Mkk. Eltanani et Cd. Green, 2 SEPARATE MECHANISMS FOR LIGAND-INDEPENDENT ACTIVATION OF THE ESTROGEN-RECEPTOR, Molecular endocrinology, 11(7), 1997, pp. 928-937
Transient transfection experiments in which three different estrogen r
esponse element-containing reporter genes were cotransfected into HeLa
cells, together with constitutively expressed estrogen receptor (ER)
constructs, demonstrate that activation of the transcription of the re
porter genes by epidermal growth factor (EGF) and by cholera toxin wit
h 3-isobutyl-1-methyl-xanthine, which elevate cellular cAMP, is depend
ent upon the presence of functional ER, Cotransfection of the reporter
genes with truncated versions of the ER shows that the two non-ligand
activators of ER require different regions of the receptor to produce
their effects on transcription, EGF acts primarily by means of transa
ctivation domain AF-1, whereas cAMP acts via transactivation domain AF
-2 of the ER. A point mutation that removes a major site of inducible
phosphorylation within the AF-1 domain of the ER abolishes the respons
e to EGF, but the response to estradiol and cAMP is retained, Specific
inhibition of cAMP-activated protein kinase (protein kinase A) preven
ts the response to elevated cAMP but does not affect EGF or estradiol
responses. Overexpression of the protein kinase A catalytic subunit in
HeLa cells results in an amplified response to estradiol, similar to
that induced by cholera toxin with 3-isobutyl-1-methyl-xanthine. Compa
rable experiments performed using COS-1 cells produce similar results
but also reveal cell type- and promoter-specific aspects of the activa
tion mechanisms. Apparently, the ER may be activated by three differen
t signal molecules, estradiol, EGF, and cAMP, each using a mechanism t
hat is distinguishable from that of the others.