A Novel Radiosensitizer of the Functional Food Component, Monogalactosyl Diacylglycerol (MGDG) From Spinach, Enhanced the Cytotoxic Effects for the Pancreatic Cancer In Vitro and In Vivo

Abstract

Monogalactosyl diacylglycerol (MGDG), a major chloroplast membrane galactolipid extracted from spinach, has been reported as a potent inhibitor of DNA replication and repair. The purpose of the current study is to test whether the MGDG might increase the cytotoxic effects of gemcitabine (GEM) or radiation therapy (IR) in vitro and in vivo by using human pancreas cancer cell lines. Several human cancer cell lines including pancreas cancer cell lines were used in the study. In vitro, DNA polymerase (pol) assay was performed, and cytotoxic effects and cell viability of MGDG and/or GEM and/or IR were evaluated by MTT assay and comet assay. Moreover, detection of apoptosis was assessed by a flow cytometer using the APO-Direct kit. In vivo, BALB/cAJcl-nu/nu mice were subjected to the experiments. Equal numbers of MIAPaCa2 cells were injected subcutaneously and assessed in size, according to 4-groups, such as PBS (sham), MGDG alone, IR alone and a combination of the same doses of MGDG and IR. In vitro, the inhibitory effects of MGDG and/or GEM on mammalian pols were observed clearly. The combination of phosphorylated GEM with MGDG significantly enhanced the inhibition of pols α and γ activities. The cell growth suppressive effects of combined GEM and MGDG treatments were determined using various timed treatments in which three human pancreatic cancer cell lines were treated with mixtures of (a) GEM and MGDG, (b) GEM → MGDG or (c) MGDG → GEM. The effects of schedules (a) and (b) were almost the same strength as MGDG alone, whereas schedule (c) showed synergistic effects on overall cytotoxicity. The synergistic effect of schedule (c) on BxPC-3, MIAPaCa2 and PANC-1 cells was 6.60, 28.7 and 13.0-fold stronger, respectively, than MGDG treatment alone. Moreover, MGDG and IR showed induction of apoptosis in MIAPaCa2 cells (control: 2.6%, MGDG: 5.2% and IR: 8.8%). Interestingly, the combination between MGDG and IR significantly increased the proportion of apoptotic cells (21.5%). In vivo studies, tumor growth inhibition was observed in the xenograft treated with either IR or MGDG alone, and the inhibitory effect was significantly increased by the combined treatment. MGDG showed cytotoxic effects and enhanced the effects of GEM or IR in vitro and in vivo. The combination, as MGDG + GEM + IR, could be a future strategy against pancreas cancer.