Abstract 3340: Small molecule mediated stabilization of PP2A modulates the homologous recombination pathway and potentiates DNA damage-induced cell death

Abstract

High Grade Serous Carcinoma (HGSC) is the most lethal ovarian cancer subtype and accounts for approximately 60% of all ovarian tumors. Despite recent advances in drug development and increased understanding of genetic alterations that drive HGSC progression, mortality has not declined, highlighting the need for novel therapies. PARP inhibitors (PARPi) have become the mainstay of HGSC targeted therapy research given that these tumors are driven by a high degree of genomic instability resulting from the combination of fast DNA replication rates and numerous defects in the DNA-damage response (DDR) pathway. Nonetheless, only ~25% of these patients initially respond to treatment and a significant percentage eventually relapses with resistant disease. Here, we discovered that a Small Molecule Activator of Protein Phosphatase 2A (PP2A) (SMAP-061) induces apoptosis in both established and patient-derived HGSC cell lines as well as in genetically distinct Patient-Derived Xenograft (PDX) mouse models. Interestingly, we also uncovered that several genes that make-up the heterotrimer PP2A tumor suppressor protein are heterozygously lost in more than 95% of HGSC tumors, second only to p53. Mechanistically, we show that stabilization of PP2A protein by SMAP-061 inhibits the Homologous Recombination (HR) pathway via the direct inhibition of RAD51, ultimately leading to chronic accumulation of DNA damage and thus programmed cell death. Furthermore, we found that SMAP-061’s ability to inhibit HR potentiated the effects of PARP inhibition and resulted in synergistic cell death in both HR proficient and deficient models. These studies emphasize the potential of PP2A activators to expand the patient population that can benefit from PARPi therapies and possibly overcome PARPi resistance. In sum, our data highlights a new role of PP2A in regulating the DDR pathway in HGSC and supports the use of SMAPs in both HR proficient and deficient HGSC tumors. Citation Format: Rita A. Avelar, Amy Armstrong, Goutham Narla, Analisa DiFeo. Small molecule mediated stabilization of PP2A modulates the homologous recombination pathway and potentiates DNA damage-induced cell death [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 3340.

Integrated stress response plasticity governs normal cell adaptation to chronic stress via the PP2A-TFE3-ATF4 pathway

“…We then compared the effects of DT-061 to those of thapsigargin (Tg), a potent ER stress/UPR inducer via the inhibition of the sarco/endoplasmic Ca 2+ ATPase (SERCA) pump 32 . As previously observed 21 , DT-061 efficiently abrogated the ability of OV81 to form colonies, which we herein confirmed using 5µM and 10µM DT-061 concentrations and further compared to 400nM of Tg (Fig. 2G).…”

“…Interestingly, despite the potent cytotoxic effects DT-061 produces in malignant tissues, normal cells are able to adapt and circumvent pro-death signals, resulting in a large therapeutic index and high tolerability profiles in vivo [21][22][23][24][25][26][27][28] . Herein, our studies seek to investigate the fundamental mechanism responsible for the malignant specificity displayed by this class of drugs and further investigate the molecular mechanism that dictates that selectivity.…”

“…Considering these findings, we examined whether a panel of patientderived xenograft tumors treated with DT-061 exhibited a change in p-TFE3 localization to determine the relevance of these findings in vivo. Immunohistochemistry was performed on tumors treated with 5mg/Kg of DT-061 or control DMA, which we had previously shown to significantly inhibit tumor growth 21 , and stained for phospho and total TFE3 (Fig. 5G).…”

“…Protein phosphatase 2A is a heterotrimeric serine/threonine tumor suppressive protein phosphatase that is comprised of three main components: PP2A-A -the scaffolding subunit, providing a nucleating platform through its conformational flexibility for heterotrimer formation -, PP2A-C -the catalytic subunit, which provides enzymatic activity directed against phosphorylated serine and threonine residues -, and PP2A-B -one of fifteen structurally distinct regulatory subunits that directs substrate specificity [15][16][17] . Given that cellular homeostasis is dysregulated in human cancers due to imbalances in protein phosphorylation, which results in global cellular signaling perturbations, the therapeutic targeting of phosphatases such as PP2A has emerged as an effective therapeutic strategy to reestablish cellular homeostasis [18][19][20][21][22][23][24][25][26][27][28] . DT-061 is a PP2A modulator that regulates this tumor suppressor's activity by selectively binding to a unique pocket at the interface of the PP2A-Aα, Cα, and B56α subunits 29 .…”

“…DT-061 is a PP2A modulator that regulates this tumor suppressor's activity by selectively binding to a unique pocket at the interface of the PP2A-Aα, Cα, and B56α subunits 29 . DT-061 demonstrates potent anticancer properties in numerous cancer models with high tolerability profiles, even in long-term in vivo dosing studies [21][22][23][24][25][26][27][28] . However, the role of DT-061 and other PP2A modulators in modulating human stress-mediated responses has yet to be elucidated, remaining unclear whether this could explain the promising safety profiles previously observed across numerous cancer models.…”