Abstract 2939: Canonical and noncanonical survivin isoforms translocate to the plasma membrane and are secreted in exosomes

Abstract

Abstract Background: Previously we found that survivin is expressed on the surface of cancer cells in vitro, and is targetable using certain survivin-specific antibodies. The survivin gene is alternatively spliced into two alternative isoforms, ΔEX3 and 2B, which diverge structurally from the canonical isoform in their C-termini but retain a conserved BIR (Baculovirus IAP Repeat) domain. Methods: In order to characterize plasma membrane survivin (PM-SVN), tagged proteins representing canonical survivin (isoform 1), Δex3 and 2B were transiently expressed in multiple cancer cell line backgrounds. Relative surface expression was measured by flow cytometry staining of intact cells. Mutant constructs were used to determine the structural requirements for translocation. Imaging flow cytometry (Imagestream) was used to visualize survivin isoform localization, and to detect co-localization on the cell surface. Secreted exosomes were examined for expression of survivin isoforms. Results: Isoforms 1, Δex3 and 2B could be detected on the surface of HEK293T embryonic kidney, HeLa cervical adenocarcinoma, U87 glioblastoma, and A1207 glioblastoma cells. PM-SVN expression was variable, however Δex3 showed robust membrane expression across all cell lines. Deletion of a predicted transmembrane sequence in Δex3's unique C-terminus did not impair its translocation, and deletion of the first 10 amino acids, thought to be required for dimerization, disrupted surface expression only in HEK293T cells. Imaging flow cytometry analysis of cells revealed polar localization of PM-SVN, with specific punctate areas seen in 13-20% of cells. Staining with antibodies targeting both isoform 1 and Δex3 showed co-immunofluorescence in cell membrane puncta, indicating the formation of heterodimers. Stressing cells by heat shock increased the proportion of PM-SVN in 293T and A1207 cells; in 293T, this was accompanied by enrichment of survivin protein in secreted exosomes. Conclusions: In addition to canonical survivin, isoforms Δex3 and 2B are displayed on the surface of cancer cells, and can be secreted as exosomal cargo. Membrane translocation may be stress-induced and appears to require the conserved BIR domain. PM-SVN shows a polar distribution on the cell membrane, and isoforms 1 and Δex3 heterodimerize within these puncta. Citation Format: Sheila Figel, Meaghan Birkemeier, Sanam Dharma, Emma Steinmetz, Michael Ciesielski, Robert Fenstermaker. Canonical and noncanonical survivin isoforms translocate to the plasma membrane and are secreted in exosomes [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 2939.