Germline, and more importantly somatic, mutations of PTEN have been implicated in breast cancer. PTEN is a dual specificity protein and lipid phosphatase that recognizes both protein and phosphatidylinositol substrates. PTEN’s role as an antagonist of the phosphatidylinositol-3-kinase/AKT pathway has been intensely studied; however, the role of PTEN in other lipid-regulated events has not been thoroughly investigated. Because the substrate and product of PTEN (PIP3 and PIP2, respectively) are involved in a variety of lipid mediated cellular events, we hypothesized that PTEN plays a role in regulating lipid metabolism and homeostasis. To examine the role of PTEN in this capacity, we attempted to observe if there was a correlation between PTEN and phospholipase D (PLD) activity. We found that in unstimulated MCF-7 cells, increased expression of PTEN, through an inducible vector, results in a 51% increase in phosphatidic acid, suggesting that PTEN may regulate PLD. In addition, overexpression of PTEN results in a 30% increase in PLD activity in unstimulated cells, with a concomitant decrease in phosphatidylcholine. As overexpression of PTEN that lacks lipid phosphatase activity results in a decrease in basal PLD activity, it appears that it is the lipid phosphatase activity of PTEN that is responsible for activating PLD. Since PLC has also been shown to be involved in the activation of PLD, we investigated the interaction between PTEN and PLC. Stimulation of MCF-7 with estrogen resulted in translocation of PLC-gamma, which required active PTEN. By studying the role of PTEN in regulation of lipid homeostasis, such as moderating PLD activity, the importance of phospholipid signaling will continually become clearer. Taken together, our data suggests that PTEN may regulate lipid signaling events mediated by PLD. The products of PLD catalyzed reactions, phosphatidic acid and choline, are implicated in several cellular processes that are common in cancer development. For instance, PLD appears to be a key component in several signal transduction pathways implicated in cellular proliferation, apoptosis, and ultimately cancer development. These results suggest that PTEN plays a role in modulating the PLC:PLD activation pathways. By understanding the role of PTEN in lipid-mediated signaling events, it may be possible to create novel targets for cancer therapy and prevention.
[Proc Amer Assoc Cancer Res, Volume 45, 2004]