Signaling through the small G‐protein Cdc42 is involved in insulin‐like growth factor‐I resistance in aging articular chondrocytes

Abstract

During aging, chondrocytes become unresponsive to insulin‐like growth factor‐I (IGF‐I). This study examined the role of Cdc42 (cell‐division‐cycle 42) in IGF‐I signaling during aging. Experiments were performed using cartilage and chondrocytes isolated from horses ages 1 day–25 years. Northern analysis was used to examine expression of the small GTPases Cdc42, Rac, and RhoA. Western analysis was utilized to assess total Cdc42 (GTP + GDP‐bound); active, GTP‐Cdc42 was assessed using a pulldown assay with Western analysis. GTP‐Cdc42 was also measured following IGF‐I treatment. Gene expression for Cdc42 and Rac were decreased in mature samples, but there was no difference in total Cdc42 (GTP + GDP‐bound) protein expression due to age. GTP‐Cdc42 was significantly greater in prepubescent samples compared to other age groups. IGF‐I diminished the GTP‐bound state of Cdc42 in prepubescent chondrocytes; however, this effect was lost during aging. No differences in results were observed due to sample type; that is, cartilage tissues versus isolated chondrocytes. These studies suggest that loss of IGF‐I‐mediated regulation of Cdc42 activation may be a mechanism for the chondrocyte unresponsive state during aging. Further, the activation state of Cdc42, measured in native and IGF‐I‐treated cartilage tissue for the first time, is similar to that of isolated chondrocytes, indicating that the activation state of small G‐proteins is not affected by isolation of chondrocytes from the extracellular matrix. Continued studies will identify the upstream regulators of Cdc42, which will further elucidate the molecular mechanism of IGF‐I resistance during aging thereby providing insight into targeted strategies for age‐related osteoarthritis. © 2006 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 24:1765–1772, 2006

Hsp90 mediates insulin‐like growth factor 1 and interleukin‐1β signaling in an age‐dependent manner in equine articular chondrocytes

“…These data support previous work that demonstrates what has been coined a resistance to IGF-1 stimulation with age (14)(15)(16)48). Hsp90 inhibition at the 2 higher concentrations of geldanamycin tested (100 nM and 500 nM) blocked IGF-1-induced COL2A1 expression.…”

The Role of Growth Factors in Cartilage Repair

“…In animal models, IGF-I has led to enhanced repair of extensive cartilage defects and protection of the synovial membrane from chronic inflammation [32,40]. However, there is a decreased capacity of chondrocytes to respond to IGF-I with age [4,12,31,53,55,59] and in OA [23,53,55,80]. Evidence suggests an uncoupling of IGF-I responsiveness in OA with IGF-I having the ability to robustly simulate matrix synthesis with the inability to decrease matrix catabolism [70].…”

Age-Related Changes in the Musculoskeletal System and the Development of Osteoarthritis

“…There is evidence that the decline in IGF-I response (or IGF-I resistance) is due to altered cell signaling. A reduced ability of IGF-I to activate cell signaling was noted in aging rat cartilage70 and in aged equine chondrocytes72, 73. Because IGF-I is an important autocrine survival factor in cartilage74 the age-related decline in IGF-I signaling may play a role in age-related cell death.…”