EGR1 expression: A calcium and ERK1/2 mediated PPARγ-independent event involved in the antiproliferative effect of 15-deoxy-Δ12,14-prostaglandin J2 and thiazolidinediones in breast cancer cells

Our aim was to get new information about the Peroxisome Proliferator Activated Receptor gamma (PPAR gamma)-independent pathway involved in the antiproliferative action of PPAR-gamma ligands in breast cancer cells. We investigated the effects of Troglitazone (TGZ), Ciglitazone (CGZ), Rosiglitazone (RGZ) and, 15-deoxy-Delta 12,14-prostaglandin J2 (15d-PGJ(2)) on the hormone-dependent breast cancer cell line MCF7. The early transcription factor EGR1 (Early Growth Response gene 1) mRNA and protein levels peaked after 3 h of incubation with 25 mu M TGZ, CGZ or 15d-PGJ(2) and then gradually decreased. RGZ, the most potent activator of PPAR gamma, did not show this effect. The PPAR gamma antagonist GW 9662 did not block EGR1 mRNA induction which also still occurred in case of PPAR gamma silencing as well as in case of treatment with the PPAR gamma-inactive compound Delta 2-TGZ. EGR1 mRNA induction required ERK1/2 phosphorylation which was not blocked by EGF Receptor (EGFR) inhibition. The ERK1/2 pathway was also involved in Delta 2-TGZ-induced EGR1 mRNA expression in the hormone-independent breast cancer cell line MDA-MB-231. Using the fluorescent dye Fura2, we showed in MCF7 that TGZ or Delta 2-TGZ induced an immediate increase in cytosolic calcium which was required for ERK1/2 phosphorylation and EGR1 mRNA induction as demonstrated by calcium chelation experiments. Furthermore, in MCF7 transfected with siRNA targeting EGR1, Delta 2-TGZ inhibited less efficiently cell proliferation. (C) 2011 Elsevier Inc. All rights reserved.