Paclitaxel is a chemotherapeutic agent that is commonly used to treat diverse solid tumours and has been associated with cognitive decline (30). However, the effect of paclitaxel in the context of cancer and cancer resection surgery has not been described. To determine if there is a cumulative long-term impact on cognition, we developed a mouse model of cancer survivorship to evaluate long-term memory in mice after the resection of a cancer and treatment with adjuvant chemotherapy. Evaluation of the model demonstrated a sustained impact of cancer, surgery and adjuvant paclitaxel on the brain. 37 days after cessation of chemotherapy, mice showed memory impairment. This was found in tests that examine spatial memory (Y Maze, NOPR) and recognition memory (NOPR). Previous studies by us and others show that memory and inflammation is impacted by each of these single events: having a cancer (21, 28, 31–34), cancer surgery (35, 36), and treatment with paclitaxel (18–20, 22). The findings of the current study extend these observations by demonstrating a cumulative effect of cancer and chemotherapy on memory and brain inflammation that results in long-term impairment.
Using this model of paclitaxel chemotherapy in cancer naïve mice, we previously observed no change in brain cytokine mRNA or protein (22). The findings here indicate that cancer and surgery sensitise the brain to chemotherapy-induced neuroinflammation, which persists long after chemotherapy treatment has ended. The discovery that paclitaxel exacerbated Tnfa and Il6 mRNA expression in the hippocampus, supports the contention of a synergistic effect of cancer and chemotherapy on brain inflammation. These findings are supported by previous research. Consistent with this, using a mouse model of mammary carcinoma, Pyter et al. (2014) demonstrated that peripheral tumours can prime or sensitise the brain to inflammation which resulted in an exacerbated inflammatory response to lipopolysaccharide (32). Here, we confirm the capacity for tumours and cancer surgery to prime the brain and extend these findings to a clinically relevant context by demonstrating that a peripheral tumour and cancer surgery can prime the brain for an exaggerated neuroinflammatory response to chemotherapeutic agents such as paclitaxel long after the tumour has been resected. To control for the impact of tumour burden on inflammation, in this study we ensured uniform tumour sizes between paclitaxel and placebo treatment groups, and resected all tumours at the same time. Future studies should examine whether cancer stage or tumour size influences the magnitude of brain sensitization to chemotherapy.
The findings suggest that brain-resident macrophages may contribute to neural inflammation in long-term cancer survivor mice. While paclitaxel specifically exacerbated Tnfa and Il6 mRNA, all cytokines assessed were elevated in mice that previously had tumours, and this was sustained 58 days after tumour resection compared with cancer-naïve mice. Iba1 was similarly increased following cancer and cancer treatment, suggesting increased proliferation or activation of macrophages in the hippocampus consistent with findings demonstrating a synergistic effect of mastectomy and cancer in another preclinical breast cancer model (35). Spleen weights did not differ by experimental endpoint suggesting that peripheral myeloid cell expansion and inflammation caused by 67NR tumours may have subsided following primary tumour resection. Comparison between 19 week and 8 week old cancer naïve mice indicated that age did not contribute to neuroinflammation. Taken together, these findings suggest that brain resident macrophages remain activated and may drive sustained elevations in hippocampal cytokines thus creating a new “baseline” level of brain cytokines.
In future studies it will be important to understand the role of hippocampal TNFa and IL6 in cancer-related CI (Fig. 1). Both TNF-α and IL-6 or their receptors have been reported to still be elevated in the blood of breast cancer survivors 1–12 years after chemotherapy, and are associated with reduced hippocampal volume and cognitive impairment (2). Those clinical findings suggest that the sustained increase in circulating TNF-α and IL-6 in breast cancer survivors may serve as biomarkers for elevated hippocampal inflammation long after cancer treatment. TNF-α and IL-6 play important roles in hippocampal-dependent memory by impeding long-term potentiation (LTP). IL-6 can inhibit LTP by reducing MAPK/ERK through prolonged activation of STAT3 (37, 38). High levels of TNF-α can affect neural transmission by causing an imbalance between excitatory and inhibitory transmission by enhancing PI3K-dependent AMPA trafficking to increase Ca2+ influx and decreasing incorporation of GABAA receptor to reduce inhibitory transmission. This can lead to apoptosis of hippocampal neurons (20, 38–40). In the current study neither of the neuronal and neurogenesis related genes Rfox3 or Bdnf were reduced in paclitaxel treated mice, suggesting that the elevations in Tnfa and Il6 may not be severe enough to cause long-term neuronal death. However, future studies should confirm these mRNA findings using protein homogenate and immunohistochemistry techniques. Nevertheless, these findings support the evaluation of TNF-α and IL-6 neutralizing antibodies as potential therapeutics for cancer survivors who suffer paclitaxel chemotherapy-induced cognitive impairment. In support of this contention, treatment with the TNF-α inhibitor thalidomide during paclitaxel treatment has been shown to prevent spatial memory impairment and neuron apoptosis in the hippocampus (20).
In contrast to clinical studies, which use cognitive function after cancer diagnosis or cancer surgery as the baseline to determine dysfunction induced by chemotherapy, here, we compared changes to a true pre-cancer baseline, in addition to between-subjects analysis of paclitaxel treated mice with placebo-treated controls. Paclitaxel treatment prolonged spatial memory impairment measured by the Y maze test whereas placebo treated mice eventually returned to baseline function. However, regardless of chemotherapy treatment, cancer and cancer treatment caused memory impairment in the Y maze test for several weeks and impairment in the NOPR test was sustained in both paclitaxel and placebo treated mice. In conjunction with the widespread hippocampal inflammatory response caused by tumours, these memory findings suggest that cancer and cancer surgery contribute to the profound long-term changes in brain function in addition to chemotherapy. However, the findings of this study may be confounded by additional risk factors including 3 months of ageing across the course of the experiment, and the stress of multiple injections. For instance, studies have shown that prior stresses amplify subsequent inflammatory responses: mice exhibited higher inflammatory responses to stress (induced by injection of saline or tail suspension) 14 days after primary tumour resection compared with their control groups (35, 36). Nevertheless, the findings of this study suggest that longitudinal clinical studies that assess cognitive changes in cancer patients or survivors after adjuvant chemotherapy compared to a pre-chemotherapy baseline, may fail to observe quantifiable cognitive decline relative to cognition at cancer diagnosis. This bias may lead some studies to underestimate or underdiagnose cognitive impairment in cancer survivors after adjuvant chemotherapy.