Lipophorin

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. 

FIGURE 24.6 Model of the molecular function of CBP. CBP moves in the cytosol and relays carotenoid in combination with the lipophorin in the hemolymph. At the membrane, other factors might be involved in the carotenoid transport (magnification, see the text for explanation). 

We consider that there must be CBPs other than CBP in the silkworm. Lipophorin binds carotenoids (Tsuchida et al. 1998). Fujii et al. isolated a CBP of 60kDa from the larval hemolymph, and... 

Lipophorin acts as a reusable shuttle between the membrane-bound lipophorin receptors in tissues (Tsuchida and Wells 1990, Gopalapillai et al. 2006) and is not generally endocytosed in the cells (Law and Wells 1989, Arrese et al. 2001, Canavoso et al. 2001). Thus, the intracellular CBP alone seems not to be able to pick up carotenoid from the lipophorin that resides outside of the cell. Cell surface components are thought to be necessary to allow intracellular delivery of carotenoids (Figure 24.6, magnification) (Arrese et al. 2001). The lipid transfer particle (LTP) (Blacklock and Ryan 1994, Tsuchida et al. 1997) on the outer surface of membranes and unknown membrane-spanning factors that specifically transfer carotenoids might be candidates. 

Chino, H. 1985. Lipid transport Biochemistry of hemolymph lipophorin. In Comprehensive Insect Physiology, Biochemistry, and Pharmacology, eds. Kerkut G. A. and Gilbert L. I., Oxford Pergamon Press, 10 115-134. 

Gopalapillai, R., Kadono-Okuda, K., Tsuchida, K. et al. 2006. Lipophorin receptor of Bombyx mori cDNA cloning, genomic structure, alternative splicing, and isolation of a new isoform. J. Lipid Res., 47(5) 1005-1013. 

Tsuchida, K., Arai, M., Tanaka, Y. et al. 1998. Lipid transfer particle catalyzes transfer of carotenoids between lipophorins of Bombyx mori. Insect Biochem. Mol. Biol., 28(12) 927-934. 

The site of pheromone production in flies and cockroaches that utilize hydrocarbons is similar to that of the moths. Oenocyte cells produce the hydrocarbon pheromone which is transported by lipophorin in the hemolymph to epidermal cells throughout the body for release from the cuticular surface in general [20,21]. 

Several families of moths utilize hydrocarbons or epoxides of hydrocarbons as their sex pheromone. Oenocyte cells produce hydrocarbons that are transported through the hemolymph by lipophorin [71]. In a study using arctiid moths it was shown that sex pheromone hydrocarbons are transported on the same lipophorin particle as the hydrocarbons destined for the cuticular surface [ 17]. Therefore, specific uptake of the sex pheromone hydrocarbon occurred in pheromone glands [17]. Similar findings have been found with other moths [72-74]. The mechanism behind this specific uptake of one hydrocarbon from a potential pool of other hydrocarbons is unknown. 

Drosophila melanogaster is another dipteran where pheromone biosynthesis has been studied [92]. Adult sexually mature female D. melanogaster utilizes primarily Z7,Z11-27 H as a contact sex pheromone. The biosynthesis of this compound follows the biosynthesis of other hydrocarbon-derived pheromones (Fig. 3). It is biosynthesized in oenocytes [93], transported through the hemo-lymph by lipophorin [94], and deposited on the cuticle surface. Biosynthesis in the oenocytes follows a similar pathway [95] as that described for the house fly... 

Fig. 7 Transport of hydrocarbons and other lipids by lipophorin from site of synthesis (oenocytes) to various tissues. In the case of pheromone glands specific hydrocarbons are unloaded to be used directly as a pheromone or modified with the addition of oxygen and then released as a pheromone... 

The biosynthesis of Me3,Mel 1-29 H takes place in oenocyte cells, released into the hemolymph and transported by lipophorin to peripheral tissues (Fig. 7) [71, 231, 232]. Direct evidence for oenocytes biosynthesizing hydrocarbon has come recently with the dissociation of oenocytes from epidermal cells and in vitro incubation with labeled propionate [233]. Differential uptake of some hydrocarbons in different tissues has also been documented although the exact mechanism behind the differential placement of hydrocarbons is unknown [20,128,230,232,234]. Although the biosynthesis of hydrocarbons may not be under direct endocrine regulation supply of precursor hydrocarbon that is converted to the sex pheromone is a requirement. 

As with the other insects studied that utilize hydrocarbon sex pheromones, once Z9-23 H is produced by oenocyte cells it is released into the hemolymph. Lipophorin is the transport protein that will move the hydrocarbon to cuticu-lar tissue [21]. It was found that about 24 h were required once Z9-23 H was induced to actual deposition on the cuticular surface [237]. As is the case with other insects selective partitioning of the sex pheromone was observed with relatively larger proportions of Z9-23 H being found on the cuticular surface than in other tissues [21]. 

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.,... 

It remains to be determined whether epidermal cells are involved, and how. Also, mechanisms that regulate the uptake and deposition of pheromone by lipophorin remain unexplored. The dynamics of lipophorin-facilitated shuttling of lipids in B. germanica were last reviewed by Schal et al. (1998, 2003). 

Fan, Y., Chase, J., Sevala, V. and Schal, C. (2002). Lipophorin-facilitated hydrocarbon uptake by oocytes in the German cockroach, Blattella germanica (L.). Journal of Experimental Biology 205 781-790. 

Gu, X., Quilici, D., Juarez, P, Blomquist, G. J. and Schal, C. (1995). Biosynthesis of hydrocarbons and contact sex pheromone and their transport by lipophorin in females of the German cockroach (Blattella germanica). Journal of Insect Physiology 41 257-267. 

Sevala, V., Shu, S., Ramaswamy, S. B. and Schal, C. (1999). Lipophorin of female Blattella germanica (L.) characterization and relation to hemolymph titers of juvenile hormone and hydrocarbons. Journal of Insect Physiology 45 431 141. 

Schal, C., Sevala, V. and Card6, R. T. (1998). Novel and highly specific transport of a volatile sex pheromone by hemolymph lipophorin in moths. Naturwissenschaften 85 339-342. 

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... 

BOX 21-A LIPOCALINS, FATTY ACID-BINDING PROTEINS, AND LIPOPHORINS... 

A third group of lipid-binding proteins have a four-helix bundle structure. They include the insect lipophorins, which transport diacylglycerols in the hemolymph (see main text), and nonspecific lipid carriers of green plants.q An 87-residue four-helix protein with a more open structure binds acyl-coenzyme A molecules in liver.r... 

In contrast, the hydrocarbon contact sex pheromone of the German cockroach is synthesized in Class II oenocytes associated with the abdominal stemites (Young et al., 1999 Fan et al., 2002). These large oenocytes, ranging up to 50 p in in diameter in B. germanica, have abundant mitochondria and extensive smooth endoplasmic reticulum (Fan et al., 2002). Although the oenocytes are associated with abdominal stemites, the hydrocarbons are released into the hemolymph and loaded, probably across a plasma membrane reticular system, into high-density lipophorin. The lipophorin then likely transports the hydrocarbon to epidermal cells for release onto the cuticle (Fan et al., 2002). 

Young H. P., Bachmann J. A. S., Sevala V. and Schal C. (1999) Site of synthesis, tissue distribution, and lipophorin transport of hydrocarbons in Blattella germanica (L.) nymphs. J. Insect Physiol. 45, 305-315. 

Moths in the families Geometridae, Arctiidae, and some Noctuidae utilize hydrocarbons or epoxides of hydrocarbons as their sex pheromones. Hydrocarbon biosynthesis occurs in oenocyte cells that are associated with either epidermal cells or fat body cells (Wigglesworth, 1970). Once the hydrocarbons are biosynthesized, they are transported to the sex pheromone gland by lipophorin (Schal et al., 1998). The hydrocarbons can be released directly in the case of some moths or they are transformed into epoxides by addition of oxygen across one of the double bonds. 

Figure 3.7 Production of the sex pheromone in the gypsy moth, Lymantria dispar. The oenocyte cells located in the abdomen biosynthesize the alkene hydrocarbon precursor to the pheromone, 2me-Z7-18 Hc. It is transported through the hemolymph by lipophorin. The alkene is taken up by pheromone gland cells where it is acted upon by an epoxidase to produce the pheromone disparlure, 2me-18 7,8Epox.

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