Eigenbrodt E, Reinacher M, Scheefers-Borchel U, Scheefers H, Friis R
Institut für Biochemie und Endokrinologie, Giessen, Federal Republic of Germany.
Crit Rev Oncog. 1992;3(1-2):91-115.
As a common characteristic of tumor cells, as well as of normal proliferating cells in the G1-phase of cell cycle, one finds constitutive high levels of all the glycolytic metabolites arising between glucose 6-phosphate and phosphoenolpyruvate. Thus, it is that the phosphometabolites fructose 1,6-bisphosphate, ribose 5-P, P-ribose-PP, NAD, GTP, CTO, UTP, UDP-glucose, glycerol 3-P, glycerol phosphocholine and glycerol phosphoethanolamine are useful in the 31P-nuclear magnetic resonance (NMR) detection of solid tumors in animals and man. This expansion of phosphometabolites is achieved during tumor formation as a result of reductions in levels of enzymes degrading phosphometabolites, owing to the decline in the glycerol 3-P hydrogen shuttle, and as a consequence of alterations in the glycolytic isoenzyme equipment. Tumor cells typically express a particular isoenzyme of pyruvate kinase called type M2 (K) at high levels. This isoenzyme is subject to a complex regulation by amino acids, by fructose 1,6-bisphosphate, and by hormonal- and oncogene-dependent phosphorylation. Pyruvate kinase type M2 is a substrate for the oncogene encoded PP60v-src-tyrosine kinase. A drastic decrease in the affinity for its substrate phosphoenolpyruvate found after transformation by the src-oncogene can be explained as a consequence of the phosphorylation of pyruvate kinase in serine and tyrosine. These phosphorylations induce the breakdown of tetrameric pyruvate kinase to the trimeric and dimeric forms. Unlike the tetrameric form, the dimeric form as a low affinity for phosphoenolpyruvate. Partial inactivation of pyruvate kinase and enolase on the one hand, and a hyperactivation of hexokinase and phosphofructokinase on the other hand, lead to an expansion of all metabolites. Only when these metabolites attain high levels, thereby assuring a sufficient supply of metabolites for RNA, DNA, lipid, and complex carbohydrate synthesis, can cell proliferation proceed. This accumulation of metabolites in the G1-phase cells has been termed a "metabolic budget system" because it senses not only the actual nutrient levels, but also the supply over a period of time. Monoclonal antibodies specific for the dimeric form of pyruvate kinase type M2 can be used for the immunohistological detection of tumor cells. The amount of the dimeric form in tumor cells closely correlates with the degree of malignancy and can be used for a nonspecific detection of tumors based on assays performed with patient's plasma.
作为肿瘤细胞以及处于细胞周期G1期的正常增殖细胞的共同特征，人们发现从6-磷酸葡萄糖到磷酸烯醇丙酮酸之间的所有糖酵解代谢产物都持续处于高水平。因此，磷酸代谢产物1,6-二磷酸果糖、5-磷酸核糖、磷酸核糖焦磷酸、烟酰胺腺嘌呤二核苷酸（NAD）、鸟苷三磷酸（GTP）、胞苷三磷酸（CTP）、尿苷三磷酸（UTP）、尿苷二磷酸葡萄糖、3-磷酸甘油、甘油磷酸胆碱和甘油磷酸乙醇胺可用于通过磷-31核磁共振（NMR）检测动物和人类的实体瘤。在肿瘤形成过程中，由于降解磷酸代谢产物的酶水平降低、3-磷酸甘油氢穿梭的减少以及糖酵解同工酶组成的改变，实现了磷酸代谢产物的这种增加。肿瘤细胞通常高水平表达一种称为M2型（K）的丙酮酸激酶特定同工酶。这种同工酶受到氨基酸、1,6-二磷酸果糖以及激素和癌基因依赖性磷酸化的复杂调节。M2型丙酮酸激酶是癌基因编码的PP60v-src-酪氨酸激酶的底物。src癌基因转化后，其对底物磷酸烯醇丙酮酸的亲和力急剧下降，这可以解释为丙酮酸激酶丝氨酸和酪氨酸磷酸化的结果。这些磷酸化导致四聚体丙酮酸激酶分解为三聚体和二聚体形式。与四聚体形式不同，二聚体形式对磷酸烯醇丙酮酸的亲和力较低。一方面丙酮酸激酶和烯醇化酶部分失活，另一方面己糖激酶和磷酸果糖激酶过度活化，导致所有代谢产物增加。只有当这些代谢产物达到高水平，从而确保为RNA、DNA、脂质和复合碳水化合物合成提供足够的代谢产物供应时，细胞增殖才能进行。G1期细胞中这些代谢产物的积累被称为“代谢预算系统”，因为它不仅能感知实际的营养水平，还能感知一段时间内的供应情况。对M2型丙酮酸激酶二聚体形式具有特异性的单克隆抗体可用于肿瘤细胞的免疫组织化学检测。肿瘤细胞中二聚体形式的量与恶性程度密切相关，可用于基于患者血浆检测的肿瘤非特异性检测。
