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.
Highlights
Fat body cells of insects exhibit a high-affinity lipoprotein binding site at their cell surfaces
Lipid mobilization is pertinent especially during sustained flight activity of migratory insects [4, 7], when High density lipophorin (HDLp) is loaded at the fat body to the larger molecular mass low density lipophorin (LDLp) by adding DAG derived from the lipid stores in the fat body cells and several molecules of an exchangeable apolipoprotein present in the hemolymph, apoLp-111, in response to adipokinetic hormones (AKHs) [8].fat body cells are able to selectively extract DAG from HDLp but they can load HDLp with DAG cargo
We have recently demonstrated that the number of cell surface-localized binding sites increases during the adult stage whereas the total number of binding sites appears to remain constant [11].Based on these observations a model was postulated that implies that a part of the HDLp binding sites is present on intracellular membranes, whereas afterwards, ca. day 7 after the imaginal ecdysis, a redistribution of binding sites occurs from an intracellular pool to the plasma membrane
Summary
Fat body tissue from young and older adults, 4 and 15days after the imaginal ecdysis,respectively,and from. DAG, cholesterol, and apolipoprotein uptake were teins were incubated with a large molar excess of fluo- measured by incubating fat body tissue in medium A at rescein succinimidylester (protein/fluorescein: 1/100) 30% with HDLp that was radioactively labeled in the in 0.1 M boric acid (pH 8.5) for 1h at 4°C. Degradation of HDLp was quantified by incubating tion of hemolymph collected from adult locusts that fat body tissue with HDLp tritiated in the protein moiety had been fed 2 pCi [9,10-‘-Hltriolein The incubation medium and an isolation from hemolymph of locusts that had been in- aliquot of the tissue homogenate were precipitated with jected 10 min prior to collection with 2 pCi [1,2- 10% trichloroacetic acid for 30 min at 4°C and centri-. The amount of fat body tissue used in the incubations was quantified by determining total protein content after a chloroform-methanol extraction as described previously (1l ) using the protein determination method of Schacterle and Pollack [17]
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
