Gutierrez-Galindo, E.; Jursik, K.; Frey, Y.; Meyer, F.; Hausser, A.

Protein kinase D3 (PKD3) is an important regulator of triple-negative breast cancer (TNBC) progression by promoting invasion, proliferation, and stem cell maintenance. However, the mechanism underlying these cellular functions has remained unclear. Here, we report that endogenous PKD3 localizes to Rab7/Lamp1-positive vesicles in MDA-MB-231 cells cultured on stiff matrices. Notably, upon PKD3 depletion the size of Rab7-positive vesicles is smaller. This correlates with impaired endosomal acidification, which is associated with dysregulated Wnt signaling and a decline in stemness. Our data thus unveil a previously unrecognized role of PKD3 in regulating endolysosomal dynamics that contributes to the maintenance of the cancer stem cell population.