Hypoxia-inducible factor-1β (HIF-1β) is upregulated in a HIF-1α-dependent manner in 518A2 human melanoma cells under hypoxic conditions

Solid tumors include hypoxic areas due to excessive cell proliferation. Adaptation to low oxygen levels is mediated by the hypoxia-inducible factor (HIF) pathway promoting invasion, metastasis, metabolic alterations, chemo-resistance and angiogenesis. The transcription factor HIF-1, the major player within this pathway consists of HIF-1 alpha and HIF-1 beta. The alpha subunit is continuously degraded under normoxia and becomes stabilized under reduced oxygen supply. In contrast, HIF-1 beta is generally regarded as constitutively expressed and being present in excess within the cell. However, there is evidence that the expression of this subunit is more complex. The aim of this study was to investigate the role of HIF-1 beta in human melanoma cells. Among a panel of five different cell lines, in 518A2 cells exposed to the hypoxia-mimetic cobalt chloride HIF-1 beta was rapidly elevated on protein level. Knockdown experiments performed under cobalt chloride-exposure and hypoxia revealed that this effect was mediated by HIP-la. The non-canonical relationship between these subunits was further confirmed by pharmacologic inhibition of HIF-1 alpha and by expression of a dominant-negative HIF mutant. Overexpression of HIF-1 alpha showed a time delay in HIP-1 beta induction, thus arguing for HIF-1 beta de novo synthesis rather than protein stabilization by heterodimerization. A Hen's egg test-chorioallantoic membrane model of angiogenesis and invasion indicated a local expression of HIF-1 beta and implies a biological relevance of these findings. In summary, this study demonstrates the HIF-1 alpha-dependent regulation of HIF-1 beta under hypoxic conditions for the first time. The results indicate a novel cell specific mechanism which might prevent HIF-1 beta to become a limiting factor. (C) 2013 Elsevier Inc. All rights reserved.