The Role of MTK1 in Cell Migration and Extracellular Acidification

Abstract

Mitogen activated protein kinase (MAPK) signaling consists of a phosphorylation cascade leading to the phosphorylation of an effector that can translocate to the nucleus and regulate transcription. MAPK signaling can be activated by diverse stimuli such as osmotic stress and hormones. Additionally, MAPK signaling can be activated by receptor tyrosine kinase (RTK) signaling in response to a growth factor binding to the RTK. It has been shown that heterodimerization of the RTKs human epidermal growth factor receptor 2 (HER2) with HER3 is highly correlated to tumor growth and metastasis in breast cancer. Previous research showed that in breast cancer cells, Mitogen Three Kinase 1 (MTK1) is a protein kinase that functions in association with the HER3 receptor upon heregulin (HRG) stimulation. Here, MTK1 has been shown to form a complex in breast cancer cells consisting of Grb2, Shc, GIT1 and ERK1/2. Furthermore, MTK1 was shown to induce extracellular acidification due to lactate secretion and cell migration. A shift from aerobic metabolism to glycolysis in cancer cells was first observed by Otto Warburg. Since then, increased glycolysis and the resulting lactate secretion have been described in multiple types of cancers and is recognized as one of the hallmarks of cancer. This dissertation aims to establish the function of MTK1 in HRG induced RTK signaling in breast cancer cells. To accomplish this goal, we identified proteins of known function that interact with MTK1 to infer the role of MTK1. Applying this strategy, we provide evidence for the first time of mitochondrial regulation of oxidative phosphorylation by MTK1, independent of glycolytic regulation. In addition, our results demonstrate that GIT1 regulates glycolysis-mediated lactic acidosis (higher extracellular acidification caused by increased lactic acid efflux) in MCF-7 cells, independent of mitochondrial function. For the first time, our research shows that the glycolytic enzyme phosphoglycerate kinase 1 (PGK1) associates with MTK1 and GIT1 in MCF-7 cells. Furthermore, it is demonstrated that PGK1 is regulated by HRG stimulation through both phosphorylation of PGK1 and its dissociation from the MTK1 complex. The protein complex consisting of MTK1, GIT1, and PGK1 favors the conversion of pyruvate to lactate resulting rather than oxidative phosphorylation of pyruvate inside the mitochondria. The resulting lactate is secreted and drives extracellular acidification, a hallmark of cancer.