Developmental down-regulation of receptor-mediated endocytosis of an insect lipoprotein

Abstract

Fat body cells of insects exhibit a high-affinity lipoprotein binding site at their cell surfaces. In the present study, the lipoprotein binding site was identified as an endocytotic receptor involved in receptor-mediated uptake of its lipoprotein ligand, high density lipophorin. After an initial period of high endocytotic uptake of high density lipophorin in the adult stage, this process strongly diminished. In the same period, a dramatic increase in cell surface-associated lipoproteins was observed. When animals were starved, however, internalization of lipoproteins was maintained. The pathway followed by the internalized lipoproteins appears to be different from the endosomal/lysosomal pathway, as the vast majority of apolipoproteins seemed to escape from lysosomal hydrolysis. In addition, no substantial intracellular accumulation of apolipoproteins was observed, suggesting that internalized lipoproteins were resecreted. It is unlikely that internalization is required for transport of the two major lipid components of insect lipoproteins, diacylglycerol and cholesterol, as inhibition of endocytosis neither affected the exchange of these lipids between lipoproteins and fat body cells nor influenced the loading of diacylglycerol onto lipoproteins in response to adipokinetic hormone. We postulate that the endosomal environment may facilitate transport of components which, unlike diacylglycerol and cholesterol, cannot be transported by simple aqueous diffusion.