The Tumor Suppressive Effects of T Cell Death-Associated Gene 8 in Blood Cancers

Abstract

In the early twentieth century Otto Warburg recognized a metabolic phenomenon that transpired in cancer cells, currently known as the Warburg Effect. Warburg discovered cancer cells favor glycolysis rather than oxidative phosphorylation for energy production, even in the presence of oxygen. As a result of increased glycolytic flux in cancer cells the tumor microenvironment is acidic. Extracellular acidosis has pleiotropic effects on tumor growth and cancer progression. Therefore, it is important to understand how cancer cells sense extracellular acidosis and respond to it. Cancer cells can sense extracellular acidosis through the proton-sensing G-protein-coupled receptor family, which includes GPR65 (TDAG8), GPR4, GPR68 (OGR1), and GPR132 (G2A). In this study, a bioinformatics analysis revealed T cell death-associated gene 8 (TDAG8) expression is significantly reduced in human hematologic malignancies when compared with normal immune cells and leukocyte-rich tissue. This observation prompts the hypothesis that TDAG8 gene expression is unfavorable for cancer progression of hematologic malignancies. To test the hypothesis, TDAG8 gene expression was restored in U937 acute myeloid leukemia cells. Upon investigation, severe extracellular acidosis inhibited U937 cell proliferation while mild acidosis stimulated it. However, restoration of TDAG8 gene expression reduced U937 cell proliferation. Restoring TDAG8 gene expression in various blood cancer cell lines also reduced tumor growth in severe combined immuno-deficient mice, which correlated with a significant reduction in c-myc oncogene expression. Investigations demonstrated TDAG8 activates G[alpha]13/RhoA signaling to reduce c-myc oncogene expression in U937 cells. G[alpha]13/RhoA signaling is crucial for cell attachment, migration, and metastasis. Consequently, metastasis was investigated with U937 cells. Restoring TDAG8 gene expression reduced U937 cell attachment to matrigel, migration toward a chemoattractant, and metastasis in severe combined immuno-deficient mice. Overall, this dissertation provides evidence that TDAG8 delivers a growth disadvantage to cancer cells of hematologic origin and acts as a contextual tumor suppressor.