On molecular level cells sense to the change in oxygen availability through the prolyl-4-hydroxylase domain enzymes (PHDs), which in turn regulates the protein stability of the α-subunit of the transcription factor hypoxia inducible factor (HIF). By using molecular oxygen PHDs 1 - 3 hydroxylate two specific proline residues thereby marking the HIFα for proteasomal degradation. Among the three PHD isoforms the constitutively expressed PHD2 is the main regulator of HIF-α stability and thus hypoxia-inducible gene expression in normoxia. PHD3 is highly induced under hypoxic conditions and serves as a negative feedback regulator.

For PHD3 a connection to apoptotic cell death mechanisms has been reported. In neuronal cells PHD3 is required and is sufficient to induce apoptosis after growth factor removal, while in neutrophils PHD3 seems to induce anti-apoptotic mechanisms. To investigate the role of PHD3 specifically in macrophages, myeloid specific PHD3 knockout mice were created. To this end PHD3^fl/fl mice were crossed with mice expressing the Cre recombinase under control of the endogenous M lysozyme locus (LysMcre). Cre-mediated recombination therefore results in deletion of PHD3 in the myeloid cell lineage, including monocytes, macrophages, granulocytes and in part dendritic cells.

Unlike PHD2 knockout bone marrow derived macrophages PHD3-deficient cells showed no HIF-1α stabilization in normoxia. Compared to macrophages from wild type littermates PHD3 knockout macrophages exhibited a significant reduction in TUNEL positive cells after serum withdrawal. Under the same conditions the PHD3 knockout macrophages also show less Annexin V staining which detects the membrane disruption indicating a reduced early apoptosis. Additionally, MTT results indicate that, over long term culture of PHD3 knockout macrophages are more viable. Above experiments indicates a pro apoptotic function of PHD3 in macrophages. The underlying molecular mechanisms have to be analyzed.