THE ROLE OF METABOTROPIC GLUTAMATE RECEPTOR 1 IN HUMAN MELANOMA

Abstract

Metabotropic glutamate 1 (mGlu1) receptor has been proposed as a target for the treatment of metastatic melanoma. In this thesis, we investigated the role of mGlu1 receptors in melanoma cell growth and death, and aim to characterize the signal transduction and pharmacology of mGlu1 receptors in melanoma. In chapter 1, we demonstrate that mGlu1 behaves like a dependence receptor in melanoma by creating a dependence on glutamate for sustained cell viability and proliferation. Using genetic and pharmacological inhibitors we established that this effect of glutamate is mediated by activation of mGlu1 receptors. We confirmed this in vivo using both targeted knockdown of mGlu1 receptors in a melanoma cell xenograft model, and a selective mGlu1 receptor antagonist.

In chapter 2, we investigated the canonical signal transduction cascade and pharmacology of mGlu1 receptors in melanoma. Glutamate stimulates melanoma cell growth, but failed to stimulate phosphoinositide (PI) hydrolysis in several human melanoma cell lines. Quisqualate, another mGlu1 receptor agonist, failed to stimulate PI hydrolysis as well as cell growth in mGlu1 receptor-expressing melanoma cells. Taken together, this demonstrates that the tools commonly used to study mGlu1 receptor function and signaling in other systems may be inappropriate in melanoma cells.

In chapter 3, we continued investigating the pharmacology and signaling of mGlu1 receptors in melanoma cells using various mGlu1 receptor ligands and pharmacological inhibitors. We demonstrated that the previously characterized, non-canonical signal transduction cascade downstream of mGlu1 receptors, which depends on mGlu1 receptor internalization and MAPK signaling, may be involved in glutamate-stimulated melanoma cell growth. However, ligands known to activate mGlu1a receptors and subsequently mediate non-canonical signaling cascade in other systems, failed to increase proliferation in mGlu1 receptor-expressing melanoma cells. Therefore, additional studies are required to further identify the pharmacology and signal transduction cascade(s) downstream of mGlu1 receptors in melanoma.

In chapter 4, we demonstrate the translational significance of our studies. We exploit the dependence receptor properties of mGlu1 to propose and justify three promising approaches for melanoma treatment: glutamate depletion, mGlu1 receptor antagonism, and targeting of mGlu1 receptor signaling.