肿瘤环境因子乳酸对肿瘤细胞代谢的作用及其在治疗中的意义

Despite that metabolic adaptation of solid tumor to glucose shortage is an important issue, it has not yet attracted sufficient attention. Due to the disorganized vasculature and mostly dysfunctional capillary bed, solid tumor is short of glucose supply. Since glucose is the essential nutrient for tumor growth, how solid tumor under constant or temporary glucose deprivation can progressively grow remains elusive. We recently revealed that lactic acidosis, a major tumor microenvironmental factor, is a culprit that assists tumor to metabolically adapt to glucose starvation or even deprivation. We sought to resolve 2 key scientific issues. (1) How does lactic acidosis regulate glucose metabolism especially when glucose supply is scarce. Currently, Warburg effect or aerobic glycolysis is the focus of cancer glucose metabolism, i.e., how and why most cancer cells in culture medium with sufficient glucose supply persistently convert incoming glucose to lactate even with presence of ample oxygen. We recently found that, without lactic acidosis, cancer cells use glucose via aerobic glycolysis, whereas under lactic acidosis, cancer cells behaved differently: they exhibited very low glucose consumption without net lactate generation. The observation indicates that lactic acidosis can switch glucose metabolic mode in cancer cells. Nevertheless, how lactic acidosis regulates the metabolic switch remains unclear hence is a major aim of this research. (2) Can intervention of lactic acidosis be a novel strategy to treat tumor? Solid tumours are dependent on glucose, but are generally glucose-deprived due to poor vascularization. Nevertheless, cancer cells can generally survive glucose deprivation better than their normal counterparts. Thus, to render cancer cells sensitive to glucose depletion may potentially provide an effective strategy for cancer intervention. We found that lactic acidosis conferred cancer cells with resistance to glucose deprivation-induced death, whereas converting lactic acidosis to lactosis by elevating acidic pH to basic pH could resume cancer cells' sensitivity to glucose depletion and lead cancer cells to rapid death as soon as glucose was deprived. This finding may shed light on how cancer cells in solid tumors tolerate glucose deprivation. The tumor anatomy dictates that with the exception of a fraction of cells surrounding the functional capillary, the majority of solid tumour cells are under glucose starvation. Under lactic acidosis, these cells might survive when glucose is deprived and resume proliferation when glucose is provided. Lactic acidosis also switch cancer cells to a much more economical mode to use glucose, and this is particularly important for cancer cells in solid tumours, where glucose is scarce. In the proposed study, we would intervene with tumor lactic acidosis by elevating intratumoral pH and prove the feasibility of this approach to treat tumor.

实体肿瘤对葡萄糖匮乏的代谢适应是肿瘤生物学的重要理论问题，但一直未受到应有的关注。由于实体肿瘤血管结构功能紊乱，葡萄糖供应不足，大部分肿瘤细胞处于糖饥饿状态。鉴于葡萄糖是肿瘤生长的必需营养素，处于糖饥饿状态的肿瘤何以能进行性生长？我们对该问题的研究有了突破性的进展，揭示了肿瘤微环境因子乳酸是肿瘤细胞适应低糖代谢的主要原因。本课题拟解决的主要科学问题是：（1）乳酸对肿瘤细胞代谢的调控及其机制。我们前期发现乳酸能调控肿瘤的糖代谢模式，使肿瘤细胞从极度浪费葡萄糖的有氧糖酵解转变成缓慢经济高效利用葡萄糖。解析这一调控规律和机制能解释肿瘤对糖匮乏的代谢适应。（2）乳酸对肿瘤细胞适应糖剥夺的生物学和临床意义。我们发现乳酸能保护肿瘤细胞免于糖剥夺引起的死亡，而中和乳酸即刻恢复肿瘤细胞对糖剥夺的敏感性而死亡。利用肿瘤这一自身的代谢弱点来治疗肿瘤是我们提出的新策略。我们将证明这一策略的可行性。

Warburg effect是肿瘤细胞代谢的一大特征，也是当前肿瘤代谢研究的热点。Warburg effect是指肿瘤细胞即使在有氧条件下将大部分葡萄糖代谢转化成乳酸，被称为有氧糖酵解。我们在前期的研究中发现了肿瘤细胞代谢的双重性：在某些条件下，肿瘤细胞的表型为Warburg effect，在另一些条件下，肿瘤细胞的表型为低速率的糖酵解和产生少量的乳酸。因此，我们提出了了肿瘤细胞代谢的双重性。..我们进而对肿瘤细胞代谢双重性的机理做了研究：肿瘤细胞在培养基中大量消耗葡萄糖，产生大量的乳酸，乳酸堆积产生两个效应，其中的质子抑制糖酵解酶，其抑制效应呈量效关系；乳酸根的堆积则影响乳酸脱氢酶催化反应的正方向和反方向的速率，当乳酸积累到一定程度时，该反应趋于平衡，因此乳酸不再净产生。这两者共同的作用是将肿瘤细胞从Warburg effect 转化到节约型代谢的基础。这两种代谢模式是可以相互转化的，这种动态变化适应肿瘤细胞适应环境变化。事实上，肿瘤内环境是不断变化的。..我们运用乳酸对肿瘤细胞抵抗葡萄糖剥夺和对肿瘤细胞糖酵解调控的原理，对原发性肝细胞肝癌做了开拓性的临床转化，突破了大肝癌的治疗瓶颈，论文发表在2016年的eLife杂志上。..我们对肿瘤细胞内的PKM2做了研究，从热力学、酶学、代谢流量控制、细胞生物学等方面详细验证了PKM2是否是肿瘤细胞糖酵解的限速酶，我们得出的结论是： PKM2不是肿瘤细胞糖酵解的限速酶，提出了新的观点，与国际主流的观点完全相反。论文发表在J Biol Chem。..肿瘤糖酵解的一个关键问题是能量代谢。糖酵解和氧化磷酸化对肿瘤细胞能量贡献的大小一直模糊。我们做了一个定量的工作，在常规培养下，肿瘤细胞表现为Warburg模式，不同的肿瘤细胞有不同，糖酵解贡献的能量在23.7% – 52.2 %之间，氧化磷酸化贡献的能量在 47.8% - 76.3% 之间；当在培养基中加入20 mM 乳酸，pH 6.7时，糖酵解贡献的能量在5.7% - 13.4%之间，氧化磷酸化贡献的能量在86.6% - 94.3% 之间。这对糖酵解和氧化磷酸化对肿瘤细胞能量贡献做了量的定义。..我们发现，乳酸的积累还能导致肿瘤细胞内ROS（reactive oxygen species）的大量增加，这是一个新的发现。对于为何ROS会大量增加，我们已做了一些研究，发现一些代谢酶