Mechanistic insights into the crosstalk between iron metabolism and diabetes

在美国，10.5%的人口患有糖尿病，占死亡人数的3%以上，2017年估计造成3270亿美元的损失。铁超载是糖尿病发病和糖尿病并发症进展的重要危险因素。减少铁含量可改善糖尿病并发症和胰岛素敏感性。因此，更好地了解糖尿病中的铁代谢以及铁有害作用的确切机制可以确定糖尿病控制的新治疗方式。我们已发表的工作和新的初步数据表明，与糖尿病相关的高活性蛋白激酶Cα(PKCα)信号通路通过上调铁进口蛋白二价金属转运蛋白1(DMT1)和铁出口蛋白铁蛋白(FPN)促进膳食铁的吸收。PKCα在调节肠道铁吸收中的重要性得到了PKCα-/-小鼠体内研究结果的支持，该研究显示，PKC FPN表达减少，肠道上皮铁滞留增加，肝脏和血清铁减少。肝脏是铁沉积的主要部位，在全身性葡萄糖稳态中起着重要作用。我们进一步观察到，铁负荷通过诱导肝细胞铁下垂加重糖尿病相关的肝损伤和炎症。我们假设，PKCα是糖尿病和铁代谢之间的一个重要调节因子，它通过介导高血糖诱导的铁负荷和加重肝损伤来实现。我们将利用多种基因工程小鼠模型、培养的肠道上皮细胞和糖尿病患者的肠道活检来解决以下相互关联的特定目标。目的1：了解高血糖-蛋白激酶Cα轴增加肠道铁输入的机制。我们将展示E3泛素连接酶的底物受体Cereblon结合并介导肠道DMT1的泛素化。高活性的蛋白激酶Cα抑制DMT1与小脑蛋白的相互作用和DMT1的泛素化/内化，促进DMT1的膜表达和铁摄取。目的2：了解高活性蛋白激酶Cα上调肠道铁离子转运和稳定性的机制。我们将确定一种新的调节fpn稳定性的机制，其中糖尿病诱导的蛋白激酶Cα的激活增加了fpn的磷酸化和乙酰化，对抗了海普西丁诱导的泛素化和fpn的降解。糖尿病患者的十二指肠活检也将检查FPN的上调及其对PKCα的依赖。目的3：探讨铁负荷加重糖尿病肝损伤的机制。我们将确定，在糖尿病引起的自噬损害的背景下，铁负荷诱导肝细胞铁下垂是加重糖尿病肝损伤的一种新机制。我们将进一步测试PKCα功能丧失是否通过降低肝脏铁含量来减轻肝损伤。这些研究将突出蛋白激酶Cα在糖尿病引起的铁负荷和组织损伤中的重要作用以及精确的分子机制，为开发糖尿病控制的新的治疗方式奠定基础。

In the United States 10.5% of the population has diabetes, accounting for over 3% of deaths and costs an estimated $327 billion in 2017. Iron overload is a strong risk factor both the incidence of diabetes and the progression of diabetic complications. Reduction of iron content improves diabetic complications and insulin sensitivity. Therefore, a better understanding of iron metabolism in diabetes as well as the precise mechanisms underlying the detrimental role of iron could identify novel treatment modalities for diabetic control. Our published work and new preliminary data have shown that hyperactive protein kinase Cα (PKCα) signaling associated with diabetes promotes dietary iron absorption by up-regulating iron importer divalent metal transporter 1 (DMT1) and exporter ferroportin (Fpn). The importance of PKCα in the regulation of intestinal iron absorption is supported by in vivo findings in PKCα-/- mice showing decreased Fpn expression and increased iron retention in intestinal epithelium as well as diminished liver and serum iron. Liver is the primary site of iron deposition, and plays a major role in systemic glucose homeostasis. We further observed that iron loading exacerbates diabetes-associated liver injury and inflammation through induction of hepatocyte ferroptosis. We hypothesize that PKCα is an important regulator of the crosstalk between diabetes and iron metabolism by mediating hyperglycemia-induced iron loading and exacerbated liver injury. We will utilize multiple genetically engineered mouse models, cultured intestinal epithelial cells, and intestinal biopsies of diabetic patients to address the following interconnected specific aims. Aim 1: To understand the mechanisms by which hyperglycemia-PKCα axis increases intestinal iron import. We will demonstrate that cereblon, a substrate receptor of E3 ubiquitin ligase, binds and mediates the ubiquitination of intestinal DMT1. Hyperactive PKCα inhibits DMT1-cereblon interaction and the ubiquitination/internalization of DMT1, enhancing membrane expression of DMT1 and iron uptake. Aim 2: To understand the mechanism by which hyperactive PKCα up-regulates Fpn stability and iron export in the intestine. We will identify a novel mechanism underlying the regulation of Fpn stability, wherein diabetes-induced activation of PKCα increases Fpn phosphorylation and acetylation counteracting hepcidin-induced ubiquitination and degradation of Fpn. Diabetic up-regulation of Fpn and its PKCα dependency will be also examined using duodenal biopsies of diabetic patients. Aim 3: To identify the mechanism by which iron loading exacerbates diabetic liver injury. We will establish that iron loading, in the context of diabetes-induced impairment in autophagy, induces hepatocytes ferroptosis as a novel mechanism exacerbating liver injury in diabetes. We will further test if the loss-of-function of PKCα alleviates liver injury through reduction of hepatic iron content. These studies will highlight the important role of PKCα in diabetes-induced iron loading and tissue injury as well as the precise molecular mechanisms, laying the foundation for the development of novel treatment modalities in diabetic control.