Diabetes and insulin resistance alters ligamentum flavum–derived fibroblast responses in an AKT2-dependent manner

Fibroblasts participate in inflammatory responses and play a critical role in the switch from acute to persistent inflammation. Whether fibroblast responses are modulated by signals from their microenvironment is not well established. Insulin signaling and insulin resistance modulate responsiveness of innate immune cells to inflammatory signals. Herein, we investigated whether fibroblast responsiveness is affected by the tissue microenvironment. As a source of fibroblasts, we used ligamentum flavum-derived fibroblasts, being a tissue that is inflamed in the context of ligamentum flavum hypertrophy. The results showed that fibroblasts from patients with ligamentum flavum hypertrophy were hyporesponsive to TLR2 signals. Since ligamentum flavum hypertrophy is associated with obesity, we utilized ligamentum flavum-derived fibroblasts from obese and lean mice. Fibroblasts from insulin-resistant obese mice expressed increased Collagen1a1 and produced more IL-6 in response to TLR2 and TLR4 signals. Insulin signaling was altered in ligamentum flavum-derived fibroblasts from obese mice, resulting in reduced insulin-induced AKT1 phosphorylation and increased insulin-induced AKT2 phosphorylation. Ligamentum flavum-derived fibroblasts from AKT2-deficient mice were hyporesponsive to TLR signals, in contrast to these from obese mice, suggesting that active AKT2 signaling is required to support responsiveness of fibroblasts. Basal respiration and stress-induced glycolysis were elevated in fibroblasts from AKT2-/- and obese mice, suggesting that even though their response to TLR signaling differs, they exhibited similar metabolic changes. The results suggest that responsiveness of fibroblasts is altered in the context of obesity and insulin resistance and is controlled by the balance of AKT1/AKT2 activation, which may be critical to the development of hypertrophy.