Anti-estrogenic effects of conjugated linoleic acid through modulation of estrogen receptor phosphorylation.
We previously showed that conjugated linoleic acids (CLA) can inhibit transcriptional activation mediated by estrogen response elements (EREs) and that this activity can, at least in part, account for the reported anti-tumor effects of these compounds on breast cancer cells. Using estrogen receptor positive (ER+) MCF-7 cells, we now demonstrate that CLA inhibited both the transactivation of artificial reporter constructs driven by canonical EREs, and the expression of endogenous progesterone receptors, a gene which is transcriptionally regulated by estrogen through novel ER-binding sites. This inhibition was accompanied by downregulation of ER alpha expression and decreased ER alpha-ERE binding activity. These effects on ER alpha were not causally linked since transfection of an ER alpha expression plasmid in MCF-7 cells failed to antagonize CLA downregulation of ER alpha-ERE binding. Immunoprecipitation/Western blot studies revealed that CLA dose-dependently suppressed the degree of phosphorylation of ER alpha, a modification known to inhibit receptor-ERE interactions. As a mechanism that may account for this induced dephosphorylation of ER alpha in MCF-7, we found that CLA specifically stimulated protein phosphatase 2A (PP2A) activity. Experiments using the PP2A inhibitor okadaic acid (OA) showed that OA antagonized both the dephosphorylation effects of CLA on ER alpha and its inhibition of ER alpha-ERE binding. These results provide evidence that the anti-estrogenic activity of CLA is caused by inducing the dephosphorylation of ER alpha through stimulation of PP2A activity.
Liu
Jingbo
J
Department of Gynecology and Obstetrics, Division of Research, Emory University School of Medicine, Atlanta, Georgia 30322, USA.
Sidell
Neil
N
D001943:Breast Neoplasms / Q000188:drug therapy* / Q000473:pathology; D045744:Cell Line, Tumor / Q000187:drug effects; D004305:Dose-Response Relationship, Drug; D020847:Estrogen Receptor Modulators / Q000008:administration & dosage / Q000494:pharmacology* / Q000627:therapeutic use; D047628:Estrogen Receptor alpha / Q000187:drug effects* / Q000235:genetics; D005260:Female; D006801:Humans; D044243:Linoleic Acids, Conjugated / Q000008:administration & dosage / Q000494:pharmacology* / Q000627:therapeutic use; D009376:Neoplasms, Hormone-Dependent / Q000188:drug therapy* / Q000473:pathology; D010766:Phosphorylation / Q000187:drug effects; D012333:RNA, Messenger / Q000032:analysis; D020133:Reverse Transcriptase Polymerase Chain Reaction
D020847:Estrogen Receptor Modulators; D047628:Estrogen Receptor alpha; D044243:Linoleic Acids, Conjugated; D012333:RNA, Messenger