Pain May Promote Tumor Progression via Substance P-Dependent Modulation of Toll-like Receptor-4.

BACKGROUND: In a previous study, persistent pain was suggested to be a risk factor for tumor patients. However, the mechanism underlying this phenomenon is still unclear. Substance P (SP), a pain-related neuropeptide secreted by the neural system and the immune system, plays an important role in the induction and maintenance of persistent pain. METHODS: In this study, in order to explore whether SP participates in the influence of pain on tumor progression, the serum samples of lung cancer and breast cancer patients were collected and tested. An elevated expression of SP was found in patients with pain. RESULTS: Cell pharmacological experiments revealed that SP can upregulate the expression of Toll-like receptor-4 (TLR-4) in tumor cells and increase the proliferation, migration, and invasive activity of tumor cells. As high expression of TLR-4 has the ability to enhance the biological activity of tumor cells, TLR-4 is thought to be involved in SP-induced tumor proliferation, migration, and invasion. Treatment of tumor cells with Aprepitant, a specific blocker of the NK-1 receptor, could reduce the expression of TLR-4 and reduce the proliferation, invasion, and migration activities of tumor cells; further proof of the influence of SP on TLR-4 expression depends on the NK-1 receptor located in tumor cells. CONCLUSIONS: Based on the results above, we proposed a possible mechanism underlying pain affecting tumor progression: The presence of pain increases the content of SP in patients' blood, and elevated SP increases the expression of tumor TLR-4 by acting on the NK-1 receptor, which ultimately affects the biological activity of the tumor.