The nutrient sensing role of mitochondria in cell growth

細胞增生(cell proliferation)在個體層級(organism level)上受到相當嚴謹的調控，過程中必須保有忠實性(fidelity)，否則為避免錯誤被持續放大，細胞命運(cell fate)則會走向細胞凋亡(cell apoptosis)或細胞老化(cell senescence)。而細胞增生過程必須是細胞本體的內含物 (ex:粒線體、ribosomes數目)和大小(cell size)增加為原本的兩倍之多，進而連同以染色體(chromosomes)為主的細胞週期(cell cycle)分裂為兩個，其中細胞本體複製並累積的過程為細胞生長(cell growth)。細胞生長期間細胞會積極從外界送入養分後用以合成新巨分子同時增加細胞內含物，此時代謝狀態進行重整(metabolic reprogramming)：以粒線體(mitochondria)為主之TCA cycle做為提供細胞合成巨分子前驅物的中心、期間能量問題由大大提升的糖解作用(glycolysis)因應，而胺基酸glutamine做為重要的TCA cycle補充(anaplerosis)來源之一。 於人類纖維母細胞WI38中，處理粒線體malate-aspartate shuttle之抑制物AOA(aminooxyacetate)，會造成細胞生長停止、伴隨mTORC1活性急遽下降、mTORC2活性高度上升，長時間更導致細胞老化(cell senescence)，顯示粒線體做為生長時代謝重整所需。而若將AOA和α-KG或NEAA共同處理下會阻卻AOA所造成的現象：α-KG做為TCA cycle intermediates，在細胞生長時期是相當重要的anaplerosis來源之一，推測因此而阻卻AOA效應；另外NEAA中只有aspartate與asparagine有阻卻AOA的效果，又aspartate較asparagine有效，進一步探討後，發現aspartate阻卻AOA效果最佳濃度為3mM且處理asparagine synthetase inhibitor(L-DON)無法影響aspartate阻卻AOA的效用，由D-malate結果顯示aspartate阻卻AOA效應透過與AOA競爭aspartate aminotransferase，以OAA的形式做TCA cycle的補充效果(anaplerosis)回復細胞生長。 此研究透過探討aspartate阻卻AOA效應的機制，凸顯出粒線體在細胞生長代謝的重要性，並且藉此思考粒線體做為營養感知的重要角色。To maintain the homeostasis of organism, the process of cell proliferation should be promised the cell fidelity as the original cell. If not, the cell fate would be turned into cell apoptosis or cellular senescence in order to avoid amplified mistakes. Normally, cell proliferation is promoted by cell growth which is stimulated by growth factors while the extracellular nutrients are sufficient. Proliferating cells often take up a large amount of nutrients and shunt metabolites into pathways that support macromolecules biosynthesis and thereby increase in size and mass. In the meantime, cells reorganize the whole metabolic activity which is the process called “metabolic reprogramming”. Proliferating cells switch the role of mitochondria from producing energy to providing the macromolecule precursors for macromolecule biosynthesis and the energy demand during cell growth provided by upreglulated glycolysis. Moreover, glutamine-dependent anaplerosis is one of the major sources of TCA cycle intermediates. In human embryonic fibroblast WI38 cells, the malate-aspartate shuttle inhibitor, AOA(aminooxyacetate) induced cell cycle arrest, reduced mTORC1 activity, promoted mTORC2 activity and even caused cellular senescence in the long time. All AOA- induced effects were blocked by co-treating with α-KG or NEAA. We speculated α-KG blocked AOA-induced effects by anaplerosis because α-KG serves as one of the important TCA cycle intermediates that support macromolecule biogenesis. Among NEAA, only aspartate and asparagine could block the AOA-induced effects and the ability of aspartate was better than asparagine. In the further research, the most effective concentration of aspartate was 3 mM and the asparagine synthetase inhibitor (L-DON) couldn’t interfere with the ability of aspartate . We found that aspartate blocked the AOA-induced effects may be via competing with AOA and increasing anaplerosis through OAA. In this study, we explore the important metabolic role and the nutrient sensing role of mitochondria during cell growth