Microsomal antiestrogen binding site (AEBS) (AEBS)

The microsomal antiestrogen binding site (AEBS) is a high-affinity, non-estrogen receptor protein complex located in the endoplasmic reticulum of various tissues (de Medina et al., 2010, Genes & Dev). It is a hetero-oligomeric complex primarily composed of two enzymes involved in the late stages of cholesterol biosynthesis: 3β-hydroxysteroid-Δ8,Δ7-isomerase (also known as emopamil binding protein or EBP) and 3β-hydroxysteroid-Δ7-reductase (Silvente-Poirot & Poirot, 2012, Cancer Cell). AEBS was originally identified through its high affinity for antiestrogenic drugs like tamoxifen, which bind to it independently of their interaction with the estrogen receptor (UniProt P49728). Binding of ligands to AEBS inhibits sterol isomerase activity, leading to the accumulation of specific sterol precursors that can induce oxidative stress, autophagy, and apoptosis in cancer cells. This mechanism contributes to the anti-tumor effects of certain drugs, particularly in ER-negative cancers or as a secondary mechanism in ER-positive breast cancer. Beyond oncology, AEBS plays a critical role in cellular sterol homeostasis and is implicated in rare metabolic disorders related to cholesterol synthesis, such as Conradi-Hünermann-Happle syndrome.

Other names

AEBSAntiestrogen binding siteEBP complex3-beta-hydroxysteroid-delta(8),delta(7)-isomerase complex

Inhibition of 3β-hydroxysteroid-Δ8,Δ7-isomerase activity within the AEBS complex, leading to the accumulation of 8-en-sterols and subsequent induction of oxidative stress and cell death pathways.