HIV-1 is a lentivirus of the family of Retroviridae and the aetiological cause of AIDS. An estimated 33 million people are infected with HIV worldwide. HIV/AIDS persists as a major cause of morbidity in developed and non-developed countries. In the absence of a protective vaccine or a cure, prevention and access to antiretroviral treatments are the best options against HIV-1 (Simon et al., 2006). Significant advances in antiretroviral therapy have been made since the introduction of zidovudine (AZT) in 1987. However, these drugs frequently cause severe side effects and the development of drug resistant HIV is rapidly emerging. Globally, with the lack of effective treatment regimens HIV/AIDS continues to be a major public health crisis. It is therefore important to develop more potent and conceptually novel drugs and therapies for the treatment of this infection.
In several different studies green tea EGCG has been reported to have antiviral effects against HIV-1 infection. Interestingly, various mechanisms for this inhibitory effect have been proposed (Nance and Shearer, 2003). Nakane and Ono (1989) initially demonstrated inhibition of HIV-1 replication by EGCG in human peripheral blood mononuclear cells (PBMCs) in vitro (Nakane and Ono, 1989). EGCG was shown to block the enzymic activity of the HIV-1 reverse transcriptase (RT) resulting in a decrease in p24 antigen concentration. Recently, it was confirmed that EGCG acts as an allosteric RT inhibitor, with time of addition assays revealing a similar inhibitory profile to non-nucleoside RT inhibitors (NNRTIs) (Li et al., 2011). However, the mechanism of inhibition seems to be different from those of currently approved NNRTIs, as HIV-2 with another binding pocket was inhibited and NNRTI-resistant viruses were also still susceptible to EGCG. Synergistic inhibition was also observed with AZT (Li et al., 2011). Additionally, and similar to HCV, in a number of different studies EGCG was also found to interfere with the viral envelope of HIV-1. The reduced HIV-1 infectivity in the presence of EGCG was also shown to be due to increased lysis of viral particles (Fassina et al., 2002). In another study, the possible antiviral effects of EGCG for every step of the HIV-1 life cycle were investigated (Yamaguchi et al., 2002). Again, EGCG destroyed virions in a dose- and time-dependent manner and inhibited RT activity. Mechanistically, viral lysis was facilitated via EGCG binding to the surface of the viral envelope and deforming membrane phospholipids in a manner similar to the effect of polymixin B on bacterial membranes (Ikigai et al., 1993; Yamaguchi et al., 2002).
HIV-1 entry is initiated by the attachment of the gp120 envelope protein to the CD4 receptor and subsequent interaction with the co-receptors CCR5 or CXCR4. Fusion of host and virus membrane occurs with the help of the fusion peptide located in the gp41 of HIV-1. After membrane fusion, the capsid is released into the cytoplasm. Kawai et al. investigated the effect of EGCG on the expression of CD4 molecules and noted that EGCG, but not ECG, prevented the attachment of HIV-1 virions by blocking the interaction of gp120 and CD4 on T helper cells (Kawai et al., 2003). EGCG in a concentration ranging from 25–250 μmol L−1 down-regulated the cell surface receptor expression by binding to CD4, presumably at a binding site recognized by gp120 (Kawai et al., 2003; Nance et al., 2009). Supporting this observation, EGCG was shown to compete with anti-CD4 monoclonal antibodies. Cell-surface CD4 expression is regulated via multiple mechanisms, including CD4 endocytosis, intracellular retention of the molecular complex and shedding from the cell surface (Geleziunas et al., 1994). HIV-1 infection per se induces CD4 down-regulation by proteasomal degradation (Aiken et al., 1994). Details of the molecular mechanism by which EGCG modulates CD4 down-regulation at the cell surface are not fully understood, although CD4 shedding from the cell surface, and CD4 endocytosis are unlikely to be involved (Kawai et al., 2003). However, the crucial mechanism by which EGCG inhibits HIV-1 entry seems to be its interference with gp120, a ligand for CD4, and thereby prevent the initial attachment of viruses to CD4 T cells. The characteristics of the binding of EGCG to CD4 were further investigated by NMR spectroscopy and molecular modelling (Williamson et al., 2006). The addition of CD4 to EGCG produced a linear decrease in the NMR signal intensity from EGCG, but not from the control molecule catechin, providing clear evidence for high-affinity binding of EGCG to the CD4 molecule with a Kd of approximately 10 nmol L−1 (Williamson et al., 2006). A physiologically relevant concentration of EGCG (0.2 μmol L−1) inhibited the binding of gp120 to isolated human CD4 T cells and molecular modelling studies suggested a binding site for EGCG in the D1 domain of CD4, the pocket that binds gp120 (Williamson et al., 2006). The HIV-1 integrase protein is responsible for the insertion of HIV proviral DNA into the genome of infected cells. Recently, the ability of EGCG to inhibit the HIV-1 integrase was also evaluated in an elisa (Jiang et al., 2010). It was shown that catechins with a galloyl moiety were able to reduce HIV-1 integration by binding between the integrase and the viral DNA, so disrupting this interaction. However, further studies with infectious viruses are needed to validate these in vitro data.
In conclusion, EGCG appears to interfere with several aspects of the HIV-1 life cycle, including virion destruction via interaction with the viral envelope, prevention of viral replication via inhibition of reverse transcription, inhibition of proviral genome integration and CD4 receptor down-regulation. Most conclusively, competition with gp120 for CD4 binding was validated in several independent studies. Importantly, physiological EGCG concentrations were able to reduce the attachment of gp120 to CD4 by a factor of 20-fold. Further in vivo studies are needed to determine whether EGCG has potential as a future antiretroviral therapy.