Abstract 4158: MEKK1 regulates DMP1 transcriptional activity via phosphorylation and predicts breast cancer patient outcome

Proceedings: AACR 103rd Annual Meeting 2012‐‐ Mar 31‐Apr 4, 2012; Chicago, IL Breast cancer remains a significant public health issue in the industrialized countries. Physicians depend on surgery, cytotoxic adjuvant chemotherapy, or radiation for treatment of breast cancer patients. Few novel therapies are approved for clinical use. Although existing therapies are effective in controling the disease in many patients, the cytotoxic side-effects make them limited. This is especially the case in the patients with early stage breast cancer whose prognosis with surgery alone is good and avoiding adjuvant chemotherapy would spare them toxic side effects. On the other hand, significant fraction of patients relapses and develops metastasis. Therefore, it is critical to identify markers for better breast cancer patient stratification based on their prognosis that would guide physicians in selection and aggressiveness of therapies. Recently, we linked transcription factor DMP1 (cyclin D binding protein 1, Dmtf1) as a critical tumor suppressor that blocks proliferative signals from ErbB2 and oncogenic Ras by activating p14Arf-p53 pathway. hDMP1 is hemizygously deleted in ∼50% of human breast tumors that retain wild-type ARF and p53. In this study we identified an upstream kinase, MEKK1, which cooperates with Dmp1 to activate Arf transcription. In constitutively active form (C-terminal kinase domain or CA-MEKK1), MEKK1 increased transcriptional activity of Dmp1 via direct phosphorylation, while the full length (regulatory and kinase domain) form behaved as a feedback inhibitor by increasing Dmp1 ubiquitination. Phosphorylation sites on DMP1 that increase its transcriptional activity were mapped to the C-terminal transactivation domain. Expression of CA-MEKK1 in breast cancer cell lines activated endogenous Arf-p53 pathway. hMEKK1 is located on human 5q11 chromosome, a locus frequently deleted in breast and lung cancer. Since MEKK1 appeared to be an activator of p53 tumor suppressor pathway, we wondered if MEKK1 is involved in human cancer. Using DNA from breast cancer patient tumors and matched normal tissue, we analyzed hMEKK1 gene deletion using specific loss of heterozygosity (LOH) primers. MEKK1 was found hemizygously deleted in ∼20% of patients. Immunohistochemistry confirmed reduction of MEKK1 protein intensity to LOH(+) compared to LOH (-) patients. In the independent cohort of breast cancer patients, we show that low MEKK1 mRNA expression is associated with adverse outcome. Here we show that constitutively active MEKK1 is a novel activator of Dmp1-Arf-p53 pathway via Dmp1 phosphorylation. The MEKK1 gene was frequently deleted in breast tumor tissue and its expression was correlated with disease outcome. Overall, MEKK1 is a potential prognostic/predictive indicator for breast cancer and could be used by physicians to better stratify patients. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 4158. doi:1538-7445.AM2012-4158