Department of Chemistry, Princeton University, Princeton, New Jersey, USA.
PLoS Biol. 2010 Oct 19;8(10):e1000514. doi: 10.1371/journal.pbio.1000514.
Many cells in mammals exist in the state of quiescence, which is characterized by reversible exit from the cell cycle. Quiescent cells are widely reported to exhibit reduced size, nucleotide synthesis, and metabolic activity. Much lower glycolytic rates have been reported in quiescent compared with proliferating lymphocytes. In contrast, we show here that primary human fibroblasts continue to exhibit high metabolic rates when induced into quiescence via contact inhibition. By monitoring isotope labeling through metabolic pathways and quantitatively identifying fluxes from the data, we show that contact-inhibited fibroblasts utilize glucose in all branches of central carbon metabolism at rates similar to those of proliferating cells, with greater overflow flux from the pentose phosphate pathway back to glycolysis. Inhibition of the pentose phosphate pathway resulted in apoptosis preferentially in quiescent fibroblasts. By feeding the cells labeled glutamine, we also detected a "backwards" flux in the tricarboxylic acid cycle from α-ketoglutarate to citrate that was enhanced in contact-inhibited fibroblasts; this flux likely contributes to shuttling of NADPH from the mitochondrion to cytosol for redox defense or fatty acid synthesis. The high metabolic activity of the fibroblasts was directed in part toward breakdown and resynthesis of protein and lipid, and in part toward excretion of extracellular matrix proteins. Thus, reduced metabolic activity is not a hallmark of the quiescent state. Quiescent fibroblasts, relieved of the biosynthetic requirements associated with generating progeny, direct their metabolic activity to preservation of self integrity and alternative functions beneficial to the organism as a whole.
哺乳动物的许多细胞都处于静止状态，其特征是可逆地退出细胞周期。静止细胞的体积、核苷酸合成和代谢活性普遍较低。与增殖淋巴细胞相比，静止淋巴细胞的糖酵解速率要低得多。相比之下，我们在这里展示了，当通过接触抑制诱导原代人成纤维细胞进入静止状态时，它们仍保持较高的代谢率。通过监测代谢途径中的同位素标记并从数据中定量确定通量，我们发现，接触抑制的成纤维细胞以类似于增殖细胞的速率利用葡萄糖的所有中心碳代谢分支，戊糖磷酸途径回流到糖酵解的溢出通量更大。戊糖磷酸途径的抑制导致静止成纤维细胞优先凋亡。通过给细胞喂食标记的谷氨酰胺，我们还检测到从α-酮戊二酸到柠檬酸的三羧酸循环中的“反向”通量，在接触抑制的成纤维细胞中增强；这种通量可能有助于将 NADPH 从线粒体穿梭到细胞质，用于氧化还原防御或脂肪酸合成。成纤维细胞的高代谢活性部分指向蛋白质和脂质的分解和再合成，部分指向细胞外基质蛋白的排泄。因此，代谢活性降低不是静止状态的标志。静止的成纤维细胞摆脱了与产生后代相关的合成要求，将其代谢活性导向自我完整性的维持和对整个生物体有益的替代功能。
