Inhibition of HER-kinase activation prevents ERK-mediated degradation of PPARγ

R-etodolac, a nonsteroidal anti-inflammatory drug, inhibits the progression of CWRSA6 androgen-independent and LuCaP-35 androgen-dependent prostate cancer xenograft growth through downregulation of cyclin D1 expression via the PPARγ pathway. PPARγ protein degradation, observed post-R-etodolac treatment, resulted from phospho-MAP kinase (p44/42) induction by R-etodolac negatively regulating PPARγ function. Negative regulation of PPARγ was overcome by a combination regimen of R-etodolac with the HER-kinase axis inhibitor, rhuMab 2C4, which demonstrated an additive antitumor effect. We further show that the inhibition of HER-kinase activity by rhuMab 2C4 is sufficient to inhibit PPARγ protein degradation. This study introduces a novel concept of an in vivo crosstalk between the HER-kinase axis and PPARγ pathways, ultimately leading to negative regulation of PPARγ activity and tumor growth inhibition.