The peroxisome proliferator-activated receptor γ is an inhibitor of ErbBs activity in human breast cancer cells

One of the most interesting recent developments in the nuclear receptor field has been the identification of natural and synthetic agonists of the peroxisome proliferator-activated receptor (PPAR) family, coupled with a growing recognition that the γ isoform (PPARγ) affects pathways important in a variety of human diseases. Here we show that the activation of PPARγ through the 15-deoxy-Δ-12,14-prostaglandin J₂ (PG-J₂) ligand causes a dramatic inhibition of ErbB-2 and ErbB-3 tyrosine phosphorylation caused by neuregulin 1 (NRG1) and neuregulin 2 (NRG2) in MCF-7 cells. This effect is accompanied by a very efficient blocking of ErbBs effects upon proliferation, differentiation and cell death in these cells. Preincubation of MCF-7 cells with PG-J₂ before addition of NRG1 and NRG2 had a dramatic growth-suppressive effect accompanied by accumulation of cells in the G0/G1 compartment of the cell cycle, and a marked increase in apoptosis. NRG1 and NRG2 induce G1 progression, which was associated with stimulation of the phosphatidylinositol-3 kinase (PI 3-K) pathway, whereas survival was dependent on ERK1/ERK2 activation. Both pathways were inhibited by PG-J₂. Furthermore, PG-J₂ can abolish the NRG1 and NRG2-induced increase in anchorage-independent growth of these cells. PG-J₂ also blocks phosphorylation of other receptor tyrosine kinases, such as IGF-IR, in MCF-7 cells, and suppress proliferation of other breast cancer cell lines. In summary, our data show a specific inhibitory action of PG-J₂ on the activity of the ErbB receptors in breast cancer cells.