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Peripheral nerves constitute an essential component of cellular microenvironments. With the exception of cartilage and lens, all human tissues are infiltrated by nerves of sensory, autonomic (sympathetic and parasympathetic), and/or motor origin. Nerves connect all body parts to the central nervous system (CNS) and are essential not only to locomotion, sensation, and cognition, but also to physiologic regulation of internal organs. However, nerves also have a trophic effect during tissue development, repair, and regeneration, a role that has been underestimated. Nerve dependence in tissue growth was initially established over 200 years ago in the context of limb regeneration in the salamander, where denervation of the limb prevents regeneration (1). This was later confirmed in embryogenesis and various processes of tissue repair where it was shown that the outgrowth of nerve endings (axonogenesis) in the cellular microenvironment is required for tissue growth (2). Although nerve endings are known to release a variety of neurotransmitters, hormones, and growth factors, the growth-stimulatory mechanisms of nerves during development and regeneration have remained unclear (3). Illustrative of the compartmentalization in science and medicine, the role of nerves in cancer growth has been understudied, and until recently nerves were not regarded as major contributors in tumorigenesis. Although nerves were known to be eventually surrounded and invaded by cancer cells, a process called perineural invasion (4), the perception was nevertheless that nerves were essentially passive bystanders in cancer. However, in the last 5 years, there has been a series of pioneering studies that have demonstrated the driving role of nerves in cancer initiation and progression. In this review, we describe the evidence for the role of nerves in cancer and discuss how it could affect both research and clinical practice.
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An overview of the current evidence demonstrating the impact of nerves in tumorigenesis is presented in Table 1. The initial demonstration that denervation can inhibit cancer progression was performed in mouse models of prostate cancer (5). The prostate gland is essentially innervated by autonomic nerves of sympathetic (adrenergic) and parasympathetic (cholinergic) origin, and surgical or chemical denervation of the prostate was found to result in a complete inhibition of prostate cancer growth and dissemination (5). On the one hand, denervation of adrenergic nerves or knockout of adrenergic receptors beta 2 (ADRβ2) and beta 3 (ADRβ3) inhibited the proliferation of stromal and cancer cells at early stages of prostate tumor development. On the other hand, denervation of cholinergic nerves or knockout of type 1 muscarinic acetylcholine receptors (CHRM1) inhibited tumor cell dissemination at latter stages of the disease. The crucial role played by stromal cells, which express both ADRβ and CHRM1 receptors, as a relay that promotes cancer cell growth was also noted. The authors also reported that the density of nerve infiltration in the tumor microenvironment of prostate cancer, indicative of axonogenesis, was increased in high-grade cancers compared with low-grade cancers or benign prostatic hyperplasia (5). The conclusion of this early study was that the activation of adrenergic signaling by the release of catecholamines from sympathetic nerves stimulated tumor growth, whereas cholinergic signaling that was activated by parasympathetic nerves stimulated tumor dissemination (5). Incidentally, this landmark demonstration of nerve dependence in cancer also provided a rationale for the long-reported lower incidence of prostate cancer in patients with spinal cord injuries where a functional denervation of the prostate occurs (6, 7).
Together, the pioneering studies described above have revealed the active role of nerves in cancer (Fig. 1). It is also clear that nerves can stimulate cancer growth and dissemination either directly or indirectly through the tumor microenvironment. Given the presence of nerves in most tissue microenvironments, it is anticipated that nerves may play a role in other if not all solid tumors, but this remains to be experimentally demonstrated.
Molecular basis and functional impact of tumor innervation. The outgrowth of nerves in the tumor microenvironment (axonogenesis) is driven by the secretion of neurotrophic factors (NTF) by cancer cells and takes place from peripheral nerves in the surrounding tissues that emerge from the CNS and associated neural ganglia. In return, nerve endings in the tumor microenvironment, which can be of adrenergic, cholinergic, or sensory origin, release neurotransmitters (NT) that stimulate corresponding receptors in stromal cells, immune cells, and cancer stem cells, resulting in the regulation of cancer growth and metastasis. Therefore, the stimulation of cancer cell growth can be direct and indirect through the stimulation of other cell types in the tumor microenvironment. Of note, the stimulation of endothelial cells by noradrenalin (NA) released from adrenergic nerves induces an angiogenic switch that fuels tumor growth and metastasis. The presence of sensory nerves in the tumor microenvironment can also participate in cancer pain.
As nerves were not considered to be important for tumor progression, the outgrowth of nerves in the tumor microenvironment, or axonogenesis, has been overlooked. The other reason why nerves have been understudied in cancer is that they are difficult to observe in regular histology. Big nerve trunks can be seen in regular histology and constitute the basis for assessing perineural invasion in pathologic examination (4). However, most nerves in the tumor microenvironment are small trunks or even individual axons that require specific neuronal biomarkers to be detected in IHC. The pan-neuronal marker PGP9.5 (protein gene product 9.5/UCH-L1/PARK5) can be used as an IHC marker to detect all nerve types. Other neuronal biomarkers include peripherin, a type III intermediate filament protein expressed mainly in neurons of the peripheral nervous system, as well as tubulin beta-3. Autonomic nerves can be differentiated by using tyrosine hydroxylase for adrenergic nerves and vesicular acetylcholine transporter (VAChT) for cholinergic nerves. Additionally, peripheral glial cells (Schwann cells) can also be identified in the tumor microenvironment by immunostaining for glial fibrillary acidic protein or S100. Assessment and quantitation of nerves by IHC in the tumor can be done by direct microscopic observation, but quantification of nerve density may also necessitate digital computer-based analysis as illustrated in prostate cancer (20). The discovery of the regulatory role of nerves in cancer progression has led to more recent insightful explorations of the distribution of nerve subtypes in human tumors, using the above neuronal biomarkers and methodologies.
Importantly, the stimulatory influence of sympathetic nerves on tumor angiogenesis provides a rationale for the reported potential impact of beta blockers on the survival of patients with cancer. Beta blockers are currently prescribed for cardiovascular diseases and anxiety disorders, but some retrospective studies have suggested a positive impact on the survival of patients with prostate (43), breast (44, 45), and ovarian cancers (46), as well as multiple myeloma (47). Up until now, it was unclear how beta blockers could improve the survival of patients with cancer, but the results of Zahalka and colleagues (38) provide a possible explanation: The inhibition of ADRβ2 signaling induced by sympathetic nerves in endothelial cells by beta blockers results in the inhibition of angiogenesis. Evidence supporting this mechanism has been demonstrated in pancreatic cancer, where the use of beta blockers was associated with significantly improved survival of patients with pancreatic cancer undergoing surgery, compared with no beta blocker use (10). Therefore, prospective clinical trials are investigating the therapeutic value of beta blockers in prostate (NCT02944201 and NCT03152786), gastrointestinal (NCT03245554), and breast (NCT01847001) cancers, as well as melanoma (NCT02962947) and multiple cancers (NCT02013492). The outcome of these clinical trials is highly awaited, as it may lead to the repurposing of this commonly used class of drugs as anticancer therapeutics.
The discovery that sympathetic nerves drive tumor angiogenesis reveals that the regulation of angiogenesis is more complicated than previously thought. To date, the concept was that tumor angiogenesis was essentially driven by the secretion of angiogenic growth factors, such as vascular endothelial growth factor (VEGF), by cancer cells and secondarily immune cells in the tumor microenvironment. The novel concept introduced by Zahalka and colleagues (38) is that the neural compartment of the tumor microenvironment, which was previously regarded as inert, is also involved. This may partially explain why anticancer strategies based on the inhibition of angiogenic growth factors, such as VEGF, have shown limited therapeutic utility (48). In the development of future antiangiogenic strategies, the essential participation of sympathetic nerves and noradrenaline in angiogenesis should be considered.