Background:
For breast cancer patients, metastasis is the primary cause of treatment failure and results in 90% of cancer-related deaths. Thus far, therapeutic approaches have largely focused on treatments that inhibit growth and proliferation of primary tumors, but few have achieved prevention of metastasis. Understanding the critical factors that drive cells through the complex metastatic cascade will allow for new targeted therapies.
The phospholipase D (PLD) isoforms 1 and 2 generate the lipid signaling molecule phosphatidic acid through the hydrolysis of phosphatidylcholine. This pleotropic second messenger is involved in multiple tumorigenic pathways. PLD1 and PLD2 have been shown to promote several early metastatic steps including migration and invasion from the primary tumor, neoangiogenesis, and survival in circulation. Together, they have also been linked to 5 of the 6 hallmarks of cancer. Although PLD1 and PLD2 are highly related in amino acid sequence and enzymatic function, their cellular roles are not always redundant or additive, but can also be contradictory. As such, it is necessary to elucidate their separate and combined effects.
Materials and Methods:
Double knockout (DKO) mice lacking PLD1 and PLD2 were crossed with mice containing a mouse mammary tumor virus (MMTV) polyoma middle-T antigen (PyMT) transgene in a C57B6 background. These MMTV-PyMT mice spontaneously develop breast cancer that regularly metastasizes to the lung. The development of tumors is first noted by palpation around 8 weeks of age, and then monitored by caliper measurements when possible, typically around 12 weeks of age. The mice are euthanized at 22 weeks of age to collect mammary fat pads and lungs for measurements of size and weight, histological assessment, and the presence of metastasis.
Results:
PLD1-/- mice exhibit reduced metastasis relative to wild-type mice. We show here a further reduction in DKO mice that results in a dramatic suppression of lung metastasis. Furthermore, DKO mice have a decrease in tumor burden compared to both wild-type and PLD1-/- mice.
Conclusions:
Although PLD1 and PLD2 have been shown to have both complimentary and conflicting roles in cancer cell development, these and other studies offer promising results in support of their therapeutic inhibition. The overtly normal phenotype of DKO mice suggests that PLD inhibitors should have minimal toxicity, allowing them to be added to current-best therapeutic approaches.