美国冷泉港实验室（CSHL）研究人员在肿瘤细胞中发现3种阻抑蛋白，可影响丙酮酸激酶的两个亚型的剪接，从而改变细胞代谢机制。该发现有助于科学家理解并克服困扰医学界长达80年的难题——“瓦伯格效应”，并将有助于找到一种抑制肿瘤细胞代谢和肿瘤生长的新方法。相关研究成果刊发在最近的《美国国家科学院院刊》网络版上。
20世纪30年代，德国生物化学家奥托·瓦伯格发现，肿瘤和正常成体组织存在着代谢差异，它们通过糖酵解产能，并产生大量的副产品——乳酸。这种代谢性质使得肿瘤细胞的耗糖速度远大于正常细胞。这种肿瘤细胞对糖酵解通路产能依赖增强的现象，称为“瓦伯格效应”，它会极快地促进细胞增生和肿瘤生长。而最近有研究表明，是一种叫做PK-M2（丙酮酸激酶M2）的蛋白促进了肿瘤细胞的这种代谢，对肿瘤的形成和生长起着至关重要的作用。
PK-M2是丙酮酸激酶的一个亚型，而其另一个亚型PK-M1，则与M2不同，是无害的。这两种亚型都源于同一基因——PK-M基因。该基因以一种独一无二的方式进行可变剪接（即基因的mRNA前体按不同的方式剪接，产生出两种或更多种mRNA），生成M1和M2两种亚型。在肿瘤细胞中，PK-M基因的可变剪接为何会只产生危险的M2，而不产生无害的M1，则一直是个谜团。
美国纽约冷泉港实验室阿德里安·克莱内尔教授领导的研究小组，对多种类型癌症细胞中的众多剪接因子进行了筛查，最终发现了决定着M1和M2开关的3种剪接阻抑蛋白。研究人员发现，这3种蛋白在肿瘤细胞中的含量很高，是它们抑制了M1亚型的剪接，使肿瘤细胞只产出M2。通过降低细胞中这3种蛋白的水平，可降低M2水平和乳酸生成量，恢复M1的生成，从而在很大程度上逆转“瓦伯格效应”。
克莱内尔指出，这3种阻抑蛋白被阻断后，细胞并不会完全停止M2的生产，这表明可能还有其他的剪接因子影响着M1和M2的开关。
目前该研究团队正在寻找其他可能的剪接因子。而这种恢复正常的代谢状态是否会阻碍肿瘤细胞的快速生长，则有待进一步研究。
克莱内尔表示，虽然对于“瓦伯格效应”还有几个基本问题尚未解决，目前也还不十分清楚该效应的作用机制，但关于细胞代谢机制的研究或可有助于揭开这一谜题，从而发现新的分子药物标靶，开发出剪接因子抑制药物和逆转“瓦伯格效应”的药物，通过改变肿瘤细胞的代谢机制来治疗癌症。（生物谷Bioon.com）
生物谷推荐原始出处：
PNAS January 19, 2010, doi: 10.1073/pnas.0914845107
The alternative splicing repressors hnRNP A1/A2 and PTB influence pyruvate kinase isoform expression and cell metabolism
Cynthia V. Clowera,1, Deblina Chatterjeeb,c,1, Zhenxun Wangb,d, Lewis C. Cantleya, Matthew G. Vander Heidena,e,3, and Adrian R. Krainerb,c,d,2
aDivision of Signal Transduction, Beth Israel Deaconess Medical Center and Department of Systems Biology, Harvard Medical School, Boston, MA 02115;
bCold Spring Harbor Laboratory, Cold Spring Harbor, NY 11724;
cGraduate Program in Molecular and Cellular Biology, Stony Brook University, Stony Brook, NY 11794;
dWatson School of Biological Sciences, Cold Spring Harbor, NY 11724; and
eDepartment of Medical Oncology, Dana-Farber Cancer Institute, Boston, MA 02115
Cancer cells preferentially metabolize glucose by aerobic glycolysis, characterized by increased lactate production. This distinctive metabolism involves expression of the embryonic M2 isozyme of pyruvate kinase, in contrast to the M1 isozyme normally expressed in differentiated cells, and it confers a proliferative advantage to tumor cells. The M1 and M2 pyruvate-kinase isozymes are expressed from a single gene through alternative splicing of a pair of mutually exclusive exons. We measured the expression of M1 and M2 mRNA and protein isoforms in mouse tissues, tumor cell lines, and during terminal differentiation of muscle cells, and show that alternative splicing regulation is sufficient to account for the levels of expressed protein isoforms. We further show that the M1-specific exon is actively repressed in cancer-cell lines—although some M1 mRNA is expressed in cell lines derived from brain tumors—and demonstrate that the related splicing repressors hnRNP A1 and A2, as well as the polypyrimidine-tract-binding protein PTB, contribute to this control. Downregulation of these splicing repressors in cancer-cell lines using shRNAs rescues M1 isoform expression and decreases the extent of lactate production. These findings extend the links between alternative splicing and cancer, and begin to define some of the factors responsible for the switch to aerobic glycolysis.
