Inhibition of NAMPT by PAK4 Inhibitors

The serine/threonine kinase PAK4 plays a crucial role in regulating cell proliferation, survival, migration, and invasion. Overexpression of PAK4 correlates with poor prognosis in some cancers. KPT-9274, a PAK4 inhibitor, significantly reduces the growth of triple-negative breast cancer cells and mammary tumors in mouse models, and it also inhibits the growth of several other types of cancer cells. Interestingly, although it was first identified as a PAK4 inhibitor, KPT-9274 was also found to inhibit the enzyme NAMPT (nicotinamide phosphoribosyltransferase), which is crucial for NAD (nicotinamide adenine dinucleotide) synthesis and vital for cellular energy and growth. These results made us question whether growth inhibition in response to KPT-9274 was due to PAK4 inhibition, NAMPT inhibition, or both. To address this, we tested several other PAK4 inhibitors that also inhibit cell growth, to determine whether they also inhibit NAMPT activity. Our findings confirm that multiple PAK4 inhibitors also inhibit NAMPT activity. This was assessed both in cell-free assays and in a breast cancer cell line. Molecular docking studies were also used to help us better understand the mechanism by which PAK4 inhibitors block PAK4 and NAMPT activity, and we identified specific residues on the PAK4 inhibitors that interact with NAMPT and PAK4. Our results suggest that PAK4 inhibitors may have a more complex mechanism of action than previously understood, necessitating further exploration of how they influence cancer cell growth.