Tumor growth increases neuroinflammation, fatigue and depressive-like behavior prior to alterations in muscle function

Abstract

Cancer patients frequently suffer from fatigue, a complex syndrome associated with loss of muscle mass, weakness, and depressed mood. Cancer-related fatigue (CRF) can be present at the time of diagnosis, during treatment, and persists for years after treatment. CRF negatively influences quality of life, limits functional independence, and is associated with decreased survival in patients with incurable disease. Currently there are no effective treatments to reduce CRF. The aim of this study was to use a mouse model of tumor growth and discriminate between two main components of fatigue: loss of muscle mass/function and altered mood/motivation. Here we show that tumor growth increased fatigue- and depressive-like behaviors, and reduced body and muscle mass. Decreased voluntary wheel running activity (VWRA) and increased depressive-like behavior in the forced swim and sucrose preference tests were evident in tumor-bearing mice within the first two weeks of tumor growth and preceded the loss of body and muscle mass. At three weeks, tumor-bearing mice had reduced grip strength but this was not associated with altered expression of myosin isoforms or impaired contractile properties of muscles. These increases in fatigue and depressive-like behaviors were paralleled by increased expression of IL-1β mRNA in the cortex and hippocampus. Minocycline administration reduced tumor-induced expression of IL-1β in the brain, reduced depressive-like behavior, and improved grip strength without altering muscle mass. Taken together, these results indicate that neuroinflammation and depressed mood, rather than muscle wasting, contribute to decreased voluntary activity and precede major changes in muscle contractile properties with tumor growth.

Keywords: Cancer; Cytokines; Depression; Fatigue; Minocycline; Neuroinflammation.

Figure 1. Tumor growth was associated with muscle mass loss and muscle weakness

Mice were inoculated s.c. with PBS (Control) or PBS with C26 adenocarcinoma cells (Tumor). A) Tumor mass, B) body mass, and C) gastrocnemius muscle mass was determined 1, 2, and 3 weeks after inoculation (n=6). D) Grip strength (Newtons) was determined at baseline (day 0) and then again at 2 and 3 weeks (n=8). E) The normalized grip strength (Newtons of grip strength to kilograms of body mass) was also determined at the 3 week endpoint (n=8). Data are expressed as mean ± SEM. Means with * are different from control mice (p<0.05).

Figure 2. Tumor growth increased fatigue- and depressive-like behavior

A) Voluntary wheel running activity (VWRA) was determined overnight at baseline and again at 1, 2, and 3 weeks after tumor cell inoculation. B) Sucrose preference was determined 1, 2, and 3 weeks after tumor cell inoculation. C) Home cage activity was determined 2 weeks after tumor inoculation. Control and tumor mice were exposed to the forced swim test (FST) at 2 weeks and D) latency to become immobile and E) total time immobile were determined (n=6). Bars represent the mean ± SEM. Means with * are different from control (p<0.05).

Figure 3. Increased IL-1β and IL-6 mRNA expression in the brain of tumor-bearing mice

IL-1β mRNA expression was determined in the A) hippocampus (HPC) and B) cortex (CX) 2 and 3 weeks after tumor cell injection. IL-6 mRNA was also determined in the C) HPC and D) CX (n=6). E) Representative images of Iba-1 labeling of microglia in the HPC and CX collected 3 weeks after tumor cell inoculation. Inset includes enlarged image of Iba-1⁺ cell indicated by white arrow. Proportional area for Iba-1 labeling in the F) HPC and G) CX (n=6). Data are expressed as mean ± SEM. Means with * are different from control mice (p<0.05) and means with ‡ are different from Tumor-Week 2 (p<0.05).

Figure 4. Minocycline attenuated depressive-like behavior and restored grip strength in tumor-bearing mice

Control and Tumor-bearing mice were administered vehicle or minocycline (Mino) in the drinking water starting one day after tumor cell injection. A) Control and Tumor mice were exposed to the forced swim test at 2 weeks and total time immobile was determined. B) Grip strength was assessed and normalized to body mass at the 3 week endpoint. C) Tumor mass, D) muscle mass, and E) plasma IL-6 levels were determined at 3 weeks. Means with * are different from control mice (p<0.05), means with ‡ are different from Tumor-Vehicle mice (p<0.05) (n=8–12).

Figure 5. Minocycline attenuated neuroinflammation in tumor-bearing mice

Control and Tumor-bearing mice were administered vehicle or minocycline (Mino) in the drinking water. A–D) IL-1β and IL-6 mRNA expression was determined in the HPC and CX at 3 weeks. Data are expressed as mean ± SEM. Means with * are different from control mice (p<0.05), means with + tend to be different from control mice (p=0.07) (n=8–12).