Locally Produced IGF-1 Promotes Hypertrophy of the Ligamentum Flavum via the mTORC1 Signaling Pathway

BACKGROUND/AIMS: Narrowing of the lumbar spinal canal is a condition called lumbar spinal stenosis (LSS) and is a high-morbidity problem in the elderly. LSS is commonly caused by hypertrophy of the ligamentum flavum (HLF). Previous studies showed that fibrosis of the ligamentum flavum (LF) largely contributed to HLF. However, the underlying pathomechanism remains unclear. Insulin-like growth factor-1 (IGF-1) is known to have an intimate relationship with fibrosis in various tissues. Nevertheless, currently, there are few studies regarding IGF-1 in HLF. In this study, we investigated the role of IGF-1 in HLF and its potential molecular mechanism of action. METHODS: First, the IGF-1, phosphorylation of IGF-1 receptor (pIGF-1R), phosphorylation of AKT (pAKT), phosphorylation of S6(pS6), collagen I and collagen III expression levels were examined via immunohistochemistry and Western blotting in LF tissues from patients with LSS or Non-LSS. Second, primary LF cells were isolated from adults with a normal LF thickness and were cultured with different concentrations of IGF-1 with or without NVP-AEW541/rapamycin. RESULTS: The results showed that IGF-1, pIGF-1R, pAKT, pS6, collagen I and collagen III protein expression in the LSS group was significantly higher than that in the Non-LSS group. Meanwhile, pIGF-1R, pAKT, pS6, collagen I and collagen III protein expression was significantly enhanced in LF cells after IGF-1 exposure, which can be notably blocked by NVP-AEW541. In addition, pS6, collagen I and collagen III protein expression was blocked by rapamycin. CONCLUSIONS: Enhanced IGF-1 promotes the synthesis of collagen I and collagen III via the mTORC1 signaling pathway, which eventually contributes to hypertrophy of the ligamentum flavum.