Reversal of metformin's anti-proliferative effect in fission yeast efr3 and dnm1 (DRP1) mutants with elongated mitochondria.

Metformin is a well-tolerated drug frequently prescribed for managing type 2 diabetes. Extended metformin use has been linked to a significant decrease in cancer incidence across both diabetic and non-diabetic populations. Here we investigate the anti-proliferative effects of metformin on fission yeast S. pombe. Our findings demonstrate that metformin's inhibitory impact on cell proliferation is effective in the absence of AMP-activated protein kinase (AMPK). Using an unbiased genetic screen we identified the plasma membrane signalling scaffold Efr3, critical for phosphatidylinositol signalling and the generation of PI4Ps, as a key determinant of resistance to the anti-proliferative effect of metformin. Deletion of efr3 resulted in both AMPK-dependent and AMPK-independent resistance to metformin. We show that Efr3 does not influence cell proliferation by controlling Ras1 activity or its cellular localization in yeast. We observe that dnm1 (DRP1) mutants with elongated mitochondria are also resistant to the anti-proliferative effect of metformin and that metformin treatment promotes mitochondrial fusion. Metabolic measurements after prolonged metformin exposure demonstrated a reduction in respiration in both wild type and the efr3 deletion, however, that reduction is less pronounced in the efr3 deletion, which also contained elongated mitochondria. It is likely that mitochondrial fusion enhances yeast fitness in response to metformin exposure. Together we provide a new perspective on the cellular response to metformin.