High fat diets elevate adipose tissue-derived tumor necrosis factor-alpha activity

Adipose tissue-derived tumor necrosis factor-alpha (AT-TNF) has been associated with genetic models of insulin resistance and obesity. It is presently unknown if secreted AT-TNF protein is bioactive or whether it can be increased by environmentally induced obesity. In this study, male Wistar rats were fed either a low fat (LF; 12% of energy from corn oil) or a high fat (HF; 45% of energy from corn oil) diet for 5 weeks. From previous data, it is known that after 3 weeks, HF fed animals are obese and insulin resistant compared with the LF group. Hence, animals were killed at 1 week of HF feeding, during the acute response to the diet, and at 5 weeks, when differences in body fat are manifest. Weight gain was significantly increased by diet (P = 0.03) and time (P < 0.0001). AT-TNF bioactivity was measured on secreted protein collected from medium of minced, incubated epididymal (EPI), mesenteric (MES), and retroperitoneal (RETRO) fat pads. AT-TNF bioactivity was significantly increased by diet (P = 0.003) in the RETRO pad and tended to increase (P = 0.07) in EPI. AT-TNF activity was unaffected by diet or time in the MES pad. In the RETRO pad, TNF activity correlated negatively with RETRO fat cell number (r = -0.46, P = 0.002). Secreted AT-TNF protein did not correlate with AT-TNF activity but instead decreased in RETRO with time but not diet. In EPI, secreted AT-TNF protein decreased with the HF diet. Thus, these data suggest that high fat diets and obesity can influence AT-TNF bioactivity and secretion but in an apparent fat pad-specific manner.