Ponka P, Schulman H M
Lady Davis Institute for Medical Research, Sir Mortimer B. Davis Jewish General Hospital, Montreal, Quebec, Canada.
Stem Cells. 1993 May;11 Suppl 1:24-35. doi: 10.1002/stem.5530110607.
Our previous research has demonstrated that in hemoglobin-synthesizing cells, as compared with nonerythroid cells, a step in iron transport from transferrin localized between the transferrin receptor and ferrochelatase is rate-limiting for the synthesis of heme. In this communication we report our more recent studies on the mechanisms involved in the regulation of the transferrin receptors and ferrochelatase in differentiating erythroid cells. Our studies indicate that transferrin receptor gene expression is regulated differently in hemoglobin synthesizing as compared with uninduced murine erythroleukemia (MEL) cells: 1) With nuclear run-on assays our experiments showed increased transferrin receptor mRNA transcription cells of MEL following induction of erythroid differentiation with dimethylsulfoxide (DMSO). 2) DMSO treatment of MEL cells does not increase iron-responsive element binding protein (IRE-BP) activity which is, however, increased in uninduced MEL cells by Fe chelators. 3) Following induction of MEL cells there is an increase in the stability of transferrin receptor mRNA whose level is only slightly affected by iron excess. Using murine ferrochelatase cDNA as a probe, two ferrochelatase transcripts having lengths of 2.9 kb and 2.2 kb were found in extracts of mouse liver, kidney, brain, muscle and spleen, the 2.9 kb transcript being more abundant in nonerythroid tissues and the 2.2 more predominant in spleen. In MEL cells, the 2.9 ferrochelatase transcript is also more abundant; however, following induction of erythroid differentiation by DMSO there is a preferential increase in the 2.2 kb transcript which eventually predominates. With mouse reticulocytes, the purest immature erythroid cell population available, over 90% of the total ferrochelatase mRNA is present as the 2.2 kb transcript. Our further experiments indicate that the 2.2 kb transcript results from the utilization of the upstream polyadenylation signal and suggest that the preferential utilization of the upstream polyadenylation signal may be an erythroid-specific characteristic of ferrochelatase gene expression. These results provide further evidence for the idea that iron metabolism and heme synthesis are controlled by distinct mechanisms in erythroid versus nonerythroid cells.
我们之前的研究表明，在血红蛋白合成细胞中，与非红细胞系细胞相比，转铁蛋白中铁的转运过程（该过程发生在转铁蛋白受体和亚铁螯合酶之间）的一个步骤是血红素合成的限速步骤。在本通讯中，我们报告了我们最近关于分化中的红细胞系细胞中转铁蛋白受体和亚铁螯合酶调控机制的研究。我们的研究表明，与未诱导的小鼠红白血病（MEL）细胞相比，血红蛋白合成细胞中转铁蛋白受体基因的表达调控方式不同：1）通过核转录分析，我们的实验表明，用二甲基亚砜（DMSO）诱导MEL细胞向红细胞系分化后，其转铁蛋白受体mRNA转录增加。2）用DMSO处理MEL细胞不会增加铁反应元件结合蛋白（IRE-BP）的活性，然而，铁螯合剂会增加未诱导的MEL细胞中IRE-BP的活性。3）MEL细胞诱导后，转铁蛋白受体mRNA的稳定性增加，其水平仅受铁过量的轻微影响。用小鼠亚铁螯合酶cDNA作为探针，在小鼠肝脏、肾脏、大脑、肌肉和脾脏的提取物中发现了两种长度分别为2.9 kb和2.2 kb的亚铁螯合酶转录本，2.9 kb的转录本在非红细胞系组织中更丰富，2.2 kb的转录本在脾脏中更占优势。在MEL细胞中，2.9 kb的亚铁螯合酶转录本也更丰富；然而，用DMSO诱导红细胞系分化后，2.2 kb的转录本优先增加，最终占主导地位。在小鼠网织红细胞（可获得的最纯的未成熟红细胞系细胞群体）中，超过90%的总亚铁螯合酶mRNA以2.2 kb的转录本形式存在。我们的进一步实验表明，2.2 kb的转录本是由上游聚腺苷酸化信号的利用产生的，并表明上游聚腺苷酸化信号的优先利用可能是亚铁螯合酶基因表达的红细胞系特异性特征。这些结果为红细胞系细胞与非红细胞系细胞中铁代谢和血红素合成受不同机制控制这一观点提供了进一步的证据。
