Triple negative breast cancer (TNBC) is difficult to treat due to lack of druggable targets. We have found that treatment with the small molecule inhibitor KPT-9274 inhibits growth of TNBC cells and eventually leads to cell death. KPT-9274 is a dual specific inhibitor of PAK4 and Nicotinamide Phosphoribosyltransferase (NAMPT). The PAK4 protein kinase is often highly expressed in TNBC cells and has important roles in cell growth, survival, and migration. Previously we have found that inhibition of PAK4 leads to growth inhibition of TNBC cells both in vitro and in vivo. Likewise, NAMPT has been shown to be dysregulated in cancer due to its role in cell metabolism. In order to understand better how treating cells with KPT-9274 abrogates TNBC cell growth, we carried out an RNA sequencing of TNBC cells treated with KPT-9274. As a result, we identified Rictor as an important target that is inhibited in the KPT-9274 treated cells. Conversely, we found that Rictor is predicted to be activated when PAK4 is overexpressed in cells, which suggests a role for PAK4 in the regulation of Rictor. Rictor is a component of mTORC2, one of the complexes formed by the serine/threonine kinase mTOR. mTOR is important for the control of cell growth and metabolism. Our results suggest a new mechanism by which the KPT-9274 compound may block the growth of breast cancer cells, which is via inhibition of mTORC2 signaling. Consistent with this, sequencing analysis of PAK4 overexpressing cells indicates that PAK4 has a role in activation of the mTOR pathway.
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