Analysis of Platelets in Solid Tumor Patients: Profound Changes and Unexpected Findings.

Platelets are important contributors to several hallmarks of tumor pathophysiology including immune escape of disseminated malignant cells, metastasis, and neoangiogenesis. Following activation, platelets release their granule content, which contains a plethora of factors that may both inhibit and stimulate angiogenesis, immunosurveillance, plasmatic coagulation, or tumor growth. Targeted deletion of thrombospondin-1 and -2 from the platelet proteome leads to altered secretion of CXCL12 and matrix metalloproteinases (MMPs) in mice (Kopp HG, Hooper AT et al., J Clin Invest 2006), and platelets derived from 25 patients with lymphoma, colorectal and ovarian cancer displayed a diminished tsp-1/VEGF-ratio (Gonzalez FJ, Rueda A et al., Int J Biol Markers, 2004). We therefore set out to analyze a carefully selected patient cohort for in vivo platelet activation and platelet contents in order to define phenotypic changes in the setting of disseminated malignancy.

Platelet activation in percent of P-selectin positive platelets and platelet contents (i.e. absolute platelet count-corrected plasma/serum levels of VEGF-A, CXCL12 [SDF-1], CXCL4 [pf4], and thrombospondin-1) were analyzed. Patients were eligible after written informed consent if they had a first diagnosis of metastatic malignancy, were not taking medication on a regular basis, and had not taken any medication during the last 14 days before venipuncture. Healthy age- and sex-matched subjects were used as controls.

Tumor patients had increased platelet counts (301,558 ± 17,489/μl vs. 234,111 ± 5,946/μl) and slightly lower hematocrits. In addition, cancer patients displayed significantly elevated percentages of activated platelets as measured by CD62P expression (1.14 ± 0.20% vs. 0.24 ± 0.03%). In order to obtain absolute platelet contents per 100 million platelets, we took tumor-associated thrombocytosis and anemia in consideration and calculated as follows: ([serum VEGF/CXCL12/TSP1/CXCL4 – plasma VEGF/CXCL12/TSP1/CXCL4] * [1-hematocrit] ÷ platelets per ml whole blood) * 100×10⁶. The results showed highly significant changes, including increased contents of VEGF (143.24 ± 15.87 pg vs. 63.02 ± 6.28 pg; p < 0.001) and decreased amounts of thrombospondin-1 (2,925.13 ± 217.78 ng vs. 3,764.34 ± 184.64 ng; p = 0.005), CXCL4 (1,547.60 ± 44.54 ng vs. 1,132.32 ± 55.62 ng, p<0.001), and CXCL12 (38.94 ± 5.07 pg vs. 83.59 ± 12.13 pg; p < 0.001).

Platelets in cancer patients cannot any longer be considered innocent bystanders. Instead, they may become activated upon their contact with intratumor/metastasis vascular walls and thereby secrete their quantitatively highly abnormal contents. While our data corroborate previous findings such as increased VEGF- and decreased tsp-1 content, we unexpectedly detected an additional decrease of CXCL12 and CXCL4. Whether megakaryocytes in solid tumor patients are responsible for their daughter cells' altered contents or whether platelets by means of their scavenger activity acquire the “malignant” phenotype remains to be established.

No relevant conflicts of interest to declare.