Abstract
The ability of insulin-like growth factor I (IGF-I) to stimulate cartilage matrix synthesis is reduced in aged and osteoarthritic cartilage. Aging and osteoarthritis are associated with an increase in reactive oxygen species, which we hypothesized would interfere with normal IGF-I signaling. We compared IGF-I signaling in normal and osteoarthritic human articular chondrocytes and investigated the effects of oxidative stress induced by tert-butylhydroperoxide (tBHP). In normal human chondrocytes, IGF-I initiated a strong and sustained phosphorylation of IRS-1 (Tyr-612) and Akt (Ser-473) and transient ERK phosphorylation. In contrast, in osteoarthritic chondrocytes, which possessed elevated basal IRS-1 (Ser-312) and ERK phosphorylation, IGF-I failed to stimulate IRS-1 (Tyr-612) or Akt phosphorylation. In normal human chondrocytes, tBHP triggered strong IRS-1 (Ser-312 and Ser-616) and ERK phosphorylation and inhibited IGF-I-induced IRS-1 (Tyr-612) and Akt phosphorylation. Lentivirus-mediated overexpression of constitutively active (CA) Akt significantly enhanced proteoglycan synthesis, whereas both dominant negative Akt and CA MEK inhibited proteoglycan synthesis. CA Akt also promoted type II collagen and Sox9 expression, whereas tBHP treatment and CA MEK inhibited aggrecan, collagen II, and Sox9 mRNA expression. In osteoarthritic chondrocytes, the antioxidants Mn(III) tetrakis(4-benzoic acid)porphyrin and N-acetylcysteine increased the ratio of Akt to ERK phosphorylation and promoted IGF-I-mediated proteoglycan synthesis. Chemical inhibition of ERK significantly enhanced IGF-I phosphorylation of Akt and alleviated tBHP inhibition of Akt phosphorylation. These results demonstrate opposing roles for phosphatidylinositol 3-kinase-Akt and MEK-ERK in cartilage matrix synthesis and suggest that elevated levels of reactive oxygen species cause chondrocyte IGF-I resistance by altering the balance of Akt to ERK activity.
Highlights
Insulin-like growth factor I (IGF-I)2 plays a critical role in regulating normal growth and tissue formation during both
Because our results suggested that the ratio of Akt to ERK phosphorylation was altered in OA cells and the relative activation of these two pathways regulates proteoglycan synthesis, immunoblotting was performed to determine the phosphorylation status of Akt and ERK1/2 expressed as a ratio as
This study delineated the roles of two major IGF-I-stimulated signaling pathways, insulinreceptor substrate (IRS)-1-PI 3-kinase-Akt and ERK MAPK, in the regulation of proteoglycan and type II collagen production by human articular chondrocytes and provided evidence that oxidative stress and increased levels of reactive oxygen species (ROS) can alter the balance in the activity of these pathways, leading to IGF-I resistance (Fig. 7)
Summary
Insulin-like growth factor I (IGF-I)2 plays a critical role in regulating normal growth and tissue formation during both. OA Chondrocytes Possess a High Basal Level of IRS-1 Serine and ERK Phosphorylation and Reduced IGF-I Activation of Akt Phosphorylation—A time course study was performed to compare IGF-I signal transduction in chondrocytes isolated from normal human articular and osteoarthritic cartilage.
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
