Insulin-like growth factor-I (IGF-I) attenuation of growth hormone is enhanced by overexpression of pituitary IGF-I receptors.

Insulin-like growth factor-I (IGF-I) attenuates GH gene expression by a receptor-mediated mechanism in pituitary cells. We, therefore, isolated neomycin-resistant stable GC cell transfectants over-expressing human IGF-I receptor cDNA (IGFIR-cDNA) cloned in an Rous sarcoma virus-directed expression vector. A transfection control contained the IGFIR-cDNA cloned in the reverse orientation. Southern analysis confirmed incorporation of human IGFIR-cDNA sequences into rat genomic DNA. Immunoprecipitation of metabolically labeled 35S]methionine stably transfected cells revealed a 200-kDa human IGF-I receptor precursor protein. Growth rate and basal GH secretion were not altered in transfected cells. Although transfected and control cells had a similar Kd for IGF-I binding (0.43 and 0.40 nM, respectively), IGF-I-binding sites were induced 17-fold (384,000 vs. 22,000 sites/cell). Treatment of cells with IGF-I (6.5 nM) maximally attenuated GH secretion by 80% compared to 40% attenuation in control cells (P less than 0.0001). Maximal suppression of GH in transfectants occurred within 15 h of treatment, and GH secretion by control cells was only maximally suppressed after 42 h. The ED50 of IGF-I suppression of GH secretion in transfectants after 15 h was 0.5 nM. These results demonstrate that transfectants overexpressing human IGF-I receptor are hyperresponsive to exogenous IGF-I. These data indicate that IGF-I receptor number plays an important role in mediating the signal transduction of IGF-I to the GH gene.