BACKGROUND - Lung cancer and glioblastoma multiforme are highly angiogenic and, despite advances in treatment, remain resistant to therapy. Cytosolic phospholipase A2 (cPLA(2)) activation contributes to treatment resistance through transduction of prosurvival signals. We investigated cPLA(2) as a novel molecular target for antiangiogenesis therapy.
METHODS - Glioblastoma (GL261) and Lewis lung carcinoma (LLC) heterotopic tumor models were used to study the effects of cPLA(2) expression on tumor growth and vascularity in C57/BL6 mice wild type for (cPLA(2)α(+/+)) or deficient in (cPLA(2)α(-/-)) cPLA(2)α, the predominant isoform in endothelium (n = 6-7 mice per group). The effect of inhibiting cPLA(2) activity on GL261 and LLC tumor growth was studied in mice treated with the chemical cPLA(2) inhibitor 4-[2-[5-chloro-1-(diphenylmethyl)-2-methyl-1H-indol-3-yl]-ethoxy]benzoic acid (CDIBA). Endothelial cell proliferation and function were evaluated by Ki-67 immunofluorescence and migration assays in primary cultures of murine pulmonary microvascular endothelial cells (MPMEC) isolated from cPLA(2)α(+/+) and cPLA(2)α(-/-) mice. Proliferation, invasive migration, and tubule formation were assayed in mouse vascular endothelial 3B-11 cells treated with CDIBA. Effects of lysophosphatidylcholine, arachidonic acid, and lysophosphatidic acid (lipid mediators of tumorigenesis and angiogenesis) on proliferation and migration were examined in 3B-11 cells and cPLA(2)α(-/-) MPMEC. All statistical tests were two-sided.
RESULTS - GL261 tumor progression proceeded normally in cPLA(2)α(+/+) mice, whereas no GL261 tumors formed in cPLA(2)α(-/-) mice. In the LLC tumor model, spontaneous tumor regression was observed in 50% of cPLA(2)α(-/-) mice. Immunohistochemical examination of the remaining tumors from cPLA(2)α(-/-) mice revealed attenuated vascularity (P ≤ .001) compared with tumors from cPLA(2)α(+/+) mice. Inhibition of cPLA(2) activity by CDIBA resulted in a delay in tumor growth (eg, LLC model: average number of days to reach tumor volume of 700 mm(3), CDIBA vs vehicle: 16.8 vs 11.8, difference = 5, 95% confidence interval = 3.6 to 6.4, P = .04) and a decrease in tumor size (eg, GL261 model: mean volume on day 21, CDIBA vs vehicle: 40.1 vs 247.4 mm(3), difference = 207.3 mm(3), 95% confidence interval = 20.9 to 293.7 mm(3), P = .021). cPLA(2) deficiency statistically significantly reduced MPMEC proliferation and invasive migration (P = .002 and P = .004, respectively). Compared with untreated cells, cPLA(2)α(-/-) MPMEC treated with lysophosphatidylcholine and lysophosphatidic acid displayed increased cell proliferation (P = .011) and invasive migration (P < .001).
CONCLUSIONS - In these mouse models of brain and lung cancer, cPLA(2) and lysophospholipids have key regulatory roles in tumor angiogenesis. cPLA(2) inhibition may be a novel effective antiangiogenic therapy.