Differential sub-nuclear distribution of hypoxia-inducible factors (HIF)-1 and-2 alpha impacts on their stability and mobility

Cellular adaptation to hypoxia occurs via a complex programme of gene expression mediated by the hypoxia-inducible factor (HIF). The oxygen labile alpha subunits, HIF-1 alpha/-2 alpha, form a heterodimeric transcription factor with HIF-1 beta and modulate gene expression. HIF-1 alpha and HIF-2 alpha possess similar domain structure and bind to the same consensus sequence. However, they have different oxygen-dependent stability and activate distinct genes. To better understand these differences, we used fluorescent microscopy to determine precise localization and dynamics. We observed a homogeneous distribution of HIF-1 alpha in the nucleus, while HIF-2 alpha localized into speckles. We demonstrated that the number, size and mobility of HIF-2 alpha speckles were independent of cellular oxygenation and that HIF-2 alpha molecules were capable of exchanging between the speckles and nucleoplasm in an oxygen-independent manner. The concentration of HIF-2 alpha into speckles may explain its increased stability compared with HIF-1 alpha and its slower mobility may offer a mechanism for gene specificity.