Receptor tyrosine kinases have a single transmembrane (TM) segment that is usually assumed to play a passive role in ligand-induced dimerization and activation of the receptor. However, mutations within some of these receptors, and recent studies with the epidermal growth factor (EGF) and ErbB2 receptors have indicated that interactions between TM domains do contribute to stabilization of ligand-independent and/or ligand-induced receptor dimerization and activation. One consequence of the importance of these interactions is that short hydrophobic peptides corresponding to these domains should act as specific inhibitors. To test this hypothesis, we constructed expression vectors encoding short fusion peptides encompassing native or mutated TM domains of the EGF, ErbB2, and insulin receptors. In human cell lines overexpressing the wild-type EGF receptor or ErbB2, we observed that the peptides are expressed at the cell surface and that they inhibit specifically the autophosphorylation and signaling pathway of their cognate receptor. Identical results were obtained with peptides chemically synthesized. Mechanism of action involves inhibition of dimerization of the receptors as shown by the lack of effects of mutant nondimerizing sequences, completed by density centrifugation and covalent cross-linking experiments. Our findings stress the role of TM domain interactions in ErbB receptor function, and possibly for other single-spanning membrane proteins.

The influence of dietary (n-3) compared with (n-6) polyunsatured fatty acids (PUFA) on the lipid composition and metabolism of adipocytes was evaluated in rats over a period of 1 week. Isocaloric diets comprised 16.3 g/100 g protein, 53.8 g/100 g carbohydrate and 21.4 g/100 g lipids, the latter containing either (n-3) PUFA (32.4 mol/100 mol) or (n-6) PUFA (37.8 mol/100 mol) but having identical contents of saturated, monounsaturated and total unsaturated fatty acids and identical polyunsaturated to saturated fatty acid ratios and double bond indexes. Despite comparable food intake, significantly smaller body weight increments and adipocyte size were observed in rats of the (n-3) diet group after feeding for 1 wk. Rats fed the (n-3) diet also had significantly lower concentrations of serum triglycerides, cholesterol and insulin compared with those fed the (n-6) diet, although levels of serum glucose and free fatty acids did not differ in the two dietary groups. In the (n-6) diet group, the (n-6) and (n-3) PUFA contents of plasma triglycerides, free fatty acids and phospholipids were 30-60% higher and 60-80% lower, respectively, than in the (n-3) diet group, whereas adipocyte plasma membrane phospholipids showed a significantly higher unsaturated to saturated fatty acid ratio and greater fluidity. Glycerol release in response to noradrenaline was significantly higher in the adipocytes of rats fed the (n-3) diet, whereas the antilipolytic effect of insulin generally did not differ in the two groups. Finally, insulin stimulated the transport of glucose and its incorporation into fatty acids to a lesser extent in adipocytes of (n-3) diet fed rats compared with (n-6) diet fed rats. This reduction in the metabolic effects of insulin in rats fed a (n-3) diet for 1 wk could be related to smaller numbers and a lower binding capacity of the insulin receptors on adipocytes and/or to a lesser degree of phosphorylation of the 95 kDa beta subunit of the receptor. In conclusion, dietary intake for 1 wk of (n-3) rather than (n-6) PUFA is sufficient to induce significant differences in the lipid composition and metabolic responses to insulin of rat adipocytes.

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