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Breast cancer (BC) is a serious global challenge, and depression is one of the risk factors and comorbidities of BC. Recently, the research on the comorbidity of BC and depression has focused on the dysfunction of the hypothalamic–pituitary–adrenal axis and the persistent stimulation of the inflammatory response. However, the further mechanisms for comorbidity remain unclear. Epoxide metabolism has been shown to have a regulatory function in the comorbid mechanism with scattered reports. The imbalance in epoxide metabolism and its downstream effect shared by BC and depression, including overexpression of inflammation, upregulation of toxic diols, and disturbed lipid metabolism is disclosed. These downstream effects are mainly involved in the construction of the breast malignancy microenvironment through liver regulation.
Cancer is one of the malignant diseases with the highest mortality in the world, and its incidence continues to grow rapidly [1][2]. The leading cause of cancer-related mortality among the female population is breast cancer (BC) [3]. Since 2020, BC has been the major cause of cancer incidence worldwide, accounting for 11.7% of all cancer cases [3][4]. BC is also the fifth highest cause of cancer deaths around the world [3]. In addition, BC is an obstacle to improving life expectancy in each country, causing a heavy economic burden and health and social challenges globally [3]. BC is a highly heterogeneous disease whose development is associated with genetic, dietary, and environmental factors [5]. Various types of BC can be broadly classified as hormone receptor status (estrogen receptor, ER, or progesterone receptor, PR), human epidermal growth factor receptor status (HER2), and triple-negative status (TNBC) [5][6][7][8]. The current mainstream treatment options include conventional chemotherapy, monoclonal antibodies, and coupled systemic administration [5]. Although increased levels of diagnosis of BC over the years have led to increased survival rates, the side effects of treatment, the impact of stress, and the unsatisfactory quality of survival have still attracted public concern [9]. BC has a high rate of physical and mental comorbidity, mainly due to chronic stress [10]. Depression, as a vital risk factor and comorbidity of BC, has plagued women with BC for decades. In the absence of the precise management of individuals, families, and professional domains, female BC patients are placed under mental stress, which eventually brings a heavier physical burden. Studies have illustrated that depression is an independent predictor of higher frequency hospitalization, longer hospitalization, lower quality of life, and lower treatment compliance [11]. Depression has also been demonstrated to be an important predictor for the diagnosis of advanced BC patients, and the suicide rate of BC patients has highly correlated with clinical symptoms of depression phenotypes [12]. It was reported that comorbid depression is associated with poor prognosis and increased mortality in cancer patients [13]. A study demonstrated that the prevalence of depression in BC patients is 15% during and after medical cancer treatment. The treatment of anxiety and depression are associated with decreased neurocognitive function and reduced hippocampal volume following chemotherapy [9][14][15][16][17]. What is more, in the context of the current global prevalence of infectious diseases, BC patients are prone to emotional disturbances and cognitive dysfunction due to the impact of work and employment [18]. A meta-analysis demonstrates that negative emotions significantly increase the risk for the incidence of BC [19]. Thus, the comorbidity of BC and depression is an inescapable biomedical problem.
Hitherto, most studies addressing the comorbidity of BC and depression have focused on four aspects: inflammation and oxidative/nitrosative stress, reduced immune monitoring, abnormal activation of the autonomic nervous system, and the hypothalamic–pituitary renal axis (HPA) [20]. In fact, the imbalance of peripheral dopamine (DA) and kynurenine (KYN) are proposed to positively predict depression in BC patients [21]. Moreover, the persistent activation of the HPA and sympathetic nervous system is believed to promote BC growth. Unfortunately, due to the dispersion of information, the bridging mechanism between depression and BC is still unclear since the etiology and final effect of the comorbidity have only been partly discussed. Epoxide metabolism is an important metabolic process that mediates inflammation, tumor, and immune surveillance, which mainly occurs in the liver, kidney, and blood vessels [22].
Epoxide metabolism is noted to play a significant regulatory role in BC. Soluble epoxide hydrolase (sEH) is an essential intermediate enzyme in epoxide metabolism and has a vital effect on the pathogenesis of depression and BC [22][23]. Several studies have indicated that upregulation of sEH is closely related to neurological disorders [24]. A decrease in sEH level is also found in BC tissues, whereas an increase in sEH level inhibited BC proliferation. Other scholars have shown that sEH can promote BC cell proliferation by hydrolyzing toxic epoxides, which is inconsistent with previous studies [25][26]. Therefore, sEH-mediated epoxide metabolism might be a crucial area for investigation and one of the critical comorbid mechanisms of BC and depression. However, the presented evidence is controversial. According to the study, epoxide metabolism mainly occurs in the liver, and sEH may have different effects on different subtypes of BC. Furthermore, epoxide metabolism is involved in mediating immune responses and regulating lipid homeostasis in the tumor microenvironment (TME) [27]. Researchers have demonstrated that the levels of plasma interleukin 6 (IL-6) in patients with BC and depression are higher and are also regulated by sEH [28][29]. The epoxide metabolism mediated by sEH might be related to a deeper mechanism, which is the key point of the controversy.
Depression, one of the reported risk factors for cancer, is known as a comorbidity of BC. Researchers have found that BC survivors experience a high rate of depression, and the incidence of depression during and after treatment is 15% [9][30]. Current research has underlined the neurohormonal signaling system as the major shared mechanism of BC and depression. The sympathetic nervous system (SNS) and HPA are two stress responses that affect the nervous system and contribute to BC development [31]. When depression occurs, chronic stressors activate the HPA axis, resulting in adrenaline and catecholamines release. Following the HPA axis activation, adrenaline activates BC-adrenergic receptors, accumulates myeloid-derived suppressor cells (MDSCs), and promotes BC development [32]. Cortisol secreted by the adrenal cortex promotes BC cell development by activating the glucocorticoid receptor (GR) signaling pathway, serum/glucocorticoid-regulated kinase 1 (SGK1), and mitogen-activated protein kinase phosphatase 1 (MKP1)/dual-specific phosphatase 1 (DUSP1) [33]. At the same time, cortisol leads to a reduction in tumor immunosurveillance by suppressing immune function with decreased natural killer (NK) cell activity and T cell proliferation [34] (Figure 1).
Further, depression is associated with BC partly due to the increase in macrophage activity induced by depressive phenotypes. The M1 macrophages are an important factor in inflammation in patients with severe neurological disorders [35]. Research on major depressive disorder found elevated levels of circulating cytokines in peripheral blood mononuclear cells (PBMCs), as well as increased levels of NF-kB in PBMCs [36]. Adipocytes and BC tumor cells release chemokines (e.g., C-C motif chemokine ligand 2 (CCL2), C-C motif chemokine ligand 5 (CCL-5), or colony-stimulating factor (CSF-1)) to promote the migration of monocytes and macrophages into the BC microenvironment [37][38]. These macrophages contain M1 and M2 phenotypes, while the M1 macrophages are always transformed into M2 within the BC microenvironment, and so are the monocytes [39][40][41][42]. Adipocytes in the breast stroma are an important source of interleukin 10 (IL-10), which also contributes to the polarization of macrophages to the M2 phenotype in BC [43][44][45]. Clinical studies have also indicated that IL-10 is an independent factor in poor prognosis in TNBC, ER-negative, or PR-negative cases [46][47].
Chronic systemic inflammation induced by prolonged stress in depression has clearly been shown to be an initiating factor in carcinogenesis [48]. IL-6, one of the proinflammatory cytokines, is a signaling promoter and pathological product of depression [49]. Studies have shown that high levels of IL-6 are related to the chronic course of depression, and the severity of depression in patients with high expression of IL-6 is increased as well. The research results of elderly patients with depression show higher levels of IL-6 than in healthy elderly people [50]. Likewise, IL-6 also plays an instrumental procancer role in BC. Clinical evidence indicates that IL-6 induction is associated with a poor prognosis for a patient with BC, with plasma IL-6 levels showing a positive correlation with pathological grade. A preclinical study derived that the IL-6/IL-6R/gp130 pathway promotes the growth and metastasis of BC, while inhibiting the pathway is not conducive to the development of BC. Therefore, IL-6 may contribute to BC and depression in comorbid states. Additionally, TNF-α is a pathogenic cytokine in depression. One study found that the levels of proinflammatory cytokines TNF-α and IL-6 in patients with major depression increased significantly [51]. Meanwhile, anti-TNF-α drugs are found to be antidepressants [52]. The dual effect of TNF-α on BC is discussed as well [53]. The immune response further suggests that chronic inflammation is an important basis for depression and BC comorbidity. The fact of the hormone regulation and cytokine effect have been widely mentioned, but the intermediate stage of the pathogenesis of the comorbidity is still unclear.