Discovery of PTPRJ Agonist Peptides That Effectively Inhibit <i>in Vitro</i> Cancer Cell Proliferation and Tube Formation

PTPRJ is a receptor protein tyrosine phosphatase involved in both physiological and oncogenic pathways. We previously reported that its expression is strongly reduced in the majority of explored cancer cell lines and tumor samples; moreover, its restoration blocks <i>in vitro</i> cancer cell proliferation and <i>in vivo</i> tumor formation. By means of a phage display library screening, we recently identified two peptides able to bind and activate PTPRJ, resulting in cell growth inhibition and apoptosis of both cancer and endothelial cells. Here, on a previously discovered PTPRJ agonist peptide, PTPRJ-pep19, we synthesized and assayed a panel of nonapeptide analogues with the aim to identify specific amino acid residues responsible for peptide activity. These second-generation nonapeptides were tested on both cancer and primary endothelial cells (HeLa and HUVEC, respectively); interestingly, one of them (PTPRJ-19.4) was able to both dramatically reduce cell proliferation and effectively trigger apoptosis of both HeLa and HUVECs compared to its first-generation counterpart. Moreover, PTPRJ-pep19.4 significantly inhibited <i>in vitro</i> tube formation on Matrigel. Intriguingly, while ERK1/2 phosphorylation and cell proliferation were both inhibited by PTPRJ-pep19.4 in breast cancer cells (MCF-7 and SKBr3), no effects were observed on primary normal human mammary endothelial cells (HMEC). We further characterized these peptides by molecular modeling and NMR experiments reporting, for the most active peptide, the possibility of self-aggregation states and highlighting new hints of structure–activity relationship. Thus, our results indicate that this nonapeptide might represent a great potential lead for the development of novel targeted anticancer drugs