Ana L. Gomes

Because metastasis is associated with the majority of cancer-related deaths, its prevention is a clinical aspiration. Prostanoids are a large family of bioactive lipids derived from the activity of cyclooxygenase-1 (COX-1) and COX-2. Aspirin impairs the biosynthesis of all prostanoids through the irreversible inhibition of both COX isoforms. Long-term administration of aspirin leads to reduced distant metastases in murine models and clinical trials, but the COX isoform, downstream prostanoid, and cell compartment responsible for this effect are yet to be determined. Here, we have shown that aspirin dramatically reduced lung metastasis through inhibition of COX-1 while the cancer cells remained intravascular and that inhibition of platelet COX-1 alone was sufficient to impair metastasis. Thromboxane A2 (TXA2) was the prostanoid product of COX-1 responsible for this antimetastatic effect. Inhibition of the COX-1/TXA2 pathway in platelets decreased aggregation of platelets on tumor cells, endothelial activation, tumor cell adhesion to the endothelium, and recruitment of metastasis-promoting monocytes/macrophages, and diminished the formation of a premetastatic niche. Thus, platelet-derived TXA2 orchestrates the generation of a favorable intravascular metastatic niche that promotes tumor cell seeding and identifies COX-1/TXA2 signaling as a target for the prevention of metastasis.

Hypercholesterolemia is associated with elevated peripheral blood leukocytes and increased platelet levels, generally attributed to cholesterol-induced proinflammatory cytokines. Bone marrow (BM) cell mobilization and platelet production is achieved by disrupting the SDF-1:CXCR4 axis, namely with granulocyte colony-stimulating factor and/or CXCR4 antagonists. Here we show that high cholesterol disrupts the BM SDF-1:CXCR4 axis; promotes the mobilization of B cells, neutrophils, and progenitor cells (HPCs); and creates thrombocytosis. Hypercholesterolemia was achieved after a 30-day high-cholesterol feeding trial, resulting in elevated low-density lipoprotein (LDL) cholesterol levels and inversion of the LDL to high-density lipoprotein cholesterol ratio. Hypercholesterolemic mice displayed lymphocytosis, increased neutrophils, HPCs, and thrombocytosis with a lineage-specific decrease in the BM. Histologic analysis revealed that megakaryocyte numbers remained unaltered but, in high-cholesterol mice, they formed large clusters in contact with BM vessels. In vitro, LDL induced stromal cell-derived factor-1 (SDF-1) production, suggesting that megakaryocyte delocalization resulted from an altered SDF-1 gradient. LDL also stimulated B cells and HPC migration toward SDF-1, which was blocked by scavenger receptor class B type I (cholesterol receptor) inhibition. Accordingly, hypercholesterolemic mice had increased peripheral blood SDF-1 levels, increased platelets, CXCR4-positive B lymphocytes, neutrophils, and HPCs. High cholesterol interferes with the BM SDF-1:CXCR4 axis, resulting in lymphocytosis, thrombocytosis, and HPC mobilization.