Monogalactosyl diacylglycerol, a replicative DNA polymerase inhibitor, from spinach enhances the anti-cell proliferation effect of gemcitabine in human pancreatic cancer cells. 
Gemcitabine (GEM) is used to treat various carcinomas and represents an advance in pancreatic cancer treatment.  In the screening for DNA polymerase (pol) inhibitors, a glycoglycerolipid, monogalactosyl diacylglycerol (MGDG), was isolated from spinach.  Phosphorylated GEM derivatives were chemically synthesized.  In vitro pol assay was performed according to our established methods.  Cell viability was measured using MTT assay.  Phosphorylated GEMs inhibition of mammalian pol activities assessed, with the order of their effect ranked as: GEM-5'-triphosphate (GEM-TP) > GEM-5'-diphosphate > GEM-5'-monophosphate > GEM.  GEM suppressed growth in the human pancreatic cancer cell lines BxPC-3, MIAPaCa2 and PANC-1 although phospholylated GEMs showed no effect MGDG suppressed growth in these cell lines based on its selective inhibition of replicative pol species.  Kinetic analysis showed that GEM-TP was a competitive inhibitor of pol alpha activity with nucleotide substrates, and MGDG was a noncompetitive inhibitor with nucleotide substrates.  GEM combined with MGDG treatments revealed synergistic effects on the inhibition of DNA replicative pols alpha and gamma activities compared with GEM or MGDG alone.  In cell growth suppression by GEM, pre-addition of MGDG significantly enhanced cell proliferation suppression, and the combination of these compounds was found to induce apoptosis.  In contrast, GEM-treated cells followed by MGDG addition did not influence cell growth.  GEM/MGDG enhanced the growth suppression of cells based on the inhibition of pol activities.  Spinach MGDG has great potential for development as an anticancer food compound and could be an effective clinical anticancer chemotherapy in combination with GEM.