In this study, we found that surgical resection of the HNSCC tumors down-regulated the plasma angiopoietin-1 level and up-regulated the plasma angiopoietin-2 in conjunction with an increase in plasma NO level and reduced phosphorylation level of Tie2 in erythrocyte membrane. Importantly, the angiogenic and NO signaling was significant only in stage III and IV HNSCC. The dynamic change of angiopoietin-Tie2 and NO signaling in erythrocyte membrane in response to surgical trauma suggests that erythrocytes have an important role in modulating surgery-induced angiogenesis, which may provide a novel marker for cancer surveillance and control.
Previous studies showed that surgical injury increases the plasma levels of angiopoietin-2 and VEGF in lung, breast, and colorectal cancer.9,10 Our results further showed the plasma angiopoietin-2 was also upregulated by surgery in HNSCC. Angiopoietin-1 and angiopoietin-2 are two important cytokines and function with the vascular Tie2 receptor in regulating the complex process of angiogenesis.22,23 Accumulating evidences have demonstrated that like angiopoietin-1, angiopoietin-2 can induce the phosphorylation of Tie2 receptor and promote chemotaxis, tube formation, and sprouting of endothelial cells.24,25 The expression of angiopoietin-2 is upregulated at sites of tumor angiogenesis in multiple types of cancer, and overexpression of angiopoietin-2 promoted angiogenesis and tumor growth in experimental models.26–28 Importantly, overexpression of angiopoietin 2 accelerated the carcinogenesis of oral squamous cell carcinoma through promoting epithelial-mesenchymal transition-induced angiogenesis.28 In our study, the increased angiopoietin 2 in plasma after surgery suggests that surgical injury may trigger the release of proangiogenic factors and facilitate the proliferation and metastasis of residual tumor cells in HNSCC.1,9 Angiopoietin-2 has been proposed as a potential target for antiangiogenic drug development.29 Our results may provide an important implication for postoperative cancer control in HNSCC.
Our results showed patients with HNSCC have a higher plasma level of NO before surgery compared to non-cancer controls. Besides, the raised plasma NO level in cancer patients was further enhanced after surgical resection of tumors. Study has showed that NO production induced by ethanol and tobacco may initiate inflammatory response, dysregulate antioxidant protection system, and contribute to tumor growth in head and neck cancer.30,31 Investigators have showed surgical trauma stimulates the release of angiopoietin-2 and VEGF in plasma and promotes the process of angiogenesis in lung, breast, and colorectal cancer.9,10 Our results further suggested a novel mechanism of surgery-induced angiogenesis via NO signaling pathway. NO has been shown to regulate the process of angiogenesis and promote tumor progression through the mechanism of vessel dilatation by eNOS, release of VEGF, activation of cyclooxygenase-2 stimulating the production of proangiogenic factors, and increased production of prostaglandin E2 inducing tumor vasculature hyperpermeability.8 NO may also have clinical significance as a biomarker of inflammation and risk stratification of malignant transformation in patients with oral pre-cancer.8 Researchers have proposed NO as a novel potential therapeutic target in resistant cancer by sensitizing cancer cells to chemotherapy and immunotherapy.32 However, further studies are needed to evaluate the therapeutic applications of NO in cancer.
In this study, we found the baseline phosphorylation level of Tie2 in erythrocyte membrane was significantly higher in cancer patients compared with non-cancer controls and was correspondingly reduced after surgery in connection with the decreased angiopoietin-1 level and increased angiopoietin-2 level in plasma. Tie2 is a receptor tyrosine kinase expressed principally on vascular endothelium and interacts with its ligand angiopoietin-1 and angiopoietin-2 in regulating vessel branching and maintaining endothelial homeostasis.33 Our prior study has showed that osmopressor response activates the Tie2/Akt/eNOS signaling pathway in erythrocytes and stimulates the secretion of angiopoietin-1 in plasma.12 The present study further demonstrated surgical injury activates the angiopoietin-Tie2 and NO signaling axis in erythrocyte membrane, indicating that peripheral erythrocytes may serve as a potential diagnostic and therapeutic target for cancer patients.
There are limitations to our study. First, our findings should be interpreted with caution due to small patient sample. Second, we did not measure the risk of cancer recurrence after surgical resection and therefore could not evaluate the relationship between our biological findings and clinical outcomes. Third, more studies are need to clarify how Tie2/Akt/eNOS signaling pathway interacts between vascular beds and erythrocytes in surgical injury. Fourth, we did not measure the long-term change of angiogenic response (e.g. 30 days after surgery), which is important for postoperative adjuvant therapy.27
In conclusion, the dynamic change of angiopoietin-Tie2 and NO signaling in erythrocyte membrane in response to surgical resection of HNSCC suggests that erythrocytes have a significant role in modulating surgery-induced angiogenesis, which may provide a novel marker for cancer surveillance and control. We need more studies to evaluate the relationship between changes in angiopoietin-Tie2 and NO expression on cancer outcomes after tumor resection and to explore potential clinical applications of angiopoietin-Tie2 and NO signaling in cancer prevention and treatment.