A possible role of insulinlike growth factor I (IGF-I) and IGF-I receptors in the proliferation of human colon cancer has, to the best of the authors' knowledge, not been examined as yet. To determine a role of IGF-I in colon cancer, several human colon cancer cell lines and colon cancers were screened for specific binding to 125I-IGF-I. Almost all the human colon cancer cells examined were variably positive for specifically binding 125I-IGF-I. Almost half the colon cancer cell lines examined showed significant growth response to 6.6 nmol/L IGF-I. Dose-dependent growth effects of exogenously added IGF-I (0.05-3.3 nmol/L) were shown in a representative human colon cancer cell line (Colo-205). To determine if IGF-I binding sites on colon cancer cells were similar or different from that described on other cells, the binding affinity, binding specificity, and molecular size of the IGF-I binding sites were characterized in representative human colon cancer cell lines (HCT-116 and Colo-205). The optimal binding assay conditions for measuring maximum number of 125I-IGF-I binding sites on the cells, in vitro, were determined and found to be similar to that described on other cells. Scatchard analysis of the specific binding data showed the presence of a single class of high-affinity binding sites [disassociation constant (Kd) = approximately 1.0-2.0 nmol/L], with a binding capacity of 1.0-2.0 x 10(5) sites per cell. The molecular weight of IGF-I receptors on cell membranes was determined by gel electrophoresis of the affinity cross-linked proteins, followed by autoradiography. A single band of binding proteins with a molecular mass of approximately 300 kilodaltons was evident under nonreducing conditions and separated out into two bands of approximately 240 and approximately 130 kilodaltons under reducing conditions. The 130-kilodalton band represents the alpha subunit of IGF-I receptors, and the 240-kilodalton band may represent aggregates of the receptor subunits that were not reduced completely. The widespread existence of high-affinity binding sites for IGF-I in established human colon cancer cell lines and freshly resected human colon cancers and the proliferative effect of IGF-I in several colon cancer cell lines, in vitro, reflect that IGF-I may be an important endocrine/paracrine/autocrine factor in the growth regulation of colon cancers in situ.