| Summary: | 碩士 === 國立中興大學 === 動物科學系所 === 107 === Abstract
Plasma VLDL biology in laying hens differs distinctively from that of immature hens, including a smaller VLDL particle diameter, dramatic increases of actual and fractional of levels of VLDL-TG (triacylglycerol) and -PL (phospholipid) concentration, and the presence of an avian specific apolipoprotein; VLDL-II (Apo VLDL-II). These estrogen-induced alterations were suggested to provide TG-rich VLDL for yolk formation for embryo development, and thereby termed VLDLy. Precious studies have shown the peripheral role of Apo VLDL-II by hampering LPL (lipoprotein lipase) activity to escort intact VLDL to the ovary. Despite lacking direct evidences, due to the presence of Apo VLDL-II in VLDLy simultaneously with smaller particle size, Apo VLDL-II was postulated to mediate smaller particle diameter of VLDLy. . Microsomal triglyceride transfer protein (MTP) functions to prevent apo B from degradation and facilitate TG mobilization and traffic into vesicles for secretion. However, no studies have been tried to elucidate altered lipid compositions and massive secretion of VLDLy in relation to the role of MTP functionality in TG mobilization for VLDL secretion. In this study, chick primary hepatocytes and LMH-2A cell line in combination with Apo VLDL-II overexpression and estrogen induction and/or pulse stimulation by oleate to impose VLDL assembly and secretion were used to elucidate the role of Apo VLDL-II in mediating the change of VLDL size and its relationship with MTP in hepatic TG mobilization. Under scanning electron microscope (SEM), primary cells had VLDL particle size as Control (C); 91-100 nm, Control+ OA (oleic acid; C+); 111-120 nm, Transfection with apo VLDL-II expressing vector (T): 31-40 nm, T+OA(T+): 21-30 nm. In LMH-2A cells, VLDL particle was sized as Control (C): 121-130 nm, Control+ OA (C+): 81-90 nm, Transfection (T): 91-110 nm, Transfection +OA (T+): 51-60 nm, Estradiol induction (E): 111-120 nm, E+ OA(E+): 51-60 nm. Accordingly, oleate increased VLDL size in primary hepatocytes but decreased it in LMH-2A cells. Enforced expression of Apo VLDL-II and estrogen induction decreased the VLDL particle size in both chick primary hepatocytes and LMH-2A cells. No differences of hepatic MTP expression between laying hens and roosters, and so as by apo VLDL-II overexpression and estrogen induction in LMH-2A cells. However, Apo VLDL-II overexpression and oleate treatment increased MTP expression in chick primary hepatocytes. Immunoprecipitation analysis confirmed that Apo VLDL-II binds to MTP and apoB in laying hen hepatocytes, suggesting allosteric regulation of MTP activity by Apo VLDL-II along the assembly process of VLDL with Apo B and cargo lipids In summary, the presence of Apo VLDL-II indeed mediates the decrease of VLDLy particle size and intracellular it may interact with MTP and Apo B to facilitate VLDL assembly and secretion in the laying hens.
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