Key Roles for the Lipid Signaling Enzyme Phospholipase D1 in the Tumor Microenvironment During Tumor Angiogenesis and Metastasis

Abstract

Angiogenesis inhibitors confer only short-term benefits on tumor growth. We report that ablation of the lipid signaling enzyme phospholipase D1 (PLD1) in the tumor environment compromises neovascularization and growth of tumors. PLD1 deficiency suppressed activation of AKT and mitogen-activated protein kinase signaling pathways by vascular endothelial growth factor (VEGF) in vascular endothelial cells, resulting in decreased integrin-dependent cell adhesion to and migration on extracellular matrixes and reduced tumor angiogenesis in a xenograft model. In addition, mice lacking PLD1 incurred significantly fewer lung metastases. Bone marrow transplantation and binding studies identified a platelet-derived mechanism involving decreased tumor cell:platelet interaction due to impaired activation of platelet αIIbβ3 integrin, which decreased seeding of tumor cells into the lung parenchyma. Treatment with a small molecule inhibitor of PLD1 phenocopied PLD1 deficiency, efficiently suppressing both tumor growth and metastasis in mice. These findings reveal that PLD1 in the tumor environment promotes tumor growth and metastasis, and taken together with prior reports of PLD1 roles in tumor cell intrinsic adaptations to stress, suggest potential utility for PLD1 inhibitors as cancer therapeutics.

Cellular and Physiological Roles for Phospholipase D1 in Cancer

“…The phenotype could be rescued in part by transplantation of non-targeted mesenchymal notochord tissue, suggesting that the defect arose from loss of a notochord morphogen that stimulated angiogenesis. Although intriguing, mice lacking PLD1 do not appear to exhibit this developmental phenotype, and the phenotype may not relate well to the role of PLD1 in neoangiogenesis in adult mice, which directly connects to the vascular endothelial cells (22,26), as will be discussed subsequently.…”

“…A very interesting finding was obtained, however, through implantation of wild-type melanoma and lung tumors into mice lacking PLD1. In brief, the xenografts grew slowly and exhibited virtually no tumor angiogenesis (22). This was traced to blunted signaling through the VEGF receptor, leading to minimal neoangiogenesis in response to VEGF signaling as the tumor grew hypoxic.…”

“…Moreover, a PLD1 single nucleotide polymorphism (SNP) associates with the risk of non-small cell lung cancer and increased PLD1 expression and/or PLD activity have been reported in multiple types of cancer (27)(28)(29)(30)(31)(32)(33), although the mechanisms underlying this increase and the specific advantage this confers to the tumor cells are not known. As will be discussed as well, roles for PLD1 in the tumor microenvironment have also been uncovered, specifically in relationship to platelet activation (34 -36) and angiogenesis (22,26,37).…”

“…To date, the inhibitors have not been associated with off-target or nonspecific effects, excepting one of the PLD1-selective compounds (66). FIPI has also been employed in vivo to study cancer growth and metastasis and platelet activation (22,35,67), which was achievable given its 5.5 h half-life and acceptable bioavailability and solubility (54). The subtypespecific PLD inhibitors currently available are less amenable for in vivo studies with shorter half-lives (45 min to 1 h) and efficient first-pass clearance (56), but with continued development could become invaluable tools for study in animal models and as potential therapeutics.…”

“…Increased cell motility is similarly important in most types of cancer. PLD1 has been linked to cell migration in multiple settings for non-transformed cells (21)(22)(23)(24)(25)(26) and may have similar roles in some types of cancer cells. The most interesting finding related to PLD1 and motility of cancer cells, however, addresses a setting in which loss of PLD1 activity results in decreased motility that enhances the oncogenic process.…”