Abstract
The HT‐29 human colon cancer cell line has previously been shown to secrete high amounts of insulin‐like growth factor II (IGF‐II). The recent demonstration that soluble IGF‐II/ mannose 6‐phosphate receptor was present in fetal serum prompted us to search for a release of type‐II IGF receptor by these human colonic carcinoma cells. Serum‐free conditioned medium from the HT‐29 cell line was gel filtered on Sephadex G‐200. There was significant binding of [^125^I]IGF‐II to the void volume fractions in addition to binding to the 40‐kDa IGF‐binding protein (IGF‐BP) fractions. Competitive binding studies using [^l25^I]IGF‐II and the void volume pool showed a pattern typical of the type‐II receptor. It exhibited a high affinity for IGF‐II (K~D~ = 0.4 nM), but had a low affinity for IGF‐I (K~D~ = 6.8 nM), and no detectable affinity for insulin. Additional evidence was provided by affinity cross‐linking of [^l25^I]IGF‐II to the same high‐molecular‐weight material which demonstrated a major specific band at 250kDa after reduction of disulfide bonds. In contrast, the type‐I IGF receptor was undetectable. The extracellular type‐II IGF receptor was not a significant carrier for IGF‐II since virtually all IGF‐II secreted by HT‐29 cells was associated with IGF‐BP. The presence of a soluble IGF‐II/mannose 6‐phosphate receptor in the culture medium from colonic cancer cells suggests that it may play an important role in tumor pathogenesis.