Universität Tübingen
Metformin-induced Effects on Human Skeletal Muscle Metabolism and Function
Abstract
Metformin, the primary oral antidiabetic agent for type 2 diabetes (T2D), has been FDA-approved for three decades. Today more than 150 million people worldwide benefit from metformin via effectively reduced blood glucose levels with low risk of hypoglycemia. However, the precise mechanism of action has not yet been conclusively clarified. Skeletal muscle is an important player in insulinmediated glucose uptake, but the number of studies on the effects of metformin on this tissue is limited. This thesis aims to improve the understanding of the effects of metformin on skeletal muscle by investigating two specific questions: 1. What are the molecular mechanisms by which metformin leads to increased lactate production? This phenomenon is associated with the known adverse effects of hyperlactatemia and metabolic acidosis. At the same time, it has been hypothesized that moderate increases in plasma lactate levels may contribute to the pleiotropic beneficial effects of metformin. 2. Which molecular mechanisms provide an explanation for the attenuated muscular adaptations to regular performed physical exercise during metformin therapy as reported in some clinical studies? This question arises in the context that regular exercise is an integral part of T2D therapy and skeletal muscle plays a central role in physical activity. Both questions were addressed by a 48-hour treatment of primary human myotubes with a range of different metformin concentrations (16–776 µM) to cover both pharmacological and suprapharmacological doses of the drug. Global effects were investigated by RNA sequencing analysis. For the determination of lactate production and glucose consumption cell culture supernatant was analyzed. Individual mechanisms were investigated on the basis of immunoblots, immunohistochemistry, respirometry and luminescence assays. Mimicking exercise in vitro, electro-pulse stimulation was applied and cellular energy status was investigated by quantification of adenosine nucleotides via capillary electrophoresis (CE). Increasing concentrations of metformin led to its intracellular accumulation. As from 78 µM, the metformin treatment resulted in a dose-dependent increase in extracellular lactate, predominantly due to increased lactate production covered by increased glucose uptake and glycolysis. Lactate production was forced through a shift in the equilibrium of the LDH reaction in myotubes. First, metformin inhibited mitochondrial respiration in complex I causing a shift in cellular redox state represented by the accumulation of NADH. Second, metformin mediated an accumulation of pyruvate through the inhibition of PDH complex. Which of the two mechanisms is at work in the skeletal muscle of metformin-treated patients with T2D needs to be investigated. Assessing the second question, reduced mTOR-C1/S6K1 signaling was found in metformin-treated myotubes, indicating a blunted hypertrophic response mediated by metformin. In addition, even at low concentrations, metformin impaired the electro-pulse-stimulated contraction of myotubes. No evidence of structural changes could be found, so that this does not appear to be the cause of the limited functionality of the myotubes. The hypothesis that a metformin-induced energy deficit is the underlying cause could not be conclusively confirmed. Nucleotide quantification using CE did not reveal any differences, but in contrast, an increased activation of the energy sensor AMPK was found. In conclusion, the results suggest a novel mechanism by which metformin can increase lactate production through altered activity of the PDH complex. The impaired contractility of metformin-treated myotubes provides in vitro evidence that metformin may interfere with biochemical processes during exercise in vivo. Given the high prevalence of metformin-exercise combination therapies in patients with T2D, further in vivo studies are required.
Author and committee
dc:creator, dc:contributor.*- Author
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- Maurer, Jennifer
Identifiers
dc:identifier.*- Identifier
- hdl:10900/157523