Insulin-Like Growth Factor I Receptors Are Expressed by the Enteroendocrine Cell Line STC-1: Relationship with Proliferation and Cholecystokinin Expression

Abstract

Receptors for insulin-like growth factors (IGF-I and IGF-II) are expressed in mammalian intestinal epithelium. No information on the presence of IGF receptors in intestinal endocrine cells is available. We tested for IGF-I receptors the endocrine cell line STC-1, which synthesizes and processes cholecystokinin (CCK) among other peptides, and assessed the effects of IGF-I on cell growth and CCK content. Cell monolayers in serum-free culture medium specifically bound [¹²⁵I]IGF-I. Scatchard analysis was consistent with a single class of high affinity binding sites (K_D = 0.91 nM; B_max = 4,700 sites/cell). In competitive binding assays, unlabeled IGF-I, IGF-II and insulin displaced in a dose-dependent manner [¹²⁵I]IGF-I binding with the following potencies (K_I): IGF-I (0.74 nM) > IGF-II (3 nM) >> insulin (1 µM). Affinity cross-linking with [¹²⁵I]IGF-I using disuccinimidyl suberate and SDS-PAGE under reducing conditions yielded a polypeptide band with apparent M_r 130,000, consistent with the α-subunit of the IGF-I receptor. IGF-I and IGF-II (0.3–30 nM) dose-dependently stimulated [³H]thymidine incorporation, with a maximal response of 110% above basal. IGF-II was approximately 10-fold less potent than IGF-I, suggesting a mediation through IGF-I receptors. In addition, the numbers of cells treated with 3 nM IGF-I amounted to 116, 130 and 159% of control values after 1, 2 and 4 days of incubation, respectively (p < 0.05). A significant increase in the cell CCK contents was observed after a 48-hour exposure to 3 or 30 nM IGF-I. These results demonstrate IGF-I receptor expression by the enteroendocrine cell line STC-1. IGF-I stimulates proliferation in short-term experiments, and increases intracellular levels of CCK.