Microglia contribute to cancer cachexia through affecting PVN neurons and POMC neurons
Authors: Kashihara Soichiro , Shinohara Keisuke , Ikeda Shota , Tsutsui Hiroyuki 
Published: April 2020

Background

Cancer cachexia is a state of involuntary weight loss, muscle wasting, and severe malnutrition. Browning of adipocytes and anorexia are major contributors to cancer cachexia through thermogenesis‐induced increasing energy consumption and decreasing food intake, respectively. Increased neural activity of hypothalamic paraventricular nucleus (PVN) is crucially involved in sympathoexcitation causing the browning of adipocytes. Stimulation of pro‐opiomelanocortin (POMC) neurons in arcuate nucleus (ARC) of hypothalamus negatively regulates feeding. Activation of microglia in the hypothalamus can be associated with activity of PVN neurons and POMC neurons. Although microglial activation has been observed in systemic inflammatory animal models, it remains unclear whether microglia contribute to cachexia through affecting PVN neurons and POMC neurons in the cancer model with cancer‐associated systemic inflammation.

Methods and Results

We implanted AH‐130 hepatoma cells intraperitoneally in Wistar‐Hannover rats to create an established cancer cachexia model exhibiting systemic inflammation. To examine the activity of microglia in hypothalamus, Iba1 immunostaining was performed at day 10 after implantation. In both PVN and ARC, a decreased cell perimeter and an increased cell circularity of microglia were observed in tumor‐bearing rats compared with controls (perimeter, 920.5±77.5 vs 1395±49.7 μm; circularity, 1.20±0.11 vs 0.80±0.04 *10−2; n=2, 5; p<0.05), suggesting microglial activation in cancer cachexia. Then, to test whether the microglial activation contributes to cachexia and prognosis in tumor‐bearing rats, continuous intracerebroventricularinfusion of minocycline, an inhibitor of microglial activation, or vehicle was initiated 3 days before tumor implantation (tumor‐mino, n=8; tumor‐vehicle, n=8). As a control, the vehicle intracerebroventricularinfusion was performed without tumor implantation (control, n=6). We evaluated survival rate up to 12 days after implantation and obtained the tissues from the survived rats at day 12. Minocycline showed a trend increasing survival in tumor‐bearing rats. Food intake had been lower in tumor‐vehicle group than in control group from day 1. Minocycline attenuated the decrease of food intake in tumor‐bearing rats (food intake at day 11, 12.3±1.04 vs 5.45±1.30 g/day; n=7, 6; p<0.05 vs vehicle treatment). Although we obtained the tissues from a limited number of tumor‐bearing rats due to the low survival rate, body weight, heart weight and skeletal muscle weight (gastrocnemius and soleus) were obviously smaller in tumor‐vehicle group than in controls. The immunostaining study demonstrated that, in tumor‐vehicle group compared with control group, microglial activity assessed by morphological analysis was increased in the PVN and ARC, and c‐Fos, a marker of neuronal activity, was also increased in the PVN neurons and ACTH‐positive POMC neurons. Minocycline attenuated these tumor‐induced body and tissue wasting, microglial activation, and neuronal activation.

Conclusion

These preliminary data suggest that microglia contribute to cancer cachexia probably through affecting sympathoexcitatory PVN neurons and anorexigenic POMC neurons.