Goralska M, Harned J, Fleisher L N, McGahan M C
Department of Anatomy, Physiology and Radiology, College of Veterinary Medicine, North Carolina State University, Raleigh, NC, 27606, USA.
Exp Eye Res. 1998 Jun;66(6):687-97. doi: 10.1006/exer.1997.0466.
Ferritin is the major intracellular iron storage protein which has been shown to protect cells against oxidative damage. Recent reports that an inherited abnormality in human ferritin synthesis is associated with early bilateral cataracts underscore the importance of understanding ferritin synthesis and iron storage in lens epithelial cells. We previously demonstrated that ascorbic acid greatly increases de novo synthesis of ferritin in lens epithelial cells. The objectives of the present study were to determine: (1) the effects of ascorbic acid and ferric ammonium citrate on iron uptake by canine lens epithelial cells from iron bound to transferrin and from ferric chloride and (2) the incorporation of this element into ferritin. Iron uptake by lens epithelial cells from 59ferric chloride was 20 times higher than from 59iron-transferrin and iron deposition into ferritin was 8-fold higher when 59ferric chloride was the source. Ascorbic acid had a stimulatory effect on iron uptake from transferrin and on incorporation of this element into ferritin. The ascorbic acid-induced increase of iron uptake required de novo protein synthesis but not specifically de novo ferritin biosynthesis. Although ferritin is not directly involved in iron uptake, the level of ferritin protein could control the pool of intracellular iron. The present results indicate that iron homeostasis in lens epithelial cells is affected mainly by changes in apoferritin synthesis, which is greatly stimulated by ascorbic acid, rather than by altering the rate of protein degradation, which is very slow in these cells under all circumstances. Ferric ammonium citrate activates iron uptake from transferrin in a wide range of cell lines by generation of free radicals. Ferric ammonium citrate also increased iron uptake from Tf in lens epithelial cells. Ferric ammonium citrate treated cells incorporated 5 times more iron and deposited 2 times more iron into ferritin than control cells. Increased incorporation of iron into ferritin was due to ferric ammonium citrate-induced stimulation of de novo ferritin synthesis rather than an increased rate of iron deposition into pre-existing ferritin. Ferric ammonium citrate had a different effect on iron uptake from ferric chloride; total iron uptake was not significantly increased while deposition into ferritin was significantly decreased. These results demonstrate that iron homeostasis in lens epithelial cells is regulated by ascorbic acid and by changes in the rate of de novo ferritin synthesis. In addition, the differences in iron uptake from transferrin and ferric chloride and its subsequent incorporation into ferritin suggests that the mechanisms by which iron is incorporated into ferritin are source dependent.
铁蛋白是主要的细胞内铁储存蛋白，已被证明可保护细胞免受氧化损伤。最近有报道称，人类铁蛋白合成的遗传性异常与早期双侧白内障有关，这突出了了解晶状体上皮细胞中铁蛋白合成和铁储存的重要性。我们之前证明，抗坏血酸可大大增加晶状体上皮细胞中铁蛋白的从头合成。本研究的目的是确定：（1）抗坏血酸和柠檬酸铁铵对犬晶状体上皮细胞从与转铁蛋白结合的铁以及从氯化铁中摄取铁的影响，以及（2）该元素掺入铁蛋白的情况。当以59氯化铁为铁源时，晶状体上皮细胞从59氯化铁中摄取的铁比从59铁 - 转铁蛋白中摄取的铁高20倍，并且铁沉积到铁蛋白中的量高8倍。抗坏血酸对从转铁蛋白摄取铁以及该元素掺入铁蛋白有刺激作用。抗坏血酸诱导的铁摄取增加需要从头合成蛋白质，但并非特异性地需要从头合成铁蛋白。虽然铁蛋白不直接参与铁摄取，但铁蛋白的水平可以控制细胞内铁池。目前的结果表明，晶状体上皮细胞中的铁稳态主要受脱铁铁蛋白合成变化的影响，抗坏血酸可极大地刺激脱铁铁蛋白合成，而不是通过改变蛋白质降解速率来影响，在所有情况下这些细胞中的蛋白质降解速率都非常缓慢。柠檬酸铁铵通过产生自由基在多种细胞系中激活从转铁蛋白摄取铁。柠檬酸铁铵也增加了晶状体上皮细胞从转铁蛋白摄取铁。与对照细胞相比，用柠檬酸铁铵处理的细胞摄取的铁多5倍，沉积到铁蛋白中的铁多2倍。铁掺入铁蛋白的增加是由于柠檬酸铁铵诱导的脱铁铁蛋白从头合成增加，而不是铁沉积到预先存在的铁蛋白中的速率增加。柠檬酸铁铵对从氯化铁摄取铁有不同的影响；总铁摄取没有显著增加，而沉积到铁蛋白中的铁显著减少。这些结果表明，晶状体上皮细胞中的铁稳态受抗坏血酸和脱铁铁蛋白从头合成速率变化的调节。此外，从转铁蛋白和氯化铁摄取铁及其随后掺入铁蛋白的差异表明，铁掺入铁蛋白的机制取决于铁源。
