Hausser, Angelika

Protein kinase D3 (PKD3) is upregulated in triple‐negative breast cancer (TNBC) and associated with cell proliferation and metastasis development but its precise pro‐oncogenic function is unknown. Here we show that PKD3 is required for the maintenance of the TNBC stem cell population. The depletion of PKD3 in MDA‐MB‐231 cells reduced the cancer stem cell frequency in vitro and tumor initiation potential in vivo. We further provide evidence that the RhoGEF GEF‐H1 is upstream of PKD3 activation in TNBC stem cells. Most importantly, pharmacological PKD inhibition in combination with paclitaxel synergistically decreased oncosphere and colony formation efficiency in vitro and tumor recurrence in vivo. Based on our results we propose that targeting the GEF‐H1/PKD3 signaling pathway in combination with chemotherapy might provide an effective therapeutic option for TNBC.

Increases in reactive oxygen species (ROS) have been implicated in age-related diseases, including cancer. The serine/threonine kinase protein kinase D1 (PKD1) is a stress-responsive kinase and sensor for reactive oxygen species, which can initiate cell survival through NF-κB signaling. We have previously shown that in response to ROS, PKD1 is activated at the mitochondria. However, the initial signaling events leading to localization of PKD1 to the mitochondria are unknown. Here, we show that formation of mitochondrial diacylglycerol (DAG) and its binding to PKD1 is the means by which PKD1 is localized to the mitochondria in response to ROS. Interestingly, DAG to which PKD1 is recruited in this pathway is formed downstream of phospholipase D1 (PLD1) and a lipase-inactive PLD1 or inhibition of PLD1 by pharmacological inhibitors blocked PKD1 activation under oxidative stress. To date it has been viewed that monosaturated and saturated DAG formed via PLD1 have no signaling function. However, our data describe a role for PLD1-induced DAG as a competent second messenger at the mitochondria that relays ROS to PKD1-mediated mitochondria-to-nucleus signaling.