New Strategy Against Virus: Using Host Cellular Proteins As Drug Targets

Targeting viral enzymes is an effective approach leading to potent drugs; however, newly discovered antiviral agents that act at different binding sites on the enzymes or different viral proteins generate new drug-resistant mutations. It has become very crucial to identify novel drug targets for the control of drug-resistance. This study explored the possibility of using cellular molecules as drug targets to control viral infection. The in vitro investigation has been divided into two parts, for HCV and HIV-1, respectively.Part I. It has been known that viruses interact with host cellular proteins in their replication cycle, and employ some of them for viral proliferation. Recent study has indicated host Hsc70 protein is packaged into the hepatitis C viral (HCV) particles as a structural component of the virus in the assembly process. It helps HCV RNA releasing into the cytoplasm in the next infection cycle. We investigated whether chemically down-regulating host Hsc70 expression could be a novel strategy to interrupt HCV replication. Using Hsc70 mRNA assay, compounds were screened. IMB-DM122 was found to be an effective and safe inhibitor for Hsc70 mRNA/protein expression in human hepatocytes. IMB-DM122 inhibited HCV replication through destabilizing Hsc70 mRNA, and the T1/2 of host Hsc70 mRNA was reduced by 78% after the compound treatment. The Hsc70 mRNA 3'UTR sequence is the element responsible for IMB-DM122' s effect on Hsc70 mRNA. The compound appears to be highly efficient to the Hsc70-related HCV replication. Treatment of the HCV infected hepatocytes with IMB-DM122 reduced the virion encapsidation of Hsc70, and therefore made a break in HCV replication and infection cycle. IMB-DM122 showed a considerably good safety in vitro as well as in vivo with no indication of harmful effect on liver and kidney functions. It is our conclusion that Hsc70 might be a new drug target and mechanism to inhibit HCV proliferation.Part II. On the other hand, host innate immunity system fights against viral intruders in a variety of strategies. Human APOBEC3G (hA3G) is part of this system of host cell and has cytidine deaminase activity. It specifically incorporates into the virion during HIV-1 replication. In the HIV-1 reverse transcription process, hA3G deaminates dC to dU in the first minus strand cDNA, and then induce extensive hypermutation in the viral genome. Unfortunately, HIV-1 Vif counteracts the activity of hA3G by an ubiquitin-proteasome-mediated degradation of hA3G, suggesting that the interaction between Vif and hA3G might be a potential target for novel anti-HIV-1 drugs. In this part of work, we first established a screening system to discover compounds that protect hA3G from Vif-mediated degradation. After screening over 8634 samples, two active compounds, IMB-26 and IMB-35, were identified to be blockers for the Vif/hA3G interaction. The compounds specifically inhibited the degradation of hA3G by Vif without interference of the proteasomal function, and efficiently inhibit HIV-1 replication in hA3G-containing cells but not those null of hA3G The anti-HIV activity of IMB-26/35 positively correlated with the endogenous hA3G level in host cells. BIAcore examination found that IMB-26/35 protected hA3G by inhibiting the interaction between Vif and hA3G, and the hA3G protein appeared to be the receptor molecule for binding of the compound. The compounds were safe with a therapeutic index over 200 in vitro. LD50 of IMB-26 in mice was over 1000 mg/kg (ip) with no toxicity in liver and kidney. These data together indicate that Vif/hA3G interaction a new anti-HIV target, and IMB-26/35 potential leads for the discovery of new mechanism anti-HIV drugs.In summary, host cellular proteins that relate to viral replication could be novel drug targets to inhibit viruses, if their intracellular concentration is not a key factor for cell survival. It might be a potential strategy against drug resistance.