DEVELOPMENTAL DOWN-REGULATION OF RECEPTOR-MEDIATED ENDOCYTOSIS OF AN INSECT LIPOPROTEIN

Fat body cells of insects exhibit a high-affinity lipoprotein binding site at their cell surfaces. In the present study, the lipoprotein bin ding site was identified as an endocytotic receptor involved in recept or-mediated uptake of its lipoprotein ligand, high density lipophorin. After an initial period of high endocytotic uptake of high density li pophorin in tile adult stage, this process strongly diminished. In the same period, a dramatic increase in cell surface-associated lipoprote ins was observed. When animals were starved, however , internalization of lipoproteins was maintained. The pathway followed by the internali zed 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 accum ulation of apolipoproteins was observed, suggesting that internalized lipoproteins were resecreted. It is unlikely that internalization is r equired for transport of the two major lipid components of insect lipo proteins, diacylglycerol and cholesterol, as inhibition of endocytosis neither affected the exchange of these lipids between lipoproteins an d fat body cells nor influenced the loading of diacylglycerol onto lip oproteins in response to adipokinetic hormone. We postulate that the e ndosomal environment may facilitate transport of components which, unl ike diacylglycerol and cholesterol, cannot be transported by simple aq ueous diffusion.