Hybrid channels containing the dupα7 subunit mediate resistance to α7-nAChR targeting therapy in metastatic melanoma.

Nicotinic acetylcholine receptor of α7 type (α7-nAChR) is a ligand-gated ion channel composed of five identical α7 subunits. Secreted lymphocyte antigen-6 urokinase-type plasminogen activator receptor (Ly6/uPAR)-related protein-1 (SLURP-1) controls carcinoma progression by negative modulation of oncogenic α7-nAChR. In this study, we observed dramatic decrease of SLURP-1 plasma level in patients with metastatic melanoma. We suggested usage of recombinant analog of human SLURP-1 (rSLURP-1) to compensate this deficiency for metastatic melanoma treatment. rSLURP-1 did not affect viability of different patient-derived metastatic melanoma cells, but reduced migration of some of them. Metastatic melanoma cells of other lines were resistant to rSLURP-1. Antimigratory rSLURP-1 effect was mediated by α7-nAChR, whereas resistance to rSLURP-1 correlated with overexpression of human-specific CHRFAM7A gene, which encodes the α7 subunit with truncated N-terminal region (dupα7) able to form hybrid α7/dupα7-nAChR channels. Electrophysiological study in Xenopus laevis oocytes showed that rSLURP-1 inhibits α7/dupα7-nAChR weaker than α7-nAChR. In contrast, "Oncotag" peptide, which mimics the loop I of SLURP-1, inhibited α7/dupα7- and α7-nAChRs with similar efficiency. Oncotag suppressed metastatic melanoma cell migration independently on dupα7 expression. Computer modeling provided rationale for altered activities of rSLURP-1 and Oncotag on α7/dupα7-nAChR. The Cancer Genome Atlas Program (TCGA) database analysis revealed correlation between CHRNΑ7 and CHRFAM7A gene expression and worse survival prognosis for patients with metastatic melanoma. Thus, 1) low plasma SLURP-1 level may be a specific marker of metastatic melanoma development, 2) metastatic melanoma progression can be controlled by α7-nAChR inhibition, and 3) dupα7 overexpression is a new molecular mechanism of melanoma resistance to internal cholinergic control and new target for melanoma treatment.NEW & NOTEWORTHY Metastatic melanoma is aggressive skin tumor often resistant to standard therapies. High α7-nAChR expression negatively correlates with survival of patients with metastatic melanoma, who are characterized by SLURP-1 drop in the plasma. Targeting α7-nAChR with recombinant SLURP-1 could significantly suppress metastatic melanoma cell migration; however, overexpression of human-specific dupα7 subunit forming hybrid α7/dupα7-nAChRs causes melanoma resistance to SLURP-1. This resistance can be overcome by the SLURP-1 mimicking peptide Oncotag, which exhibits activity against hybrid α7/dupα7-nAChRs.