Imaging of the hypoxia-inducible factor pathway: insights into oxygen sensing

The transcription factor complex hypoxia-inducible factor (HIF)-1 controls the expression of most genes involved in adaptation to hypoxic conditions. HIF-1 is a heterodimer composed of oxygen-labile HIF-alpha and constitutively expressed HIF-alpha subunits. The oxygendependent regulation of HIF-a is a multistep process that includes degradation under normoxia but stabilisation, translocation into the nucleus and activation under hypoxic conditions. The present paper summarises the contributions of optical methods to the understanding of oxygen-dependent regulation of the HIF-1 pathway. The tissue- and cell-specific distribution of HIF-alpha was visualised immunohistochemically and by immunofluorescence. Transcriptional activity of HIF-1 was monitored using green fluorescent protein as a reporter under control of hypoxia response elements in living cells, spheroids and tumour tissues in living mice. With cyan and yellow variants of green fluorescent protein fused to HIF subunits and regulatory proteins, subcellular distribution, migration and interaction were imaged in vivo by means of fluorescence recovery after photo-bleaching and fluorescence resonance energy transfer. Noninvasive imaging of these cellular and molecular processes by laser scanning microscopy complements ex vivo molecular biology assays and provides an additional spatial and temporal dimension to the understanding of the HIF-1 pathway.