Environment Dictates Dependence on Mitochondrial Complex I for NAD+ and Aspartate Production and Determines Cancer Cell Sensitivity to Metformin

Abstract

Summary Metformin use is associated with reduced cancer mortality, but how metformin impacts cancer outcomes is controversial. While metformin can act cell autonomously to inhibit tumor growth, the doses of metformin that inhibit proliferation in tissue culture are much higher than what has been described in vivo. Here, we show that environment drastically alters sensitivity to metformin and other complex I inhibitors. We find that complex I supports proliferation by regenerating NAD+, and metformin’s anti-proliferative effect is due to loss of NAD+/NADH homeostasis and inhibition of aspartate biosynthesis. However, complex I is only one of many inputs that determine cellular NAD+/NADH ratio, and dependency on complex I is dictated by the activity of other pathways that affect NAD+ regeneration and aspartate levels. This suggests that cancer drug sensitivity and resistance are not intrinsic properties of cancer cells, and demonstrates that environment can dictate sensitivity to therapies that impact cell metabolism.

Targeting Metabolism for Cancer Therapy

“…Although the precise mechanism of metformin action remains controversial (Luengo et al, 2014), recent work has shown that the anti-tumorigenic effect of metformin can at least be partially accounted for by direct mitochondrial complex I inhibition in tumors (Gui et al, 2016; Wheaton et al, 2014). Consistent with this notion, other complex I inhibitors have shown efficacy as anti-tumor agents (Appleyard et al, 2012; Schockel et al, 2015) and may show selective toxicity against oncogene-ablation resistant cells (Viale et al, 2014) and cancer stem cells (Sancho et al, 2015).…”

“…Additionally, NAD+ serves as a critical redox cofactor required to generate oxidized molecules, such as amino acids and nucleotides necessary for biomass accumulation (Birsoy et al, 2015; Sullivan et al, 2015; Titov et al, 2016). Proliferating cells often require a high NAD+/NADH ratio to support anabolic reactions, and in some contexts the NAD+/NADH ratio directly correlates with proliferation rate (Gui et al, 2016). Higher NAD+/NADH ratios appear important for proliferation while lower ratios favor mitochondrial ATP production.…”

The effects of metformin on gut microbiota and the immune system as research frontiers

“…More recently, it has been recognised that complex I inhibition has additional consequences related to accumulation of NADH relative to NAD + , which of course would also be expected to influence cellular biochemistry. One example of this is the inhibitory effect of biguanides on aspartate synthesis [13].…”

Androgen Deprivation Therapy (ADT), Metabolic Syndrome and Metastatic Prostate Cancer: In Vivo and In Vitro Assessments of Effects of Metformin

“…In assessing the cellular metabolic capacity with Seahorse XF24, the cells exhibited different maximal respiratory capacity that may explain the findings. Gui et al recently attempted to explain the discrepancy between in vitro and in vivo doses of metformin by providing evidence that breast cancer cell sensitivity was dependent on tumour microenvironments such as low NAD+/NADH ratio secondary to lower serine levels [305]. Therefore, when considering metformin doses and various cell type sensitivities, the microenvironment needs to be considered in addition to the cell types.…”

“…The upstream regulators of GTPase Pac1 such as P-Rex1, cAMP, and CXCL12/CXCR4 were also downregulated by metformin. Further, the drug decreased lipogenic proteins such as acetyl-CoA carboxylase, fatty acid synthase and sterol regulatory element binding protein-1c resulting in inhibition of lipogenesis of LNCaP and DU145 [305]. Lipogenesis is a key self-propagating mechanism for PCa cell lines which has been observed to be affected by insulin and metformin [5].…”