HUMAN APOLIPOPROTEIN-E MEDIATES PROCESSIVE BUOYANT LIPOPROTEIN FORMATION IN INSECT LARVAE

The expression of human apolipoprotein E in tobacco hornworm larvae causes a dramatic change in the buoyant density of the insect's endogenous lipoproteins. Larvae without apoE have lipoproteins that are found exclusively in the high-density range. Baculovirus-mediated apoE expression results in the conversion of approximately one-fourth of the endogenous lipoproteins to low-density species. This density conversion is progressive and parallels a similar change in apoE density distribution. ApoE is secreted from the lipoprotein producing fat body tissue in a lipid-poor form, but readily associates with circulating insect lipoproteins in the hemolymph where the density conversion takes place. Analysis of the buoyant lipoprotein particles indicates that they contain apoE and insect apolipophorins I and II with few or no other proteins present. Immunoprecipitation of apolipophorins I and II results in coprecipitation of apoE. This association is disrupted by detergent, consistent with the three proteins sharing the same lipoprotein particles. The ability of apoE to influence buoyant lipoprotein formation in an invertebrate system leads us to suggest that small apolipoproteins such as apoE may play a role in buoyant lipoprotein production in mammals.