Multiple interactions between insect lipoproteins and fat body cells: extracellular trapping and endocytic trafficking.

The binding and internalization of a circulating insect lipoprotein, high density lipophorin (HDLp), by insect fat body cells was studied at the electron-microscopic level using ultrasmall gold-labeled HDLp and DiI-labeled HDLp, which were visualized by silver enhancement and diaminobenzidine photoconversion, respectively. Internalization of HDLp seems to conflict with the selective process by which the lipids are transported between HDLp and fat body cells. The pathway followed by the internalized lipoproteins was investigated. In addition, the localizations of HDLp in fat body cells of young and older adult locusts were compared because of the previously reported age-related differences in distribution of cell-associated and internalized HDLp. In the present study, internalized labeled HDLp was observed in early endosomes, late endosomes, and putative lysosomes. In older adults, these labeled structures were much less abundant than in young adults. Moreover, in these animals, the labeled endosomal/lysosomal vesicles were located close to the plasma membranes. A more intense labeling was observed in the extracellular matrix in older adults compared to young adults. In both developmental stages, an apparent accumulation of labeled HDLp was found in extracellular spaces. We propose that this entrapment of HDLp may be essential for selective lipid transport between HDLp and fat body cells.