs) modulates the biological actions of IGF-I and IGF-II. Since IGF-II has been shown to be an autocrine regulator of Caco-2 cells, we investigated the effects of these PUFAs on IGF-II and IGFBP secretion in association with Caco-2 cell proliferation. Immunoblot analysis of serum-free conditioned medium using a monoclonal anti-IGF-II antibody showed that concentrations of both mature 7,500 M r and higher M r forms of pro IGF-II were lower in conditioned medium by cells treated with EPA or DHA compared with LA. Ligand blot analysis revealed that the secretion of IGFBP-6 was significantly higher in cells treated with 100 μM EPA or DHA compared to LA. Northern blot analysis demonstrated that the steady state levels of IGFBP-6 mRNA were higher in cells cultured with EPA or DHA compared to the controls. Exogenously added IGFBP-6 inhibited Caco-2 cell proliferation. We propose that low IGF-II/IGFBP-6 ratios may have resulted in less free IGF-II and, consequently, the slower proliferation of Caco-2 cells treated with EPA or DHA.