Bending of the estrogen response element by polyamines and estrogen receptors α and β: A fluorescence resonance energy transfer study

Abstract

Estrogenic regulation of gene expression is mediated by the binding of the hormone to its receptors (ERα and ERβ) followed by their binding to estrogen response element (ERE). Previous studies showed that natural polyamines – putrescine, spermidine, and spermine – facilitated ERα·ERE recognition. We determined the effects of natural and synthetic polyamines on the bending of a 27-mer oligonucleotide (ODN) harboring the ERE (ERE-ODN), using fluorescence resonance energy transfer (FRET) technique. Complementary strands of the ERE-ODN were labeled with fluorescein and tetramethylrhodamine, as donor and acceptor, respectively. The ERE-ODN was intrinsically bent with an end-to-end distance of 76±2Å, compared to a theoretical value of 98Å. The end-to-end distance of the ERE-ODN was reduced to 64Å in the presence of 250μM spermine. A control ODN with scrambled sequence did not show intrinsic bending or spermine-induced bending. Alkyl substitution at the pendant amino groups reduced the ability of spermine to bend the ERE-ODN. Both ERα and ERβ decreased the end-to-end distance of the ERE-ODN, although ERα was more efficient than ERβ in inducing ERE bending. Spermine-induced bending of the ERE-ODN was significantly increased by ERα. Fluorescence anisotropy measurement showed that the equilibrium association constant of ERα–ERE binding increased by 12-fold in the presence of 250μM spermine compared to control. The free energy change (ΔG) of ERα·ERE complex formation was −13.1kcal/mol at 22°C in the presence of spermine. Our results suggest that polyamine-induced bending of the ERE might be a mechanism for enhancing ERα–ERE binding affinity and thereby fine-tuning the transcriptional response of estrogen-responsive genes.