In vitro evidence for an inhibitor of lipogenesis in serum from overfed obese rats

Involvement of a humoral agent in regulation of energy balance has been demonstrated by parabiosis experiments. Overfed obese rats produce a blood-borne factor that inhibits adipose fatty acid synthesis in their partners, resulting in loss of body fat without significant inhibition of food intake. An in vitro bioassay was developed to test small serum samples for antilipogenic activity. Epididymal adipocytes from ad libitum-fed rats were preincubated for 12 h with 2% serum. Basal adipocyte fatty acid synthesis, measured in a subsequent serum-free incubation, was inhibited by obese serum. Insulin response was not changed. Characterization studies indicated that the factor was probably a protein, larger than 30 kDa, but not a protease or a low-density lipoprotein and was not associated with serum albumin. Physiological experiments demonstrated that the agent was produced when body weight was raised substantially above 'set-point'. Inhibitory activity was neither species specific nor pituitary dependent. Structure, origin, and physiological significance of the factor are unknown, but it may be involved in the control of body fat content.