Figure 4. S1P stabilizes HIF-1α independently of pVHL binding.

(A) S1P prolongs HIF-1α half-life. Cells were either left untreated, treated with S1P (100 nM) for 6 h, incubated in hypoxia (1% O₂) for 6 h or treated with CoCl₂ (150 µM) for 3 h before the cycloheximide chase (Chx, 5 µg/ml). S1P, hypoxic conditions or CoCl₂ were present throughout the chase. Time points are mean ± SEM, n = 3–10. Curve fit was done with the one phase exponential decay equation. (B) S1P does not inhibit binding of pVHL to HIF-1α. Cells were treated with S1P (100 nM) for 6 h. The level of co-immunoprecipitated HIF-1α was compared with the level of immunoprecipitated pVHL and IgG bands were used as a loading control. **P < 0.01 indicates statistically significant difference between S1P treatment and vehicle control.