Metformin is a first-line defense therapy for individuals with type 2 diabetes mellitus (T2D). Metformin is an inexpensive, well-tolerated, and widely prescribed drug which led to re-purposing appeal for metformin use in other diseases beyond T2D [88
]. Mechanistically, metformin action is complex and may be dose and tissue dependent, but has been observed to signal through AMPKα and PGC-1α in many cell types, including hepatocytes and skeletal muscle [89
]. The pleiotropic effects of metformin make it difficult to delineate an exact mechanism but contribute to excitement as a therapy for targeting multiple facets dysregulated with disease and aging [88
]. A similar line of thinking in lieu of the evidence lend to metformin being a useful therapeutic option to promote recovery from disuse in older adults.
Currently, research exploring metformin to prevent disuse atrophy and promote muscle recovery in aging at least in rodents appears to be strong. During 7 days hindlimb immobilization in young adult rats, metformin treatment ameliorated muscle atrophy and prevented tissue fibrosis [97
]. In another study, rats treated with metformin during 16 weeks of high fat diet was effective to counter myofiber atrophy, fibrosis and increased E3 ubiquitin ligases expression compared to the non-metformin treated group [98
]. During recovery from burn injury, metformin protected against myofiber atrophy and muscle fat infiltration while increasing satellite cell abundance [95
]. In another study, 21 days of metformin prior to cardiotoxin injury prevented muscle damage without altering embryonic myosin heavy chain or central nuclei content [99
]. A total of 60 days of metformin treatment in mice improved aerobic capacity while 3 days of metformin exposure in C2C12 muscle cells promoted differentiation, anabolic signaling, and SOD2 protein expression [100
]. Moreover, metformin treatment prevented mouse satellite cell exhaustion in vitro and in single myofibers [101
], ROS emission in obese rats [102
], human T cell inflammation in vitro [103
], and enhanced muscle membrane stability through AMPKα in dysferlin deficient mice [104
]. Together, these studies strongly suggest that metformin treatment may be suitable to prevent muscle damage or promote recovery through influencing multiple phenotypes and signaling pathways that are commonly altered with disuse.
Metformin is known to increase PGC-1α in skeletal muscle tissue and cells [91
], but AMPKα dependance is unknown. In C2C12 myotubes, metformin is able to increase PGC-1α mRNA [94
]. In mouse muscle, metformin increased AMPKα and PGC-1α expression in slow- and fast-twitch fibers, indicating that metformin can increase PGC-1α regardless of muscle fiber type [91
]. Studies in hepatocytes support that metformin works through AMPKα to promote PGC-1α expression. In hepatocytes derived from liver-specific AMPKα1/2 null mice, the normal increase in PGC-1α with metformin are blunted [105
]. Furthermore, blocking AMPKα with compound C prevented the metformin-induced increase in PGC-1α expression in hepatocytes [106
]. These studies indicate that metformin can increase PGC-1α in skeletal muscle and may require AMPKα for this action but many mechanistic metformin studies in skeletal muscle are lacking.
When considering metformin therapy as a target of PGC-1α, one must consider the cell type, species (rodent vs. human), and dosing. For instance, in primary mouse hepatocytes treated with dexamethasone, 8 h of suprapharmacological metformin doses (1 and 2 mM) given with cyclic AMP decreased PGC-1α mRNA expression [105
], whereas human primary hepatocytes exposed to 1 mM metformin for 48 h increased PGC-1α mRNA [106
]. Further, in C2C12 myotubes, exposure of a single suprapharmacological metformin dose (2 mM) for 4, 8, 12 or 24 h increased PGC-1α mRNA expression, where a pharmacological dose (30 μM) did not during the same time course [94
]. Metformin dosing also appears to alter mitochondrial function. Initially it was thought that metformin inhibits mitochondrial complex I to ameliorate enhanced glucose production in individuals with T2D [107
]. More recent reports support that mitochondrial inhibition is caused by high metformin concentrations (≥1 mM) whereas clinically prescribed (50–80 μM) doses likely work through mechanisms independent of mitochondrial inhibition [92
] and may actually improve mitochondrial function dependent on AMPKα [92
]. However, therapeutic, prescribed doses of metformin (1.5–2 g/d) given to older adults blunted exercise-induced improvements in aerobic capacity through impaired mitochondrial respiration [108
] and impaired resistance training muscle adaptations [109
], suggesting that metformin may interfere with exercise training. Overall, given that metformin at higher doses may be consequential to mitochondrial function, it may be more beneficial to investigate lower metformin doses to prevent disuse atrophy and promote muscle recovery in aging.
4.2. Metformin Combination Therapies
The use of compounds discussed above (leucine, HMB, or resveratrol) combined with metformin or in combination with vitamins (vitamin D) has appeal to not only enhance treatment outcomes by achieving a synergistic effect, but also to lower metformin dosing and decrease the likelihood of metformin-induced side effects (primarily gastrointestinal distress). Metformin combined therapies have not been studied in the context of muscle disuse atrophy or recovery in aging. However, outside the focus of this review, some metformin combination therapies have been shown to improve muscle insulin sensitivity and alleviate metabolic dysfunction. Importantly, metformin combined therapies reveal SIRT1–AMPKα–PGC-1α signaling as a common target mechanism, suggesting that a combined nutraceutical-pharmaceutical therapy approach may be worth investigating to prevent disuse-induced muscle atrophy and promote recovery in aging.
For instance, metformin combined with vitamin D for 8 weeks in rats with hyperglycemia (2 week HFD + 1 streptozotocin (STZ) injection) resulted in increased muscle PGC-1α mRNA expression as well as decreased E3 ubiquitin ligases, fibrosis and sarcolemma abnormalities compared to metformin or vitamin D alone [110
]. In insulin stimulated C2C12 cells, metformin combined with leucine for 2 or 24 h enhanced SIRT1 and AMPKα activity whereas metformin and leucine monotherapy did not. Moreover, the influence of metformin and leucine combination on AMPKα was blocked by SIRT1 pharmacological or siRNA-induced inhibition [111
]. Metformin and leucine given during 6 weeks of HFD in mice enhanced glucose tolerance compared to a higher dose metformin monotherapy [112
]. Metformin and resveratrol combination therapy increased AKT activation in triceps muscle, and this treatment resulted in improved glucose tolerance 4 weeks after a 9 week HFD intervention in mice [113
]. Lastly, metformin combined with HMB and resveratrol resulted in increased oxygen consumption rate and AMPKα phosphorylation in C2C12 cells [114
] which similarly also occurred with metformin-leucine combined therapy [111
While understudied in skeletal muscle and during disuse and recovery in aging, metformin combination therapies may be promising SIRT1–AMPKα–PGC-1α signaling agonists to improve muscle function. Indeed, this field of research is in its infancy and other metformin combinations (such as with the compound sildenafil [115
]) may prove to be interesting translational therapeutic targets.