Identification of certain chemokine receptors as fusion/entry co- factors for HIV has substantially extended our understanding of HIV pathogenesis. CCR5, a receptor for the beta chemokines RANTES, MIP-1- alpha, and MIP-1-beta, serves as a co-receptor for macrophage (M)- tropic strains of HIV, while CXCR4, a receptor for the alpha-chemokine SDF-1, is critical for fusion/entry of T cell (T)-tropic strains. Recently, an orphan receptor, STRL33 has been shown to serve as a fusion/entry cofactor for various HIV and simian immunodeficiency virus strains. The presence of a 32 base pair deletion in the CCR5 gene confers resistance to HIV infection. This finding proved the significance of CCR5 in HIV pathogenesis; however, in addition to genetic factors affecting expression of HIV co-receptors, immunoregulatory mechanisms may affect the functional availability of co-receptors. In order to investigate the molecular and cellular mechanisms that control expression of HIV co-receptors and their ligands, we cloned and characterized the promoter regions for the genes encoding these proteins, namely RANTES, CCR5, CXCR4, and STRL33. We have identified cis-acting elements involved in regulation of those promoters, and determined molecular mechanisms whereby those promoters are regulated.(A)Regulation of the CCR5 promoterWe have demonstrated that several transcriptional factors such as Oct2 and GATA1 can transactivate CCR5 promoter through promoter elements that are responsive to these factors. We have also demonstrated that retinoic acid, which can induce CCR5 expression in promonocytic cell lines, is able to upregulate CCR5 promoter activity, and that lipopolysaccharide, a bacterial product, downregulates CCR5 promoter activity in macrophages.(B)Regulation of CXCR4 promoterTranscription factors USF1 and USF2 up-regulate gene expression by interacting with an E box in their target promoters; this E box is also a binding site for c-Myc. The c-Myc oncoprotein is important in regulating cellular proliferation and differentiation; c-Myc can also interact with Yin-Yang 1 (YY1), another important regulator of cellular and viral gene expression. c- Myc and YY1 mutually inhibit their respective biological functions. We have demonstrated that USF/c-Myc up-regulates, while YY1 down-regulates the promoter activity of CXCR4. An E box and a YY1 site were identified within the CXCR4 promoter region. The Tax protein of the human lymphotropic virus type 1 (HTLV-I) interacts with cellular transcription factors such as the cAMP-responsive element (CRE) binding factors or the NF-kappaB/Rel family of proteins. We found that the HTLV-I Tax protein trans-activates the CXCR4 promoter through its interaction with nuclear respiratory factor 1 (NRF1). The promoter region for CXCR4 contains an NRF1 binding site. We demonstrated that Tax binds to NRF1 and that this interaction enhances the DNA binding activity of NRF1. (C)Regulation of STRL33 promoterWe have cloned and characterized STRL33 promoter region, and identified several cis-acting elements on the promoter. We have demonstrated that stimulation with either phorbol myristate acetate/ionomycin, lipopolysaccharide or prostaglandin E2 upregulates STRL33 promoter activity. - HIV co- receptor; chemokines; promoters; transcription; gene regulation
