Chemotherapy-induced cognitive impairment is associated with decreases in cell proliferation and histone modifications

Abstract

Background: In this study, we examined the effects of cyclophosphamide, methothrexate, and 5-Fluorouracil (CMF) drug combination on various aspects of learning and memory. We also examined the effects of CMF on cell proliferation and chromatin remodeling as possible underlying mechanisms to explain chemotherapy-associated cognitive dysfunction. Twenty-four adult female Wistar rats were included in the study and had minimitter implantation for continuous activity monitoring two weeks before the chemotherapy regimen was started. Once baseline activity data were collected, rats were randomly assigned to receive either CMF or saline injections given intraperitoneally. Treatments were given once a week for a total of 4 weeks. Two weeks after the last injection, rats were tested in the water maze for spatial learning and memory ability as well as discrimination learning. Bromodeoxyuridine (BrdU) injection was given at 100 mg/Kg intraperitoneally 4 hours prior to euthanasia to determine hippocampal cell proliferation while histone acetylation and histone deacetylase activity was measured to determine CMF effects on chromatin remodeling.

Results: Our data showed learning and memory impairment following CMF administration independent of the drug effects on physical activity. In addition, CMF-treated rats showed decreased hippocampal cell proliferation, associated with increased histone acetylation and decreased histone deacetylase activity.

Conclusions: These results suggest the negative consequences of chemotherapy on brain function and that anti-cancer drugs can adversely affect the self-renewal potential of neural progenitor cells and also chromatin remodeling in the hippocampus. The significance of our findings lie on the possible usefulness of animal models in addressing the clinical phenomenon of 'chemobrain.'

Figure 1

Study Design. Legends: CMF - cyclophosphamide, methothrexate, and 5-Fluorouracil.

Figure 2

Physical Activity and Body Weight. General physical activity significantly decreased in the CMF group compared to the saline group during the treatment period but fatigue decreased once the drugs were discontinued (A). In contrast, no weight loss was seen in the CMF group during the treatment period (B). However, the CMF group failed to normally gain weight compared to the saline group although they remained healthy throughout the study and no overall reduction in food intake was observed. *p < 0.05.

Figure 3

Spatial Learning and Memory. Significantly increased mean swim latency (A) and path taken to reach the goal (B) was seen in the CMF group. However, these rats eventually performed as well as the saline group in the spatial learning and memory tasks on the last day of testing. *p < 0.05.

Figure 4

Discrimination-Learning Test. Significantly increased mean swim latency (A) and errors (B) were made by the CMF group compared to saline controls. However, all rats learned to perform the task. *p < 0.05.

Figure 5

Cell Proliferation. Representative samples of BrdU labeling seen in the dentate gyrus of the hippocampal region in the CMF and saline groups (upper panel). Scale bar = 60 μm. CMF administration significantly decreased cell proliferation in the hippocampal region (lower panel). *p < 0.05.

Figure 6

Histone Modifications in the Hippocampus. Representative samples of acetyl-H3 immunoreactivity in the hippocampus in the CMF and saline groups (upper panel). Scale bar = 40 μm. CMF administration significantly increased histone acetylation in the hippocampus, striatum, and prefrontal brain regions (lower panel, left). However, overall decreased in histone deacetylase activity was seen in the hippocampus of CMF-treated rats when compared to the saline group (lower panel, right). *p < 0.05.