Phosphorylation of ERK1/2 mitogen-activated protein kinase is associated with poor response to anti-hormonal therapy and decreased patient survival in clinical breast cancer.

It is believed that growth factor phosphorylation cascades interact closely with oestrogen receptor (ER) signaling to regulate breast cancer growth, and that alterations in these pathways may underlie resistance to anti-hormones such as tamoxifen. There is an increasing body of experimental evidence implicating the mitogen-activated protein kinase extracellular signal-regulated-kinases ERK1 and ERK2 (ERK1/2 MAPK) in these events. The present study is the first to address the relationship between ERK1/2 MAPK phosphorylation (p-MAPK) and response to anti-hormonal agents in clinical breast cancer (n = 90). Immunocytochemical analysis using a phosphorylation state-specific ERK1/2 MAPK antibody revealed 72% of breast tumors to have considerable nuclear p-MAPK immunostaining (designated p-MAPK positive), whereas staining was barely detectable or absent in the remaining 28% (designated p-MAPK negative). Comparison with staining in normal breast material obtained from reduction mammoplasty patients (n = 10) demonstrated an increased frequency of higher intensity p-MAPK immunostaining cells within carcinomas (p = 0.002). Significant relationships were revealed between p-MAPK positivity and poorer quality (p = 0.001) and shortened duration (p = 0.006) of anti-hormonal response, as well as with decreased survival time from the initiation of therapy (p = 0.022). These associations were retained in ER positive disease (p = 0.013, p = 0.037 and p = 0.048 respectively), where multivariate analysis demonstrated p-MAPK status to be a significantly independent predictor for response duration (p = 0.034) and patient survival (p = 0.029). Phosphorylated ERK1/2 MAP kinase is thus potentially prognostic for prediction of response to anti-hormonal agents and survival, data providing further evidence that ERK1/2 MAP kinase plays a role in circumvention of anti-hormonal response in breast cancer.