Similarly to normal organs, tumors establish their own microenvironment to fulfill their dynamic growing demands. This microenvironment may include the extracellular matrix, vessels, lymphangions and, as an increasingly recognized compartment, nerves (1). The role of vessels in tumor microenvironments has been widely explored (2,3). However, so for the curative effects of anti-angiogenesis treatments have been proven dissatisfying, and more and more studies find that the nerves play a pivotal role in various cancer (4,5).

Oral cancer is a common malignant cancer type in the head and neck region with over 50,000 new cases and 10,000 new deaths every year in the U.S.A (6). Oral squamous cell carcinoma (OSCC) constitutes 90% of cases of oral cancer (6). OSCC is associated with severe disease and treatment-associated morbidity, and is frequently reported as having high rates of recurrence despite advances in cancer treatment (7). The oral cavity is innervated by cranial nerves with a high density of sensory nerves, particularly the trigeminal nerve (8). Therefore, pain is the most frequent complaint at the primary site (9), and the prevalence and intensity of OSCC-associated pain are higher than those in all other types of cancer (10,11). The mechanisms involved in the OSCC-nerve microenvironment are likely to differ from those of prostate, pancreatic and breast cancer, which are predominantly innervated by autonomic nerves (4,12).

As the most abundant neuropeptide in the trigeminal ganglion, the calcitonin gene-related peptide (CGRP) exerts a dual effect on both cancer development and cancer-associated pain in various types of cancer, such as osteosarcoma (13) and breast cancer (14). The present review discussed the potential molecular mechanisms by which CGRP is implicated in the development of OSCC and associated pain, suggesting that CGRP may be a promising therapeutic target in OSCC (Fig. 1).

CGRP is a 37-amino acid peptide that has a ubiquitous distribution, particularly in trigeminal ganglions (TGs) (15). CGRP has two major isoforms: α- and β-CGRP, which have similar structures and biological activities, but are encoded by separate genes(CALCA for α-CGRP and CALCB for β-CGRP) (16). In general, it is thought that α-CGRP is the cardinal form occurring in the central and peripheral nervous system, whereas β-CGRP, the only product of the CALCB gene, acts mainly in the enteric nervous system (16). Along with the wide innervation of C and Aδ fibers, CGRP exerts various physical effects, including the regulation of energy metabolism, cardiovascular function and pain conduction (17). CGRP has crucial functions in pathological situations, including hypertension, migraine, atherosclerosis and vessel remodeling, which has been elaborated on in a previous review (17).

The CGRP receptor is present in both central and peripheral nerves (18). It is composed of three proteins, namely the calcitonin receptor-like receptor (CLR), receptor activity-modifying protein 1 (RAMP1) and receptor component protein (RCP) (18). The CLR belongs to the class B ‘secretin-like’ family of G-protein-coupled receptors (GPCRs) (18). RAMP1s are small membrane proteins that possess a large extracellular NH₂ terminus of 100 amino acids, a single transmembrane domain and a short intracellular domain of 10 amino acids (19). RCP is a 16-kDa protein that interacts with the second intracellular cytoplasmic loop of the CLR (20). It is hypothesized that the CLR, RAMP1 and RCP compose a high-affinity receptor complex for CGRP that may be antagonized by the short peptide CGRP_8-37 and non-peptide antagonists, such as rimegepant (21).

With the revelation of the pivotal role of the CGRP/CLR axis in migraines (22), successful therapeutic strategies using humanized monoclonal antibodies or non-peptide antagonists directed against CGRP or CLR have emerged. Antibodies against CGRP, such as fremanezumab (23) and galcanezumab (24), or against CLR, such as erenumab (25), have been approved by the Food and Drug Administration (FDA) for the treatment of migraine. Non-peptide antagonists of CLR, such as ubrogepant (26,27), were also approved by the FDA, and rimegepant has been through phase III clinical trials (28,29) for the treatment of migraine. These antibodies and antagonists have been useful for the study of the CGRP/CLR axis.

Pain is one of the most common factors affecting the quality of life in patients with cancer and 75–95% of patients with metastatic or advanced-stage cancer will experience considerable amounts of cancer-induced pain (30). The incidence rate of cancer related pain varies for different types of cancers. For instance, breast adenocarcinoma is frequently characterized as being painful only after it metastasizes to the bone (31). However, melanomas (even in the head and neck) are typically not painful at the primary site or metastatic sites (32). In OSCC, pain is the most frequent complaint at the primary site (9), and the prevalence and intensity of OSCC-associated pain are higher than in all other types of cancer (10,11). In a prospective study, the pretreatment pain in OSCC has been able to predict peripheral nerve invasion (33), and was identified as an independent predictor of survival in 2,340 patients with OSCC in a retrospective study (10). The dorsal root ganglion (DRG) and TG neurons are pseudounipolar, which means that the axon bifurcates and sends projections to both the dorsal horn of the spinal cord or brainstem, and to the periphery (34). This unique morphology allows the release of transmitters at central and peripheral sites, and allows for bidirectional communication between the two terminals (35). When peripheral noxious stimuli (including high hydrogen, high temperature, capsaicin or prostaglandin E2) act on peripheral nociceptors, CGRP serves a dual role in afferent and efferent nerves (36). On one hand, CGRP is released in the synapse of primary afferent nerves through anterograde axoplasmic transport and acts on its receptors on the postsynaptic neurons in the DRG and TG, thus transmitting algesia to pain centers. On the other hand, it is also released from peripheral nerve terminals and exerts paracrine effects on the surrounding tissues by retrograde axoplasmic transport (36).

CGRP modulates pain in two major ways. A relatively well-researched mechanism is that following the vasodilation effect of CGRP, the accumulated proinflammatory factors irritate peripheral nociceptors. In addition, CGRP may directly influence peripheral and central sensitization, resulting in hyperalgesia and allodynia (37) (Fig. 2). A recent study demonstrated that CGRP induces differential regulation of cytokines, mostly interleukin (IL)-1β and IL-6, from satellite glial cells in TG, causing increased pain (37). In most situations, these two effects often co-exist and mutually promote each other (36).

The present review summarized the existent studies on the role of CGRP in cancer-associated pain (Table SI). Most studies used in vivo experiments to reveal that there are higher numbers of CGRP(+) sensory nerves sprouting adjacent to an ectopic cancer and in the DRG or TG than elsewhere, and mechanical allodynia and thermal hyperalgesia exist in various types of tumor, such as gingival cancer, metastatic bone cancer and breast cancer (35,38–45). Animal models should be optimized to study the role of CGRP in OSCC-associated pain. Nagamine et al (44) established a rat model of oral cancer pain by inoculating cancer cells into the lower gingiva, and instead of hind paw withdrawal, used head withdrawal as the evaluation index; in addition, it was revealed that the mechanical allodynia and thermal hyperalgesia in the ipsilateral maxillary and mandibular nerve area were accompanied by nerves with upregulated CGRP expression in the TG. Although this was an innovative discovery, perhaps adding another assessment index, such as food intake, may help to confirm this result.

Overall, CGRP exerts considerable roles in cancer-associated pain, as well as in OSCC. The active sites of CGRP are the peripheral nociceptors and central nerve endings in the DRG or TG. However, the current in vivo models and antagonists require further improvements.

Initially, high levels of CGRP were identified in tumor tissues and serum of patients with medullary thyroid carcinoma (46). Subsequent studies emphasized the diagnostic value of CGRP in non-small cell lung carcinoma (47,48), neuroendocrine tumors (49), prostate cancer (50,51) and particularly medullary thyroid carcinoma (46,52). Recently, Angenendt et al (53) analyzed the expression levels of the gene encoding the CLR, CALCRL, in >1,500 patients with well-characterized acute myeloid leukemia from five international cohorts, revealing that increasing transcript levels of CALCRL were associated with decreasing complete remission rates, 5-year overall survival and event-free survival. Furthermore, CRISPR-Cas9-mediated knockout of CALCRL markedly impaired colony formation in human myeloid leukemia cell lines (53).

One way in which CGRP is able to influence tumor behavior is secondary to its potent vasodilation effect. A previous study on CGRP-knockout mice revealed that endogenous CGRP facilitated tumor-associated angiogenesis and tumor growth in Lewis lung carcinoma-bearing mice and that the administration of the CGRP antagonist CGRP8-37 or denervation markedly suppressed tumor growth and tumor-associated angiogenesis (54).

A number of studies have revealed that CGRP can directly modulate development of various types of cancer, with a large amount of literature reporting the expression of the CGRP receptor complex in cancer cells (55–57). CGRP was demonstrated to be able to modify the chemokinetic abilities of a metastatic breast cancer cell line and increase the expression of its receptors (58). In addition, it has been demonstrated that CGRP increased the invasive ability of prostate cancer PC-3 cells and an osteosarcoma cell line (13,50). Recently, Dallmayer et al (56) revealed that targeting the CALCB/RAMP1 (the receptor complex of β-CGRP) axis inhibited the growth of Ewing sarcoma cells. Despite the lack of studies on the role of CGRP in OSCC, several studies have revealed a wide distribution of CLR/RAMP1 in oral soft tissues, bones and dental tissues (59). Therefore, the present review discussed the mechanisms that CGRP may utilize in cancer development.

Clinical and pathological features confirm that nerves serve an important role in OSCC, as well as in other types of cancer. However, sensory nerves, rather than autonomic nerves, appear to have a pivotal role in OSCC. CGRP is mostly expressed in sensory neurons in the TG and DRG (94,95). CGRP exerts a significant role in both cancer development and cancer-associated pain. CGRP promotes cancer development through metabolic reprogramming, anomalous receptor internalization or inhibition of antitumor immune responses. In addition, CGRP may augment pain by inducing mechanical allodynia peripherally and centrally. Based on the current knowledge, the present review suggested that CGRP may represent an important bridge between cancer development and cancer-associated pain in OSCC, and this hypothesis should be further investigated in future studies.