Platelet Depletion Reduces Tumor Hypoxia and Metastasis Mediated by Met Signaling Pathway

Abstract 3321

Platelets play a fundamental role in maintaining hemostasis and have been shown to participate in hemorrhagic metastasis. However, the role of platelets in the tumor growth, angiogenesis, and metastasis initiation remains undefined. The B16/F10 melanoma cancer cells model of metastasis and the Lewis lung carcinoma (LLC) spontaneous pulmonary metastasis model were used for this purpose. Using induction of thrombocytopenia, primary tumor growth was monitored and every 3 days anti-GPIbα or rat IgG injections were initiated when tumor reached ∼500mm³and continued until tumor reached to 3 weeks. We showed that platelet depletion had no change in tumor growth but reduced metastasis. Platelet depletion significantly increased pericyte coverage and reduced vascular density compared with control mice. We evaluated the ratio of fluorescence intensities within the plasma and tumor following injection of mice with FITC-dextran. We found that the FITC-dextran was similarly deposited into the tumor tissue in either platelet-depleted or control mice, indicating that tumor vessel perfusion did not differ in either platelet-depleted or control mice.

To further gain insight into the molecular mechanisms associated with reduced metastasis resulting from platelet depletion, we assessed hypoxia levels by examining pimonidazole adduct formation in the tumors of platelet-depleted and control mice and found decreased hypoxic levels in the platelet-depleted tumors. In addition, expression of the hypoxia-inducible transcription factor HIF-1α was also significantly reduced in the tumors of platelet-depleted mice. Tumor hypoxia is strongly associated with deposition of hemoglobin. We measured the intratumor hemoglobin content, reflecting the level of erythrocytes extravasation. The hemoglobin content in the tumors of mice with platelet-depletion was significantly higher than that of control mice (172.11 ± 20.2 g/L/g Vs. 110.28 ± 12.4 g/L/g, p<0.05) Based on the known induction effects of hypoxia and cancer invasiveness on the expression and activation of the proinvasive tyrosine kinase receptor Met, we analyzed total protein and tyrosine phosphorylation levels of Met in both platelet-depleted and control mice. Western blotting analysis revealed that platelet-depletion caused a significantly decrease of both total Met and phosph-Met in tumors when compared to tumors from control mice. To evaluate intratumoral growth factor level, microdialysis was performed after 3 weeks and there was a significant decrease of extracellular VEGF and TNF-β in platelet depletion mice compared with control mice.

Recent studies demonstrated that abundant platelets were detected in the tumor microenvironment apart from the vasculature. Based on the finding platelets in contact with tumor cells outside the bloodstream, we examined the functional effects of co-implantation of B16/F10 tumor cells with platelets on tumor progression and metastasis. B16/F10 melanoma cancer cells were implanted into back of wild type mice. During a 3-weeks growth, co-implantation of B16/F10 with platelets not only led to promoted tumor volume (3968 ± 296 mm³Vs. 2956 ± 180 mm³, p<0.05) and weight (5.529 ± 0.35 g Vs. 3.943 ± 0.738 g, p<0.05 ) compared with B16/F10 alone implantation, but also led to an increase in metastasis. Furthermore, in vitro co-culture of B16/F10 cancer cells with platelets showed a significant increase in B16/F10 cancer cells invasion compared with B16/F10 cancer cells alone.

In conclusion, our findings demonstrate for the first time that platelets play a critical role in the initiation of tumor metastasis. Moreover, our findings suggest that platelets within the primary tumor microenvironment are likely involved in tumor progression and metastasis.