Activation of GPR4 by acidosis inhibits tumor cell migration, invasion and metastasis.

Abstract

Melanoma and Prostate Cancer are two of the most diagnosed forms of cancer in the United States. The ability to metastasize and colonize in a distant location is one of the most devastating hallmarks of cancer as modes of treatment are severely limited and often inefficient. As cancer accounts for one out of every four deaths in the United States, there is no doubt as to the necessity to develop novel strategies to combat cancer progression. Cancer research has transformed to not only target the actual tumor cells, but also the tumor microenvironment as it is known to play a strong role in tumorigenesis and cancer progression. The defining characteristics of the tumor microenvironment are hypoxia, acidosis and high interstitial fluid pressure, and these characteristics actively select for resistant tumor cells, conferring a progressive phenotype. GPR4, a G-protein coupled receptor, has recently been shown to act as a proton sensor, and as acidosis is a defining characteristic of the tumor microenvironment and GPR4 expression is detected in a wide array of human tissues, it is thought GPR4 may influence tumor cell progression. Therefore, it is the purpose of this study to analyze the biological function acidosis activation of GPR4 has on the ability of tumor cells to acquire a motile and metastatic phenotype. Through a series of cell motility, migration and invasion assays we have determined that GPR4 is activated by acidosis, and that this activation inhibits the motility, migratory and invasive ability of GPR4 overexpressing tumor cells. Furthermore, using a pulmonary melanoma metastasis model, we have shown that GPR4 overexpression significantly decreases the ability for in vivo metastasis of tumor cells. To deduce a possible molecular mechanism by which the inhibitory phenotype is acquired, it was found that GPR4 activation increases the occurrence of actin stress fibers in tumor cells, indicative of upregulation of the G12/13-Rho pathway. Furthermore, treatment with CN01, a known cell permeable Rho activator, significantly inhibited both the motility and in vivo metastasis of treated tumor cells. In conclusion, it was found that GPR4 is activated by acidosis, and that this activation inhibits the ability for motility, migration and invasion in tumor cells. Furthermore, GPR4 overexpression inhibits the ability for metastasis in a murine model, and it appears that activation of the small GTPase Rho is at least partially involved in the observed motility and metastasis inhibitory phenotypes.