Wang X, Boonyaratanakornkit V, Husovsky M, Gutierrez G, Zastrow C, Wilkins S, Strong DD, Linkhart TA. "Regulation of human insulin-like growth factor binding protein-5 (IGFBP-5) promoter activity in human osteoblasts by glucocorticoids and transcription activating factor-2 (AP-2)." Biochemica et Biophysica Acta, Gene Structure and Expression . (): -. (*)
Glucocorticoids inhibit bone formation by inhibiting osteoblast proliferation and differentiation, and stimulating osteoblast apoptosis. Glucocorticoids inhibit expression of cytokines and growth factors, including IGF I and IGF binding protein-5 (IGFBP-5). IGFBP-5 enhances IGF activities on osteoblasts in many conditions, stimulates bone formation in vivo, and may function in part as a growth factor that affects cells by IGF-independent mechanisms. To investigate glucocorticoid inhibition of IGFBP-5 expression, we identified sequences in the proximal human IGFBP-5 gene promoter that mediate Dexamethasone inhibition of transcription. Promoter deletion and mutation constructs in pGL3 luciferase reporter were tested by transient transfection of human U2-OS and TE-85 osteosarcoma cells. An activator protein-2 (AP-2) binding site sequence contributed to dexamethasone inhibition of transcription. AP-2a bound to this sequence and overexpression of AP-2a protein in TE-85 cells increased promoter activity and prevented dexamethasone inhibition. Dexamethasone induced glucocorticoid receptor (GF) binding to AP-2a as determined by GST-GR fusion protein pull down, co-immune precipitation, and mammalian two-hybrid interaction assay. GR did not bind directly to a promoter DNA sequence containing the AP-2 site, or prevent AP-2 from binding. Results suggest that glucocorticoids induce GR binding to AP-2 and inhibit human IGFBP-5 transcription in part by decreasing AP-2 transactivation of the promoter.