Regulation and function of mitochondrial Hep27: A novel modulator of the Mdm2-p53 pathway

The ever-expanding role of p53 in cellular metabolism, apoptosis, and cell cycle control has led to increasing interest in defining the stress response pathways that regulate Mdm2. In an effort to identify novel Mdm2 binding partners, we performed a large-scale immunoprecipitation of Mdm2 in the osteosarcoma U2OS cell line. One significant binding protein identified was Hep27, a member of the short-chain alcohol dehydrogenase/reductase (SDR) family of enzymes. Here we demonstrate the Hep27 pre-protein contains an N-terminal mitochondrial targeting signal that is cleaved following mitochondrial import, resulting in mitochondrial matrix accumulation of mature Hep27. A fraction of mitochondrial Hep27 translocates to the nucleus, where it binds to Mdm2 in the central domain, resulting in attenuation of Mdm2 mediated p53 degradation. In addition, Hep27 is regulated at the transcriptional level by the proto-oncogene c-Myb and is required for c-Myb induced p53 activation. Breast cancer gene expression analysis correlated estrogen receptor (ER) status with Hep27 expression and p53 function, providing a potential in vivo link between estrogen receptor signaling and p53 activity. Our data demonstrate a unique ER-c-Myb-Hep27-Mdm2-p53 mitochondria-to-nucleus signaling pathway that may have functional significance for ER positive breast cancers.