ABSTRACT

Using switchable mouse models to assess the

therapeutic potential of MdmX and E2F3

by

Daniel Garcia

Diverse as tumors are, they share some common features that are hallmarks of cancer. Two of these hallmarks are the evasion of tumor surveillance mechanisms and deregulated proliferation. Aiming to address these problems, we have used novel switchable mouse models to test the therapeutic potential of MdmX and E2F3, two genes directly involved in allowing tumor cells to evade tumor suppression and to proliferate, respectively.

MdmX is a critical negative regulator of p53 whose over-expression in many cancers is thought to block p53 tumor suppressor function. Consequently, inhibiting MdmX has emerged as an intriguing approach to restoring p53 function in established cancers. However, MdmX-deficient mice exhibit p53-dependent death early in embryogenesis, which has precluded elucidation of the consequences of systemic MdmX inhibition. To determine directly the effects of systemic MdmX inhibition in normal adult tissues and in tumors, we crossed mdmX-/- mice into the p53ERTAM knock-in background to obtain adult MdmX-null mice in which p53 function can be restored. p53 became active when transiently restored in these mice, but the effects of this activity were non-lethal and reversible. Moreover, absence of MdmX enhanced p53 restoration therapy in a mouse model of lymphoma. Hence, systemic inhibition of MdmX is a feasible therapeutic strategy for restoring p53 function in tumors that retain wild type p53.

The E2F family of transcription factors plays a key role in a number of cellular processes, including cellular proliferation and tumorigenesis. E2Fs are deregulated in the vast majority of cancer and appear to be essential conduits for upstream oncogenic lesions. Recent genetic studies in mice have established E2F3 as the most essential activating E2F for cellular proliferation and embryonic viability. However, the embryonic lethality of E2F3-null mice has prevented the study of the contribution of E2F3 to tumor proliferation and the assessment of E2F3 inhibition as a therapeutic avenue. To directly study E2F3 biology in adult tissues and tumors, we established a novel tet-repressor system that allows control of endogenous e2f3 expression in adult mice.

Although induction of blood vessel growth is acknowledged as a pivotal requirement for the evolution of macroscopic tumors, the events that trigger onset of tumor angiogenesis remain largely obscure. The pervasive Myc oncoprotein is itself a potent inducer of angiogenesis in a wide range of tissues. We have used a reversibly switchable mouse transgenic model of Myc-dependent beta-cell carcinogenesis to delineate the kinetics and causal sequence of angiogenic processes following acute Myc activation. We show that onset of endothelial cell proliferation is induced shortly after Myc-induced cell cycle entry of beta cells. Endothelial cell proliferation is not indirectly induced by local tissue hypoxia but instead via a diffusible angiogenic signal produced by Myc-expressing beta cells. This signal triggers the release of pre-existing, sequestered VEGF from the islet extracellular matrix, that then homes to the endothelial compartment where it induces endothelial cell proliferation. Myc activation in beta cells rapidly induces expression and release of the proinflammatory cytokine interleukin 1 beta (IL-1 beta). We show that IL-1 beta is the principal effector downstream of Myc responsible for triggering rapid onset of islet angiogenesis. Together, our data delineate a complete pathway in vivo by which the highly pleiotropic Myc oncoproteins elicits coexpansion of the vascular compartment during tumorigenic progression.