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We investigated tissue turnover in healthy and osteoarthritic cartilage.
Aspartic acid
racemization (AAR) is one of the vital types of non-enzymatic covalent
modification that escorts an age-dependent accumulation of abnormal protein
human tissues frequently. An anabolic response was revealed by the knee
osteoarthritis cartilage which was not existing in the hip osteoarthritis
cartilage.
We investigated
tissue turnover in healthy and osteoarthritic cartilage. We challenge long held
views that osteoarthritis (OA) is dominated by a similar turnover process in
all joints and present evidence that hip and knee cartilage respond very
differently to OA.
d-and l-Aspartate (Asp) were quantified for
whole cartilage, collagen and non-collagenous components of cartilage obtained
at the time of joint replacement. We computed the Asp racemization ratio
(Asp-RR = d/d + l Asp), reflecting the proportion of old to total protein, for
each component.
Compared with hip
OA, knee OA collagen fibrils (P < 0.0001), collagen (P = 0.007), and
non-collagenous proteins (P = 0.0003) had significantly lower age-adjusted mean
Asp-RRs consistent with elevated protein synthesis in knee OA. Knee OA collagen
had a mean hydroxyproline/proline (H/P) ratio of 1.2 consistent with the
presence of type III collagen whereas hip OA collagen had a mean H/P ratio of
0.99 consistent with type II collagen. Based on Asp-RR, the relative age was
significantly different in knee and hip OA (P < 0.0005); on average OA knees
were estimated to be 30yrs ‘younger’, and OA hips 10yrs ‘older’ than non-OA.
The metabolic
response to OA was strikingly different by joint site. Knee OA cartilage
evinced an anabolic response that appeared to be absent in hip OA cartilage.
These results challenge the long held view that OA cartilage is capable of only
minimal repair and that collagen loss is irreversible.
Osteoarthritis Cartilage. 2016 Feb;24(2):374-81
Aspartic acid racemization reveals a high turnover state in knee compared with hip osteoarthritic cartilage
J.B. Catterall et al.
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