Protein Phosphatase 2A Inhibition with LB100 Enhances Radiation-Induced Mitotic Catastrophe and Tumor Growth Delay in Glioblastoma

Abstract

Protein Phosphatase 2A (PP2A) is a tumor suppressor whose function is lost in many cancers. An emerging, though counterintuitive, therapeutic approach is inhibition of PP2A to drive damaged cells through the cell cycle, sensitizing them to radiation therapy. We investigated the effects of PP2A inhibition on U251 glioblastoma cells following radiation treatment in vitro and in a xenograft mouse model in vivo. Radiation therapy alone augmented PP2A activity, though this was significantly attenuated with combination LB100 treatment. LB100 treatment yielded a radiation dose enhancement factor of 1.45 and increased the rate of post-radiation mitotic catastrophe at 72 and 96 hours. Glioblastoma cells treated with combination LB100 and radiation therapy maintained increased γ-H2AX expression at 24 hours, diminishing cellular repair of radiation-induced DNA double-strand breaks. Combination therapy significantly enhanced tumor growth delay and mouse survival and decreased p53 expression 3.68-fold, compared to radiation therapy alone. LB100 treatment effectively inhibited PP2A activity and enhanced U251 glioblastoma radiosensitivity in vitro and in vivo. Combination treatment with LB100 and radiation significantly delayed tumor growth, prolonging survival. The mechanism of radiosensitization appears to be related to increased mitotic catastrophe, decreased capacity for repair of DNA double-strand breaks, and diminished p53 DNA damage response pathway activity.

Inhibition of Protein Phosphatase 2A Sensitizes Mucoepidermoid Carcinoma to Chemotherapy via the PI3K-AKT Pathway in Response to Insulin Stimulus

“…It has been conclusively shown that PP2A inhibition in tumor cells leads to a downregulation of the DNA-damage response [15,18,34] and the abrogation of cell cycle checkpoints to achieve preferential killing [15,34,35]. Consistent with those findings, we have shown that LB100 inhibited proliferation and induced a G2/M arrest in MEC cells.…”

Targeting PP2A in cancer: Combination therapies

“…Cells progress to mitosis, but spindle formation is disordered and mitotic catastrophe occurs [84, 95, 96, 98, 99]. Disrupted cell cycle progression was further evident in cell cycle spread analyses, where treatment with a DNA damage inducer led to predominant S-phase arrest (more cells in G1/S), but when combined with PP2A inhibition by LB100, cells progress through S-phase and accumulated in G2/M due to disordered mitosis [84, 95, 96, 98-100]. PLK1 inhibition of TCTP, a protein that stabilizes microtubules and has anti-apoptotic functions, may contribute to abnormal mitotic figures and mitotic catastrophe [84, 96].…”

“…In addition to irregular cell cycle regulation, a few studies reported persistence of DNA damage with LB100 treatment, which was indicated by presence of γ-H2AX foci [96, 98-100]; likewise, LB100 inhibited Rad51 foci formation as part of DDR [97]. This suggests that homologous recombination (HR) repair may also be impaired by LB100, which is unsurprising given PP2A’s additional roles in regulating damage repair.…”

Safety, Tolerability, and Preliminary Activity of LB-100, an Inhibitor of Protein Phosphatase 2A, in Patients with Relapsed Solid Tumors: An Open-Label, Dose Escalation, First-in-Human, Phase I Trial

“…As with fostriecin, the lead compound, LB-100, and its lipid-soluble homolog, LB-102, inhibited proliferation of cell lines from a variety of human solid tumors at low micromolar concentrations (9). More strikingly, both compounds potentiated the activity, without significantly increasing the toxicity, of cisplatin, doxorubicin, and temozolomide against xenografts of pancreatic (10) and hepatocellular carcinoma (11), fibrosarcoma (12), pheochromocytoma (13), neuroblastoma (14), and glioblastoma (14) and of focal X-ray against pancreatic (15), nasopharyngeal (16), and glioblastoma xenografts (17). In addition, LB-100 reversed resistance to cisplatin in ovarian carcinoma (18) and medulloblastoma (19) xenografts.…”

“…Wei and colleagues (15) demonstrated that inhibition of PP2A by LB-100 sensitized human pancreatic cell lines in culture and as xenografts to radiation by interfering with DNA repair. Moreover, Gordon and colleagues (17) showed that LB-100 enhanced radiation inhibition of glioblastoma xenografts by interfering with mitotic exit, leading to increase mitotic catastrophe. Additional support for the potential value of PP2A inhibition for enhancing cytotoxic therapy was provided by Chang and colleagues (18), who demonstrated that LB-100 sensitized human ovarian cancer xenografts to cisplatin, at least in part by altering cell-cycle checkpoint regulation.…”