Hypoxia-inducible factors 1 alpha and 2 alpha (H1F-1 alpha and HIF-2 alpha) determine cancer cell fate under hypoxia. Despite the similarities of their structures, HIF-1 alpha and HIF-2 alpha have distinct roles in cancer growth under hypoxia, that is, HIF-1 alpha induces growth arrest whereas HIF-2 alpha promotes cell growth. Recently, sirtuin 1 (Sirt1) was reported to fine-tune cellular responses to hypoxia by deacetylating HIF-1 alpha and HIF-2 alpha. Yet, the roles of Sirt1 in HIF-1 alpha and HIF-2 alpha functions have been controversial. We here investigated the precise roles of Sirt1 in HIF-1 alpha and HIF-2 alpha regulations. Immunological analyses revealed that HIF-lot K674 and HIF-2 alpha K741 are acetylated by PCAF and CBP, respectively, but are deacetylated commonly by Sirt1. In the Gal4 reporter systems, Sirt1 was found to repress HIF-la activity constantly in ten cancer cell-lines but to regulate HIF-2 alpha activity cell type-dependently. Moreover, Sirt1 determined cell growth under hypoxia depending on HIF-1 alpha and HIF-2 alpha. Under hypoxia, Sirt1 promoted cell proliferation of HepG2, in which Sirt1 differentially regulates HIF-1 alpha and HIF-2 alpha. In contrast, such an effect of Sirt1 was not shown in HCT116, in which Sirt1 inactivates both HIF-1 alpha and HIF-2 alpha because conflicting actions of HIF-1 alpha and HIF-2 alpha on cell growth may be offset. Our results provide a better understanding of the roles of Sirt1 in HIF-mediated hypoxic responses and also a basic concept for developing anticancer strategy targeting Sirt1. (C) 2014 Elsevier Inc. All rights reserved.