Madreiter-Sokolowski Corina T, Gottschalk Benjamin, Sokolowski Armin A, Malli Roland, Graier Wolfgang F
Molecular Biology and Biochemistry, Gottfried Schatz Research Center, Medical University of Graz, Graz, Austria.
Department of Health Sciences and Technology, ETH Zurich, Schwerzenbach, Switzerland.
Front Cell Dev Biol. 2021 Feb 4;9:614668. doi: 10.3389/fcell.2021.614668. eCollection 2021.
Cancer cells have increased energy requirements due to their enhanced proliferation activity. This energy demand is, among others, met by mitochondrial ATP production. Since the second messenger Ca maintains the activity of Krebs cycle dehydrogenases that fuel mitochondrial respiration, proper mitochondrial Ca uptake is crucial for a cancer cell survival. However, a mitochondrial Ca overload induces mitochondrial dysfunction and, ultimately, apoptotic cell death. Because of the vital importance of balancing mitochondrial Ca levels, a highly sophisticated machinery of multiple proteins manages mitochondrial Ca homeostasis. Notably, mitochondria sequester Ca preferentially at the interaction sites between mitochondria and the endoplasmic reticulum (ER), the largest internal Ca store, thus, pointing to mitochondrial-associated membranes (MAMs) as crucial hubs between cancer prosperity and cell death. To investigate potential regulatory mechanisms of the mitochondrial Ca uptake routes in cancer cells, we modulated mitochondria-ER tethering and the expression of UCP2 and analyzed mitochondrial Ca homeostasis under the various conditions. Hence, the expression of contributors to mitochondrial Ca regulation machinery was quantified by qRT-PCR. We further used data from The Cancer Genome Atlas (TCGA) to correlate these findings with expression patterns in human breast invasive cancer and human prostate adenocarcinoma. ER-mitochondrial linkage was found to support a mitochondrial Ca uptake route dependent on uncoupling protein 2 (UCP2) in cancer cells. Notably, combined overexpression of Rab32, a protein kinase A-anchoring protein fostering the ER-mitochondrial tethering, and UCP2 caused a significant drop in cancer cells' viability. Artificially enhanced ER-mitochondrial tethering further initiated a sudden decline in the expression of UCP2, probably as an adaptive response to avoid mitochondrial Ca overload. Besides, TCGA analysis revealed an inverse expression correlation between proteins stabilizing mitochondrial-ER linkage and UCP2 in tissues of human breast invasive cancer and prostate adenocarcinoma. Based on these results, we assume that cancer cells successfully manage mitochondrial Ca uptake to stimulate Ca-dependent mitochondrial metabolism while avoiding Ca-triggered cell death by fine-tuning ER-mitochondrial tethering and the expression of UCP2 in an inversed manner. Disruption of this equilibrium yields cancer cell death and may serve as a treatment strategy to specifically kill cancer cells.
癌细胞因其增强的增殖活性而对能量的需求增加。这种能量需求，除其他外，由线粒体ATP产生来满足。由于第二信使Ca维持为线粒体呼吸供能的三羧酸循环脱氢酶的活性，适当的线粒体Ca摄取对癌细胞存活至关重要。然而，线粒体Ca过载会诱导线粒体功能障碍，并最终导致凋亡性细胞死亡。由于平衡线粒体Ca水平至关重要，多种蛋白质组成的高度复杂机制负责管理线粒体Ca稳态。值得注意的是，线粒体优先在内质网（ER）（最大的内部Ca库）与线粒体的相互作用位点处隔离Ca，因此，线粒体相关膜（MAMs）被认为是癌细胞繁荣与细胞死亡之间的关键枢纽。为了研究癌细胞中线粒体Ca摄取途径的潜在调控机制，我们调节了线粒体与ER的连接以及UCP2的表达，并分析了各种条件下的线粒体Ca稳态。因此，通过qRT-PCR对线粒体Ca调节机制相关因子的表达进行了定量。我们进一步使用来自癌症基因组图谱（TCGA）的数据，将这些发现与人类乳腺浸润癌和人类前列腺腺癌中的表达模式进行关联。发现ER-线粒体连接支持癌细胞中依赖解偶联蛋白2（UCP2）的线粒体Ca摄取途径。值得注意的是，Rab32（一种促进ER-线粒体连接的蛋白激酶A锚定蛋白）和UCP2的联合过表达导致癌细胞活力显著下降。人为增强ER-线粒体连接进一步引发UCP2表达突然下降，这可能是一种适应性反应，以避免线粒体Ca过载。此外，TCGA分析显示，在人类乳腺浸润癌和前列腺腺癌组织中，稳定线粒体-ER连接的蛋白质与UCP2之间存在反向表达相关性。基于这些结果，我们推测癌细胞通过以相反的方式微调ER-线粒体连接和UCP2的表达，成功地管理线粒体Ca摄取，以刺激Ca依赖性线粒体代谢，同时避免Ca触发的细胞死亡。破坏这种平衡会导致癌细胞死亡，这可能成为一种特异性杀死癌细胞的治疗策略。
