Mechanism of Metformin Action in Dermal Fibroblasts
Burn injury is a significant problem that affects approximately half a million people in the U.S. annually. One of the major complications emerging from burn injury is hyperglycemia, which can last for weeks following the initial trauma. The primary cause of hyperglycemia is dysregulated AMPK signaling and mitochondrial complex activity, which regulates both glucose uptake and cellular energy status. Current therapeutic methods to counter hyperglycemia include tight euglycemic control and/or intensive insulin treatment, which are associated with increases in hypoglycemia and mortality, but also accelerate wound healing rates, potentially decreasing scarring. Alternatives to insulin therapy include biguanide drugs such as metformin. Metformin downregulates gluconeogenesis, and reduces blood glucose levels by activating glucose transport via GLUT4 in muscle cells. Importantly metformin has been shown to decrease mitochondrial respiratory complex I activity in hepatocytes. However its effects in dermal fibroblasts are unknown. Furthermore, up to 90 percent of burn patients develop hypertrophic scars due to dysregulated collagen secretion, which can lead to functional impairment due to excessive collagen deposition associated with hypertrophic scarring. In this study the hypothesis is that in dermal fibroblasts metformin will alter mitochondrial oxidative phosphorylation and alter cAMP and AMPK signaling activity, resulting in increased wound healing and improved mitochondrial function. Our experimental data shows that metformin induced AMPK phosphorylation, reduced mTOR phosphorylation, and downregulated cAMP accumulation in dermal fibroblasts. Fibroblast proliferation and migration were downregulated in hyperglycemic conditions with metformin exposure. Expression of mitochondrial biogenesis genes were altered by metformin treatment. Significantly, metformin specifically upregulated oxygen flux activity in fibroblasts, which was independent of AMPK activity. Metformin also altered expression of epithelial-mesenchymal transition (EMT) genes and markers. The primary EMT pathway regulated by metformin was SMAD3, which was decreased by metformin treatment resulting in decreased collagen I gene expression. Metformin also reduced total collagen secretion in low glucose conditions. These results indicate that metformin alters metabolism in fibroblasts through mitochondrial respiratory chain independently of AMPK. This will allow a better understanding of the effects of metformin on fibroblasts leading to new therapeutic options to prevent burn injury complications for wound healing in burn patients.