(A) siRNA knockdown of multiple transcription factors was performed in HeLa cells to examine its effect on basal expression of HIF-α. Graphs depict mean + SEM. *P ≤ 0.05, **P ≤ 0.01, ***P ≤ 0.001. (B) Endogenous SINHCAF immunoprecipitations in HeLa cells were conducted to determine SINHCAF interaction with SP1, p52, and E2F1. (C) Control, SINHCAF, and SP1 were singly or doubly knocked down in HeLa cells with or without treatment with hypoxia for 24 h and expression of the HIF system isoforms was determined by immunoblot. (D) RNA expression of the HIF-α isoforms was examined by qPCR following single or multiple knockdown of control, SINHCAF, and SP1. Representative images from at least three experiments are shown. N = 3. Graphs depict mean + SEM. *P ≤ 0.05, **P ≤ 0.01, ***P ≤ 0.001. See also Supplementary Figure S2.