Biological Functions of Iron Responsive Elements

描述(申请人提供)：铁代谢障碍，无论是由先天遗传错误、对疾病的不适应反应或饮食引起的，都是美国和世界各地的主要公共卫生问题。营养性缺铁与认知发育受损和工作量减少有关，从而导致严重的负面经济和社会后果。除了铁代谢本身，细胞和机体对缺铁的适应性反应所涉及的生理途径仍然不清楚。除了饮食外，贫血还可能是由于与肾功能衰竭、正常衰老、血红蛋白或其他蛋白质的基因突变相关的氧气感知通路的障碍，以及由于未完全切换到出生后控制促红细胞生成素的产生机制而导致的早产。总而言之，仅在美国，这些病态就影响了数百万人。恰当地控制铁和氧代谢的能力对于整个生命周期的最佳健康至关重要。在脊椎动物中，铁调节蛋白1(Irp1)和IRP2是细胞铁代谢的中枢调节因子。IRP决定了编码蛋白质的mRNA的命运，这些蛋白质是维持铁的稳态和响应铁状态的适应性变化所必需的。低氧诱导因子2？(HIF2？)是基因组对氧和铁的反应的核心转录因子，其编码的mRNA是Irp1在培养细胞中的特异性靶标。我们证明在IRP1-/-小鼠中HIF2？mRNA翻译被激活，但在IRP2-/-小鼠中不被激活。IRP1-/-小鼠有严重的红细胞生成障碍，包括一过性严重红细胞增多症，并表现出在HIF2功能增强的人类或HIF2调节改变的动物模型中观察到的其他症状。对Irp1-/-小鼠肠道粘膜细胞的转录分析显示，多个HIF2基因靶点上调，提示铁吸收能力增强，就像当HIF2激活时观察到的缺氧时所观察到的那样。到目前为止，我们的结果还表明，HIF2基因的5‘非翻译区包含多个先前未被识别的假定的翻译调控元件，这些元件强烈表明翻译控制是一个关键的额外水平，在这个水平上，铁和氧水平的变化与HIF2？的表达和作用水平相结合。我们认为，Irp1-HIF2调节轴是哺乳动物铁和氧代谢整合所必需的。因此，本研究的具体目的是：1)确定IRp1依赖的HIF2？调节在小鼠对铁和氧状态改变的适应性反应中的组织特异性作用；2)确定铁和氧对HIF2？mRNA翻译的调控机制及其对人肾细胞促红细胞生成素分泌的HIF2？作用的影响。为了阐明Irp1-HIF2轴在铁和氧代谢中枢通路的适应性和非适应性控制中的作用，我们的研究可能最终对开发治疗常见疾病的新治疗策略产生变革性的影响。

DESCRIPTION (provided by applicant): Disorders of iron metabolism, whether caused by inborn genetic errors, maladaptive responses to disease or diet are major public health issues in the U.S. and throughout the world. Nutritional iron deficiency is associated with impaired cognitive development and reduced work output resulting in significant negative economic and social consequences. Other than iron metabolism per se, the physiological pathways involved in the adaptive cellular and organismal response to iron deficiency remain poorly defined. In addition to diet, anemia can be caused by disorders in oxygen sensing pathways associated with renal failure, normal aging, genetic mutations in hemoglobin or other proteins, and as a consequence of preterm delivery when the full switch to post-natal mechanisms of controlling erythropoietin production has not occurred. In sum, these pathological states affect millions of people in the U.S. alone. The ability to properly integrate the control of iron and oxygen metabolism is essential for optimal health throughout the life cycle. In vertebrates, iron regulatory protein 1 (IRP1) and IRP2 are central regulators of cellular iron metabolism. IRP dictate the fate of mRNA encoding proteins required for the maintenance of iron homeostasis and for the adaptive changes in response to iron status. The mRNA encoding hypoxia inducible factor 2¿ (HIF2¿), a transcription factor central to the genome wide responses to oxygen and iron, has been previously shown to be a specific target of IRP1 in cultured cells. We demonstrate that HIF2¿ mRNA translation is activated in IRP1-/- mice but not in IRP2-/- mice. IRP1-/- mice have profound disturbances in erythropoiesis, including a transient severe polycythemia, and display other symptoms observed in humans with HIF2¿ gain-of-function or in animal models of altered HIF2¿ regulation. Transcriptomic analysis of intestinal mucosal cells in IRP1-/- mice revealed up-regulation of multiple HIF2¿ gene targets suggesting enhanced iron absorption capacity as would be observed in hypoxia when HIF2¿ is activated. Our results to date also demonstrate that the 5' untranslated region of HIF2¿ mRNA contains multiple previously unrecognized putative translational regulatory elements that strongly suggest that translational control is a critical additional level at which changes in iron and oxygen level are integrated with the level of expression and action of HIF2¿. We propose that the IRP1-HIF2¿ regulatory axis is required for the integration of iron and oxygen metabolism in mammals. Consequently, the specific aims are to: 1) determine the tissue-specific roles of IRP1-dependent regulation of HIF2¿ in the adaptive response to altered iron and oxygen status in mice; 2) determine the mechanisms regulating HIF2¿ mRNA translation by iron and oxygen and their impact on HIF2¿ action in erythropoietin-secreting human kidney cells. In elucidating the role of the IRP1-HIF2¿ axis in the adaptive and maladaptive control of central pathways of iron and oxygen metabolism our studies may ultimately have a transformative effect on the development of new therapeutic strategies for treatment of common disorders.