Dichek H L, Brecht W, Fan J, Ji Z S, McCormick S P, Akeefe H, Conzo L, Sanan D A, Weisgraber K H, Young S G, Taylor J M, Mahley R W
Gladstone Institute of Cardiovascular Disease, University of California, San Francisco 94141-9100, USA.
J Biol Chem. 1998 Jan 23;273(4):1896-903. doi: 10.1074/jbc.273.4.1896.
To determine the mechanisms by which human hepatic lipase (HL) contributes to the metabolism of apolipoprotein (apo) B-containing lipoproteins and high density lipoproteins (HDL) in vivo, we developed and characterized HL transgenic mice. HL was localized by immunohistochemistry to the liver and to the adrenal cortex. In hemizygous (hHLTg+/0) and homozygous (hHLTg+/+) mice, postheparin plasma HL activity increased by 25- and 50-fold and plasma cholesterol levels decreased by 80% and 85%, respectively. In mice fed a high fat, high cholesterol diet to increase endogenous apoB-containing lipoproteins, plasma cholesterol decreased 33% (hHLTg+/0) and 75% (hHLTg+/+). Both apoB-containing remnant lipoproteins and HDL were reduced. To extend this observation, the HL transgene was expressed in human apoB transgenic (huBTg) and apoE-deficient (apoE-/-) mice, both of which have high plasma levels of apoB-containing lipoproteins. (Note that the huBTg mice that were used in these studies were all hemizygous for the human apoB gene.) In both the huBTg,hHLTg+/0 mice and the apoE-/-,hHLTg+/0 mice, plasma cholesterol decreased by 50%. This decrease was reflected in both the apoB-containing and the HDL fractions. To determine if HL catalytic activity is required for these decreases, we expressed catalytically inactive HL (HL-CAT) in apoE-/- mice. The postheparin plasma HL activities were similar in the apoE-/- and the apoE-/-,HL-CAT+/0 mice, reflecting the activity of the endogenous mouse HL and confirming that the HL-CAT was catalytically inactive. However, the postheparin plasma HL activity was 20-fold higher in the apoE-/-,hHLTg+/0 mice, indicating expression of the active human HL. Immunoblotting demonstrated high levels of human HL in postheparin plasma of both apoE-/-,hHLTg+/0 and apoE-/-,HL-CAT+/0 mice. Plasma cholesterol and apoB-containing lipoprotein levels were approximately 60% lower in apoE-/-,HL-CAT+/0 mice than in apoE-/- mice. However, the HDL were only minimally reduced. Thus, the catalytic activity of HL is critical for its effects on HDL but not for its effects on apoB-containing lipoproteins. These results provide evidence that HL can act as a ligand to remove apoB-containing lipoproteins from plasma.
为了确定人肝脂酶（HL）在体内促进含载脂蛋白（apo）B的脂蛋白和高密度脂蛋白（HDL）代谢的机制，我们培育并鉴定了HL转基因小鼠。通过免疫组织化学方法将HL定位于肝脏和肾上腺皮质。在半合子（hHLTg+/0）和纯合子（hHLTg+/+）小鼠中，肝素后血浆HL活性分别增加了25倍和50倍，血浆胆固醇水平分别降低了80%和85%。在喂食高脂肪、高胆固醇饮食以增加内源性含apoB脂蛋白的小鼠中，血浆胆固醇分别降低了33%（hHLTg+/0）和75%（hHLTg+/+）。含apoB的残余脂蛋白和HDL均减少。为了扩展这一观察结果，将HL转基因在人apoB转基因（huBTg）和载脂蛋白E缺陷（apoE-/-）小鼠中表达，这两种小鼠血浆中含apoB的脂蛋白水平都很高。（请注意，这些研究中使用的huBTg小鼠对于人apoB基因均为半合子。）在huBTg、hHLTg+/0小鼠和apoE-/-、hHLTg+/0小鼠中，血浆胆固醇均降低了50%。这种降低在含apoB和HDL组分中均有体现。为了确定这些降低是否需要HL催化活性，我们在apoE-/-小鼠中表达了无催化活性的HL（HL-CAT）。apoE-/-小鼠和apoE-/-、HL-CAT+/0小鼠的肝素后血浆HL活性相似，反映了内源性小鼠HL的活性，并证实HL-CAT无催化活性。然而，apoE-/-、hHLTg+/0小鼠的肝素后血浆HL活性高20倍，表明有活性的人HL得到了表达。免疫印迹显示，apoE-/-、hHLTg+/0和apoE-/-、HL-CAT+/0小鼠的肝素后血浆中均有高水平的人HL。apoE-/-、HL-CAT+/0小鼠的血浆胆固醇和含apoB的脂蛋白水平比apoE-/-小鼠低约60%。然而，HDL仅略有降低。因此，HL的催化活性对其影响HDL至关重要，但对其影响含apoB的脂蛋白则并非如此。这些结果提供了证据，表明HL可作为一种配体从血浆中清除含apoB的脂蛋白。
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