INSULIN-LIKE GROWTH-FACTOR-I REGULATES CELL-PROLIFERATION IN THE DEVELOPING INNER-EAR, ACTIVATING GLYCOSYL-PHOSPHATIDYLINOSITOL HYDROLYSIS AND FOS EXPRESSION

The role of insulin-like growth factors (IGF) was investigated during the early development of the inner ear. IGF-I stimulated growth of otic vesicles that were isolated and cultured in vitro. IGF-I induced DNA synthesis, increased cell number, and mitotic rate in a dose-dependent manner at concentrations between 0.1-10 nM. IGF-II also induced growth but with a lower potency, whereas insulin had no effect. In the presence of IGF-I, otic vesicles developed from stage 18 to stage 21 in 24-h cultures, mimicking the normal mitotic pattern and morphogenesis in vivo. IGF-I also stimulated growth in the cochleo-vestibular ganglion. Binding of I-125-IGF-I to specific receptors occurred with high affinity. An autoradiographic study of sections from otic vesicles showed radiolabeled IGF-I in the epithelium. Immunoreactivity to IGF-I was detected in the otic vesicle and in the cochleo-vestibular ganglion. Intracellular signaling mechanisms of IGF were explored by studying the turnover of glycosylated phosphatidylinositols and the expression of Fos oncoprotein. IGF-I rapidly increased Fos levels in cultured otic vesicles. Furthermore, antisense oligonucleotides complementary to c-fos were able to inhibit IGF-I-induced growth. Both IGF-I-induced cell proliferation and Fos expression were blocked by an antiinositol phosphoglycan (alpha-IPG) antibody. This work suggests that IGF-I may be a candidate to regulate proliferative growth of the otic primordium during normal development and that this action requires the sequential modulation of glycosyl-phosphatidylinositol turnover and Fos expression.