INSULIN-LIKE GROWTH-FACTORS AND THEIR BINDING-PROTEINS IN HUMAN COLONOCYTES - PREFERENTIAL DEGRADATION OF INSULIN-LIKE-GROWTH-FACTOR BINDING-PROTEIN-2 IN COLONIC-CANCERS

We have compared the expression of insulin-like growth factors (IGFs) and IGF binding proteins (IGFBPs) in ten paired samples of normal and tumour colonic tissue with regard to both mRNA and protein. We have co mpared sensitivity of these tissues to IGF-I using primary cultures of epithelial cells of colonic mucosa, and we have examined the producti on of IGFs and IGFBPs by these cells. In the tissues, IGFBP-2 mRNA was expressed in all normal and cancer samples but other IGFBPs showed va riable expression. mRNAs for IGF-I were expressed in all normal and ca ncer tissues but IGF-II mRNA was only detected in cancer tissue (3 out of 10). Immunostaining of sections of normal and cancer tissue was ne gative for IGF-I and IGF-II; IGFBP-2 was positive in 2 out of 10 cance r tissues and 7 out of 10 normal tissues; IGFBP-3 was positive in 7 ou t of 10 cancer tissues and 7 out of 10 normal tissues; and IGFBP-4 was positive in 5 out of 10 cancer tissues and 6 out of 10 normal tissues . In the cells in culture, cancer cells showed increased incorporation of [S-35]methionine into protein and [H-3]thymidine into DNA (P < 0.0 2) when treated with IGF-I. Western blotting of serum-free conditioned media from cells in culture showed that 8 out of 10 normal and 3 out of 10 cancer cultures produced a 32-kDa immunoreactive IGFBP-2. No IGF BP-3 was secreted by any culture but 24-kDa IGFBP-4 was found in 3 out of 10 normal and 5 out of 10 cancer tissues. Because of the discrepan cy between mRNA and protein expression for IGFBP-2, degradation of nat ive IGFBPs was assessed using tissue extracts. Colon cancer extracts w ere able to degrade exogenous IGFBP-2, IGFBP-3 and IGFBP-4, whereas no rmal tissue extracts were without effect on IGFBP-2. We conclude that IGFBPs are synthesized and secreted by cells of the colonic mucosa but that proteolysis of secreted IGFBP-2 occurs in colon cancer tissue. T his selective degradation may confer a growth advantage.