On May 20, 2022, the team of Professor Tian Changlin, Department of Life Science and Medicine, University of Science and Technology of China, Anhui Peptide Drug Engineering Laboratory, and the team of Professor Liu Lei, Department of Chemistry, Tsinghua University, and the Key Laboratory of Organophosphorus Chemistry and Chemical Biology of the Ministry of Education cooperated in Nature The Group's magazine Cell Discovery published a research paper entitled "Structural insights into the activation of somatostatin receptor 2 by cyclic SST analogues" online.
Somatostatin (SST14) is a 14 amino acid cyclic hormone release inhibitory peptide, which is responsible for regulating cell proliferation and the release of various hormones by activating five homologous somatostatin receptors (SSTR1-5), and inhibiting the spread of cancer and grow. SSTR2 is overexpressed in many cancer cells and tumor blood vessels, and is an important drug target in the treatment of rectal cancer, neuroendocrine tumors, meningioma, etc. Therefore, research and drugs targeting highly specific and selective ligands for SSTR2 R & D has received extensive attention from scientific research and the market. However, the short half-life of SST14 (<3 min) and non-selective activation of the five SSTR receptors limit the clinical application of SST14.
Octreotide is the cyclic octapeptide analog of SST14, which has a longer half-life (~2h) and exhibit high specificity for the SSTR2 receptor. At present, it has been approved for the treatment of diseases such as acromegaly and neuroendocrine tumors, with annual sales of more than 1 billion US dollars per year. However, the mechanism of how SST14, and octreotide bind to and activate SSTR2 is still unknown, which severely restricts the further optimization and development of specific peptide drugs targeting SSTR2.
Tian Changlin's team from University of Science and Technology of China and Liu Lei's team from Tsinghua University used techniques such as cryo-electron microscopy structure analysis and G protein function analysis to reveal the important amino acids that form the β-ring in the structures of SST14, octreotide and the middle span of the SSTR2 ligand-binding pocket. The interaction between amino acids in the membrane region is critical for the binding of ligands to receptors and the transmission of downstream signals. In this study, the NanoBiT tethering strategy was implemented to obtain the structure of SSTR2 in complex with Gi1, SSTR2 in complex with SST14, octreotide at resolutions of 2.85 Å, 2.97 Å and 2.87 Å, respectively.
Structural analysis showed that amino acids Q1022.63, D1223.32, Q1263.36, F2085.38, F2726.51, N2766.55, and Y3027.43 in the transmembrane region of the ligand-binding pocket of SSTR2 were closely related to the β-loop of the SST analog have a direct interaction. Compared with SST14, the ECL2 of SSTR2 moves down after binding Octreotide, which may further stabilize the interaction between SSTR2 and Octreotide.
Octreotide can specifically activate SSTR2, but do not bind SSTR1 or SSTR4. Through the sequence comparison and structural exploration of SSTR1-5, the author found three important amino acids Q1022.63, N2766.55, and N2947.35 related to ligand selectivity. They were mutated to the corresponding amino acids on SSTR1, and functional verification showed that the calcium ion response of the two polypeptides decreased significantly after mutation. An in-depth understanding of the correlation between the structure and pharmacological properties of SSTR2 and SST analogs will facilitate the further development of SSTR2-targeting peptide drugs with higher selectivity, stability, safety and efficacy.
The first authors of this research paper are Bai Qing, a doctoral candidate, Fan Yang, a postdoctoral fellow, and Yingge Li, a postgraduate student at the University of Science and Technology of China. Professor Tian Changlin and Associate Professor Shi Pan from University of Science and Technology of China, and Professor Liu Lei from Tsinghua University are the co-corresponding authors. The research work has been supported by the National Key R&D Program of the Ministry of Science and Technology, the National Natural Science Foundation of China, the Strategic Leader B of the Chinese Academy of Sciences, and the institutionalization research of the Chinese Academy of Sciences relying on the national major scientific and technological infrastructure.