Estrogen induction of VLDLy assembly in egg-laying hens.

The yolk of a 60-g chicken egg contains 6 g of triacylglycerols transported to the oocyte from the liver of the laying hen in apolipoprotein (apo) B-containing particles. With the onset of egg production, estrogen shifts hepatocytic lipoprotein production from generic VLDL to VLDLy (yolk targeted). These VLDLy are triacylglycerol-rich particles; they are reduced in size by one half, are resistant to lipoprotein lipase and are taken up intact by oocyte receptors. The VLDLy pathway for apoB provides sufficient energy for the caloric requirements of chick development. VLDLy size reduction occurs in spite of surplus liver triacylglycerols and is necessary for VLDL particles to pass through the granulosa basal lamina and reach the receptors located on the oocyte surface. New ultrastructural data show that some proximal tubule cells of bird kidney secrete generic VLDL, perhaps providing energy and other VLDL-associated nutrients to tissues bypassed by VLDLy. Birds are an apoB100-only species, providing a natural in vivo model with which to investigate mechanisms of apoB100 VLDL assembly. Preliminary studies of liver lipoprotein assembly intermediates isolated from the biosynthetic membranes (endoplasmic reticulum) of the laying hen are consistent with the presence of both putative first- and second-step precursor particles of VLDLy. These findings suggest that the two-step mechanism of apoB core lipidation is an ancient development in apoB biology, handed down to mammals from oviparous ancestors.