The Impact of Hypercholesterolemia on Tendon Injury Repair

Background: Hypercholesterolemia is a condition characterized by high blood cholesterol levels and is associated with tendon xanthomas (cholesterol deposits found on superficial tendons). Lipid accumulation in the tendon's extracellular spaces¹ may disrupt the parallel organization of collagen substructures within the tendon, and negatively affect its mechano-sensing and mechanical properties, which may lead to injury. We hypothesized that tendon strength and metabolism would be inferior in a high cholesterol environment and attenuate the tendon's ability to repair after injury.

Methods: 50 Sprague-Dawley (SD, control) and 50 apolipoprotein E knock-out rats (ApoE, Envigo, IL, USA) rats bred in-house were subject to a unilateral patellar tendon (PT) injury at 12 weeks of age; the uninjured limb served as a control. Animals were euthanised at 3, 14 or 42 days post-injury, time-points chosen to represent tendon inflammatory, reparative and remodelling repair phases, respectively. Animals were randomly assigned for investigating gene expression (SD/ApoE per timepoint n=4/4), tissue histology (n=6/6), biomechanical testing (n=10/10; 14 and 42 days only). Equal male/female ratios were used where possible (total SD M23/F27, ApoE M23/F26).

Results: ApoE total cholesterol was over double that of SD rats (mean 2.12 vs 0.99 mg/ml, p<0.001). Despite this, there was little histological evidence of lipid content (Oil Red-O staining) in tendons of either group, and no apparent differences in tendon mechanical or material properties. However, there were differences in gene expression. ApoE rats demonstrated significantly greater COX2 expression than SD; other inflammatory markers (TGFβ, IL-1, IL-6) did not differ between groups. COL1A1, SCX, LOXα were significantly upregulated 14 days post injury in SD compared to ApoE. A greater COL3A1 response was also observed in ApoE compared to SD rats.

Discussion: We found differences in injury repair at the cellular level, using an ApoE high cholesterol model. There was little evidence of lipid accumulation at the tissue level, therefore the PT may have been a poor choice for an injury study in this instance and it is not surprising that biomechanical differences were not found. Moreover, ApoE rats were recently found to demonstrate only modest early atherosclerotic characteristics², thus the young age of our cohort may be a primary factor in the lack of physical evidence of lipid accumulation. Nonetheless, our results indicate that even with a mild phenotype, high cholesterol significantly modulates tendon inflammatory and healing cellular activity, as indicated by altered mRNA levels, which may contribute to the known consequences of tendon cholesterol on tendons in humans.

References

¹Grewal et al. 2014. Accumulation of oxidized LDL in the tendon tissues of C57BL/6 or apolipoprotein E knock-out mice that consume a high fat diet: potential impact on tendon health. PLoS One 9(12): e114214.

²Rune et al., 2018. Long-term Western diet fed apolipoprotein E-deficient rats exhibit only modest early atherosclerotic characteristics. Sci Rep 8(1): 5416.