Autophagy is a protective mechanism in normal cartilage, and its aging-related loss is linked with cell death and osteoarthritis.
OBJECTIVE
Autophagy is a process for turnover of intracellular organelles and molecules that protects cells during stress responses. We undertook this study to evaluate the potential roles of Unc-51-like kinase 1 (ULK1), an inducer of autophagy, Beclin1, a regulator of autophagy, and microtubule-associated protein 1 light chain 3 (LC3), which executes autophagy, in the development of osteoarthritis (OA) and in cartilage cell death.


METHODS
Expression of ULK1, Beclin1, and LC3 was analyzed in normal and OA human articular cartilage and in knee joints of mice with aging-related and surgically induced OA, using immunohistochemistry and Western blotting. Poly(ADP-ribose) polymerase (PARP) p85 expression was used to determine the correlation between cell death and autophagy.


RESULTS
ULK1, Beclin1, and LC3 were constitutively expressed in normal human articular cartilage. ULK1, Beclin1, and LC3 protein expression was reduced in OA chondrocytes and cartilage, but these 3 proteins were strongly expressed in the OA cell clusters. In mouse knee joints, loss of glycosaminoglycans (GAGs) was observed at ages 9 months and 12 months and in the surgical OA model, 8 weeks after knee destabilization. Expression of ULK1, Beclin1, and LC3 decreased together with GAG loss, while PARP p85 expression was increased.


CONCLUSIONS
Autophagy may be a protective or homeostatic mechanism in normal cartilage. In contrast, human OA and aging-related and surgically induced OA in mice are associated with a reduction and loss of ULK1, Beclin1, and LC3 expression and a related increase in apoptosis. These results suggest that compromised autophagy represents a novel mechanism in the development of OA.
Caramés
Beatriz
B
The Scripps Research Institute, 10550 North Torrey, Pines Road, La Jolla, CA 92037, USA.
Taniguchi
Noboru
N
Otsuki
Shuhei
S
Blanco
Francisco J
FJ
Lotz
Martin
M
D000368:Aged; D000375:Aging / Q000502:physiology*; D000818:Animals; D051017:Apoptosis Regulatory Proteins / Q000032:analysis / Q000502:physiology*; D001343:Autophagy / Q000187:drug effects / Q000502:physiology*; D000071189:Autophagy-Related Protein-1 Homolog; D000071186:Beclin-1; D015153:Blotting, Western; D002358:Cartilage, Articular / Q000502:physiology*; D016923:Cell Death / Q000502:physiology*; D019902:Chondrocytes / Q000502:physiology; D006801:Humans; D007150:Immunohistochemistry; D047908:Intracellular Signaling Peptides and Proteins / Q000032:analysis / Q000502:physiology*; D008565:Membrane Proteins / Q000032:analysis / Q000502:physiology*; D051379:Mice; D008810:Mice, Inbred C57BL; D008869:Microtubule-Associated Proteins / Q000032:analysis / Q000502:physiology*; D010003:Osteoarthritis / Q000503:physiopathology*; D017346:Protein Serine-Threonine Kinases / Q000032:analysis / Q000502:physiology*; D055815:Young Adult
D051017:Apoptosis Regulatory Proteins; C491997:BECN1 protein, human; D000071186:Beclin-1; C115010:Becn1 protein, mouse; D047908:Intracellular Signaling Peptides and Proteins; C477012:MAP1LC3A protein, human; D008565:Membrane Proteins; D008869:Microtubule-Associated Proteins; D000071189:Autophagy-Related Protein-1 Homolog; D017346:Protein Serine-Threonine Kinases; C484754:ULK1 protein, human