Transcriptional regulation in neuroblastoma cells under normoxic and hypoxic conditions

The childhood malignancy neuroblastoma develops from early cells of the sympathetic nervous system (SNS), and the tumors often produce catecholamines. Neuroblastoma cells retain several characteristics of immature sympathetic cells including the expression of a number of transcription factors normally expressed during embryogenesis. The genesis of the SNS requires the correct expression pattern of several transcription factors of the basic helix-loop-helix (bHLH) family, such as HASH-1 a pro-neuronal gene. HASH-1 expression is negatively regulated by another bHLH factor, HES-1. In these studies, we show that HASH-1 form transcription activating dimers with the ubiquitously expressed bHLH factor E2-2 in neuroblastoma cells. Furthermore, we establish that the Id proteins, which lack the basic DNA-binding domain and act as dominant negative inhibitors in the bHLH network, bind to HES-1. By dimerization with HES-1 the Id proteins may act to repress the HES-1 mediated transcriptional repression. In addition, HES-1 can alleviate the negative effect of Id proteins on bHLH factor induced transcription, by sequestration of Id. These findings reveal a novel regulatory level of the bHLH network.

Solid tumors most often contain regions with impaired circulation and hypoxia. Two transcription factors of the bHLH/PAS subgroup, the hypoxia inducible factors HIF-1a and HIF-2a, are key regulators of the cellular response to oxygen deprivation. Interestingly, HIF-2a is also expressed in the developing SNS and required for catecholamine production, and HIF-2a deficient mice die with bradycardia before birth. The dual implications of HIF-2a in neuroblastoma tumors, involvement in SNS development as well as in adaptation to the tumor microenvironment , prompted us to investigate how neuroblastoma cells respond to growth under hypoxic conditions. Unexpectedly, we found that hypoxia (1% oxygen) drive the neuroblastoma cells toward an immature and neural crest-like phenotype. Several neuronal and neuroendocrine marker genes, such as chromogranin A/B, NPY, and HASH-1, were down-regulated in response to oxygen deprivation, whereas a number of genes expressed during early neural crest development were up-regulated, examplified by c-kit, Notch-1, and Id2. To further delineate the the hypoxic phenotype of human neuroblastoma cells, we have analyzed their gene expression after 72 h exposure to hypoxia employing microarray analysis harboring 35 000 clones. Almost one percent of the represented transcripts were up-regulated more than three-fold and about 0.5 % were down-regulated more than three-fold in response to hypoxia. The microarray results strenghten our view of hypoxic neuroblastoma cells as less mature. To test whether hypoxia-induced dedifferentiation is a neuroblastoma specific event or may occur also in other solid tumors, we have analyzed a panel of ductal breast carcinoma in situ. Also in these tumors, were hypoxia associated with a less mature phenotype of the tumor cells. We propose hypoxia-induced dedifferentiation as one means by which intra-tumor hypoxia drives tumor propagation.