Abstract P1-05-03: Isoform specific targeting of insulin receptor

Abstract

The insulin receptor (InsR) exists in both an A and B isoform. InsR-B differs from InsR-A by the inclusion of exon 11, which encodes 12 amino acid residues at the C-terminus of the InsR alpha-subunit. Increased InsR-A expression is associated with mitogenic signaling pathways while InsR-B is linked to insulin-mediated metabolic functions. Predominant InsR-A expression may therefore be important in growth and fetal development of embryos, whereas predominant InsR-B expression has a role in metabolic insulin action in adult life. Increased InsR-A expression is seen in breast cancer. In endocrine resistant breast cancer, InsR-A is expressed at high levels (Gradishar, et al. Clin Cancer Res 22:301 2016 PMID: 26324738). Thus, developing InsR-A specific inhibitors could be a useful therapy for breast cancer. We have previously published InsR specific binders using a T7 phage gene 2 protein (Gp2), a small protein scaffold (Chan, et al. Mol Cancer Ther 16:1324 2017 PMID: 28468775), with the long-term goal of creating effective InsR inhibitors and diagnostics. Using yeast display and directed evolution, we identified three Gp2 variants (Gp2 #1, #5, and #10) with low nanomolar affinity and specific binding to cell surface InsR. We have shown that these Gp2 variants inhibited insulin-mediated monolayer proliferation in both endocrine-sensitive and resistant breast cancer, but did not downregulate InsR expression. To further characterize the specificity of Gp2 variants, we used two techniques. HEK293T cells were infected with lentiviral vectors expressing either InsR-A tagged with mCherry or InsR-B tagged with eGFP. Using these cells, we performed “mock panning” and showed the Gp2 #5 variant bound both InsR-A and InsR-B, but had higher affinity for InsR-B. We also incorporated Gp2 #5 into the capsid of a tropism-null adeno-associated virus (AAV). Using this Gp2-AAV, we infected HEK293T-InsR-A or InsR-B cells at a number of different multiplicities of infection. These data were consistent with panning data and showed specific Gp2-AAV infection of cells expressing high levels of InsR-B, but not InsR-A. Thus, our data show that Gp2 variants we created have a higher affinity for InsR-B than InsR-A. Despite this preferred affinity, these Gp2 binders have sufficient binding to InsR-A to disrupt the biological effects of insulin in breast cancer cells. Thus, even relatively low affinity binding to InsR-A can disrupt its function. Further development of InsR-A Gp2 binders may be developed and provide more specific targeting of the breast cancer specific isoform of InsR. Citation Format: LaPara K, Chan JY, Zdechlik A, Ljunggren K, Schmidt D, Hackel B, Yee D. Isoform specific targeting of insulin receptor [abstract]. In: Proceedings of the 2018 San Antonio Breast Cancer Symposium; 2018 Dec 4-8; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2019;79(4 Suppl):Abstract nr P1-05-03.