PTB-associated splicing factor regulates growth factor-stimulated gene expression in mammalian cells

An insulin-like growth factor I (IGF-I) response element (IGFRE) in the porcine P-450 cholesterol side-chain cleavage gene (P450scc) binds two transcription factors, Sp1 and polypyrimidine tract-binding protein-associated splicing factor (PSF). In this study, we investigated expression of these transcription factors in mouse Y1 adrenal cells, a cell line that does not increase P450scc expression in response to IGF-I. Western blot analysis showed a greater expression of PSF in Y1 cells when compared with a mouse fibroblast cell line (NWTb3) in which IGF-I stimulates the P450scc IGFRE. The two cell lines expressed Sp1 equally, and IGF-I did not increase expression of either transcription factor. Chromatin immunoprecipitation analysis with Y1 chromatin confirmed that PSF and Sp1 bound to the IGFRE. When increasing amounts of Sp1 were expressed in Y1 cells, the IGFRE became responsive to IGF-I. Moreover, a mutant oligonucleotide IGFRE reporter construct that lacks PSF binding was responsive to IGF-I. In conclusion, Y1 adrenal cells are a physiological example of PSF repression of growth factor-stimulated (IGF-I) gene expression (P450scc). The dynamic nature of this repression is consistent with PSF functioning as a regulator of growth factor-stimulated gene expression in mammalian cells.