Synthesis rates of HIF-1α are unchanged in Psen1-/- fibroblasts. Psen1+/+ (A) and Psen1-/- (B) fibroblasts were treated with the proteasome inhibitor MG132 for the indicated times (min). Lysates were prepared and Western blotting was performed using an anti-HIF-1α antibody. The lower panels show the blots reprobed for β-tubulin. In panels C and D, the rate of accumulation of HIF-1α normalized to β-tubulin levels from the experiments in panels A and B is shown either as the raw data (C) or normalized to basal levels of HIF-1α in the respective cell types (D). Note that although basal levels of HIF-1α are lower in Psen1-/- fibroblasts, HIF-1α accumulated at similar rates in Psen1+/+ and Psen1-/- fibroblasts. A representative example is shown from experiments that were performed multiple times.