The role of mTORC2 in mammary morphogenesis and HER2-mediated tumorigenesis

Abstract

CANCER BIOLOGY

The role of mTORC2 in mammary morphogenesis and HER2-mediated tumorigenesis

Meghan Melinda Morrison Dissertation under the direction of Professor Rebecca Cook

The phosphatidyl inositol-3-kinase (PI3K)/Akt signaling pathway is aberrantly activated in nearly 60% of breast cancers, through HER2 amplification, PIK3CA mutation, PTEN inactivation, and other alterations. The mTOR complexes mTORC1 and mTORC2 operate as activators (mTORC2) or effectors (mTORC1) of Akt and control key cellular processes, including growth, survival and metabolism. While most studies of mTOR in breast cancer have focused on mTORC1, little is known about the distinct role of mTORC2 in breast cancer. As molecular pathways that are essential for normal development are often hijacked by cancer cells, we sought to test the requirement of Rictor/mTORC2 in untransformed mammary epithelial cells (MECs) and transformed MECs. We show that genetic disruption of Rictor decreased Akt phosphorylation, ductal length, secondary branching, cell motility, and cell survival, effects that were recapitulated with a pharmacological dual inhibitor of mTORC1/mTORC2, but not upon genetic disruption of mTORC1 function via Raptor deletion. Surprisingly, Akt re-activation was not sufficient to rescue cell survival, invasion, or branching of mTORC2-impaired MECs. However, PKCα-mediated signaling through the small GTPase Rac1 was necessary for mTORC2-dependent mammary epithelial development during puberty, revealing a novel role for Rictor- PKCα-Rac1 signaling axis in MEC survival and motility during branching morphogenesis. These data lead us to hypothesize that Rictor/mTORC2 drives PI3K/Akt-mediated cell survival of HER2-amplified breast cancer cells. RICTOR gene ablation in a transgenic mouse model of HER2-amplified breast cancer delayed tumorigenesis and decreased lung metastasis, Akt-S473 phosphorylation, cell growth and survival. In human HER2-amplified breast cancer cells, Rictor loss, but not Raptor loss, decreased Akt-S473 phosphorylation, reducing cell survival and motility/invasion. Interestingly, Rictor/mTORC2 loss or treatment with a dual mTORC1/2 inhibitor improved lapatinib-induced cell killing in parental and resistant tumor cells to a greater extent than mTORC1 inhibition. Akt re-activation rescued cell survival, but not motility/invasion, in Rictor-depleted cells. However, Rictor loss caused accumulation of the Rac inhibitor RhoGDI2, thus impairing Rac1-dependent invasion. We conclude that HER2-amplified breast cancers use Rictor/mTORC2 to drive Akt-mediated cell survival and Rac1-mediated cellular invasion. These studies support additional studies into mTORC2-specific inhibitors.

Approved Rebecca Cook 6/26/15