Interrogating the Molecular Mechanisms of Breast Cancer Metastasis: The Contributions of RhoC GTPase and p38y MAPK.

Abstract

Breast cancer lethality is primarily due to cancer cell metastasis away from the primary tumor to vital organs. Understanding and targeting the molecular mechanisms contributing to metastasis is therefore essential to combating the disease. Here we focus on two main mechanisms by which molecules contribute to metastasis—as either metastatic initiators (genes that can catalyze the shift from tumorigenic to metastatic) or metastatic perpetuators (genes that maintain, but cannot independently initiate, the metastatic state).

We discovered that RhoC GTPase is both necessary and sufficient for breast cancer stem cell metastasis. Surprisingly, RhoC was capable of initiating metastasis independent of primary tumor formation, which attests to its strength as a metastatic initiator. In addition to governing breast cancer stem cell metastatic potential, we also found that RhoC affects the abundance of breast cancer stem cells within a population. Together these data establish RhoC as the first identified initiator of breast cancer stem cell metastasis.

In separate studies, we discovered that p38γ MAPK perpetuates breast cancer metastasis and specifically impacts the mesenchymal-like behaviors of aggressive breast cancer cells. Interestingly, we found that p38γ is connected to RhoC, as changes in p38γ expression concurrently alter RhoC expression by affecting RhoC ubiquitination and lysosomal degradation. These findings are clinically relevant, as high p38γ expression was associated with both the basal-like breast cancer subtype as well as poorer overall patient survival.

Using computational modeling alongside cell biology we revealed that p38γ modulates cell motility by regulating actin stress fiber organization. In doing so we also discovered a novel physical behavior of motile cells—leading edge protrusion oscillations—which we verified both experimentally and computationally. Through in silico modeling we were able to deduce a likely sequence of events leading from p38γ knockdown to the observed motility phenotype.

Taken together, this body of work defines a new subset of breast cancer stem cells that act as metastatic initiators—metastatic breast cancer stem cells, driven by RhoC expression—and characterizes a novel metastatic perpetuator in p38γ. This work opens many new research avenues which we discuss and elaborate on through several proposed directions for future investigation.