Melanoma differentiation-associated gene-7 (mda-7/interleukin [IL]-24), a unique tumor suppressor gene, induces selective apoptosis in tumor cells. Secreted IL-24 binds to heterodimeric receptor complexes of IL-20R1/IL-20R2, IL-22R1/IL-20R2, or sigma-1 receptor (Sig1R) that consequently enhances apoptosis. However, this mechanism is not well understood and most likely involves different pathways. Targeting of cytokine by tumor homing peptides (THPs) to the tumor cell surface molecule-like integrin shows to be beneficial in gene immunotherapy approaches. In this study, the in silico targeting of RGD/NGR-modified IL-24 to tumor cells was conducted. In this regard, the sequences of six new synthetic IL-24s that have been modified by RGD (Arg-Gly-Asp) or NGR (CRNGRGPDC) were aligned and their structures were modeled through homology modeling to evaluate their attachment potential to cognate receptor complexes such as IL-20R1/IL-20R2, IL-22R1/IL-20R2, or Sig1R. The results of homology modeling showed that modification of IL-24 with RGD motif in N-terminal and middle of this protein exhibited stronger interaction with cognate receptors. These results also demonstrated that modified IL-24 with RGD motif in the C-terminal has lost native activity. However, the interaction of THP-modified IL-24 with Sig1R would not be affected to that extent, interestingly. Conclusively, in silico analysis showed that modification of IL-24 with THPs needs a more detailed study as these modifications may disrupt native interaction with receptors and reduce apoptosis induction property. This structural analysis gives us a better understanding of mda-7/IL-24 interaction with cognate receptors and helps a more rational design for further cytokine modification.
Keywords: NGR peptide; RGD peptide; computational modeling; mda-7/IL-24.