HnRNP proteins controlled by c-Myc deregulate pyruvate kinase mRNA splicing in cancer

Abstract

Cancer cells avidly take up glucose and convert it to lactate while eschewing oxidative phosphorylation. This phenomenon is critical for maximal tumorigenicity, and is in part explained by the almost universal reversion of tumours to the embryonic form of pyruvate kinase, PKM2. Here, David et al. now show that aberrant expression of the splicing factors PTB, hnRNPA1 and hnRNPA2, which are themselves regulated by the c-Myc oncogene, is responsible for the PKM1 to PKM2 switch in cancer. This work adds to our understanding of alternative splicing and its role in cancer cell growth. Expression of the embryonic M2 isoform of pyruvate kinase (PKM2) by tumour cells promotes aerobic glycolysis, whereas the normal adult isoform, PKM1, promotes oxidative phosphorylation. Expression of these isoforms is regulated by alternative splicing; here, aberrant expression of three heterogeneous nuclear ribonucleoprotein splicing factors, which are themselves regulated by the c-Myc oncogene, is shown to be responsible for the M1 to M2 switch in cancer. When oxygen is abundant, quiescent cells efficiently extract energy from glucose primarily by oxidative phosphorylation, whereas under the same conditions tumour cells consume glucose more avidly, converting it to lactate. This long-observed phenomenon is known as aerobic glycolysis1, and is important for cell growth2,3. Because aerobic glycolysis is only useful to growing cells, it is tightly regulated in a proliferation-linked manner4. In mammals, this is partly achieved through control of pyruvate kinase isoform expression. The embryonic pyruvate kinase isoform, PKM2, is almost universally re-expressed in cancer2, and promotes aerobic glycolysis, whereas the adult isoform, PKM1, promotes oxidative phosphorylation2. These two isoforms result from mutually exclusive alternative splicing of the PKM pre-mRNA, reflecting inclusion of either exon 9 (PKM1) or exon 10 (PKM2). Here we show that three heterogeneous nuclear ribonucleoprotein (hnRNP) proteins, polypyrimidine tract binding protein (PTB, also known as hnRNPI), hnRNPA1 and hnRNPA2, bind repressively to sequences flanking exon 9, resulting in exon 10 inclusion. We also demonstrate that the oncogenic transcription factor c-Myc upregulates transcription of PTB, hnRNPA1 and hnRNPA2, ensuring a high PKM2/PKM1 ratio. Establishing a relevance to cancer, we show that human gliomas overexpress c-Myc, PTB, hnRNPA1 and hnRNPA2 in a manner that correlates with PKM2 expression. Our results thus define a pathway that regulates an alternative splicing event required for tumour cell proliferation.