Complex mediation of uterine endometrial epithelial cell growth by insulin-like growth factor-II (IGF-II) and IGF-binding protein-2

The coexpression of IGF (-I and -II) peptides, corresponding receptors, and IGF binding proteins (IGFBPs) in uterine endometrium suggests that a signi ficant component of IGF action in this tissue is via autocrine or paracrine pathways, or both. The present study examined whether IGF-II and a major u terine-expressed IGF-II binding protein, IGFBP-2, modulate endometrial epit helial cell mitogenesis. Serum-deprived porcine endometrial glandular epith elial (GE) cells of early pregnancy were treated with various concentration s of IGFs, recombinant porcine (rp) IGFBP-2, or both, and examined for chan ges in cellular mitogenesis by incorporation of [H-3]thymidine into DNA. Re combinant human (rh) IGF-II stimulated DNA synthesis in a dose-dependent ma nner. Human [Leu(27)]-IGF-II, an analog with selective affinity for the IGF -II (type II) receptor, increased thymidine uptake by twofold compared with untreated GE cells. When added in combination with an equimolar concentrat ion of rhIGF-I, [Leu(27)]-IGF-II or rhIGF-II stimulated thymidine incorpora tion to a greater extent than did rhIGF-I alone. Ligand blot analysis of GE cell conditioned medium revealed the presence of four IGFBPs with molecula r masses of 48, 31, 23, and 15 kDa. Physiological concentrations of rpIGFBP -2 (nM range) increased both basal and IGF-induced DNA synthesis in GE cell s. At equimolar concentrations, Des(1-6)IGF-II (an IGF-II analog with much reduced affinity for IGFBPs) and rpIGFBP-2 had additive effects on GE cell mitogenesis, suggesting that the IGFBP-2 modulation of uterine cell growth may involve both IGF-dependent and IGF-independent pathways. Our results de monstrate the complex interplay of IGF system components in uterine endomet rial epithelial growth regulation in vitro, identify IGF-II and IGFBP-2 as locally coexpressed uterine epithelial cell mitogens, and suggest the prese nce of a functional signaling pathway by which IGF-II stimulates epithelial cell proliferation via the type II receptor.