Abstract 1707: PACE4-dependent cellular uptake and retention of the Multi-Leucine peptide inhibitor into cancer cells

Abstract The proprotein convertase PACE4 is strongly overexpressed in prostate cancer and plays an important role in tumor progression by promoting cell proliferation and tumor angiogenesis. Because the high expression levels and the necessity of this enzyme for cancer progression, we have developed a peptide-based inhibitor known as the Multi-Leucine (ML) peptide, which is a PACE4-specific inhibitor. Just like PACE4-downregulation through stable shRNA transfections, this compound displays anti-proliferative properties when applied on cancer cells. We showed its target-specific uptake into prostate cancer xenografts by positron emission tomography, which allowed clear tumor visualization. The ML-peptide has potent effects to block the further progression of prostate cancer cells, however, the uptake mechanism of this peptide is not clear and whether it is PACE4-dependent. We therefore tested the uptake and efflux rates of the ML-peptide peptide in prostate cancer cells and correlated these with PACE4 expression levels in wild type and stable PACE4-knockdown cell lines. In addition to prostate cancer cell lines LNCaP, DU145 and PC3, we tested HepG2, Huh7 and HT1080 cells, which also express PACE4. To measure peptide uptake, the ML peptide was modified through the addition a 1,4,7-triazacyclononane-1,4,7-triacetic acid (NOTA) moiety to strongly chelate radiometal such as 64Cu. Our study shows that uptake of the ML-peptide is correlated with expression levels, but more importantly can be blocked in PACE4-knockdown cell lines, demonstrating a PACE4-dependant uptake mechanism. We also correlated uptake with the anti-proliferative effects of the ML-peptide and showed that entry into these cell lines predicted the effects of the ML-peptide on the growth of these cells. These results confirm the notion that ML-peptide entry within cell is an important requirement to exert growth inhibition properties, and further provide a mechanism for that entry. This study demonstrates further support for the use of the ML-peptide or its analogs as potential drugs in prostate cancer, as well as any other PACE4-dependent tumors. These indications of peptide uptake within tumor could allows PACE4-status to be determined by positron emission tomography and may be of great theranostics uses in the context of pharmacological intervention with PACE4 inhibitors. Citation Format: Frédéric Couture, Kevin Ly, Christine Levesque, Anna Kwiatkowska, Roxane Desjardins, Brigitte Guérin, Robert Day. PACE4-dependent cellular uptake and retention of the Multi-Leucine peptide inhibitor into cancer cells. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 1707. doi:10.1158/1538-7445.AM2015-1707