Metformin as an Anticancer Agent

Abstract

Metformin has been a frontline therapy for type 2 diabetes (T2D) for many years. Its effectiveness in T2D treatment is mostly attributed to its suppression of hepatic gluconeogenesis; however, the mechanistic aspects of metformin action remain elusive. In addition to its glucose-lowering effect, metformin possesses other pleiotropic health-promoting effects that include reduced cancer risk and tumorigenesis. Metformin inhibits the electron transport chain (ETC) and ATP synthesis; however, recent data reveal that metformin regulates AMP-activated protein kinase (AMPK) and the mechanistic target of rapamycin complex 1 (mTORC1) by multiple, mutually nonexclusive mechanisms that do not necessarily depend on the inhibition of ETC and the cellular ATP level. In this review, we discuss recent advances in elucidating the molecular mechanisms that are relevant for metformin use in cancer treatment.

Keywords: AMPK; electron transport chain; mTORC1; metformin.

Figure 1.

The Major Molecular Targets of Metformin. These are the ETC, AMPK, and mTORC1. ETC produces ATP, leading to AMPK downregulation. Metformin inhibits the ETC, resulting in reduced ATP synthesis. The elevated AMP/ATP ratio activates AMPK, which phosphorylates and inhibits mTORC1. The metformin-mediated inhibition of ATP synthesis also results in inhibition of mTORC1. Metformin also activates AMPK and inhibits mTORC1 by a mechanism that is independent of the ETC. Abbreviations: AMPK, AMP-activated protein kinase; ATP, adenosine triphosphate; ETC, electron transport chain; mTORC1, mechanistic target of rapamycin complex 1.

Figure 2.

Inhibition of the ETC Reverses the Direction of the Malate-Aspartate Shuttle. (A) In the absence of metformin, the malate-aspartate shuttle removes NADH from cytosol and regenerates it in mitochondria to be used as a substrate for the ETC. Since NADH cannot be transported across the mitochondrial inner membrane, the reducing equivalents are transported from the cytosol to the mitochondria in the form of malate (indicated by green arrows). Under these conditions, aspartate is produced in mitochondria by the mitochondrial aspartate aminotransferase mAST. (B) When the ETC is inhibited by metformin or phenformin, mitochondrial NADH accumulates and malate is transported in the opposite direction from mitochondria to cytosol (indicated by red arrows). Under these conditions, cytosolic aspartate aminotransferase cAST is required for synthesis of aspartate and cell proliferation. Abbreviations: cAST, cytosolic aspartate aminotransferase; ETC, electron transport chain; mAST, mitochondrial aspartate aminotransferase; NADH, nicotinamide adenine dinucleotide.

Figure 3.

Metformin Inhibits mTORC1 in an AMPK-Dependent Manner. Metformin-activated AMPK downregulates mTORC1 signaling by at least two mechanisms. First, AMPK inhibits mTORC1 by phosphorylating the raptor subunit of mTORC1. Second, AMPK activates the tuberous sclerosis complex (TSC), composed of TSC1, TSC2, and TBC1D7, by phosphorylating the TSC2 subunit. This, in turn, inhibits Rheb. Abbreviations: AMPK, AMP-activated protein kinase; mTORC1, mechanistic target of rapamycin complex 1; Rheb, Ras homolog enriched in the brain.

Figure 4.

Reciprocal Regulation of AMPK and mTORC1 by Metformin in the Lysosomal Pathway. mTORC1 resides at the lysosomal membrane through interacting with the Rag heterodimer (RagA and RagC) and is activated by amino acids that are sensed by the Rags, Ragulator, and v-ATPase. mTORC1 is also activated by growth factors through the GTPase Rheb. Metformin induces recruitment of AMPK to the lysosomal membrane and promotes formation of a complex consisting of Ragulator, v-ATPase, AXIN, LKB1, and AMPK. AXIN inhibits the GEF activity of Ragulator towards Rags, leading to mTORC1 dissociation from the Ragulator and lysosome. The overall effect of metformin is activation of AMPK and inhibition of mTORC1 at the lysosome. Abbreviations: AMPK, AMP-activated protein kinase; GEF, guanine nucleotide exchange factor; GTPase, guanosine triphosphatase; mTORC1, mechanistic target of rapamycin complex 1; Rag, Ras-related GTPase complex.

Figure 5.

Metformin Inhibits mTORC1 by Preventing RagC Activation. Rag heterodimer activates mTORC1 when RagA (or RagB) binds GTP and RagC (or RagD) binds GDP. In order to be activated, RagC needs to enter the nucleus, where it acquires GDP and becomes competent for mTORC1 activation. When the cellular level of ATP is reduced due to metformin inhibition of the ETC, function of the nuclear pore complex is altered, resulting in nuclear exclusion of RagC, failure of RagC to bind GDP, and, consequently, mTORC1 inhibition. Abbreviations: ATP, adenosine triphosphate; ETC, electron transport chain; GDP, guanosine diphosphate; GTP, guanosine triphosphate; mTORC1, mechanistic target of rapamycin complex 1; Rag, Ras-related GTPase complex.