Role of Hypoxia-Inducible Factor-1α in Regulating Muscle Degeneration After Rotator Cuff Tears.

Secondary muscle degeneration after a rotator cuff tear (RCT) critically affects clinical outcomes. Vascular compromise after a tendon injury creates a complex microenvironment that may be associated with the degeneration of rotator cuff muscle. The role of hypoxia-inducible factor-1α (HIF-1α), a master regulator of cellular stress responses to hypoxia, in modulating muscle abnormalities after an RCT remains undefined. To define the role of HIF-1α in stem cell differentiation and muscle degeneration after an RCT in a murine model. Controlled laboratory study. A supraspinatus tendon transection and suprascapular nerve transection (TTDN) model was established in C57BL/6J, platelet-derived growth factor receptor α (PDGFRα)-green fluorescent protein (GFP) reporter, and inducible cell-specific HIF-1α knockout mice. Vascularity and HIF-1α colocalization with fibroadipogenic progenitor (FAP) cells and satellite cells were analyzed. Fibrosis, fatty infiltration, and myofiber cross-sectional area were assessed. In vitro, HIF-1α was modulated in isolated FAP cells via CRISPR-Cas9 or prolyl hydroxylase domain inhibitors to evaluate FAP cell differentiation. TTDN induced significant capillary density reduction (CD31+) at 1, 2, and 6 weeks after an injury. Global HIF-1α expression decreased after TTDN compared to the sham side (1 week: 0.78 ± 0.22 vs 1.40 ± 0.42, respectively [P = .019]; 2 weeks: 0.74 ± 0.51 vs 1.70 ± 0.48, respectively [P = .015]). The percentage of PDGFRα+ FAP cells increased at 6 weeks after TTDN compared to the sham side (15.69% ± 1.90% vs 12.76% ± 0.78%, respectively; P = .013). The percentage of HIF-1α+ FAP cells relative to total PDGFRα+ cells significantly decreased in the late stage (6 weeks) of an RCT compared to the sham side (2.78% ± 0.90% vs 7.38% ± 2.29%, respectively; P = .003). Knocking out HIF-1α in FAP cells in vivo resulted in increased fibrosis (Cre+: 4.43% ± 2.16% vs Cre-: 1.72% ± 0.39%; P = .047), decreased fatty infiltration (Cre+: 0.62% ± 0.42% vs Cre-: 1.55% ± 0.45%; P = .016), and reduced cross-sectional area (Cre+: 664.71 ± 354.45 vs Cre-: 1195.81 ± 338.66; P = .041). Neither satellite cell-specific nor myocyte-specific HIF-1α deletion resulted in significant phenotypic changes. The downregulation of HIF-1α led to a decrease in uncoupling protein 1 expression and an increase in α-smooth muscle actin expression in FAP cells. Although vascularity was reduced after TTDN, pronounced global tissue hypoxia was not directly evidenced. Decreased global HIF-1α expression may reflect denervation-induced reductions in metabolic demand. HIF-1α emerges as a key player in FAP cell differentiation within the injury microenvironment, promoting brown adipose tissue differentiation and inhibiting fibrogenesis. Targeting HIF-1α in FAP cells offers a novel therapeutic strategy to mitigate secondary muscle atrophy and fibrosis after an RCT.