Project Summary/Abstract
Carcinogenic mutations in the RAS-RAF-MEK-ERK pathway are present in 46% of all human cancers. This has
inspired the successful development of many small molecule BRAF and MEK pathway inhibitors. These agents
are currently FDA approved for the treatment of metastatic melanomas containing BRAF mutations. Although
BRAF and MEK inhibitors have shown some benefits to patients, these “targeted” therapeutics have a very low
therapeutic index, since these small molecules also target normal cells, causing undesirable, even fatal, side
effects. Moreover, whether the clinical efficacy of these inhibitors is via direct effects on the tumor or through
modulation of the tumor stromal compartment remains elusive. In this application, we address the above critical
unmet needs and will develop a potent and highly tumor-selective MEK inactivator by engineering an anthrax
toxin-based protein delivery system. Through manipulating the expression of the toxin receptor on specific tumor
stromal cell types, we will also delineate how stromal MEK inhibition regulates tumor development.
We propose two parallel, but independent Specific Aims to achieve these goals. In Aim 1, we will generate an
anthrax toxin-based, highly tumor-selective MEK inactivator and evaluate its anti-tumor activity in a variety of
tumor models. This MEK inactivator specifically binds to the major toxin receptor CMG2 (capillary morphogenesis
protein-2) on tumor cells and tumor stromal cells. Specificity is ensured through strict reliance on the
simultaneous presence of two distinct tumor-associated proteases, MMPs and urokinase, to gain entry into tumor
cells and tumor stromal cells. Once inside, it inactivates MEK-ERK signaling, achieving potent selective targeting.
In Aim 2, we will employ our unique tumor-host-toxin system to determine the roles of MEK-ERK signaling in
tumor stromal cells in tumor development and in the toxin’s tumor targeting. We have previously established a
genetic system allowing CMG2 gain-of-function or loss-of-function in various specific cell types in the tumor
microenvironment. Thus, we hypothesize that a genetic manipulation of CMG2 expression on cancer cells and
various tumor stromal cells in the whole body of CMG2-/- mice will provide the first tractable genetic system to
delineate the role of specific cell types in the tumor microenvironment. Therefore, in this Aim 2, we will determine
the roles of selected tumor stromal cell types including tumor endothelial cells (EC), tumor-associated
macrophages (TAM), cancer-associated fibroblasts (CAF), B cells, and regulatory T cells (Treg), and the
molecular mechanisms of MEK-ERK inhibition in these cells in tumor development. Upon completion of these
studies, we expect to have developed a highly potent, tumor-selective MEK inactivator as a novel tumor-targeted
therapeutic. These studies will also unambiguously determine the role of MEK-ERK signaling in both the tumor
and the stroma, thereby validating and identifying specific tumor stromal cell types as targets for future
therapeutics development.