CREB1 and neoplasm: As metformin is thought to activate AMPK by inhibiting oxidative phosphorylation [18, 19] and because phosphorylation of CREB at Ser-133 can be observed in cultured cells that have been incubated with oxidative phosphorylation inhibitors [20], it could be argued that metformin-induced CREB activation might merely reflect metformin's ability to impair mitochondrial activity in tumor cells.