A microfluidic oxygen gradient demonstrates differential activation of the hypoxia-regulated transcription factors HIF-1α and HIF-2α.

Gas-perfused microchannels generated a linear oxygen gradient via diffusion across a 100 μm polydimethylsiloxane (PDMS) membrane. The device enabled exposure of a single monolayer of cells sharing culture media to a heterogeneous oxygen landscape, thus reflecting the oxygen gradients found at the microscale in the physiological setting and allowing for the real-time exchange of paracrine factors and metabolites between cells exposed to varying oxygen levels. By tuning the distance between two gas supply channels, the slope of the oxygen gradient was controlled. We studied the hypoxic activation of the transcription factors HIF-1α and HIF-2α in human endothelial cells within a spatial linear gradient of oxygen. Quantification of the nuclear to cytosolic ratio of HIF immunofluorescent staining demonstrated that the threshold for HIF-1α activation was below 2.5% O while HIF-2α was activated throughout the entire linear gradient. We show for the first time HIF-2α is subject to hyproxya, hypoxia by proxy, wherein hypoxic cells activate HIF in close-proximity normoxic cells. These results underscore the differences between HIF-1α and HIF-2α regulation and suggest that a microfluidic oxygen gradient is a novel tool for identifying distinct hypoxic signaling activation and interactions between differentially oxygenated cells.