Cancer activates microglia to the same extent as chronic stress throughout stress neurocircuitry in a mouse model of breast cancer

The Interplay of Chronic Stress and Cancer: Pathophysiology and Implications for Integrated Care

BACKGROUND

Cancer-associated depression is a multifaceted condition that arises from the interplay of biological, psychological, and social factors in individuals diagnosed with cancer. Understanding this condition involves exploring how cancer and its treatments can precipitate depressive symptoms and the mechanisms behind this association. Chronic stress, inflammation, and immunological responses play a crucial role in the development of both cancer and depression. The objective of this review is to describe and synthesize information on the complex interactions between chronic stress, inflammation, immunological responses, and cancer development. Additionally, it aims to review existing evidence regarding mechanisms such as neurotransmitter imbalances, structural brain changes, and genetic predispositions as key contributors to depression in cancer patients.

RECENT FINDINGS

A comprehensive literature search on Cancer-associated Depression was conducted in electronic databases, including APA PsycINFO, Medline, Google Scholar, Embase, PubMed, Scopus, and Web of Science. The research focused on understanding the potential relationship between stress-induced depression and cancer by examining neurochemical, anatomical, immunological, genetic, and psychological changes. The findings revealed a compilation of both quantitative and qualitative studies on depression in cancer patients. Evidence suggested a potential link between cancer-induced stress and depression, with increased levels of proinflammatory cytokines (such as IL-6) and dysregulation of neurotransmitters, including serotonin, contributing to the onset of depression. Furthermore, studies indicated that antidepressants, along with psychological interventions, were effective in managing depression among cancer patients.

CONCLUSION

This narrative review provides insights into the importance of integrating oncology and mental health services to address the psychosocial needs of cancer patients. Future research should focus on the bidirectional interactions between stress and cancer, aiming to improve cancer care by incorporating mental health support. Addressing the mental health aspects of cancer treatment can significantly enhance patient outcomes and overall quality of life.

Blood IL-1α and IL-6 predict specific breast cancer-induced increases in hippocampal pro-inflammatory cytokines in mice

Neuroinflammation is a key factor in cognitive and behavioral changes seen in patients with non-CNS cancers, and cytokine levels in the blood are often used as a proxy for brain inflammation. However, this approach has yielded inconsistent results, and a common inflammatory signature remains elusive. To explore whether a blood-to-brain inflammatory signature exists across breast cancer types, we assessed cytokine and glial protein responses in the hippocampus, prefrontal cortex (PFC), and their relationship to serum cytokines in mice bearing three different mammary cancers (n = 40). While cytokine profiles in both serum and brain varied by cancer type, IL-1β and IL-4 were consistently altered across brain regions. In some cases, elevated serum IL-1α and IL-6 correlated with increased hippocampal IL-6. These findings support the use of blood cytokines to identify cancer patients at risk for cognitive and psychiatric comorbidities. However, our data also suggest that relying solely on serum cytokines may lead to under-diagnosis, as some mice exhibited brain cytokine elevations without changes in serum levels. This underscores the need for a broader range of inflammatory markers in blood to better identify at-risk patients. Brain region-specific differences in the cytokine response to mammary cancer highlighted the hippocampus as more vulnerable to cancer-induced inflammation than the PFC. We observed region-specific glial cell reactivity, however, only astrocyte and oligodendrocyte markers were correlated with cytokine changes within the hippocampus. Elevated serum IL-1α and IL-6 were correlated with reduced cortical astrocyte reactivity, suggesting that these cytokines can inform glial cell-specific changes in this region.

Microglia contribute to mammary tumor-induced neuroinflammation in a female mouse model

Following diagnosis but before treatment, up to 30% of breast cancer patients report behavioral side effects (e.g., anxiety, depression, memory impairment). Our rodent mammary tumor model recapitulates aspects of these behavioral sequelae, as well as elevated circulating and brain inflammatory mediators. Neuroinflammation is a proposed mechanism underlying the etiology of mood disorders and cognitive deficits, and therefore may be contributing to tumor-associated behavioral side effects. The cellular mechanisms by which tumor-induced neuroinflammation occurs remain unknown, making targeted treatment approaches inaccessible. Here, we tested the hypotheses that microglia are the primary cells driving tumor-induced neuroinflammation and behavioral side effects. Young adult female BALB/c mice were induced with a 67NR mammary tumor; tumor-free controls underwent a sham surgery. Mammary tumors increased IBA1+ and GFAP+ staining in the amygdala and hippocampus relative to tumor-free controls. However, tumors did not alter gene expression of Percoll-enriched microglia isolated from the whole brain. While cognitive, social, and anhedonia-like behaviors were not altered in tumor-bearing mice, tumors increased central tendency in the open-field test; microglia depletion did not reverse this effect. Brain region RT-qPCR data indicated that microglia depletion attenuated tumor-induced elevations of neuroinflammatory gene expression in a region- and mediator-specific manner. These results indicate a causal role of microglia in tumor-induced neuroinflammation. This research advances our understanding of the cellular mechanisms underlying tumor-induced neuroinflammation in order to understand how brain responses (e.g., behavior) may be altered with subsequent cancer-related immune challenges.

Interplay between stress and cancer-A focus on inflammation

Stress is an integral part of life. While acute responses to stress are generally regarded as beneficial in dealing with immediate threats, chronic exposure to threatening stimuli exerts deleterious effects and can be either a contributing or an aggravating factor for many chronic diseases including cancer. Chronic psychological stress has been identified as a significant factor contributing to the development and progression of cancer, but the mechanisms that link chronic stress to cancer remain incompletely understood. Psychological stressors initiate multiple physiological responses that result in the activation of the hypothalamic-pituitary-adrenal (HPA) axis, sympathetic nervous system, and the subsequent changes in immune function. Chronic stress exposure disrupts the homeostatic communication between the neuroendocrine and immune systems, shifting immune signaling toward a proinflammatory state. Stress-induced chronic low-grade inflammation and a decline in immune surveillance are both implicated in cancer development and progression. Conversely, tumor-induced inflammatory cytokines, apart from driving a tumor-supportive inflammatory microenvironment, can also exert their biological actions distantly via circulation and therefore adversely affect the stress response. In this minireview, we summarize the current findings on the relationship between stress and cancer, focusing on the role of inflammation in stress-induced neuroendocrine-immune crosstalk. We also discuss the underlying mechanisms and their potential for cancer treatment and prevention.