IGFBP-3/transferrin/transferrin receptor 1 complexes as principal mediators of IGFBP-3 delivery to colon cells in non-cancer and cancer tissues.

Purpose: The aim of this work was to study the involvement of IGFBP-3/Tf complexes in the pathology of colorectal carcinoma (CRC), quantify them, investigate their relation to iron concentration and binding to transferrin receptor (TfR) in colon tissue (non-cancer and cancer), and to assess the priority of this pathway for internalization of IGFBP-3.

Methods: The presence of IGFBP-3/Tf complexes was analyzed in sera from healthy persons and patients with CRC, and in colon tissue by immunoblotting. Complexes were immunoprecipitated, quantified by immunoassay and structurally characterized by immunoblotting, lectin blotting and mass spectrometry. Complexes which interacted with colon cells were immunoprecipitated with anti-TfR1 antibody and studied. Colon tissue slides were subjected to immunohistochemical analysis.

Results: The concentration of IGFBP-3/Tf complexes was three times lower in patients with CRC. They were increasingly carbonylated, sialylated, contained more Galβ4GlcNAc units, expressed altered charge density and increased affinity for metal ions. Immunoprecipitation experiments revealed more TfR1 on membranes than in cytosol of colon cells, also more in cancer than non-cancer tissue. TfR1 on membranes were less occupied with IGFBP-3/Tf complexes than in cytosol. Immunofluorescent staining indicated a remarkable degree of co-localization of IGFBP-3 and TfR1, evenly distributed in non-cancer tissue and both evenly and cell surface concentrated in cancer tissue.

Conclusions: Increased expression of TfR1 on colon cell membranes in patients with CRC compensates for the reduced extracellular availability of IGFBP-3/Tf and TfR1 is the principal binding partner of extracellular IGFBP-3. IGFBP-3/Tf complexes in patients with CRC exhibit increased affinity for iron ions.