Autocrine secretion of a colorectum‐derived growth factor by HT‐29 human colon carcinoma cell line

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Abstract
The human colon cancer cell line HT‐29 produces a growth factor (CRDGF; Mr = 25,000) which inhibits EGF binding to a wide variety of different normal and tumoral cell types in culture. Scatchard analysis of EGF binding shows that CRDGF induces a decrease in EGF receptor affinity. In contrast, EGF binding to any of the human colorectal cancer cell lines tested, i.e., HT‐29, HT‐29 (clone D4), HRT‐18 or CAL‐14, remains unaltered in the presence of exogenous CRDGF. However, the inhibitory effect of CRDGF becomes apparent on HT‐29 cells after overnight exposure of these to suramin (at 37°C). A short exposure to suramin (1 hr at 4°C) or a mild acid washing of HT‐29 cells can partially restore the inhibitory activity of CRDGF. These observations suggest that the action of suramin results in an unmasking of substantial levels of CRDGF receptors on HT‐29 cells. Scatchard analysis of EGF binding on suramin‐treated HT‐29 cells shows that CRDGF inhibits EGF binding by decreasing EGF receptor affinity, as previously observed with the non‐colonic cell types. A similar unmasking of CRDGF receptors is observed when the other colorectal cell lines are exposed to suramin. These results provide evidence for a model in which the colorectal cell lines have the property of secreting a unique growth factor that binds to its receptor by an autocrine mechanism.