Upregulation Of Vitamin D Pathways Is Inversely Associated With Quadriceps Fiber Atrophy Following ACL Tear.

Abstract

Protracted quadriceps atrophy and weakness after Anterior Cruciate Ligament (ACL) injury results in poor knee mechanics and contributes significantly to the development of posttraumatic osteoarthritis. The cellular and molecular underpinnings of poor functional recovery are unknown, limiting the development of evidence-based therapies. Vitamin D is a fat-soluble vitamin with critical roles for bone and skeletal muscle health. PURPOSE: Our goal is to determine how participants’ skeletal muscle expression of vitamin D receptor (VDR), CYP24A1, and CYP27B1 is affected by anterior cruciate ligament (ACL) tear. METHODS: Fifteen participants (mean ± SD; age, 22.3 ± 5.4; BMI, 27 ± 5; 47% female) who sustained an ACL tear received muscle biopsies from the injured and noninjured vastus lateralis. Muscle biopsies were processed for western blotting and immunohistochemistry. The expression of VDR, CYP24A1, and CYP27B1 were assayed via western blot. Seven μm sections were cut on a cryostat and fiber cross-sectional area (CSA) was assessed with automated methods. Arbitrary densitometry units (AU) from western blotting and mean fiber CSA results were compared between limbs using paired t-tests. Pearson correlations were used to identify associations between VDR protein expression in the injured limb and pooled muscle fiber CSA. RESULTS: VDR expression was 57% higher in the injured limb when compared to the noninjured limb (0.11 ± 0.05 vs 0.07 ± 0.037 AU, p = 0.024). The expression of CYP24A1 were CYP27B1 were higher in the injured limb, but the differences did not reach statistical significance. Additionally, mean fiber CSA was lower in the injured limb (4259 ± 990 vs. 4976 ± 1115 μm2, p = 0.005). VDR expression in the injured limb and VDR percent difference between limbs was inversely associated with fiber atrophy (as assessed as CSA percent difference) (R = -0.575, p = 0.025; R = -0.545, p = 0.036). CONCLUSIONS: ACL injury increases VDR expression in quadriceps, and preliminary results suggest alterations in vitamin D activity and turnover. Our results support a role for vitamin D receptor expression to maintain quadriceps fiber size and highlight the need for further research into micronutrient metabolism to aid in the recovery from injury.