Differential expression of IGF-binding protein-3 in normal and malignant colon and its influence on apoptosis

IGF-binding protein-3 (IGFBP-3) has been reported to exert a protective influence on the pathogenesis of colorectal cancer. This may reflect its modulation of IGF-I bioactivity as well as IGF-I-independent effects on cell proliferation and apoptosis. Although local expression of IGF-I in the colon is increasingly recognised as having important regulatory consequences, the role of locally expressed IGFBP-3 remains unknown. The aims of the present study were: (i) to quantify and localise the expression of IGFBP-3 in human normal and malignant colon; (ii) to relate this expression to that of other components of the IGF-I axis; and (iii) to investigate the effects of IGFBP-3 on colonic epithelial cell proliferation and apoptosis. RNA was extracted from 46 paired samples of normal and malignant colonic tissue. IGFBP-3, IGF-I, IGF-I receptor and GH receptor mRNA levels were quantified using real-time RT-PCR. Laser-capture microdissection of the same samples was used to isolate mRNA from epithelium and stromal components and localise mRNA expression. Expression was confirmed at a protein level by immunohistochemistry. Human colorectal cancer HT-29 and CaCo-2 cells were cultured with IGFBP-3 (200 ng/ml), +/− IGF-I (20 ng/ml), +/− sodium butyrate (5 mM). Cell number was assessed by an MTS assay (a modification of the MTT assay), and apoptosis assessed by cell morphology and FACS analysis using both annexin and propidium iodide staining. UO146, a MAP kinase inhibitor, and wortmannin, an inhibitor of the phosphatidylinositol 3-kinase (PI-3K) pathway, were used to determine the contribution of these signalling pathways on the effects of IGFBP-3. IGFBP-3 mRNA was detected in all samples (mean copy number/μg total RNA in normal colon, 2.6 × 106 compared with 1.3 × 107 in the cancers; P