Redd1, a recently discovered stress-response gene, is regulated by hypoxia via hypoxia-inducible factor 1 (HIF-1) and by DNA damage via p53/p63; however, the signaling pathway by which its expression is induced by hypoxia has not been elucidated. In the present study, we demonstrated that the expression of Redd1 in response to hypoxia (1% O₂), hypoxia-mimetic agent, cobalt chloride (CoCl₂) and high cell density (HCD) requires coactivation of HIF-1α and Sp1. CoCl₂ and HCD induced the activation of HIF-1α and Sp1 in HeLa cells, and siRNAs targeting HIF-1α and Sp1 abrogated Redd1 expression. Inhibition of phosphatidylinositol 3-kinase (PI3K) by LY294002 and by a dominant-negative PI3K mutant reduced the expression of Redd1 and activation of HIF-1α and Sp1 by CoCl₂ and HCD. Also, suppression of Akt activation blocked the expression of Redd1 and the activation of HIF-1α and Sp1 by CoCl₂ and HCD. Furthermore, we found that the induction of Redd1 expression by CoCl₂ can be mediated by activation of Sp1 in HIF-1α-deficient cells but that a higher level of Redd1 expression is achieved when these cells are transfected with HIF-1α. These results demonstrate that hypoxic condition-and HCD-induced expression of Redd1 is mediated by coactivation of Sp1 and HIF-1α downstream of the PI3K/Akt signaling pathway.