The Neuroendocrine Impact of Chronic Stress on Cancer

Abstract

Over the past 25 years, epidemiological and clinical studies have linked psychological factors such as stress, chronic depression, and lack of social support to the incidence and progression of cancer [1,2]. Although the mechanisms underlying these observations are not completely understood, recent molecular and animal studies have begun to identify specific signaling pathways that could explain the impact of neuroendocrine effects on tumor growth and metastasis. This review will highlight the importance of known clinical, molecular, and cellular processes with regard to the neuroendocrine stress effects on tumor biology and discuss possible behavioral and pharmacological interventions to ameliorate these effects and ultimately improve cancer outcomes.

Introduction

The impact of psychosocial factors on the development of cancer has been a longstanding hypothesis. Around 200 A.D., Galen, the ancient Greek physician, recorded that women with “melancholic” disposition were more susceptible to breast cancer than “sanguine” women [3]. In 1936 Hans Selye defined stress physiologically as the state in which the autonomic nervous system (ANS) and the hypothalamic-pituitary-adrenal axis (HPA) are co-activated [4]. Today stress is considered a complex process encompassing environmental and psychosocial factors that initiates a cascade of information-processing pathways in both the peripheral and central nervous systems [5,6]. Then, the fight-or-flight stress responses in the ANS or the defeat/withdrawal responses in the HPA are created resulting in catecholamine secretion (norepinephrine and epinephrine) from the sympathetic neurons and adrenal medulla and cortisol from the adrenal cortex [7,8]. Although activation of these pathways in acute stress is necessary for adaptive processes and survival, in chronic stress these pathways have negative physiological effects due to the prolonged exposure to catecholamines and glucocorticoids [9].

Thus far, the majority of research on the deleterious effects of stress has focused on the neuroendocrine regulation of the immune response. These effects are thought to be mediated in part by the sympathetic nervous system, the HPA axis, and a variety of other hormones and peptides [8,10–13]. In both animal and human studies, chronic stress has been shown to decrease cellular immune parameters, such as natural killer (NK) cell cytotoxicity and T-cell responses to mitogen stimulation [14–16]. However, the uncertain role of the immune system in regulating solid tumor growth led us to consider an alternative hypothesis: stress mediators from the sympathetic nervous system might directly regulate the growth and metastatic potential of tumor cells, independent of the effects on the immune system [17]. Recently, there is growing evidence confirming that alterations in neuroendocrine dynamics due to chronic stress can cause alterations in tumor pathogenesis [17–21]. In this review, we will focus on these biological pathways that may be affected by stress mediators.

Neuroendocrine Influences on Biological Pathways of Cancer

In humans, tumorigenesis is a multistep process that requires many rate-limiting steps including both genetic and epigenetic processes in order for a normal cell to transform into a malignant cell [22]. According to Hanahan and Weinberg, there are six essential acquired alterations in cell physiology that dictate malignant growth: 1) self-sufficiency in growth signals, 2) insensitivity to anti-growth signals, 3) evasion of apoptosis, 4) limitless replicative potential, 5) sustained angiogenesis, and 6) tissue invasion and metastasis [22]. After establishment of the primary cancer, metastasis can occur if a series of sequential, interrelated steps including proliferation/angiogenesis, invasion, embolism/circulation, transport, arrest in organs, adherence to vessel wall, and extravasation occur [23] {Table 1}. Metastasis is a result of crosstalk between different cell types within the tumor, tumor stroma, and microenvironment [24]. Recently, research is beginning to delineate the role of neuropeptides and neurotransmitters, which are increased in certain biobehavioral states, on the multistep process of cancer development and metastasis.

Table 1.

Steps involved in metastasis

Proliferation and growth of primary tumor Detachment or shedding of cells from primary tumor into circulation Survival of cancer cells in circulation Transport of cancer cells to new organs with subsequent arrest Extravasation into surrounding tissues Angiogenesis and growth of metastatic tumor

Proliferation and growth of primary tumor and metastases

Tumor proliferation relies on nutrient and oxygen diffusion. Currently, there are limited data regarding the effects of stress hormones on proliferation. The majority of the data show that catecholamines suppress proliferation of normal cells such as keratinocytes, thereby leading to impaired wound healing in the context of stress [25]. Depending on the tumor type, adrenergic receptor, and stress-related hormones, the effects of stress-related hormones on tumor cell proliferation can be either stimulatory or inhibitory. For example, in a breast cancer model, activation of beta-adrenergic receptors (ADRB) has been shown to accelerate tumor growth [19, 20, 26]. In contrast, Carie and Sebti have demonstrated that ADRB2 agonist pirbuterol causes human tumor regression in MDA-MB-231 breast cancer in vivo by blocking the Raf-1/Mek-1/Erk1/2 pathway. However, the authors acknowledge that this approach is limited to human cancers that not only express ADRB2, but also that stimulation of this receptor results in the blockade of the Raf-1/Mek-1/Erk 1/2 pathway [27]. In addition to the stimulation by catecholamines on ADRB2, carcinogens like nicotine may interact with ADRB2 and the downstream protein kinase C/Erk1/2/cyclooxygenase 2 pathway to lead to cell proliferation of gastric cancer cells [28]. Additionally, the cyclic AMP responsive element-binding (CREB) protein is an important transcription factor that is activated by many signal transduction pathways in response to external stimuli such as stress hormones [29, 30]. Several studies have demonstrated a role for the CREB family of proteins in tumor cell proliferation, migration, angiogenesis, and inhibition of apoptosis [29–31]. Further studies will be instructive in delineating the specific signaling pathways responsible for ADRB2 mediated effects on tumor cell proliferation.

In other models, catecholamines inhibit tumor cell proliferation that may be mediated by the α-adrenergic receptor or the dopamine transporter. Treating melanoma cells with the α₁-adrenergic agonist phenylephrine led to a dose-dependent decrease in proliferation that could be reversed by using prazosin, α₁-adrenergic antagonist [32]. Pifl and colleagues found that norepinephrine treatment caused neuroblastoma cells expressing the dopamine transporter to enter into the G0/G1 phase thereby blocking proliferation [33].

The role of glucocorticoid hormones on cancer cell proliferation has been examined; however, there are limited data with regard to the effects of glucocorticoids along with neuroendocrine hormones. Zhao and associates described that cortisol, the main circulating glucocorticoid and cortisone the main metabolite, stimulated growth of prostate cancer cells in the absence of androgens and increased the secretion of prostate specific antigen. The androgen receptor was mutated and acted as a high-affinity cortisol/cortisone receptor [34]. Simon and colleagues examined the effects of several steroid hormones on mammary carcinoma cells in vitro and found that biologically relevant concentrations of glucocorticoids enhanced proliferation by almost two-fold [35].

Adhesion

The ability of a tumor cell to ultimately invade and metastasize to outlying tissues relies on adhesion to the extracellular matrix within tissues [36]. The extracellular matrix is dynamic and consists of type I and IV collagens, laminins, heparin, sulfate proteoglycan, fibronectin, and other noncollagenous glycoproteins [37]. Integrins are cell surface receptors that interact with the extracellular matrix and mediate intracellular signals. There is emerging evidence that stress hormones may modulate tumor-stroma interactions. For example, Enserink and associates have shown that the β-agonist isoproterenol promotes ovarian cancer cell spreading and adhesions via integrins through the Epac (exchange factor directly activated by cAMP)-Rap1 pathway [38, 39]. Ultimately, stress hormones may promote cell-matrix attachment of cancer cells. This mechanism would be especially relevant in ovarian cancer where the primary tumor sheds cancer cells that are then able to implant diffusely onto peritoneal surfaces and organs.

Migration and invasion

The migration of tumor cells is a prerequisite for invasion into the host stroma, blood vessels, and lymphatics and ultimately the development of metastasis, which accounts for approximately 90% of cancer mortality [40]. There is growing evidence that stress hormones affect tumor cell motility and invasion. Drell and associates showed that multiple neurotransmitters such as substance P, dopamine, and norepinephrine had a stimulatory effect on the migration of breast cancer cells, but only norepinephrine had a chemotactic effect on breast cancer cells [41]. Similarly, other groups have demonstrated that norepinephrine was a potent inducer of colon cancer and prostate cancer cell migration, which could be inhibited by beta-blockers [30, 42]. Sood and associates have studied the direct effects of catecholamines and cortisol on invasion of ovarian cancer cells using a membrane invasion culture system and the production of key matrix metalloproteinases (MMP), which are involved in tumor cell penetration of the extracellular matrix. Although stress levels of norepinephrine and epinephrine increased the in vitro invasive potential of ovarian cancer cells by 89–198% and 64–76% respectively, cortisol did not significantly affect the invasive potential of ovarian cancer cells. Propranolol, the non-specific beta-adrenergic antagonist, completely reversed the norepinephrine-induced increase in invasive potential. Additionally, norepinephrine also enhanced tumor cell expression of MMP-2 and MMP-9, and pharmacologic blockade of MMPs abrogated its effects on tumor cell invasive potential. These experimental findings provided evidence that stress hormones can enhance the invasive potential of ovarian cancer cells in vitro [43].

Angiogenesis

Angiogenesis is a critical process in the growth of most solid tumors beyond 1–2 mm in diameter, and metastasis requires the recruitment of nearby blood vessels to permeate the tumor [44]. Norepinephrine has been shown to upregulate vascular endothelial growth factor (VEGF) in adipose tissue via the beta-adrenoreceptor/cAMP/protein kinase A (PKA) pathway and in two ovarian cancer cell lines. The upregulation of VEGF by norepinephrine was blocked by a β-blocker and mimicked by the β-agonist isoproterenol thereby confirming the functional relevance of the β-adrenoreceptor in mediating these signals [45, 46]. Thaker and colleagues showed that chronic behavioral stress with daily immobilization resulted in higher levels of tissue catecholamines, greater tumor burden, and a more invasive pattern of disease [17]. Microvessel density counts as a measure of angiogenesis were significantly increased in stressed compared to control mice tumor samples. VEGF mRNA and protein levels were also significantly elevated in the stressed tumor samples. A continuous infusion of propranolol abrogated the effects of stress on tumor burden and pattern of disease; therefore, confirming the importance of beta-adrenergic receptors on ovarian cancer cells in an in vivo model. Clinical studies have also shown a correlation between higher levels of social support and lower serum VEGF levels in ovarian cancer patients [47].

Interleukin-6 (IL-6) is another important molecule in tumor progression and angiogenesis [48]. IL-6 has been shown to be secreted by ovarian cancer cells and to enhance tumor cell proliferation and migration in vitro [49, 50]. Furthermore, IL-6 is a potent angiogenic cytokine in vivo as evidenced by gelfoam sponge assays, showing a significant increase in microvessel density in the IL-6 impregnated sponges compared to control [48]. Clinically, individuals experiencing chronic stress have been shown to exhibit elevated circulating levels of IL-6 [51]. In ovarian cancer patients, elevated IL-6 is associated with poorer prognosis [52, 53]. Lutgendorf and colleagues found that social support was associated with lower levels of IL-6 in the blood and ascites samples of ovarian cancer patients, thereby playing a protective role [54]. Nilsson and associates demonstrated that in ovarian cancer cells, norepinephrine increased IL-6 mRNA expression by 45-fold at 6 hours. Significant increases in IL-6 promoter activity were also observed indicating that catecholamines regulate the IL-6 gene at the transcriptional level. The effects of norepinephrine on IL-6 production in ovarian carcinoma cells were found to be mediated through a β-adrenergic receptor/Src tyrosine kinase axis [55].

Signal transducer and activator of transcription factor-3 (STAT3) is activated by growth factors such as IL-6, promotes angiogenesis by VEGF, and suppresses apoptosis [56]. Due to cytokines such as IL-6 contributing to malignant progression through the activation of STAT3, Landen and colleagues sought to determine if stress hormones have a direct effect on expression and activation of STAT3 in ovarian cancer. They found that both norepinephrine and epinephrine are capable of activating STAT3 with subsequent translocation to the nucleus and DNA binding. STAT3 activation proceeded through β-adrenergic receptors and protein kinase A in a rapid fashion and was independent of IL-6. They concluded that stress-mediated malignant progression may be mediated in part through upregulation of STAT3 leading to the activation of multiple carcinogenic downstream effector pathways [57].

The data pertaining to the effects of glucocorticoids on angiogenesis are limited. Dexamethasone (a synthetic glucocorticoid) treatment of cancer cells resulted in a 50–60% downregulation of VEGF mRNA in a rat glioma model, and this effect was dependent on the glucocorticoid receptor function. This inhibitory effect by dexamethasone was markedly reduced by hypoxia, which is a known inducer of VEGF [58]. In ovarian cancer cell lines, cortisol had limited stimulating effects at lower doses and inhibitory effects at pharmacologic doses. Since stress causes elevations in both cortisol and catecholamines, co-stimulation experiments in ovarian cancer cell lines were performed. Although priming the cells with cortisol blunted the norepinephrine-induced VEGF production, significant increases in VEGF were still observed [46]. These results demonstrate that catecholamine effects are dominant in the production of angiogenic cytokines.

Cell Survival

Avoidance of apoptosis is critical to the metastatic cascade. Until recently, most of the data with regard to the effects of stress hormones on tumor cell survival have focused on glucocorticoids. However, Chan and colleagues found that both dopamine and norepinephrine triggered apoptosis via a G protein mediated signaling cascade in neuroblastoma cells but not in lung carcinoma cells [59]. In prostate and breast cancer cell lines, epinephrine (important in acute and chronic stress) reduced the sensitivity of cancer cells to apoptosis through interaction with ADRB2 receptors followed by protein kinase A-dependent BAD phosphorylation [60]. Although large-scale human studies are lacking, a recent epidemiological study showed a decreased incidence of prostate cancer in patients who took beta-blockers regularly thereby implying the importance of the activation of β-adrenergic receptors on the development of prostate cancer [61].

Glucocorticoids may also activate survival genes that protect cancer cells from the cytotoxic effects of chemotherapy [62, 63]. In cervical and lung carcinoma cells, glucocorticoids led to a downregulation of pro-apoptotic elements of death receptor and mitochondrial apoptosis pathways [62]. Similarly, Wu and colleagues found that dexamethasone pretreatment of breast cancer cell lines inhibits chemotherapy-induced apoptosis in a glucocorticoid receptor-dependent manner and is associated with the transcriptional induction of mitogen-activated protein kinase phosphatase-1 (MKP-1) and serum and glucocorticoid-inducible protein kinase-1 (SGK-1). Specific inhibition of these two proteins with small interfering RNA reversed the anti-apoptotic effects of glucocorticoid treatment [63]. Additionally, glucocorticoids such as cortisol may act synergistically with catecholamines to facilitate cancer proliferation. In lung carcinoma cells, cortisol potentiated the isoproterenol-induced increase in cAMP concentration, increased β-adrenergic receptor density, and dramatically increased the effects of IL-1α, IL-1β, and tumor necrosis factor-alpha [64].

Other stress mediators

Although epinephrine, norepinephrine, and cortisol are widely considered as the major stress mediators, other hormones such as prolactin, oxytocin, dopamine, and substance P are affected by stress [40, 65–67] {Table 2}. Prolactin plays a functional role in tumor cell proliferation and promotes survival of breast, prostate, endometrial, and other cancer cells [68–71]. Several epidemiological studies have shown a consistent relationship between prolactin levels and known risk factors for breast cancer such as parity and age at menarche [68]. A significant proportion of breast cancer cell lines express the prolactin receptor and exogenously added prolactin has modest trophic effects on human tumor tissues and cells in vitro [72]. Saez and coworkers demonstrated that exercise-induced stress using a forced swim model enhanced 9,10-dimethyl-1,2benz(a)anthracene mammary tumor growth rate, but not survival time or tumor multiplicity via involvement of catecholamines and prolactin in vivo, and melatonin could offset these effects [73]. However, Thompson and associates showed that in rats with 1-methyl-1-nitrosurea induced mammary tumors, which are less sensitive to prolactin, treadmill exercise reduced the mammary cancer incidence rate and tumor multiplicity, thereby underscoring the complexity of neuroendocrine interactions [74]. Additionally, prolactin may also inhibit apoptosis of mammary cancer cells via stimulation of the Akt pathway [75, 76].

Table 2.

Neurotransmitters with known function in Tumor Proliferation and Metastasis

Neurotransmitter	Tumor Cell Type	Receptor
Norepinephrine	Breast, ovarian, prostate, colon	β2-adrenoreceptor [17, 30, 42, 46, 53,54,58, 59]
Epinephrine	Ovarian	β2-adrenoreceptor [17,54]
Cortisol	Prostate, breast	Glucocorticoid receptor [34, 35, 56 61,62]
Prolactin	Breast, prostate, endometrial	Prolactin receptor [66–70]
Oxytocin	Small cell lung, Kaposi’s sarcoma	Oxytocin receptor [74, 75]
Dopamine	Breast, melanoma, neuroblastoma, head & neck	D2-receptor [41, 82–86]
Substance P	Breast, colon, lung	Neurokinin-1 receptor [40, 41,92]

This list is meant to be illustrative and not comprehensive with regard to tumor cell types and references.

Uniquely, oxytocin inhibits the growth of some epithelial cell (e.g. breast, endometrial) tumors and those of nervous or bone origin, but it has a proliferative effect on trophoblast and other tumors (e.g. small cell lung tumors, Kaposi’s sarcoma) [77, 78]. The presence of the oxytocin receptor has been described on breast and prostate cancer cells [79–81]. Dopamine is a precursor in the synthesis of catecholamines and a major neurotransmitter in the central and peripheral nervous systems [82]. Chronic stress exposure leads to decreased dopamine release in patients [83, 84]. Dopamine has been shown to have direct effects on inhibition of cancer cells including breast, melanoma, neuroblastoma, and head and neck cancer cell lines [85–89]. The inhibitory effect of dopamine is thought to occur through the dopamine receptors on the tumor cell surface or by auto-oxidation of dopamine creating the generation of reactive oxygen species. Additionally, dopamine may have anti-angiogenic properties and has been shown to inhibit cancer growth in several in vivo experimental models [90, 91]. Basu and associates demonstrated that at non-toxic levels, dopamine administration inhibited the angiogenic functions of VEGF via the D2-receptor to induce endocytosis of VEGF receptor 2 on endothelial cells thereby preventing VEGF binding, receptor phosphorylation, and subsequent signaling steps [92]. Teunis and colleagues found that both tumor size and vessel density were lower in rats with a hyperactive dopaminergic system [91]. All these findings suggest a link between dopaminergic activity, angiogenesis, and tumor development. Substance P is a peptide in the neurokinin family, is located in both the central and peripheral nervous systems, and plays a role in stress reactions, anxiety, and depression [93, 94]. Substance P promotes the migration of colon and breast carcinoma cell lines and is a chemoattractant for squamous cell lung cancer [40, 41, 95].

Summary

Cancer initiation and progression is a complex process that relies on multiple steps including genetic changes, proliferation, vascularization, invasion, embolization, and evasion of apoptosis. After the primary tumor is established, growth and metastasis can occur depending on interactions with homeostatic mechanisms. In this review, we have focused on the interrelationships between biobehavioral factors and cancer initiation and metastasis. However, our understanding of the underlying mechanisms is still in its infancy and needs to be expanded. These studies may offer new clinical strategies for therapeutic interventions utilizing behavioral and pharmacological approaches that target neuroendocrine pathways that support cancer initiation and metastasis. Already there are some biobehavioral intervention studies that have had promising results in modifying neuroendocrine dysfunction [8]. Beta-blockers have shown to block the deleterious effects of stress in both in vitro and in vivo models of several carcinomas [17, 21, 46]. Clinically, beta-blockers have been shown to both lower cancer incidence and cancer risk in a cohort of patients [61, 96]. However, in other studies the cancer risk was neutral [97–99]. Although great strides have occurred in understanding the influence of behavioral factors on cancer initiation and metastasis, further research is required to fully understand the complexity of the mechanisms along with interplay with immune system and to develop meaningful clinical trials. As cancer treatment evolves to have patient-specific therapeutic approaches, the inclusion of pharmacologic and behavioral interventions can be used in combination with conventional therapies to hopefully produce superior outcomes, but further research is warranted.