Systematic investigation of dose-dependent protein thermal stability changes to uncover the mechanisms of the pleiotropic effects of metformin

Metformin is the most popular drug to treat type II diabetes. Besides lowering blood sugar, it is proven to have more beneficial effects, including extending life expectancy. However, the underlying mechanisms remain largely elusive. Here, we systematically studied the proteome thermal stability changes induced by a wide dosage range of metformin in a liver cancer cell line using the proteome integral solubility alteration (PISA) assay. This work provides valuable and unique information about the interactions between proteins and this drug, which is not able to obtain using commonly used abundance-based proteomics. The current results demonstrated that the most acceptable target of metformin responsible for lowering blood glucose levels, complex I, was stabilized only under the high-concentration metformin treatment. Intriguingly, the complex IV subunits were significantly stabilized by low concentrations of (such as 0.2 μM) metformin, indicating that the complex IV may play an important role in the sugar-lowering effect. Moreover, low-dose metformin significantly altered ribosome and histone protein stabilities, correlated with the inhibitive effect of cell proliferation by metformin. We further found that low-concentration metformin impacted mitochondrial cargo transport and vesicle transport, while high-concentration metformin affected cell redox responses and cell membrane protein sorting. Systematic investigation of metformin-induced proteome thermal stability changes provides intriguing insights into the molecular mechanisms of lowering blood sugar and the pleiotropic effects of metformin.