Further affected proteins and pathways like the inhibition of signal transducer and activator of transcription 3 (STAT3) [37], various routes of apoptosis induction [38], or regulation of microRNA (miRNA) [39] have been suggested as potential targets for the biguanide drug, but the underlying mechanism of action of metformin’s anti-cancer activity is still not completely understood. Here, STAT3 is linked to cancer.