Enterostatin - target proteins and intracellular mechanisms. Function in food intake and energy metabolism

Sammanfattning: Hunger and satiety are the results of complex neural events that involve several neurotransmitters and peptides. Enterostatin is an appetite-regulating peptide released in the intestine in response to fat ingestion. Enterostatin specifically decreases fat intake, but has also metabolic effects like inhibition of insulin secretion. Enterostatin has both central and gastrointestinal site of action, although the gastrointestinal action is dependent on vagal transmission. The target and cellular mechanisms has however not been elucidated. In this thesis, a possible target protein for enterostatin has been found in a neuronal cell line and further identified in rat brain membranes. The target protein was identified as the beta-subunit of the F1F0-ATP synthase. Furthermore, the binding of enterostatin to the protein was verified in pure F1F0-ATP synthase and was also confirmed in the insulin producing cell line INS-1. In INS-cells, the targeting of enterostatin resulted in perturbed ATP synthesis, enhanced heat production and increased oxygen consumption. Enterostatin was also shown to decrease the insulin secretion in these cells. This indicates that enterostatin is involved in the regulation of energy metabolism within the cell. The binding of enterostatin to F1-ATPase was inhibited by µ-opiates, i.e. beta-casomorphin, but not by a kappa-opiate. The inhibition of high-fat food intake by low doses of intravenous injection of enterostatin was abolished by simultaneous equimolar administration of beta-casomorphin. Higher doses of enterostatin instead increased the high-fat food intake, and there was a synergistic increase on food intake together with beta-casomorphin. The association between insulin secretion and uncoupling protein-2 (UCP2) expression was compared in INS-1 cells after long-term exposure to oleic acid. It was concluded that fatty acid-induced increase in UCP2 expression is not always associated with decreased insulin secretion.