Requirement of Sp1 and estrogen receptor alpha interaction in 17beta-estradiol-mediated transcriptional activation of the low density lipoprotein receptor gene expression.

Estrogen is one of the most important physiological regulators of low density lipoprotein receptor (LDLR) expression. Despite many studies conducted in animals and humans showing increased expressions of LDLR messenger RNA by hormone treatment, the molecular basis of the effect of estrogen on LDLR transcription has not been clearly elucidated. By using HepG2 cells that transiently express functional estrogen receptor alpha (ERalpha) and LDLR promoter constructs, we show that the specific interaction of ERalpha with the transcription factor Sp1 bound to the LDLR promoter is responsible for the activation of LDLR transcription by estrogen. We demonstrate that 1) mutations to abrogate the binding of Sp1 to its recognition sequences present in repeat 1 and repeat 3 elements of the LDLR promoter completely abolish the ERalpha-mediated activation of the LDLR promoter activity; 2) mutations that abolish the selective DNA-binding activity or inactivate the C-terminal transcription activation function (AF2) of ERalpha had no effect on the ability of ERalpha to activate LDLR transcription; however, transcriptional activation was completely lost by deletion of the N-terminal transcription activation region (AF1); 3) a subregion of AF1 (amino acids 67-139) was further identified to be important for ERalpha to activate the LDLR promoter; and 4) ERalpha enhanced the formation of Sp1-repeat 3 DNA complexes. We also show that mutation at the sterol-responsive element-1 site diminishes the activity of ERalpha on LDLR transcription, thereby suggesting that the sterol-responsive element-1-binding protein may interact with the Sp1-ERalpha complex to trans-activate LDLR gene transcription. This study for the first time provides a molecular basis for an understanding of the regulation of LDLR transcription by estrogens.