Design of Peptide Ligands Which Mimic the Activity of Carcinoembryonic Antigen

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Palermo, Nicholas Y.
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Design of a peptide which mimics the biological activity of carcinoembryonic antigen (CEA) through interaction with its receptor, the heterogeneous ribonucleoprotein M (hnRNP M), is based on Pro-Glu-Leu-Pro-Lys, which is the minimal sequence of CEA required for activation of Kupffer cells. CEA has been found to promote colorectal cancer metastasis by inducing Kupffer cells to produce inflammatory cytokines which, in turn, make the hepatic micro-environment ideal for tumor cell implantation. Understanding the structure and biological activity of the PELPK sequence is fundamental to the design of any antagonist of the hnRNP M receptor. Ac-Pro-Glu-Leu-Pro-Lys-NH2 (PELPK), analogs thereof and a polypeptide based on a CEA receptor fragment (CEARF) were synthesized using microwave-assisted solidphase peptide synthesis. The structures of the peptides and the interaction between PELPK and CEARF were elucidated using electronic circular dichroism, vibrational circular dichroism and molecular dynamics simulations. PELPK was shown to have a stable, polyproline-II helical structure and to interact with CEARF, forming a stable complex. The binding of PELPK and analogs to hnRNP M was also investigated using molecular docking calculations. The peptide Ac-Tyr-Pro-Glu-Leu-Pro-Lys-NH2 (YPELPK) was chosen as the lead compound because of its higher calculated energy of binding than that of PELPK. Ala and D-amino acid scans of YPELPK, in which sequential residues of the peptide were replaced with Ala and D-stereoisomers, respectively, were docked to the structure of hnRNP M to determine the importance of the presence and orientation of the side-chains to the energy of binding of the peptide to the receptor. The last three residues of YPELPK, Leu-Pro-Lys, were found to be critical for the binding of the peptide to hnRNP M. The biological activity of YPELPK and its Ala-scan analogs was studied using differentiated human THP-1 cells, which express hnRNP M on their surface and secrete IL-6 when stimulated by CEA. YPELPK was found to be the most effective ligand, but, the analogs APELPK and YPELPA were also able to stimulate IL-6 production by THP-1 cells. This result is in agreement with the data from the molecular docking calculations, which showed that the APELPK and YPELPA analogs lost less binding energy than the other Ala-scan analogs of YPELPK. In conclusion, YPELPK was found to be a peptide which mimics the biological activity of CEA and can be used as a model to develop antagonists of the hnRNP M receptor to inhibit CEA-mediated colorectal cancer metastasis.
Creighton University
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