Hemolymph lipoprotein transport

FIGURE 24.1 The pathway of carotenoid transport in the silkworm. Carotenoids are absorbed from dietary mulberry leaves into the intestinal mucosa, transferred to the hemolymph (1), transported in the hemolymph by plasma lipoproteins (2), and accumulated in the silk gland (3). 

It is thought that dietary carotene is transferred to the hemolymph lipoprotein, which is called lipophorin (11), at the midgut during digestion of food. It is transported to epidermal cells, where it probably associates with a different protein inside the cells. Unlike the blue component of green coloration, insects appear to be completely dependent upon dietary carotenes for the yellow component ( ). M. sexta larvae, raised on a standard laboratory diet, are distinctly blue in color, rather than green. 

Lipids are involved in a number of essential processes in the growth and reproduction of marine invertebrates. Membrane lipids, primarily phospholipids and sterols, combine with membrane proteins to form insoluble complexes that are important in membrane structure. Many marine invertebrates have oil droplets within cells of hepatic-type tissues. These droplets are primarily triacylglycerols or wax esters and serve as energy stores. Finally, lipids occur in water-soluble lipoproteins where phospholipids, triacylglycerols, and sterols are combined with various apoproteins. Lipids are transported between various tissues via hemolymph lipoproteins. Female-specific lipoproteins occur in the hemolymph and eggs of adult females of many marine invertebrates. These egg lipoproteins provide protein and lipid for the development of larvae after they hatch from the... 

The hemolymphal transport of carotenoids by lipophorin has been elucidated and documented (Law and Wells 1989, Tsuchida et al. 1998, Arrese et al. 2001, Canavoso et al. 2001), as has plasma transport by mammalian lipoproteins (Paker 1996, Yeum and Russell 2002). Lipophorin serves as a shuttle that moves carotenoids from one tissue to another without itself entering the cells, in stark contrast to the vertebrate low-density lipoprotein (LDL) (Brown and Goldstein 1986), which is endocytosed and metabolized in the cell. Here, we focus on the recent biochemical and genetic studies of the intracellular CBP of the silkworm, which mainly transports lutein. We hope this review provides insights into the studies of CBPs in other organisms. 

The third, and perhaps least understood, mechanism regulating contact pheromone production involves its transport to the cuticular surface. The detection of large amounts of hydrocarbons and pheromone internally, within the hemolymph, prompted an examination of lipid transport in B. germanica. Gu et al. (1995) and Sevala etal. (1997) isolated and purified a high density lipoprotein, lipophorin, that carries hydrocarbons, contact pheromone, and JH within the hemolymph. The accumulated evidence supports the idea that the hydrocarbons and contact pheromone components are produced by oenocytes within the abdominal integument, carried by lipophorin, and differentially deposited in the cuticle and ovaries (Fan et al.,... 

The major lipoproteins of insect hemolymph, the lipophorins, transport diacylglycerols. The apolipo-phorins have molecular masses of -250, 80, and sometimes 18 kDa.34-37a The three-dimensional structure of a small 166-residue lipophorin (apolipophorin-III) is that of a four-helix bundle. It has been suggested that it may partially unfold into an extended form, whose amphipathic helices may bind to a phospholipid surface of the lipid micelle of the lipophorin 35 A similar behavior may be involved in binding of mammalian apolipoproteins. Four-helix lipid-binding proteins have also been isolated from plants.38 See also Box 21-A. Specialized lipoproteins known as lipovitellins... 

The transport of hydrocarbons by social insects can be involved in creating the hydrocarbon signature . Evidence was first obtained in the termite Zootermopsis nevaden-sis (Sevala et al., 2000). Comparison of cuticular lipids with internal and hemolymph hydrocarbons in different castes showed that, as in other species, the content was qualitatively similar. However, quantitative differences were observed between hemolymph and cuticular hydrocarbon profiles. Sevala et al. (2000) showed that hemolymph hydrocarbons were associated with a dimeric high-density lipoprotein (HDLp) lipophorin, similar to those described from other insects (see above). This lipoprotein consisted... 

Insects use camouflage coloration as a means of avoiding predation. The green color of the tobacco hornworm larvae, (Manduca sexta) can be separated into constituent blue and yellow components. The water soluble blue component is the biliprotein, insecticyanin. The yellow color is derived from lipoprotein bound carotenes. This lipoprotein, lipophorin, is the major lipid transport vehicle in insect hemolymph. In addition to transporting dietary lipid, lipophorin is also involved in the transport of lipophilic insecticides. Nearly all the recovered radioactivity in hemolymph from topically applied [14c] ddt is associated with lipophorin. Lipophorin of adult M. sexta is larger, less dense and is associated with small amounts of a third, adult specific, apoprotein. Alterations in adult lipophorin density, lipid content and apoprotein stoichiometry can be caused by injection of the decapeptide, adipokinetic hormone. 

The yellow carotene binding protein of M. sexta hemolymph is a more complicated case. Carotenes are extrerraTTy water-insoluble materials. They share this property with several other natural products including sterols, fats and hydrocarbons, all of > hich are important to insects. This property is also shared by many xenobiotics, including pesticides. Transport of hydrophobic materials within the aqueous compartments of living organisms, e.g. blood or hemolymph, is accomplished by lipoproteins. Extensive... 

The function of apoLp-IIl is to facilitate transport of lipid from sites of storage in the fat body to sites of utilization in certain metabolic situations, e.g., flight. The triacylglycerol stores of the fat body are converted to DG, which leaves the fat body and becomes associated with preexisting HDLp in the hemolymph. In the process, HDLp is converted to LDLp and several molecules of apoLp-III become associated with LDLp. LDLp moves to the flight muscle, where the DG is hydrolyzed by a lipoprotein lipase. As the DG is removed, LDLp is converted back to HDLp and apoLp-III dissociates. The HDLp and apoLp-III then cycle back to the fat body to carry more DG (see Section V for details). 

Teshima S-I, Kanazawa A (1978) Hemolymph lipids of the prawn. Bull Jpn Soc Sci Fish 44 925 Teshima S-I, Kanazawa A (1980) Transport of dietary lipids and role of serum lipoproteins in the prawn. Bull Jpn Soc Sci Fish 46 51-55... 

---