cAMP Enhances Estrogen-Related Receptor α (ERRα) Transcriptional Activity at the SP-A Promoter by Increasing Its Interaction with Protein Kinase A and Steroid Receptor Coactivator 2 (SRC-2)

Estrogen-related receptor (ERR alpha) plays a critical role in basal and cAMP-induced expression of the human surfactant protein-A (SP-A) gene in lung type II cells through direct binding to an ERR response element (ERRE, 5'-TGACCTTA-3') within its 5'-flanking region. Furthermore, protein kinase A (PKA) up-regulates ERR alpha activation of the hSP-A promoter. In the present study, using cultured human fetal lung type II cells, we observed that cAMP enhanced ERR alpha phosphorylation and nuclear expression levels. cAMP/PKA stimulation of ERR alpha activation of the SP-A promoter was blocked by the PKA inhibitor, H89, whereas the MAPK P38 inhibitor, SB203580, and the MAPK kinase inhibitor, PD98059, had negligible to modest effects. This suggests that cAMP acts selectively through PKA to increase ERR alpha transcriptional activity. Of several coactivators tested, steroid receptor coactivator 2 (SRC-2) had the most pronounced effect to increase ERR alpha transcriptional activity at the SP-A promoter; this was enhanced by cotransfection with PKA catalytic subunit (PKAcat). Interestingly, SRC-2, ERR alpha, and PKAcat in type II cell nuclear extracts interacted at the ERRE; this was enhanced by cAMP and inhibited by H89. cAMP increased in vivo binding of PKAcat and SRC-2 to the ERRE genomic region in lung type II cells. In mutagenesis studies, three serines (S87, S114, and S277) were found to be critical for PKA and SRC-2 induction of ERR alpha transcriptional activity. Collectively, these findings indicate that cAMP/PKA signaling enhances ERR alpha phosphorylation and nuclear localization, recruitment to the SP-A promoter, and interaction with PKAcat and SRC-2, resulting in the up-regulation of SP-A gene transcription. (Molecular Endocrinology 23: 772-783, 2009)