Combinatorial regulation of neuroblastoma tumor progression by N-Myc and hypoxia inducible factor HIF-1alpha.

In human neuroblastoma, amplification of the MYCN gene predicts poor prognosis and resistance to therapy. Because hypoxia contributes to aggressive tumor phenotypes, predominantly via two structurally related hypoxia inducible factors, HIF-1α and HIF-2α, we examined hypoxia responses in MYCN-amplified neuroblastoma cells. We demonstrate here that HIF-1α, but not HIF-2α, is preferentially expressed in both MYCN-amplified neuroblastoma cells and primary tumors in comparison to samples without MYCN amplification. Our results showed that interplay between N-Myc and HIF-1α plays critical roles in neuroblastoma. For example, high levels of N-Myc override HIF-1α inhibition of cell cycle progression, enabling continued proliferation under hypoxia. Furthermore, both HIF-1α and N-Myc are essential for the Warburg effect (aerobic glycolysis) in neuroblastomas by activating the transcription of multiple glycolytic genes. Of note, expressions of Phosphoglycerate Kinase 1 (PGK1), Hexokinase 2 (HK2), and Lactate Dehydrogenase A (LDHA) were each significantly higher in MYCN-amplified neuroblastomas than in tumors without MYCN amplification. Interestingly, MYCN-amplified neuroblastoma cells are "addicted" to LDHA enzymatic activity, as its depletion completely inhibits tumorigenesis in vivo. Thus, our results provide mechanistic insights explaining how MYCN-amplified neuroblastoma cells contend with hypoxic stress and paradoxically how hypoxia contributes to neuroblastoma aggressiveness through combinatorial effects of N-Myc and HIF-1α. These results also suggest that LDHA represents a novel, pharmacologically tractable target for neuroblastoma therapeutics.