Angiopoietin-Like Protein 2 Induced by Mechanical Stress Accelerates Degeneration and Hypertrophy of the Ligamentum Flavum in Lumbar Spinal Canal Stenosis

Abstract

Chronic inflammation and subsequent fibrosis induced by mechanical stress play an important role in ligamentum flavum (LF) hypertrophy and degeneration in patients with lumbar spinal canal stenosis (LSCS). Angiopoietin-like protein 2 (Angptl2) is a chronic inflammatory mediator induced under various pathological conditions and increases the expression of TGF-β1, which is a well-characterized mediator in LF hypertrophy. We investigated whether Angptl2 is induced by mechanical stress, and whether it contributes to LF hypertrophy and degeneration by activating the TGF-β1 signaling cascade. In this study, we investigated human LF tissue and LF fibroblasts isolated from patients who underwent lumbar surgery. We found that Angptl2 was abundantly expressed in fibroblasts of hypertrophied LF tissues at both the mRNA and protein levels. This expression was not only positively correlated with LF thickness and degeneration but also positively correlated with lumbar segmental motion. Our in vitro experiments with fibroblasts from hypertrophied LF tissue revealed that mechanical stretching stress increases the expression and secretion of Angptl2 via activation of calcineurin/NFAT pathways. In hypertrophied LF tissue, expression of TGF-β1 mRNA was also increased and TGF-β1/Smad signaling was activated. Angptl2 expression in LF tissue was positively correlated with the expression of TGF-β1 mRNA, suggesting cooperation between Angptl2 and TGF-β1 in the pathogenesis of LF hypertrophy. In vitro experiments revealed that Angptl2 increased levels of TGF-β1 and its receptors, and also activated TGF-β1/Smad signaling. Mechanical stretching stress increased TGF-β1 mRNA expression, which was partially attenuated by treatment with a calcineurin/NFAT inhibitor or Angptl2 siRNA, indicating that induction of TGF-β1 expression by mechanical stretching stress is partially mediated by Angptl2. We conclude that expression of Angptl2 induced by mechanical stress in LF fibroblasts promotes LF tissue degeneration by activation of TGF-β1/Smad signaling, which results in LF hypertrophy in patients with LSCS.

Angiopoietin-like-2: a multifaceted protein with physiological and pathophysiological properties

“…Similarly, angptl2 was shown to contribute to the acceleration of tissue fibrosis in ligamentum flavum through the activation of the TGFβ-Smad pathway (Ref. 46). In the latter study, the following cascade has been proposed: mechanical stress activates mechano-sensitive ion channels, leading to increased intracellular calcium concentration and activation of calcineurin phosphatase, stimulating the dephosphorylation and nuclear translocation of the transcription factor nuclear factor of activated T cells (NFATc) and in turn, angptl2 expression in ligamentum flavum fibroblasts (Ref.…”

“…This was demonstrated using ligamentum flavum tissue, and isolated fibroblasts from patients with lumbar spinal canal stenosis exposed to mechanical stress and/or recombinant angptl2 (Ref. 46). In these cells, mechanical stress-induced angptl2 expression and angptl2-siRNA prevented TGFβ expression (Ref.…”

“…In these cells, mechanical stress-induced angptl2 expression and angptl2-siRNA prevented TGFβ expression (Ref. 46). Because mechanical stress contributes to various diseases including heart diseases, and because angptl2 is largely expressed in cardiac tissues, the authors asked the pertinent question whether mechanical stress-induced angptl2 contributes to the progression of heart diseases (Ref.…”

“…Because mechanical stress contributes to various diseases including heart diseases, and because angptl2 is largely expressed in cardiac tissues, the authors asked the pertinent question whether mechanical stress-induced angptl2 contributes to the progression of heart diseases (Ref. 46), an unresolved issue.…”

MicroRNA transcriptome analysis on hypertrophy of ligamentum flavum in patients with lumbar spinal stenosis

“…The pathology of degenerative hypertrophy is characterized by loss of elastic fibers and the appearance of a focal disorganized collagenous matrix, which is often associated with chondrometaplasia 3,4) . Although several cytokines such as TGFβ, CTGF, FGF2, and Angptl2 have been identified to be involved in LF hypertrophy, the molecular mechanism underlying the hypertrophy still remains largely unknown 3,[5][6][7][8][9] . MicroRNAs (miRNAs) are short non-coding RNAs that predominantly bind to the coding or 3'-untranslated region of the target mRNA to inhibit translation by promoting the degradation and/or blocking of the translational complex of the target mRNA, thereby playing important roles as posttranscriptional regulators 10,11) .…”