The Journal of Nutrition Vol. 127 No. 5 May 1997, pp. 793S-794S
Copyright ©1997 by the American Society for Nutritional Sciences

An Overview of Recent Developments in Avian Lipoprotein Metabolism¹

Robert G. Elkin

Department of Animal Sciences, Purdue University, West Lafayette, IN 47907-1151

FOOTNOTES

LITERATURE CITED

Lipoproteins transport both water-insoluble nutrients and other compounds, such as cholesterol and nonessential fatty acids, through the bloodstream from their sites of absorption or synthesis to peripheral tissues. In avians, lipoproteins are intimately involved in oocyte (future egg yolk) growth and the delivery of lipids to specific tissues in response to a nutritional or physiological need. The metabolism of lipoproteins in male or immature female chickens is similar to that in mammals, and much of our understanding of this subject in birds represents an extrapolation from the very detailed knowledge of mammalian lipoprotein metabolism (Griffin and Hermier 1988). In contrast to these similarities, the elegant system that has evolved to ensure the efficient delivery of massive amounts of hepatically synthesized lipoproteins to growing oocytes makes lipoprotein metabolism in laying birds very different from that of mammals, immature hens or male birds.

We are still in the midst of what Segrist and Albers (1986) referred to over a decade ago as a "molecular revolution in our understanding of lipoprotein structure and metabolism." This is evidenced by the fact that structures of genes encoding for apolipoproteins, lipoprotein receptors and enzymes involved in the regulation of lipoprotein metabolism are still emerging, not only in mammals but in a variety of species. In addition, major advances are being made in our understanding of the synthesis, assembly and secretion of lipoproteins, particularly in the case of apolipoprotein B-containing VLDL (Adeli et al. 1995). Furthermore, the once-clear picture of lipoprotein receptors, originally drawn by Brown and Goldstein (1986) from their studies on the LDL receptor, has become much more complicated with the discovery of several new receptor proteins (Beisiegel 1995). The goal of this symposium is to provide an overview of several of these recent developments in lipoprotein metabolism from an avian perspective.

In birds, vitellogenin and VLDL, the major yolk lipoprotein precursors, are hepatically synthesized under the transcriptional control of estrogen, secreted into the bloodstream, and transported to the ovary, where they are taken up by growing oocytes via receptor-mediated endocytosis (Schneider 1996). In the first paper of this series, Davis (1997) reviews the evolution of gene products that are essential in regulating, synthesizing, assembling and secreting the lipid and protein components of lipoproteins. Using the primordial vitellogenin system as a paradigm, the author suggests that the gene products required for mammalian VLDL assembly and secretion and its regulation were derived from the vitellogenin lipoprotein system. Davis (1997) also proposes that nutritional and metabolic needs for hepatic VLDL secretion are signaled by a steroid that is not involved in the signaling required for reproduction, thus allowing the mammalian VLDL secretory pathway to respond in a more versatile manner, independent of sex-linked processes.

In laying hens, the distribution of hepatically synthesized lipoproteins into oocytes and somatic tissues occurs simultaneously. This dual task of yolk formation and systemic lipid homeostasis is achieved by efficient lipoprotein transport systems, at the center of which are receptors belonging to the LDL receptor supergene family. In the second paper of this series, Bujo et al. (1997) review current understanding of lipoprotein metabolism in laying hens and describe why it serves as an excellent model system for intercellular transport. In addition, they discuss the biological rationale for the simultaneous expression of closely related genes in a single organism.

Extrahepatic de novo fatty acid synthesis is very limited in birds. Thus, most of the fat that accumulates in avian adipose tissue is either derived from the diet or synthesized in the liver and transported to tissues via the plasma in the form of triglyceride-rich lipoproteins. Lipoprotein lipase, bound to the luminal surface of endothelial cells in peripheral tissues, catalyzes the hydrolysis of the core triglyceride from portomicrons and VLDL, thus releasing nonesterified fatty acids for oxidation or storage in muscle or adipose tissue. In the third paper of this series, Hermier (1997) provides an overview of the role of lipoproteins and lipoprotein lipase in fat deposition and the mechanisms that regulate this process in poultry. In addition, the author discusses a unique avian model for hepatic steatosis, the overfed goose, which is apparently unable to adequately incorporate triglyceride into nascent hepatic VLDL.

Because lipoproteins are essential components of the processes of egg yolk formation and body fat deposition, the manipulation of lipoprotein metabolism in poultry would be expected to have significant economic and human health implications. In this regard, utilization of the basic knowledge of avian lipoprotein metabolism, summarized in the present symposium, may ultimately lead to future improvements in the nutritional quality of poultry meat and eggs.

FOOTNOTES

LITERATURE CITED

• Adeli K., Mohammadi A., Macri J. Regulation of apolipoprotein B biogenesis in human hepatocytes: posttranscriptional control mechanisms that determine the hepatic production of apolipoprotein B-containing lipoproteins. Clin. Biochem. 1995; 28:123-130 [Medline][Medline]
• Beisiegel U. Receptors for triglyceride-rich lipoproteins and their role in lipoprotein metabolism. Curr. Opin. Lipidol. 1995; 6:117-122 [Medline][Medline]
• Brown M. S., Goldstein J. L. A receptor-mediated pathway for cholesterol homeostasis. Science 1986; 232:34-47 [Medline][Free Full Text]
• Bujo H., Hermann M., Lindstedt K. A., Nimpf J., Schneider W. J. Low density lipoprotein receptor gene family members mediate yolk deposition. J. Nutr. 1997; 127:801S-804S [Medline]
• Davis R. A. Evolution of processes and regulators of lipoprotein synthesis: from birds to mammals. J. Nutr. 1997; 127:795S-800S [Medline]
• Griffin, H. & Hermier, D. (1988) Plasma lipoprotein metabolism and fattening in poultry. In: Leanness in Domestic Birds (Leclercq, B. & Whitehead, C. C., eds.), pp. 175-201. Butterworths, London, U.K.
• Hermier D. Lipoprotein metabolism and fattening in poultry. J. Nutr. 1997; 127:805S-808S [Medline]
• Schneider W. J. Vitellogenin receptors: Oocyte-specific members of the low-density lipoprotein receptor supergene family. Int. Rev. Cytol. 1996; 166:103-137 [Medline][Medline]
• Segrist, J. P. & Albers, J. J. (1986) Preface. In: Methods in Enzymology (Segrest, J. P. & Albers, J. J., eds.), vol. 128, p. xv. Academic Press, New York, NY.

0022-3166/97 $3.00 ©1997 American Society for Nutritional Sciences