1 INTRODUCTION

For decades, oncological research was focused mainly on the genetic and immune factors related to cancer. This research led to the identification of tumor suppressor genes, proto-oncogenes, myeloid-derived suppressor cells, and other factors within the tumor micro- and macroenvironments that play an important role during carcinogenesis and cancer growth. However, evidences accumulated in the last 20 years have clearly shown that the processes associated with cancer initiation and progression are also significantly affected by the nervous system (Figure 1).1

Schematic depiction of the main pathways mediating bidirectional interactions between the nervous system and tumor. The nervous system can affect a tumor: directly, through sympathetic, parasympathetic, and sensory nerves that innervate various targets within the cancer tissue (e.g., cancer cells, immune and other stromal cells, blood, and lymphatic vessels) and indirectly by modulating the activity of the endocrine glands (e.g., adrenals) and immune organs, as well as by modulation of microbiota. The tumor can affect brain activity directly via soluble mediators released from cells within the tumor microenvironment and indirectly by altering metabolism. These metabolic effects of tumors are related also to the alteration of hypothalamic functions (e.g., hypothalamic inflammation) and may contribute to dysregulation of energy balance and the development of cancer cachexia

The role of the nervous system in the modulation of physiological processes and its role in disease development and progression are the center of interest for various neurobiological disciplines, combining neuroscientific approaches with classical disciplines of medicine. Neurobiological research of diseases is traditionally focused on neurological and psychiatric disorders. For example, the neurobiology of depression is focused on investigation of functional and morphological alterations of the brain that participate in the development of depression, the neurobiology of Parkinson's disease is focused on elucidation of processes related to the loss of dopaminergic neurons in the substantia nigra. However, in recent years, researchers have also started to study the neurobiological aspects of somatic diseases. For example, the neurobiology of obesity is focused on investigating the role of the brain and autonomic nerves in the development of metabolic alterations leading to obesity, the neurobiology of diabetes is focused on elucidating the role of hypothalamic control of energy and glucose homeostasis, and the neurobiology of hypertension is focused on investigating the neural mechanisms participating in the maintenance of high blood pressure. Analogously, research into the modulatory effect of the nervous system on tumor initiation, progression, and the formation of metastases might be referred to as the neurobiology of cancer. In this sense, the term neurobiology of cancer is related to “somatic,” non-nervous system cancers (e.g., mammary, pancreatic, ovarian, and colon cancer, in addition to hematologic malignancies) as well as cancers of the nervous system (e.g., glioblastoma). However, the term neurobiology of cancer used in this article is applied only to somatic cancers.

The concept of the neurobiology of cancer stems from interdisciplinary research situated at the borderline of oncology and neurosciences. This concept is based on several pillars: (a) psychosocial factors influence the incidence and progression of cancer diseases; (b) the nervous system affects DNA mutations and oncogene-related signaling; (c) the nervous system modulates tumor-related immune responses; (d) tumor tissues are innervated; (e) neurotransmitters released from nerves innervating tumor tissues affect tumor growth and metastasis; (f) alterations or modulation of nervous system activity affects the incidence and progression of cancers; (g) tumor tissue affects the nervous system (Figure 2).

Schematic depiction of the pillars creating a basis for the neurobiology of cancer. These pillars are based on accumulated facts demonstrating that there are bidirectional interactions between the nervous system and cancer. It has been shown that psychosocial factors affect cancer incidence and progression. These effects are mediated, at least partially, by neurotransmitters released by nerves innervating cancer tissue. Neurotransmitters released by autonomic nerves affects DNA mutations and oncogene-related pathways, stimulates cancer cell proliferation, and modulates cancer-related immunity. The role of the nervous system in cancer is further documented by findings that alterations or modulation of nervous system activity significantly affects cancer incidence and progression. Conversely, cancer also affects brain functions, which might participate in the development of cancer cachexia, for example

The aim of this article is to provide a historical overview of the findings on which the concept of neurobiology of cancer is based. These findings will not only contribute to a better understanding of the complexity of the etiopathogenesis of cancer, but also create the basis for new therapeutic and preventive approaches in oncology.

1.1 Psychosocial factors affect tumor incidence and growth

For centuries, physicians observed and discussed the role of psychosocial factors in the development and progression of various diseases, including cancer. Based on the approaches they used, three periods related to the role of psychosocial factors in cancer diseases can be recognized, reflecting the prevailing scientific methods used during a given period.

1.1.1 Period of empirical evidences (melancholy period)

Scientific and literary texts mentioning the relationship between psychosocial factors, especially melancholy mood, and cancer initiation and progression have accumulated from antiquity.2 The first preserved text describing the possible influence of psychological factors on the incidence of tumors can be traced back to the second century AD in the treatise De Tumoribus of Galen of Pergamon. Galen, who espoused the Hippocratic tradition that cancer is associated with an excess of black bile, believed that melancholy represents a factor responsible for cancer.3 This idea dominated medicine for a long time. For example, the Byzantine physician Aetius hypothesized that tumors are the result of “melancholy accumulating in the brain.” Centuries later, Arabic physicians such as Avicena and Avenzoar similarly reported that the development of tumors was associated with a melancholy mood.2 In the following centuries, a large number of physicians continued to believe that a melancholy mood formed the basis for the development of tumors. For example, at the beginning of the 17th century, the French surgeon Claude Chapuys de Saint-Amour noted in his Treatise on cancer, as occult as ulcerated that tumors are caused by grief, anger, and agitation.4 Similarly, Guillaume de Houppeville mentioned that sadness, compassion, grief, and excessive workload can exacerbate melancholy and cancer development.5 In 1740, the French surgeon Gilles Le Vacher noted in his work on breast cancer that significant and persistent grief could result in the development of breast cancer.6 Based on the above considerations, cancer and melancholy began to become synonymous, and people began to perceive cancer as a consequence of grief.2 In the early 18th century, the French physician Claude Deshais Gendron pointed to a link between serious life situations and the incidence of cancer.7 Several years later, English physicians such as J.A. Burrows, T.W. Nunn, and R. Stern described the role of psychological factors, including hypersensitivity and frustration, to the incidence of tumors in women.2, 8 In 1802, the French physician J.B.A. Burdel stated in essay Le cancer des mamelles, that women prone to breast cancer have a specific psychological profile. He hypothesized that the main cause of tumors in these women was suffering due to aging and the loss of beauty. He further noted that adverse life events, such as the dangerous situations in which women found themselves during the French Revolution, contributed to the increased incidence of breast cancer, especially in nuns.9 Similar considerations were made in 1807 by Viel Haut Mesnil, who hypothesized that aging and depression could affect the genitals and increase the likelihood of tumors in these organs.10 Description of the role of psychosocial factors in cancer was also published in the mid-19th century by Walter H. Walshe in The Nature and Treatment of Cancer.11 In 1870, an English surgeon, Sir James Paget in his classic Surgical Pathology mentioned that emotional disturbances, such as deep anxiety, deferred hope, and disappointment are quickly followed by the growth and increase of cancer.12 Fifteen years later the United States surgeon Willard Parker published a book in which he suggested that great mental depression, particularly grief, induces a predisposition to such diseases as cancer, or becomes an precipitating cause under circumstances where the predisposition had already been acquired.13

1.1.2 Period of statistical and epidemiological studies

In the last decades of the 19th century, advances in statistics and epidemiology made it possible to examine more precisely the relationship between psychosocial factors and cancer. In 1893, the London surgeon Herbert Snow conducted an epidemiological study involving 250 patients. He found that of the 250 patients diagnosed with breast or uterine tumors, 43 had a history of suspected mechanical injury, with 15 of these 43 patients reporting recent problems. Thirty-two other patients said they had jobs involving hard work and were in need. Moreover, 156 patients were identified as having recent, serious life problems, such as the loss of a loved one. Only 19 patients did not show any of the above factors.14 In 1926, psychologist Elida Evans published a book in which 100 patients with cancer had all lost or had significant emotional bonds disrupted before they developed cancer.2, 8, 15

1.1.3 Period of psychoneuroimmunological studies

Elucidation of the regulatory effects of the nervous system on the endocrine16, 17 and immune systems18 led to the establishment of a new research area, psychoneuroimmunology.19 The psychoneuroimmunological view of somatic diseases provided the basis for studies investigating the mechanisms and pathways interconnecting psychosocial factors and cancer. One of the first studies was performed by Spiegel et al. in 1989. These authors investigated the effect of psychotherapy on cancer survival and showed that group psychotherapy aimed at reducing anxiety, depression, and pain in women with metastatic breast cancer also prolonged survival.20 Later, Fawzy, et al.21 showed that a 6-week structured group psychological intervention designed to improve stress management effectiveness reduced recurrence and prolonged survival in patients with melanoma. However, studies published later in 2001 and 2007, as well as two meta-analyses published in 2004, did not find that psychotherapy had an effect on the survival of patients with breast cancer.22-26 In the following years, a large number of studies were conducted and faithful meta-analyses were published, some of which showed a positive effect of psychotherapy on the survival of cancer patients, while others did not confirm this effect. Analysis of these studies suggests that there are several factors, such as the type of intervention (group vs. individual therapy), intensity, frequency, and duration of treatment that determine their effectiveness.27 In addition, as suggested by Mirosevic et al.,28 psychotherapy may preferentially prolong survival in specific subgroups of patients, especially in socially isolated cancer patients. Due to this social isolation, these cancer patients may have a higher level of hopelessness and despair. It is known that these factors adversely affect the onset and progression of cancer, as evidenced by a series of earlier clinical studies conducted by Schmale and Iker. In one of the first studies, published in 1966, Schmale and Iker found that the presence or absence of cervical cancer could be determined in asymptomatic patients with cytologically confirmed dysplasia through an interview to determine the potential for hopelessness/despair, or the recent experience of these feelings. Based on this interview, they were able to correctly identify 8 of 14 women with cancer and 23 of the 26 healthy women.29

Whereas the first description of the role of psychosocial factors in cancer is dated almost 2 millennia ago, the mechanisms and pathways interconnecting melancholy mood, adverse life events/stress, and depression with cancer incidence and progression only started to be elucidated in more detail by researchers utilizing approaches that included neuroscientific methods since the beginning of 21st century. This research has provided a mechanistic explanation of how psychosocial factors might affect cancer.30

1.2 The nervous system affects DNA mutations and oncogene-related signaling

Alterations at the level of DNA play a crucial role in cancer initiation, progression, and metastasis. Several factors, including radiation, oncogenic viruses, and chemical carcinogens, might induce changes at the level of DNA that lead to the transformation of normal cells to cancer cells and subsequent cancer progression. Recently, it was found that the nervous system also participates in the development of DNA mutations. In addition, the nervous system attenuates DNA repair and sensitizes the cell to mutagenic factors. These data indicate that the nervous system plays a significant role in the first step responsible for initiating cancer, as well as processes related to cancer growth and metastasis. The “pro-mutagenic” effects of the nervous system are mediated mainly via systems responsible for the neuroendocrine stress response, particularly the sympathoadrenal system and hypothalamic-pituitary-adrenocortical axis.

Recently, molecular mechanisms responsible for the pro-mutagenic potential of the effector molecules released by neuroendocrine stress response systems were elucidated. It was shown that epinephrine, norepinephrine, and cortisol might induce alterations in target cells at the level of DNA via several mechanisms, including induction of DNA mutations, suppression of DNA repair, and by activation of oncogene-related intracellular pathways.

1.2.1 Nervous system effects on DNA mutations

As early as in 1925, Cramer31 published the first study investigating the role of the nervous system in chemical carcinogenesis. However, it was only at the beginning of the 21st century that the mechanisms responsible for modulating the effects of the nervous system on DNA mutations started to be elucidated at the cellular and molecular levels. It was found that the nervous system might affect DNA mutations via at least three mechanisms, including potentiation of mutagenesis, reduction of DNA repair, and sensitization of cells to mutagens.

In 2007, Flint et al.,32 measured the effect of stress hormones and neurotransmitters such as epinephrine, norepinephrine, and cortisol on DNA using in vitro murine 3T3 cells. The authors demonstrated that short-term exposure (<30 min) to physiological concentrations of these molecules significantly increased DNA damage in 3T3 cells. Moreover, cortisol and norepinephrine also interfered with the repair of DNA damage in cells exposed to UV radiation. A targeted gene array showed that cortisol, norepinephrine, and epinephrine affected the transcription of several genes, including the proto-oncogene CDC25A. A few years later, Hara, et al.33 showed that the sympathoadrenal system also plays a role in regulating the functions of tumor suppressor genes. The authors observed that activation of β-adrenergic receptors initiated a signaling cascade that induced the degradation of p53, the product of a tumor suppressor gene in mice and human cell lines. In 2012, Feng et al.34 demonstrated that chronic restrain stress greatly promoted ionizing radiation-induced tumorigenesis in p53(+/−) mice. This effect of stress was mediated by glucocorticoids, which increased mouse double minute 2 homolog (MDM2) activity and decreased p53 function. Later, Hara et al.35 showed that stress-induced DNA damage mediated by β-adrenergic signaling is preventable by an antagonist of β-adrenergic receptors. Two year later, Reeder et al.36 demonstrated that the stress hormones norepinephrine and cortisol-induced DNA damage in triple-negative breast cancer cells.

1.2.2 Nervous system's effects on oncogene-related signaling

It has been shown that nervous system-related signaling is also connected to the function of proto-oncogenes and tumor suppressor genes. The role of the sympathoadrenal system in activating proto-oncogenes was demonstrated in 1988, by Kousvelari et al.,37 who showed that stimulation of β-adrenergic receptors by isoproterenol induced expression of the proto-oncogene c-fos in rat parotid acinar cells in vitro. A few years later, Iwaki et al.38 and Okazaki et al.39 showed that activation of α- and β-adrenergic receptors induced the expression of c-fos and c-jun proto-oncogenes in rat arterial smooth muscle and myocardium. However, it is necessary to note that even if c-fos and c-jun are implicated in carcinogenesis for some cancers,40 they do not play a prominent role in human carcinogenesis.

Later, in vitro studies showed that the sympathoadrenal system also plays a role in the activation of proto-oncogenes implicated in human cancers. For example, in 2011, Shi et al.41 demonstrated that catecholamines stimulate Her2 mRNA expression and promoter activity in human breast cancer cells via β2-adrenergic receptors. Later, in 2013 Armaiz-Pena et al.42 showed that activation of β-adrenergic receptors activated Src-related phosphoproteomic signaling networks in human ovarian cancer cells.

1.3 The nervous system affects tumor-related immune response

The fact that the initiation and progression of cancer are closely and comprehensively related to the activity of the immune system has been documented by in vitro studies, in vivo experiments on animal models of cancer, as well as clinical studies. These studies have shown that the immune system recognizes cancer cells and is able to eliminate them, but it is also able to potentiate cancer initiation, progression, and the development of metastases. Research on the interactions between cancer and the immune system has shown that essentially all innate and acquired immune effector mechanisms are involved in the recognition of cancer cells and the modulation of cancer growth.43 The importance of the immune system's role in cancer documents the Nobel Prize in Physiology or Medicine in 2018, that received James P. Allison and Tasuku Honjo for their revolutionary advancement in cancer therapy based on immune checkpoint inhibitors.44

In last two decades a large number of papers have demonstrated that the nervous system modulates cancer-related immune system activity. Recently, several mechanisms and pathways that enable the nervous system to modulate cancer initiation and progression have been described in detail resulting in the emergence of potential therapeutic implications. For example, the nervous system might modulate cancer initiation/promoting chronic inflammation, potentiate or suppress anti-cancer immunity, as well as inhibit or stimulate the activity of cancer growth-promoting immune cells. Therefore, the role of the nervous system in modulating immune responses to cancer is at the center of research on the neurobiology of cancer.

1.3.1 Immune system plays crucial role in cancer

One of the first description of the role of immune system in cancer provided Rudolf Virchow. In 1863, he mentioned a relationship between inflammation and cancer based on his observation of inflammatory infiltrates in solid cancers.45 About 50 years later, Paul Ehrlich formulated the hypothesis that the host defense may prevent neoplastic cells from developing into cancers.46 However, this hypothesis was not proven experimentally as at this time experimental tools and knowledge were inadequate. Few years later, Murphy and Morton47 demonstrated significant increase of circulating lymphocytes in mice inoculated by cancer cells.

Later experiments have shown that in immunodeficient mice is increased incidence of tumors and that these animals are more susceptible to transplanted of chemical carcinogen-induced cancers. In addition, it was observed that immunosuppressed patients have increased incidence of some cancers. Based on these findings, Frank Mac Farlane Burnet suggested that immune system react to cancer cells neo-antigens. He formulated the immune surveillance theory stating that when small accumulation of cancer cells develop, the neo-antigens that they possess provoke an effective immunological reaction that lead to regression of the cancer and no clinical hint of its existence.48 This hypothesis stated that sentinel thymus-dependent cells of the body constantly surveyed host tissues for nascently transformed cells.49

However, as knowledge about the role of the immune system in cancer grew, it started to be recognized that immunosurveillance represents only one dimension of the complex relationship between the immune system and cancer. Later, Dunn et al.50 proposed a new hypothesis of cancer immunoediting. This hypothesis states that the immune system may also promote the emergence of primary tumors with reduced immunogenicity that are capable of escaping immune recognition and destruction. Cancer immunoediting represents a dynamic process that includes three phases: elimination, equilibrium, and escape. Whereas elimination represents the classical concept of cancer immunosurveillance, equilibrium is the period of immune-mediated latency after incomplete tumor destruction in the elimination phase, and escape refers to the final outgrowth of cancers that have broken the immunological restraints of the equilibrium phase.51

1.3.2 Stress and immunity

One of the first demonstrations of the modulatory effect of psychosocial factors on the immune system was provided in 1936 by Hans Selye, who described the effect of chronic stress on the thymus, spleen, and other immune organs.52 In subsequent mechanistic studies of neuroendocrine stress response, he showed that this effect is mediated by the HPA axis, specifically by glucocorticoids released from adrenals. In 1950, Slocumb et al.53 showed that glucocorticoids exert potent immunosuppressive effects in patients with inflammatory disease. Later, it was demonstrated that immune functions are also modulated by prolactin and growth hormone released from the anterior pituitary gland.54 These findings have shown that the brain might regulate the activity of immune cells via modulation of endocrine gland secretion.

1.3.3 Psychoneuroimmunology

A new view on the mechanisms interconnecting the brain and immune function was provided by an experiment published in 1975 by Robert Ader and Nicholas Cohen, who by using Pavlovian conditioning, demonstrated that immune responses might also be conditioned.18 Subsequent experiments have shown that autonomic nerves innervate all immune organs and modulate the growth and release of immune cells from bone marrow55 Mechanistic studies have demonstrated that whereas the sympathoadrenal system modulates immune function via norepinephrine released from sympathetic nerves, as well as epinephrine and norepinephrine released from adrenal medulla, the parasympathetic nervous system modulates immunity via acetylcholine.56 Moreover, it was demonstrated that sensory nerves can regulate local immune responses, for example in the skin.57

In addition to the direct modulatory effects of signaling molecules of the nervous and neuroendocrine system on immune cells, the nervous system can also affect immune functions indirectly through modulating the growth and maturation of immune cells within the bone marrow, as well as their release to systemic circulation. This is done by influencing blood flow though immune organs and vasomotor reactions in the gastrointestinal tract, as well as through the retino-hypothalamic tract and subsequent regulation of circadian rhythms, or via regulation of food intake and nutritional status of the organism.58

There are many factors that determine the effect of the nervous system on immune functions, including the type of neurotransmitter or hormone, subtype of corresponding receptors, and intensity and duration of receptor activation. For example, whereas the acute effect of psychosocial factors (stressors) on immunity seems to be predominantly stimulating, chronic exposure to adverse psychosocial factors suppresses, or dysregulates immune responses.59

Importantly, interactions between the nervous and immune systems are bidirectional. On one side, immune functions can be modulated by hormones released from neuroendocrine systems, as well as by neurotransmitters and neuromodulators released from nerve endings. On the other side, the immune system is able to affect both the peripheral and central nervous system via cytokines and other molecules synthetized by immune cells.60, 61

1.3.4 Neuroimmunology of cancer

Psychoneuroimmunological studies have shown that the nervous system exerts a complex effect on immune functions, including anti-cancer immunity. Therefore, in the last decade neuroimmunological research has also focused on investigating the effect of stressors and other factors affecting nervous system activity on immune parameters related to cancer initiation and progression. This research has uncovered mechanisms and pathways that interconnect the nervous system and cancer indirectly, via the immune system. It was shown that the nervous system, via humoral and nervous pathways, affect several components of the immune system that are related to cancer development and progression.

In 1999, Kalinichenko et al.62 showed that in vitro norepinephrine inhibited the generation of cytotoxic T lymphocytes via β-adrenergic receptor signaling. Later, it was shown that the sympathoadrenal system significantly affected the activity and distribution of NK cells,63 as well as density and survival of myeloid-derived suppressor cells in tumors and other tissues, along with the expression of immunosuppressive molecules by these cells.64

Neuroimmunological studies of cancer have provided a mechanistic explanation of how adverse psychosocial factors might affect cancer. However, the effect of the nervous system on cancer-related immunity is highly complex, as different components of nervous system affect the activity of various subtypes of immune cells differently.

1.4 Tumor tissue is innervated

Peripheral nerves play an important role during the development of an organism, participate in homeostatic regulations of innervated organs and tissues, modulate repair and regeneration of damaged tissues, and participate in compensatory reactions of tissues affected by diseases.65 In addition, recent data have demonstrated that peripheral nerves also play an important role in the progression of various pathological processes. For example, signals transmitted by peripheral nerves participate in the development of neurogenic hypertension, obesity, and diabetes. Recent data have shown that peripheral nerves innervating cancer tissue also play an important role in the initiation and progression of cancer.66, 67

Even if the first description of cancer tissue innervation was published more than 100 years ago, factors participating in the ingrowth of new nerves into cancer and mechanisms mediating the stimulatory effect of these nerves on cancer progression and the development of metastases have only recently been elucidated.

The investigation of nerves role in cancer can be divided into two periods, further subdivided into several parts. These two waves of interest in the study of cancer innervation and its role, characterized by utilization of the available research methods during a given period (especially histological methods), were interrupted by a period when significant new discoveries related to cancer biology and therapy shifted general interest in oncology to research related to chemotherapy, cancer immunity, neovascularization, and other factors.

1.4.1 Period of discovery of cancer innervation

This period, which started from the second half of the 19th century and finished at the end of the first half of the 20th century, can by subdivided into two parts.

1.4.2 Virchow period

Even if speculation about the role of nerves in cancer development might have been already begun in the 18th century, Rudolf Virchow, the founder of cellular pathology and one of the pioneers of tumor research, did not assign a more important role to nerves in the origin and growth of cancer (for review see68). Based on his opinion, it was assumed that tumor tissue was not innervated.69 However, several years later, Young published a paper demonstrating the presence of methylene-blue-stained nerves in breast and cervical cancer, as well as in sarcoma.70 In 1903, Cheatle published clinical observations indicating that a relationship exists between the spread of the primary cancer and the distribution of nerves or their trophic areas.71 The role of nerves in the development of cancer was also mentioned by Ewing in his textbook on tumors published in 1919.72

1.4.3 Silver staining period

In 1928, Oertel showed the presence of nerves in cervical cancer and adenocarcinoma of the rectum in silver-stained specimens.73 Three years later, he described the presence of silver-stained nerve structures in breast cancer, uterine myoma, and fibrosarcoma.74 In 1949, Shapiro and Warren in a combined morphological and functional study demonstrated the presence of nerves in Brown-Pearce carcinoma and mouse mesothelioma anterior ocular chamber transplants. In addition, they showed that sympathetic nerve stimulation induced the contraction of vessels in these tumors.75 Furthermore, in 1956, Winkelmann described the presence of silver-stained nerves in squamous cell carcinoma and basal cell carcinoma of the eyelid.76

1.4.4 Period of “rediscovery” of cancer innervation

This period, dated at the end of 20th century, represents a new wave characterized by the use of modern methods enabling precise investigation of cancer innervation. This period can be subdivided into two parts.

1.4.5 Electron microscopy and immunohistochemistry period

This period is characterized by more detailed descriptions of the phenotypes of nerves innervating tumor tissues. In 2001, Seifert and Spitznas demonstrated the presence of nerves in pigmented and one non-pigmented adenoma of the ciliary body epithelium using electron microscopy.77 Later, Seifert et al. showed nerve-like structures immunoreactive for protein gene product 9.5 and vasoactive intestinal neuropeptide in urinary bladder tumors.78 In 2007, Palm and Entschladen published the hypothesis that tumor cells may induce their own innervation, a process they have termed “neoneurogenesis” Moreover, they called the structures that enable interaction between peripheral neurons and tumor cells a “neuro-neoplastic synapsis”79 Importantly, further immunohistochemical studies showed that the density of innervation and size of nerves determines, or at least correlates with the aggressiveness of the tumor.80-84

1.4.6 Genetics and electrophysiology period

In 2019, Kamiya et al. employed a series of genetic techniques enabling them to selectively manipulate (stimulate) sympathetic or parasympathetic nerves innervating chemically induced breast tumors in rats.85 They showed that stimulation of sympathetic nerves in tumors accelerated cancer growth and progression, whereas stimulation of parasympathetic nerves had the opposite effect. Later, in 2020 McCallum et al. published a paper describing chronic neuronal activity recorded from breast tumors in mice showing that neural electrical activity is present within mammary tumors. As the authors stated, their results indicate a strong connection between the autonomic nervous system and tumors.86

Available data indicate that there are several sources of nerves innervating tumor tissue: (a) nerves already present in tissue before the transformation of normal tissue cells into cancer; (b) phenotypically transformed neurons that innervate the tissue of tumor origin87; (c) new branches of nerves growing to the tumor tissue from nerves localized around the tumor tissue; (d) new neurons migrating from a distant part of the central or peripheral nervous system into the tumor tissue or its vicinity88 (Figure 3). Published data indicate that cancer manipulates the nervous system by inducing the growth of new sympathetic nerves into the tumor tissue and by trans-differentiation of a sensory neuronal phenotype to adrenergic in order to utilize the stimulatory effect of adrenergic signaling for promoting cancer growth and development of metastasis. Besides sympathetic89 and parasympathetic nerves,90 sensory nerves also play a role in cancer,67, 91 even if their role in cancer development and progression has been described in less detail.

Schematic depiction of the origin of nerves innervating tumor tissue: (A) sympathetic, parasympathetic, and sensory nerves already present in tissue before the transformation of normal tissue cells into cancer; (B) sensory neurons innervating the tissue of tumor origin that have phenotypically transformed to sympathetic neurons; (C) new branches of nerves growing to the tumor tissue from nerves localized around the tumor tissue; (D) new neurons migrating from a distant part of the central or peripheral nervous system into the tumor tissue or its vicinity

Interestingly, recent data indicate that cancer cells themselves may transform to a neuron-like phenotype characterized by development of neurite-like protrusions. These protrusions might participate in the formation of synapses between neurons and cancer cells, further potentiating the stimulatory effect of nerves on cancer growth.92

Accumulated evidences have clearly shown that innervation of tumor tissue represents a complex phenomenon. In support of this, tumor tissue innervation has been found to be an important factor influencing the tumor microenvironment in several cancer types. Recent studies have shown that peripheral nerves, including sympathetic, parasympathetic, and sensory nerves, interact with tumor cells and other cells of tumor tissue, and stimulate the initiation and progression of a whole spectrum of solid and hematological tumors. In addition, the tumor itself has been found to promote its own innervation, which in turn promotes tumor growth.67 Based on these findings, tumor innervation can be included among the basic hallmarks of cancer (Figure 4).93 However, more detailed mapping of the innervation of different cancers is needed. For example, it will be necessary to determine whether cancer innervation represents a general phenomenon, which types of nerves are responsible for innervation of different types of cancers, and which factors determine it.

Based on the neurobiology of cancer, two additional hallmarks were added into the classical scheme of hallmarks of cancer as originally defined by Hanahan and Weinberg.139 These two new hallmarks include “inducing tumor tissue innervation” and “affecting nervous system” (e.g., cognitive impairment, hypothalamic inflammation-related cancer cachexia). Importantly, all original hallmarks of cancer are under the influence of the nervous system 1.5 Neurotransmitters affect tumor microenvironment

The innervation of cancer tissues is now an accepted fact. Autonomic and sensory nerves innervating cancer release neurotransmitters such as norepinephrine, acetylcholine, substance P, and others into the tumor microenvironment. Once there, these neurotransmitters can affect almost all hallmarks of cancer. In addition to their effect on the tumor microenvironment, neurotransmitters might also affect cancer via its effects on the tumor macroenvironment. The research of neurotransmitter effects on cancer has also focused on the effect of hormones released by the neuroendocrine system, especially epinephrine.

1.5.1 Early studies investigating the presence and affinity of adrenergic receptors on cancer cells

The presence of β-adrenergic receptors on cancer cells and their binding affinity to compounds including norepinephrine, epinephrine, and other agonists of β-adrenergic receptors has been investigated since the 1970’s. However, this research focused on cancers of the endocrine glands and brain and therefore cannot be seen as a precursor to research on the neurobiology of cancer.

In 1989, Marchetti et al.94 published a study describing the presence of β-adrenergic receptors on membranes of mammary cancer induced by a chemical carcinogen and their affinity to several compounds, including norepinephrine and epinephrine. The authors mentioned that the presence of β-adrenergic receptors in mammary cancers might be related to catecholamines’ effect on cancer tissue. Next year, Vandewalle et al.95 described the presence of β-adrenergic receptors expressed on breast cancer cells and their affinity of isoproterenol, epinephrine, and norepinephrine. However, even if the authors discussed the importance of stimulated cAMP production by these compounds, they stated that the role of these receptors in breast cancer needed more detailed study. In the following year, several studies were published focused on investigating the presence and affinity of adrenergic receptors on various types of cancers. However, these studies did not investigate the effect of adrenergic receptor activation on cancer initiation or progression.

1.5.2 Studies investigating the effect of neurotransmitters on cancer incidence and progression

In 1998, Tatsuta et al.96 published one of the first studies investigating the effect of adrenergic receptor stimulation on cancer. The authors showed that long-term administration of the norepinephrine-mimick