Ligand-free estrogen receptor activity complements IGF1R to induce the proliferation of the MCF-7 breast cancer cells.
BACKGROUND
Ligand-dependent activation of the estrogen receptor (ER) as well as of the insulin-like growth factor type 1 (IGF1R) induces the proliferation of luminal breast cancer cells. These two pathways cooperate and are interdependent. We addressed the question of the mechanisms of crosstalk between the ER and IGF1R.


METHODS
We evaluated the mitogenic effects of estradiol (E2; agonist ligand of ER) and of insulin (a ligand of IGF1R) in the MCF-7 cells by flow cytometry and by analyzing the cell levels of cell cycle-related proteins (immunoblotting) and mRNA (RT-QPCR). To verify the requirement for the kinase activity of Akt (a downstream target of IGF1R) in the mitogenic action of estradiol, we used shRNA strategy and shRNA-resistant expression vectors.


RESULTS
The activation of the ER by E2 is unable to induce the cell cycle progression when the phosphatidyl inositol-3 kinase (PI3K)/Akt signaling is blocked by a chemical inhibitor (LY 294002) or by shRNA targeting Akt1 and Akt2. shRNA-resistant Akt wild-type constructs efficiently complemented the mitogenic signaling activity of E2 whereas constructs with inactivated kinase function did not. In growth factor-starved cells, the residual PI3K/Akt activity is sufficient to complement the mitogenic action of E2. Conversely, when ER function is blocked by the antiestrogen ICI 182780, IGF1R signaling is intact but does not lead to efficient reinitiation of the cell cycle in quiescent, growth factor-starved MCF-7 cells. The basal transcription-promoting activity of ligand-free ER in growth factor-starved cells is sufficient to complement the mitogenic action of the IGF1R-dependent signaling.


CONCLUSIONS
The basal ER activity in the absence of ligand is sufficient to allow efficient mitogenic action of IGF1R agonists and needs to be blocked to prevent the cell cycle progression.
Inserm U938, Centre de Recherche Saint-Antoine, Hôpital Saint-Antoine, Bâtiment Kourilsky, Paris cedex, France. anne-marie.gaben@inserm.fr
Anne-Marie
AM
Gaben
Michèle
M
Sabbah
Gérard
G
Redeuilh
Monique
M
Bedin
Jan
J
Mester
D019898:Autocrine Communication; D001943:Breast Neoplasms / Q000235:genetics / Q000378:metabolism*; D002453:Cell Cycle / Q000187:drug effects; D049109:Cell Proliferation / Q000187:drug effects; D019925:Cyclin A / Q000378:metabolism; D019938:Cyclin D1 / Q000235:genetics / Q000378:metabolism; D004958:Estradiol / Q000494:pharmacology; D005260:Female; D015972:Gene Expression Regulation, Neoplastic / Q000187:drug effects; D006801:Humans; D007328:Insulin / Q000494:pharmacology; D061986:MCF-7 Cells; D019869:Phosphatidylinositol 3-Kinases / Q000378:metabolism; D010766:Phosphorylation; D051057:Proto-Oncogene Proteins c-akt / Q000235:genetics / Q000378:metabolism; D016271:Proto-Oncogene Proteins c-myc / Q000235:genetics / Q000378:metabolism; D017526:Receptor, IGF Type 1 / Q000378:metabolism*; D011960:Receptors, Estrogen / Q000378:metabolism*; D015398:Signal Transduction / Q000187:drug effects
D019925:Cyclin A; D007328:Insulin; D016271:Proto-Oncogene Proteins c-myc; D011960:Receptors, Estrogen; D019938:Cyclin D1; D004958:Estradiol; D019869:Phosphatidylinositol 3-Kinases; D017526:Receptor, IGF Type 1; D051057:Proto-Oncogene Proteins c-akt