Although the
antineoplastic activity of
metformin (MET) is well established, the underlying mechanism of the activity is not understood. Since MET activates
AMP kinase (AMPK) and
proline dehydrogenase/
proline oxidase (PRODH/POX) is stimulated by AMPK
ligands (implicated in the regulation of
cancer cell survival/apoptosis), the effect of MET on PRODH/POX-dependent apoptosis in wild-type MCF-7 cells (MCF-7WT) and POX knockdown MCF-7 cells (MCF-7crPOX cells) was studied. PRODH/POX catalyzes
proline degradation generating ROS-induced apoptosis or autophagy. Availability of
proline for PRODH/POX functions is regulated by the activity of
prolidase (
enzyme releasing
proline from imidodipeptides),
collagen biosynthesis (process consuming
proline), and metabolism of
proline,
ornithine, and
glutamic acid. We have found that MET is cytotoxic for MCF-7 cells (IC50∼17 mM), and to the lower extent for MCF-7crPOX cells (IC50∼28 mM). In MCF-7WT cells, the effect was accompanied by the inhibition of
DNA biosynthesis,
collagen biosynthesis, stimulation of ROS formation, AMPKα phosphorylation, and expression of
prolidase, p53,
caspase 8,
caspase 9, and cleaved PARP. In MET-treated MCF-7crPOX cells, the processes were less affected than in MCF-7WT cells and the expression of
caspase 9 was decreased, while cleaved
caspase 8 and cleaved PARP were not detected. The effects were accompanied by an increase in the
prolidase activity and
proline concentration. The mechanism for MET-induced apoptosis involves the up-regulation of
prolidase activity and a decrease in
collagen biosynthesis contributing to an increase in the concentration of substrate (
proline) for PRODH/POX-dependent ROS formation and activation of
caspases -9 and -8. The data suggest that PRODH/POX participates in the MET-induced intrinsic and extrinsic apoptosis in MCF-7 cells.