The American Diabetes Association recommends metformin as the first therapeutic line for type 2 diabetes mellitus and for prevention of diabetes in high risk individuals [22
]. Buformin is available in Japan and Eastern Europe. Phenformin (10 times more potent than metformin) has been withdrawn from the market since 1978, due to lack of activity and reports of fatal lactic acidosis [23
]. Buformin has a relatively limited use. Undoubtedly, improvements in our understanding of biguanides’ molecular targets will result in follow-up compounds with better properties.
The biguanides have been proposed to target complex I of the respiratory chain [11
]. In one study, high metformin dosing (30 mM) reduced the activity of complex I in skeletal muscle homogenates by about 15%. A 24-h exposure of muscles to 270 μM metformin reduced cellular respiration by 30% and increased lactate production by 80% [12
]. Similarly, exposure of rat liver mitochondria to high metformin doses (≥ 10 mM) impaired oxidations in the respiratory chain and decreased the membrane potential [13
]. The clinical significance of the high concentrations used in these studies remains to be determined since the human plasma concentrations of metformin and buformin rarely exceed 5 mg/L (about 30 μM) [24
]. Higher (up to 2-fold) drug concentrations, however, accumulate in the liver [3
]. mainly due to expression of the organic cation transporter 1 that facilitates metformin hepatocyte uptake [14
]. The level of phenformin is much lower (150 μg/L or 0.6 μM) [28
]. In this study, the biguanides were investigated at 100 μM and 1.0 mM (several fold higher than therapeutic levels) [24
The bioavailability of biguanides is about 50%, and their binding to plasma proteins is negligible. Metformin and buformin are excreted in the urine as free drugs. Phenformin is metabolized in the liver and then excreted in the urine [29
It is generally accepted that metformin treats non-insulin-dependent diabetes mellitus by suppressing hepatic glucose output. This highly-regulated activity is complex and it best tested in the targeted tissue rather than cell lines. This study investigated the effects of biguanides on murine heart muscle and liver specimens, using preparations that allowed measuring cellular energy biomarkers (cellular respiration and ATP) [16
]. The purpose of these experiments was to measure cardiomyocyte and hepatocyte bioenergetics in the presence of metformin and other biguanides.
The main finding in this study is that 100 μM metformin lowered cardiomyocyte respiration by 10% (Figure 1B) and increased cardiomyocyte ATP by 18% (Figure 2C). These two processes (↓respiration/↑ATP) may reflect a mildly improved cellular bioenergetics (metabolic energy conversion). It is worth noting that this mode-of-action is unique to this class of drugs and may be responsible for lowering the demand for hepatic gluconeogenesis. Consistently, metformin lowered hepatocyte respiration by 11-13% (Figure 3B and 4B) and decreased hepatocyte ATP by 5-10% (Figure 3C and 4C). A simple explanation of these two processes (↓respiration/↓ATP) is that metformin slightly decreases hepatocyte bioenergetics and, thus, lowing hepatic gluconeogenesis.
These data are consistent with the recent finding that metformin augments myoblast bioenergetics following energy interruption by uncouplers (dinitrophenol) or respiratory chain inhibitors (azide) [10
]. The results also are consistent with the current understanding that metformin controls blood glucose by favorably modulating metabolic pathways without directly targeting oxidative phosphorylation [1
The experiments reported here have important limitations. Heart muscle specimens were not suitable for several hours of in vitro
incubation. Their respiration, thus, was measured immediately after tissue collection (Figure 2A-D); the drug exposure was limited to about one hour. Liver specimens, on the other hand, were relatively more stable, allowing in vitro
incubation with and without the drugs for up to ~6 h (Figure 3A-D) [15
]. It is worth noting, however, that without addition, hepatocyte ATP at 0 h (immediately after tissue collection) was 235 ± 72 pmol mg-1
and at 1 h was 39 ± 15 pmol mg-1
. These results confirm significant deterioration of hepatocyte bioenergetics in vitro
Metformin has been shown to have beneficial effects on cardiomyocyte in Diabetic Mice. In the study by Xie et al. [30
] they have shown an increased AMP-activated protein kinase in cardiac myocyte in OVE26 diabetic mice after treatment with metformin in comparison to control group. These increases lead to enhancement of cellular autophagy; a process responsible for the removal of damaged organelles and aggregated proteins thus preserving cardiac function [30
Similar results have been demonstrated by in SHHF rats, an animal mode of insulin resistant diabetic rats, as activation of AMP-activated protein kinase leads to increased production of nitric oxide that plays a major rule in regulation of vascular tone and hence cardiac function. In addition, study by Cittadini et al. [31
] reveals a number of beneficial cellular alterations within including reduction in lipid accumulation and decrease in tumor necrosis factor-a expression and myocyte apoptosis. These recently highlighted cellular changes have been documented in a number of clinical studies that proposed the protective rule of use of biguanides e.g. metformin on enhancing cardiac function and decreasing risk of diabetes associated cardiomyopathy [32
Treatment with metformin increases the expression of transcription factors involved in gluconeogenesis and hence produce its diabetes therapeutic effects [34
]. In a recent study by Takayama et.al, they have added a new insight on mechanism of action of metformin. They have demonstrated a reduction in selenoprotein P, a protein secreted by hepatocytes in insulin resistant type-2 diabetes, induced by AMP-activated kinase [36
In conclusion, cardiomyocyte respiration is slightly decreased and ATP is slightly increased in the presence of metformin. The drug effects on hepatocytes are different. Hepatocyte respiration and ATP are both slightly decreased in the presence of metformin. The effects of buformin and phenformin are more prominent than metformin, pointing to the importance of investigating several biguanides in all targeted organs.