Type‐II insulin‐like growth‐factor receptor in conditioned medium from HT‐29 human colon carcinoma cell line

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 [125I]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 [125I]IGF-II and the void volume pool showed a pattern typical of the type-II receptor. It exhibited a high affinity for IGF-II (KD = 0.4 nM), but had a low affinity for IGF-I (KD = 6.8 nM), and no detectable affinity for insulin. Additional evidence was provided by affinity cross-linking of [125I]IGF-II to the same high-molecular-weight material which demonstrated a major specific band at 250 kDa 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.