Short Communication Activating Transcription Factor 4 (ATF4) Promotes HIV Type 1 Activation Elisabetta Caselli, Sabrina Benedetti, Valentina Gentili, Jessica Grigolato, and Dario Di Luca Abstract Activating transcription factor 4 (ATF4) is a central factor in the cellular response to multiple stresses, including altered metabolic conditions, anoxia and hypoxia, and redox stress. ATF4 is triggered by endoplasmic reticulum stress and consequent unfolded protein response. This report identifies for the first time ATF4 as a transcription factor upregulated by HIV-1 infection. Upregulation of ATF4 enhances HIV replication, by synergistic interac- tions with HIV Tat. Moreover, in specific cell lines ATF4 has a direct transactivating potential on the LTR, even in the absence of Tat. We also provide evidence that expression of ATF4 induces HIV reactivation in chronically infected cell lines. These results show for the first time that ATF4 induction might have an important role in HIV replication, and suggest that ATF4 might represent a convergent signaling molecule for different stressors important in regulating the HIV-1 cycle. H uman immunodeficiency virus type 1 (HIV-1) estab- lishes latent infection following integration into the host genome. Binding of host and viral proteins to the long ter- minal repeat (LTR) regulates HIV gene expression according to the host cell type and differentiation. Several studies have identified factors and cell proteins involved in HIV-1 ex- pression. 1 Sp1 and NF-jB have been extensively analyzed for their contribution to HIV-1 replication, but other transcrip- tional factors also contribute differentially to virus transcrip- tion. 2–4 Activating transcription factor 4 (ATF4) (also known as CREB-2) is a central factor in the cellular response to mul- tiple stresses, including altered metabolic conditions, anoxia and hypoxia, redox stress, and virus infection. 5,6 ATF4 is triggered by endoplasmic reticulum (ER) stress and conse- quent unfolded protein response (UPR), in a pathway col- lectively referred to as integrated stress response (ISR). 7 UPR eliminates misfolded proteins by attenuation of translation, 7,8 mainly through the phosphorylation of elF2a by PERK kinase. The repression of global protein translation is accompanied by specific translation of ATF4, which is important for recovery of the cell from stress. 5,7 Several viral infections affect the ER, causing protein mis- folding and UPR, including hepatitis C virus, 9 respiratory syncytial virus, 10 Japanese encephalitis virus, 11 adeno- viruses, 12 and human cytomegalovirus. 13,14 Viruses that in- duce ER stress are confronted with the consequences of UPR activation, with potential detrimental effects on viral repli- cation, such as attenuation of translation. However, ATF4 activation can be advantageous to viral infection, and some viruses modulate the UPR. During viral infection, several kinases, activated by dsRNA and ER stress, phosphorylate elF2a and ultimately increase ATF4. 15 This mechanism pre- vents attenuation of translation, inhibiting harmful effects of ER stress while maintaining those that may be beneficial to the virus. Furthermore, ATF4 has recently been shown to suppress IRF7 activation, inhibiting interferon (IFN) induc- tion 16 and facilitating viral replication. For example, reovirus replication requires ATF4, 17 and influenza virus replicates less efficiently in the absence of ATF4 expression. 18 Human cytomegalovirus (HCMV) infection increases ATF4 transla- tion, 13 and Epstein–Barr virus (EBV) latent membrane pro- tein 1 (LMP1) activates PERK to induce phosphorylation of elF2a, which upregulates ATF4. 19 Recently, we discovered that human herpesvirus 8 (HHV-8) also induces ATF4 ex- pression, which, in turn, enhances virus replication and promotes virus-induced angiogenesis (Caselli et al., unpub- lished observations). HHV-8 is frequently detected in HIV patients, where it is causally associated to the development of Kaposi’s sarcoma, one of the most frequent neoplasm during AIDS. In vitro, HHV-8 transactivates HIV-1 LTR, 20 leading to increased HIV replication and reactivation from latently infected cells. 21 ATF/CREB proteins have unique binding sites in the modulatory region of the HIV-1 LTR, 4 and ATF4 cooperates with Tax protein of the human T cell leukemia virus (HTLV- 1), in LTR activation. 22,23 It has also been reported that HIV Section of Microbiology, Department of Experimental and Diagnostic Medicine, University of Ferrara, Ferrara, Italy. AIDS RESEARCH AND HUMAN RETROVIRUSES Volume 28, Number 8, 2012 ª Mary Ann Liebert, Inc. DOI: 10.1089/aid.2011.0252 907