Metformin is the most widely used antidiabetic drug because of its proven efficacy and limited secondary effects. Interestingly, recent studies have reported that metformin can block the growth of different tumor types. Here, we show that metformin exerts antiproliferative effects on melanoma cells, whereas normal human melanocytes are resistant to these metformin-induced effects. To better understand the basis of this antiproliferative effect of metformin in melanoma, we characterized the sequence of events underlying metformin action. We showed that 24 h metformin treatment induced a cell cycle arrest in G0/G1 phases, while after 72 h, melanoma cells underwent autophagy as demonstrated by electron microscopy, immunochemistry, and by quantification of the autolysosome-associated LC3 and Beclin1 proteins. In addition, 96 h post metformin treatment we observed robust apoptosis of melanoma cells. Interestingly, inhibition of autophagy by knocking down LC3 or ATG5 decreased the extent of apoptosis, and suppressed the antiproliferative effect of metformin on melanoma cells, suggesting that apoptosis is a consequence of autophagy. The relevance of these observations were confirmed in vivo, as we showed that metformin treatment impaired the melanoma tumor growth in mice, and induced autophagy and apoptosis markers. Taken together, our data suggest that metformin has an important impact on melanoma growth, and may therefore be beneficial in patients with melanoma.

By targeting the tumor microenvironment to stimulate antitumor immunity, immunotherapies have revolutionized cancer treatment. However, many patients do not respond initially or develop secondary resistance. Based on the limited resources in the tumor microenvironment and competition between tumor and immune cells, the field of immune metabolism has produced extensive knowledge showing that targeting metabolism could help to modulate antitumor immunity. However, among all the different potentially targetable metabolic pathways, it remains unclear which have more potential to overcome resistance to immune checkpoint inhibitors. Here, we explore metabolic reprogramming in cancer cells, which might inhibit antitumor immunity, and strategies that can be used to favor the antitumor response.