Imaging the Warburg effect with 23Na MRI and 82Rb positron emission tomography

人们普遍认为，肿瘤或血管系统不发达或闭塞的心肌组织中的缺氧，会导致从有氧呼吸到无氧糖酵解的转换，成为产生ATP的主要燃料。肿瘤表现出对糖酵解的偏好，而不是氧化代谢，即使在氧气存在的情况下也是如此。这一观察结果最先是由Warburg描述的，此后被认为是癌症的标志之一。这种开关背后的糖酵解酶的表达和/或活性受几个细胞信号通路的控制，这些信号通路导致糖酵解和乳酸排泄上调，丙酮酸进入TCA循环。开发这些途径的抑制剂是当前分子靶向癌症治疗和开发临床可翻译的分子成像方法以报道这些过程的巨大兴趣。Na/K ATPase(EC 3.6.1.37)是膜转运体，负责将每个转化的ATP分子泵出3NA和2K到细胞内，它是ATP的主要消耗分子。由于Na/K-ATPase与糖酵解酶丙酮酸激酶、磷酸甘油酸激酶和3-磷酸甘油醛脱氢酶在空间上是共定位的，因此Na/K-ATPase的ATP供应更多地来自糖酵解。此外，在没有线粒体的人红细胞中，先前使用13C通量分析用Na离子载体莫能菌素处理后的葡萄糖消耗表明，每消耗一个葡萄糖分子，就有6个Na离子被泵出细胞，这表明糖酵解通量和Na/K ATPase活性之间存在紧密耦合，而对一系列癌细胞的研究表明，用gricidin激活Na/K泵(或用哇巴因抑制)，分别导致糖酵解增加(或减少)。在此背景下，有证据表明，Na/K ATPase可能成为癌症治疗的靶点。接受地高辛等强心苷治疗的乳腺癌患者表现出明显的肿瘤细胞形态，确诊时肿瘤体积较小，两年后随访转移减少，乳腺癌切除后五年复发率降低。这些观察结果背后的机制尚不清楚，但有新的证据表明，作为这些糖苷的主要抑制靶点的Na/K ATPase可能成为癌症治疗的靶点。肿瘤发生过程中代谢变化、Na/K ATPase活性与病理之间的关系尚不清楚，目前还没有无创性的临床验证的成像工具来检测癌症或心脏中的Na/K ATPase活性。此外，尽管我们的认识有了很大的进步，但沃伯格效应背后的机制仍然知之甚少，成像技术仍然在很大程度上依赖于18FDG PET，它报告的是葡萄糖转运，而不是下游的生物能量学。在这个CDT项目中，我们建议开发一种体内成像技术的组合，使用23Na MRI来探测细胞内和细胞外的钠离子浓度，并结合82Rb PET来探测肿瘤和心脏中的Na/K ATPase活性，以提供迄今为止尚未探索的体内代谢失调的窗口。

It is widely accepted that hypoxia in tumours or myocardium with underdeveloped or obstructed vasculature, induces a switch from aerobic respiration to anaerobic glycolysis as the main fuel for ATP production. Tumours display a preference for glycolysis over oxidative metabolism even in the presence of oxygen. This observation was first described by Warburg and has since been recognized as one of the hallmarks of cancer. The expression and/or activity of glycolytic enzymes underlying this switch are controlled by several cell signalling pathways that results in upregulated glycolysis and lactate excretion and downregulated pyruvate entry into the TCA cycle. Developing inhibitors of these pathways is of enormous current interest for molecularly targeted cancer therapeutics and for developing clinically translatable molecular imaging approaches to report on these processes in vivo.Na/K ATPase (EC 3.6.1.37) is the membrane transporter responsible for pumping 3Na out and 2K into the cell for every molecule of ATP converted, of which it is a major consumer. The ATP supply for Na/K ATPase has been suggested to be preferentially derived from glycolysis since it is spatially co-localised with the glycolytic enzymes pyruvate kinase, phosphoglycerate kinase and glyceraldehyde 3-phosphate dehydrogenase. Furthermore, in the human erythrocyte, which possess no mitochondria, previous work using 13C flux analysis of glucose consumption following treatment with the Na ionophore monensin showed that 6 ions of Na are pumped out of the cell per molecule of glucose consumed, suggesting a tight coupling between glycolytic flux and Na/K ATPase activity, while studies in a range of cancer cells have shown that activation of the Na/K pump with gramicidin (or inhibition with ouabain), leads to an increase (or decrease) in glycolysis, respectively. In this context there is evidence that Na/K ATPase could be a target for cancer therapy. Breast cancer patients receiving cardiac glycosides such as digoxin show distinct tumour cell morphologies, smaller tumour volumes at diagnosis, reduced metastasis after two years follow-up and showed reduced recurrence rate five years after mastectomy. The mechanisms underlying these observations are unclear, but there is emerging evidence that Na/K ATPase, which is the principal inhibitory target for these glycosides, could be a therapeutic target for cancer treatment.The relationship between altered metabolism during tumorigenesis, Na/K ATPase activity and pathology is not well understood and there are currently no non-invasive clinically validated imaging tools of Na/K ATPase activity in cancer or the heart. Furthermore despite significant advances in our understanding, the mechanisms underlying the Warburg effect remain poorly understood and imaging techniques still largely rely on 18FDG PET which reports on glucose transport and not downstream bioenergetics. In this CDT project we propose to develop a combination of in vivo imaging techniques employing 23Na MRI for probing intracellular and extracellular sodium ion concentration in combination with 82Rb PET for probing Na/K ATPase activity in tumours and in the heart to offer a hitherto unexplored window on dysregulated metabolism in vivo.