Davis R C, Ben-Zeev O, Martin D, Doolittle M H
Lipid Research, Veterans Administration Wadsworth Medical Center, Los Angeles, California 90073.
J Biol Chem. 1990 Oct 15;265(29):17960-6.
Newborn combined lipase-deficient (cld) mice have severe hypertriglyceridemia associated with a marked decrease of lipoprotein lipase (LPL) and hepatic lipase (HL) activities. Since the cld mutation and lipase genes reside on separate chromosomes, combined lipase deficiency cannot result from defects occurring within the LPL or HL structural genes. To elucidate the biochemical basis of this trans-acting defect, cld mice were compared to unaffected littermates for changes in lipase mRNA levels, rates of synthesis, and posttranslational processing and secretion. LPL and HL mRNA levels in cld liver and LPL in cld heart were comparable to controls; corresponding lipase synthetic rates were modestly decreased by about 30%. However, these reduced synthetic rates were not lipase-specific, since the rates of apolipoprotein (apo) A-I and apoA-II synthesis in cld liver were similarly decreased. Despite LPL synthetic rates that were 70% of controls, LPL mass in cld postheparin plasma was markedly reduced to only 7% of control values, suggesting that the majority of LPL is not secreted but remains intracellular. Consistent with a lipase secretory defect, neither the LPL nor HL oligomannosyl forms were converted to their respective complex forms in cld tissues, indicating that the lipases had failed to move from the endoplasmic reticulum/cis-Golgi to the medial/trans-Golgi network. In addition, the majority of intracellular LPL was catalytically inactive, since LPL specific activity (units/mg LPL protein) in cld heart, kidney, and brain was reduced 80-97%. In contrast to the severe impairment of lipase posttranslational processing and secretion, cld mouse plasma contained normal levels of another secretory N-linked glycoprotein, adipsin, with its oligosaccharide chains fully processed to the complex form. Thus, the cld mutation appears not to globally disrupt the secretion of all N-linked glycoproteins, but rather selectively impairs LPL and HL at points essential to their normal intracellular transport and secretion.
新生的联合脂肪酶缺乏（cld）小鼠患有严重的高甘油三酯血症，这与脂蛋白脂肪酶（LPL）和肝脂肪酶（HL）活性的显著降低有关。由于cld突变和脂肪酶基因位于不同的染色体上，联合脂肪酶缺乏不可能是由LPL或HL结构基因内发生的缺陷导致的。为了阐明这种反式作用缺陷的生化基础，将cld小鼠与未受影响的同窝小鼠在脂肪酶mRNA水平、合成速率、翻译后加工和分泌方面的变化进行了比较。cld肝脏中的LPL和HL mRNA水平以及cld心脏中的LPL与对照组相当；相应的脂肪酶合成速率适度降低了约30%。然而，这些降低的合成速率并非脂肪酶特异性的，因为cld肝脏中载脂蛋白（apo）A-I和apoA-II的合成速率也同样降低。尽管cld小鼠的LPL合成速率仅为对照组的70%，但cld肝素后血浆中的LPL质量显著降低至仅为对照值的7%，这表明大多数LPL未被分泌而是保留在细胞内。与脂肪酶分泌缺陷一致，cld组织中的LPL和HL寡甘露糖形式均未转化为各自的复合形式，这表明脂肪酶未能从内质网/顺式高尔基体转移至中间/反式高尔基体网络。此外，大多数细胞内LPL无催化活性，因为cld心脏、肾脏和大脑中的LPL比活性（单位/mg LPL蛋白）降低了80 - 97%。与脂肪酶翻译后加工和分泌的严重受损形成对比的是，cld小鼠血浆中另一种分泌性N-连接糖蛋白——脂联素的水平正常，其寡糖链已完全加工成复合形式。因此，cld突变似乎并非全面破坏所有N-连接糖蛋白的分泌，而是在LPL和HL正常细胞内运输和分泌所必需的关键环节选择性地损害它们。
