Hypoxia-inducible factors (HIF) are heterodimeric (alpha/beta)
transcription factors that play a fundamental role in cellular adaptation to low
oxygen tension. In the presence of
oxygen, the HIF-alpha subunit becomes hydroxylated at specific prolyl residues by
prolyl hydroxylases. This post-translational modification is recognized by the von Hippel-Lindau (VHL)
protein, which targets HIF-alpha for degradation. In the absence of
oxygen, HIF-alpha hydroxylation is compromised and this subunit is stabilized. We have previously shown that the
hypoxia-induced accumulation of HIF-alpha
protein is strongly impaired by the inhibitor of
diacylglycerol kinase,
R59949. Here, we have investigated the mechanisms through which this inhibitor exerts its effect. We found that
R59949 inhibits the accumulation of HIF-1/2alpha
protein without affecting the expression of their mRNAs. We also determined that
R59949 could only block the accumulation of HIF-alpha in the presence of VHL
protein. In agreement with this, the binding of VHL to endogenous HIF-alpha was significantly enhanced after
R59949 treatment, even under hypoxic conditions. In addition, we found that
R59949 could stimulate
prolyl hydroxylase both at 21% O2 as well as at 1% O2. Taken together, these results reveal that
R59949 is an activator of HIF
prolyl hydroxylases. This is of particular interest when we consider that, to date, mainly inhibitors of these
enzymes have been described.