Regulation of Carcinogenesis by Sensory Neurons and Neuromediators

Abstract

Interactions between the immune system and the nervous system are crucial in maintaining homeostasis, and disturbances of these neuro-immune interactions may participate in carcinogenesis and metastasis. Nerve endings have been identified within solid tumors in humans and experimental animals. Although the involvement of the efferent sympathetic and parasympathetic innervation in carcinogenesis has been extensively investigated, the role of the afferent sensory neurons and the neuropeptides in tumor development, growth, and progression is recently appreciated. Similarly, current findings point to the significant role of Schwann cells as part of neuro-immune interactions. Hence, in this review, we mainly focus on local and systemic effects of sensory nerve activity as well as Schwann cells in carcinogenesis and metastasis. Specific denervation of vagal sensory nerve fibers, or vagotomy, in animal models, has been reported to markedly increase lung metastases of breast carcinoma as well as pancreatic and gastric tumor growth, with the formation of liver metastases demonstrating the protective role of vagal sensory fibers against cancer. Clinical studies have revealed that patients with gastric ulcers who have undergone a vagotomy have a greater risk of stomach, colorectal, biliary tract, and lung cancers. Protective effects of vagal activity have also been documented by epidemiological studies demonstrating that high vagal activity predicts longer survival rates in patients with colon, non-small cell lung, prostate, and breast cancers. However, several studies have reported that inhibition of sensory neuronal activity reduces the development of solid tumors, including prostate, gastric, pancreatic, head and neck, cervical, ovarian, and skin cancers. These contradictory findings are likely to be due to the post-nerve injury-induced activation of systemic sensory fibers, the level of aggressiveness of the tumor model used, and the local heterogeneity of sensory fibers. As the aggressiveness of the tumor model and the level of the inflammatory response increase, the protective role of sensory nerve fibers is apparent and might be mostly due to systemic alterations in the neuro-immune response. Hence, more insights into inductive and permissive mechanisms, such as systemic, cellular neuro-immunological mechanisms of carcinogenesis and metastasis formation, are needed to understand the role of sensory neurons in tumor growth and spread.

Schwann Cells Induce Phenotypic Changes in Oral Cancer Cells

“…SCs are known to directly interact with T‐cells, macrophages, dendritic cells, myeloid‐derived suppressor cells by secreting immune regulatory mediators that affect immune cell recruitment, polarization, and activity, creating an immunosuppressive tumor environment. [ 14,19,55 ] While the direct impact of SCs on immune cells within the oral cancer tumor microenvironment waits to be studied, our data further support an indirect role of SCs in immune modulation by potentially altering the immune regulation ability of cancer cells.…”

A Novel Therapeutic Target for Small-Cell Lung Cancer: Tumor-Associated Repair-like Schwann Cells

“…Emerging evidence shows that Schwann cells may coordinate neural and neuroimmune communications with the malignant cells in the tumor microenvironment, controlling tumor growth, spreading, and metastasis [ 32 , 33 , 34 ]. Tumor-induced activation, dedifferentiation, and denervation of SCs are well-established phenotypic characteristics of so-called “repair” neurotrauma-associated SCs.…”

ß-Adrenoreceptors in Human Cancers

“…Most tissues of human body are to some extent innervated by both. The neuro-immune interactions are primarily mediated by adrenergic ligands and receptors [ 10 , 11 ]. Autonomic nerve fibers end in lymphoid organs and have important immunomodulatory roles in several disorders (including cancer) [ 12 ].…”