Subcellular localization of B apoprotein of plasma lipoproteins in rat liver.

Abstract

Multispecific antigen-binding fragments (Fab) from rabbit antisera against rat very low density lipoproteins (VLDL) and Fab against rat low density lipoproteins that were monospecific for the B apoprotein were conjugated to horseradish peroxidase. Conjugates were incubated with 6-#m frozen sections from fresh and perfusionfixed livers and with tissue chopper sections (40 am thick) from perfusion-fixed livers. In the light microscope, specific reaction product was present in all hepatocytes of experimental sections as intense brown to black spots whose locations corresponded to the distribution of the Golgi apparatus: along the bile canaliculi, near the nuclei, and between the nuclei and bile canaliculi. Perfusion fixation with formaldehyde produced satisfactory ultrastructural preservation with retention of lipoprotein antigenic determinants. In the electron microscope, patches of cisternae and ribosomes of the rough endoplasmic reticulum (ER) and particularly its smooth-surfaced ends, vesicles located between the rough ER and the Golgi apparatus, the Golgi apparatus and its secretory vesicles and VLDL particles in the space of Disse all bore reaction product. The tubules and vesicles of typical hepatocyte smooth ER did not contain reaction product, nor did the osmiophilic particles contained therein. The localization obtained in this study together with other evidence suggests a sequence for the biosynthesis of VLDL that differs in some respects from that proposed by others: (a) the triglyceride-rich particle originates in the smooth ER where triglycerides are synthesized; (b) at the junction of the smooth and rough ER the particle receives apoproteins synthesized in the rough ER; (c) specialized tubules transport the particle, now a nascent lipoprotein, to the Golgi apparatus where concentration occurs in secretory vesicles; (a t) secretory vesicles move to the sinusoidal surface where the particles are secreted into the space of Disse by fusion of the vesicular membrane with the plasma membrane of the hepatocyte.Very low density lipoproteins (VLDL) ~ of vertebrate blood plasma are triglyceride-rich particles 1Abbreviations used in this paper." DAB, 3,3'-diaminobenzidine tetrahydrochloride; EDTA, disodium (ethylenedinitrilo)-tetraacetic acid; ER, endoplasmic reticulum; Fab, fragment antigen binding; Fc, fragment 250-800 ~ in diameter. Primarily, they transport triglycerides from liver to those tissues that store their constituent fatty acids or utilize them as fuel crystallizable; LDL, low density lipoproteins; HDL, high density lipoprotcins, VLDL, very low density lipoproteins.

Lipogenesis and the synthesis and secretion of very low density lipoprotein by avian liver cells in nonproliferating monolayer culture. Hormonal effects.

“…Also visible in Fig. 3A and B are vesicles containing osmiophilic structures that are similar to VLDL particles described by others (1,20); some of the smaller triglyceride-rich vesicles have the appearance of multivesicular bodies or secondary lysosomes. Detailed examination of the electron micrographs suggests that vesicle expansion occurs by membrane fusion with both smaller lipoprotein-containing vesicles and smaller triglyceride-rich vesicles (see unbroken arrows, Fig.…”

Hepatitis c virus and host cell lipids: An intimate connection

“…By fusion of VLDL2 with luLDs, a mature VLDL particle is generated when VLDL2 acquires the exchangeable apolipoprotein ApoE by fusion with luLDs. 166 Mature VLDL is transported to the Golgi and finally secreted. 163 By using mass spectrometry analyses of affinity purified HCV particles produced in cell culture, it was found that the lipid ©2 0 1 1 L a n d e s B i o s c i e n c e .…”

Role of insulin in hepatic fatty acid partitioning: emerging concepts

“…A two-step model (see Figure 2) has also been suggested in which part of the TAG content of the mature particles is incorporated into the apoB-containing particles in the rER during the co-translational insertion of the apoprotein into the ER lumen, whereas the rest is derived through enlargement with intralumenal TAG that occurs in globules which reside within the smooth ER (sER) [102]. This is supported by electron-micrographic evidence showing that TAG globules, which are not associated with apoB, occur within the lumen of the sER [114]. A third model is an elaboration of the two-step hypothesis and envisages that the lipoprotein particles originating in the rER are relatively lipid-poor, and that core lipids are added as the particles traverse the secretory system from ER through to the Golgi [115].…”