Extended Data Fig. 1. Model of the metabolic determinants of sensitivity to low glucose and biguanides.

This diagram outlines the interplay between reserve oxidative phosphorylation (OXPHOS) capacity, sensitivity to biguanides, and sensitivity to culture in low glucose. Most cancer cell lines and normal cells tested exhibited an ability to respond to glucose limitation by upregulating OXPHOS, rendering them less sensitive to biguanides and low glucose conditions. In contrast, cell lines harboring mutations in mtDNA encoded Complex I subunits or exhibiting impaired glucose utilization have a limited reserve OXPHOS capacity and are therefore unable to properly respond to biguanides and low glucose, rendering them sensitive to these perturbations. At the extreme, cells artificially engineered to have no OXPHOS (Rho cells) exhibit extreme low glucose sensitivity, but resistance to further inhibition of OXPHOS. Thus, mtDNA mutant cancer cells exist at an intermediate state of OXPHOS functionality that renders them sensitive to treatment with biguanides in vitro and in vivo. Similarly, cell lines with impaired glucose utilization exhibit biguanide sensitivity specifically under the low glucose conditions seen in the tumor microenvironment.