Mammalian EAK-7 Activates Alternative mTOR Signaling

Abstract

Strong evidence supports the concept that mammalian target of rapamycin (mTOR) regulates essential cell processes and that dysregulation of mTOR signaling leads to the pathogenesis of several debilitating human conditions. Aberrant activation of mTORC1 signaling contributes to the malignant behavior of cancer cells by controlling proliferation, invasion, and metastasis. We identified a novel effector of the mTORC1 pathway and provisionally named the human ortholog, mammalian enhancer of akt1-7 (EAK-7) or (mEAK-7), due to its original identification in Caenorhabditis elegans. In nematodes, EAK-7 negatively affects longevity through regulation of FoxO transcription factors. While there is no functional characterization of mEAK-7 (also known as expressed sequence tag KIAA109 or TDLC1), there are examples in the literature that demonstrate upregulated mEAK-7 gene expression and copy number amplification in diverse cancer types. We discovered that some cancer cell lines derived from head and neck squamous cell carcinomas, non-small lung carcinomas, and breast carcinomas express mEAK-7 protein, while somatic cells we screened do not. Moreover, more than half of the cancer cell lines screened were mEAK-7 positive (mEAK-7+) cancers, but why these cells express mEAK-7 is unknown. When this is coupled with the fact that mTORC1 signaling is amplified in distinct human cancers, it suggests that mEAK-7 may, in part, be responsible for the pathogenesis of certain human cancers. With increasing interest in developing mTOR signaling inhibitors to treat cancer patients, this thesis explores the mechanisms by which mEAK-7 regulates mTOR signaling and functions to support the progression of human cancer. As an aspiring dentist-scientist, I am primarily interested in tumorigenesis in relation to mEAK-7 function under abnormal metabolism and DNA damage (X-ray irradiation). To this end, our central hypothesis is that mEAK-7 functions as a positive regulator of mTORC1 signaling in human cancers.

Canonical mTORC1 signaling utilizes nutrient rich conditions (insulin, amino acids, high energy levels) for activation of this evolutionarily conserved pathway. While stress conditions have been widely reported to inhibit mTOR signaling in non-cancerous cells, it has been shown that mTOR signaling can be sustained after DNA or oxidative stressors. We discovered that mTOR signaling increased after X-ray irradiation, and we provided evidence that mEAK-7 is required for sustained mTOR signaling after under DNA damage. Treatment options for cancer patients include 1) surgery and/or 2) chemo- or radio-therapy. However, patients who experience chemo- or radio- resistant cancers typically have poor clinical prognoses and there is a lack of well-defined biological markers indicative of this malignant process. Thus, the overarching goal of this thesis will be to elucidate the role of mEAK-7 in cancer, and to determine the extent to which mEAK-7 plays a role in tumorigenesis, and whether or not mEAK-7 is a cancer biomarker.

In chapter 1, we outline key facets of knowledge gathered during the last 20 years of mTOR signaling research, beginning with rapamycin’s illustrious discovery on Easter Island and leading to the identification of the molecular kinase, mTOR. We also explore ideas of development and disease in craniofacial biology and cancer. In chapter 2, we rigorously outline the molecular mechanisms regarding mEAK-7 function. In chapter 3, we determined the role of mEAK-7 in human disease, specifically radioresistant cancers and metastatic cancers. Chapter 4, the final chapter, outlines the future goals of the mEAK-7 project and its potential to impact human medicine.