Metastasis is the primary cause of cancer death. Weichand et al. describe a new mechanism explaining how tumor-associated macrophages contribute to metastatic spread, which involves promoting tumor lymphangiogenesis via S1P receptor 1 and the NLRP3 inflammasome.

Diabetic retinopathy is an important cause of blindness in the adult population1,2 and is characterized by a progressive loss of vascular cells and slow dissolution of inter-vascular junctions resulting in vascular leak and retinal edema3. Later stages of the disease are characterized by inflammatory cell infiltration, tissue destruction and neovascularization4,5. Here we identify the soluble epoxide hydrolase (sEH) as a key enzyme that initiates the pericyte “drop off” and loss of endothelial barrier function by generating a diol from docosahexaenoic acid (DHA) i.e. 19,20-dihydroxydocosapentaenoic acid (19,20-DHDP). The expression of the sEH and the accumulation of 19,20-DHDP were elevated in diabetic murine and human retinas as well as in human vitreous. Mechanistically, the diol targeted the cell membrane to alter the localisation of cholesterol-binding proteins, and interfered with the association of presenilin 1 (PS1) with N-cadherin and VE-cadherin to compromise pericyte-endothelial cell as well as inter-endothelial cell junctions. Treating diabetic mice with a specific sEH inhibitor prevented the pericyte loss and vascular permeability that are characteristic of non-proliferative diabetic retinopathy. Overexpression of the sEH in the retinal Müller glial cells of non-diabetic mice, on the other hand, resulted in vessel abnormalities similar to those seen in diabetic animals with retinopathy. Thus, increased expression of the sEH is a determinant event in the pathogenesis of diabetic retinopathy and sEH inhibition can prevent the progression of the disease.

Tumor cell-derived factors skew macrophages toward a tumor-supporting phenotype associated with the secretion of protumorigenic mediators. Apoptosing tumor cells release sphingosine 1-phosphate (S1P), which stimulates the production of lipocalin 2 (LCN2) in tumor-associated macrophages and is associated with tumor metastasis. We explored the mechanism by which S1P induces LCN2 in macrophages and investigated how this contributed to tumor growth and metastasis. Knockdown of S1P receptor 1 (S1PR1) in primary human macrophages and experiments with bone marrow-derived macrophages from S1PR1-deficient mice showed that S1P signaled through S1PR1 to induce LCN2 expression. The LCN2 promoter contains a consensus sequence for signal transducer and activator of transcription 3 (STAT3), and deletion of the STAT3 recognition sequence reduced expression of an LCN2-controlled reporter gene. Conditioned medium from coculture experiments indicated that the release of LCN2 from macrophages induced tube formation and proliferation in cultures of primary human lymphatic endothelial cells in a manner dependent on the kinase PI3K and subsequent induction of the growth factor VEGFC, which functioned as an autocrine signal stimulating the receptor VEGFR3. Knockout of Lcn2 attenuated tumor-associated lymphangiogenesis and breast tumor metastasis both in the breast cancer model MMTV-PyMT mice and in mice bearing orthotopic wild-type tumors. Our findings indicate that macrophages respond to dying tumor cells by producing signals that promote lymphangiogenesis, which enables metastasis.

Polyunsaturated fatty acids (PUFA) and their cytochrome P450 (CYP450) metabolites have been linked to angiogenesis and vessel homeostasis. However, the role of individual CYP isoforms and their endogenous metabolites in those processes are not clear. Here, we focused on the role of Cyp2c44 in postnatal retinal angiogenesis and report that Cyp2c44 is highly expressed in Müller glial cells in the retina. The constitutive as well as inducible postnatal genetic deletion of Cyp2c44 resulted in an increased vessel network density without affecting vessel radial expansion during the first postnatal week. This phenotype was associated with an increased endothelial cell proliferation and attenuated Notch signaling. LC-MS/MS analyses revealed that levels of hydroxydocosahexaenoic acids (HDHA), i.e., 10-, 17- and 20-HDHA were significantly elevated in retinas from 5day old Cyp2c44 mice compared to their wild-type littermates. Enzymatic activity assays revealed that HDHAs were potential substrates for Cyp2c44 which could account for the increased levels of HDHAs in retinas from Cyp2c44 mice. These data indicate that Cyp2c44 is expressed in the murine retina and, like the soluble epoxide hydrolase, is expressed in Müller glia cells. The enhanced endothelial cell proliferation and Notch inhibition seen in retinas from Cyp2c44-deficient mice indicate a role for Cyp2c44-derived lipid mediators in physiological angiogenesis.