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Abstract

Lipid mobilization in long-distance flying insects has revealed a novel concept for lipid transport in the circulatory system during exercise. Similar to energy generation for sustained locomotion in mammals, the work accomplished by non-stop flight activity is powered by oxidation of free fatty acids (FFA) derived from endogenous reserves of triacylglycerol. The transport form of the lipid, however, is diacylglycerol (DAG), which is delivered to the flight muscles associated with lipoproteins. In the insect system, the multifunctional lipoprotein, high-density lipophorin (HDLp) is loaded with DAG while additionally, multiple copies of the exchangeable apolipoprotein, apoLp-III, associate with the expanding particle. As a result, lipid-enriched low-density lipophorin (LDLp) is formed. At the flight muscles, LDLp-carried DAG is hydrolyzed and FFA are imported into the muscle cells for energy generation. The depletion of DAG from LDLp results in the recovery of both HDLp and apoLp-III, HDLp, identified which are reutilized for another cycle of DAG transport. A receptor for as a novel member of the vertebrate low-density lipoprotein (LDL) receptor family, does not seem to be involved in the lipophorin shuttle mechanism operative during flight activity. In addition, endocytosis of HDLp mediated by the insect receptor does not seem to follow the classical mammalian LDL pathway. Many structural elements of the lipid mobilization system in insects are similar to those in mammals. Domain structures of apoLp-I and apoLp-II, the non-exchangeable apolipoprotein components of HDLp, are related to apoB 100. ApoLp-III is a bundle of five amphipathic alpha-helices that binds to a lipid surface very similar to the four-helix bundle of the N-terminal domain of human apoE. Despite these similarities, the functioning of the insect lipoprotein in energy transport during flight activity is intriguingly different, since the TAG-rich mammalian lipoproteins play no role as a carrier of mobilized lipids during exercise and besides, these lipoproteins are not functioning as a reusable shuttle for lipid transport. On the other hand, the deviant behavior of similar molecules in a different biological system may provide a useful alternative model for studying the molecular basis of processes related to human disorders and disease.

Identification of a Novel Hemolymph Peptide That Modulates Silkworm Feeding Motivation

“…In addition, HemaP is abundant in the hemolymph (ϳ20 M). Although the complete amino acid sequence of HemaP shows no significant similarity with other proteins, similar ␣-helical characteristics and partial sequence similarities have been observed in other hemolymph proteins, such as lipophorins (insect lipoproteins) (27) and odorantbinding proteins (28). Therefore, HemaP might associate with some compounds possibly related to nutrients or metabolites.…”

Multifaceted biological insights from a draft genome sequence of the tobacco hornworm moth, Manduca sexta

“…The main insect lipoprotein, lipophorin, is the carrier of the secreted DG (Beenakkers et al, 1985). In contrast to the apolipoprotein B (apoB) containing lipoproteins of vertebrates, lipophorin particles can pick up and deliver DG molecules without compromising the integrity of the apolipoproteins (Downer and Chino, 1985; van der Horst et al, 2002). Liver and intestine of vertebrates release triglycerides (TG) into circulation, but this process requires the concomitant synthesis of apoB and the intracellular assembly of lipoproteins.…”

Lipophorin acts as a shuttle of lipids to the milk gland during tsetse fly pregnancy

“…Lipophorins (Lp) are critical in insects for lipid transport between tissues (Ryan and van der Horst, 2000; Soulages and Wells, 1994; Shapiro et al,1988; Van der Horst and Rodenburg, 2010; Van der Horst et al, 2002). The Lp gene is expressed as a single transcript that is cleaved into two separate proteins after translation (Sundermeyer et al 1996; Atella et al 2006).…”