Effect of Sp1 small interfering RNA (siRNA) on alpha-secretase activity in U87 tumor cells under normoxic and hypoxic conditions. The incubation period was 12 hours. Alpha-secretase activity was significantly increased for U87 control cells under hypoxic compared to normoxic conditions. Sp1 suppression reduced alpha-secretase activity in hypoxic conditions. *P < 0.05 compared to normoxic control. #P < 0.05 compared to hypoxic control.