Su1989 Inhibition of Protein Phosphatase 2A (PP2A) With LB-100 Enhances the Therapeutic Efficacy of Chemotherapy in Gastric Cancer Cells

Abstract

BACKGROUND& AIMS: Gastric cancer is the fourth most common cancer and the second leading cause of cancer deaths worldwide. Despite improvements in chemotherapy and introduction of molecular targeted therapy, the prognosis of patients with unresectable or metastatic gastric cancer remains poor. Novel approaches for gastric cancer treatment are thus urgently needed. Combinatorial strategy can provide dramatic improvements over monotherapies. Targeting of protein phosphatase 2A (PP2A), a ubiquitous serine/threonine phosphatase with broad substrate specificity, has shown promising efficacy in combination with chemo-therapy and radiotherapy. In this study, we systematically analyzed synergism between LB-100, a PP2A inhibitor, and chemotherapeutic agents in the treatment of gastric cancer.METHODS:The effect of LB-100 on a panel of gastric cancer cell lines was determined by cell viability assay, colony formation assay and the xCELLigence system. The synergism between LB-100 and four chemotherapeutic drugs (Cisplatin, Docetaxel, Doxorubicin and 5-Fluorouracil), as assessed by cell viability and colony formation assays, was determined by isobologram analysis and quantified using the combination index (CI). The effect of monoand combination therapieson cytokinetic parameters, including apoptosis induction and cell cycle arrest, was evaluated. Activation of the apoptosis cascade and alternations in the cell cycle-associated proteins were investigated by western blot. RESULTS: As a single agent, LB-100 inhibited the growth of gastric cancer cells with IC50 values between 0.5 and 10μM. Real time cell viability measurement by using the xCELLigence system showed that LB-100 caused rapid cell death within the first two hours of treatment. Analysis of the synergism between LB-100 and four chemotherapeutic drugs including Cisplatin (DNA damaging agent), Docetaxel (anti-mitotic agent), Doxorubicin (topoisomerase poison) and 5-Fluorouracil (anti-metabolite), revealed that LB-100 potently synergized with Docetaxel (CI: 0.13 to 0.83) and Doxorubicin (CI: 0.32 to 0.66) in inhibiting gastric cancer cell growth. On the other hand, it was ineffective as a sensitizing agent for Cisplatin and 5Fluorouracil (CI>1). For both anticancer drugs (Docetaxel and Doxorubicin), co-treatment with LB-100 significantly enhanced the induction of apoptosis and cell cycle arrest at G2/M phase. These were confirmed by the observations thatthe drug combination treatment induced an increased activation of pro-apoptosis caspase-9-dependent cascade and decreased expression of key cell cycle regulators cyclin D1 and cyclin D3. CONCLUSIONS: Our results demonstrated that LB-100 is a potent sensitizing agent for chemotherapeutic drugs and support further exploration of PP2A inhibition as a novel drug combination regimen with Docetaxel or Doxorubicin in the treatment of gastric cancer.