Metformin protects against lipoapoptosis and enhances GLP-1 secretion from GLP-1-producing cells

Metformin is the most frequently prescribed drug for treatment of type 2 diabetes. It improves insulin resistance and glycemia by reducing hepatic gluconeogenesis. In addition, diabetic patients on metformin therapy have elevated levels of the insulinotropic hormone glucagon-like peptide-1 (GLP-1) and metformin has been shown to regulate the expression of the GLP-1R in the pancreas. We have studied the direct long-term effects of metformin on apoptosis, and function of GLP-1-secreting L cells in vitro, using the murine GLUTag cell line as a model. The apoptosis of GLUTag cells was detected by DNA-fragment assay and caspase-3 activity determination. GLP-1 secretion was determined using ELISA and the expression of proglucagon mRNA was assessed by reverse transcription polymerase chain reaction. The activation of intracellular messengers was determined using western blotting. Metformin significantly decreased lipotoxicity-induced apoptosis in conjunction with increased phosphorylated AMPK. Metformin also countered the JNK2 activation evoked by lipotoxicity. In addition, long-term metformin treatment stimulated GLP-1 secretion. This study demonstrates that metformin protects against lipoapoptosis (possibly by blocking JNK2 activation), and enhances GLP-1 secretion from GLP-1-producing cells in vitro. These direct effects of the drug might explain the elevated plasma GLP-1 levels seen in diabetic patients on chronic metformin therapy. The findings may also be harnessed to therapeutic advantage in efforts aiming at enhancing endogenous GLP-1 secretion in type 2 diabetic patients.