Insulin‐like growth factor‐I diminishes the activation status and expression of the small GTPase Cdc42 in articular chondrocytes

Abstract

Insulin‐like growth factor‐I (IGF‐I) is an important anabolic growth factor in the maintenance of articular cartilage phenotypic expression. Chondrocyte morphology is also tightly linked to phenotype. The small G‐protein Cdc42 plays a key role in regulation of cell morphology and phenotypic expression in several cell types and, we show here, in articular chondrocytes. The purpose of these studies was to investigate possible links between the intracellular signaling pathways of IGF‐I and Cdc42 in articular chondrocytes. Treatment of chondrocytes with IGF‐I resulted in a rapid and sustained decrease in the activation state (decreased GTP‐bound) of Cdc42. Nucleotide exchange and hydrolysis experiments suggest that the decreased activation occurs through increased hydrolysis. Transient expression of dominant‐negative Cdc42(T17N) allowed for enhanced expression of normal chondrocyte phenotype as determined by increased mRNA expression of collagen type II (Coll II) with decreased matrix metalloproteinase‐3 (MMP‐3) expression. The results of these studies suggest a novel link between IGF‐I and Cdc42 signaling pathways. Further, an additional mechanism for the regulation of chondrocyte phenotype is defined through the IGF‐I induced down‐regulation of Cdc42 activation. © 2003 Orthopaedic Research Society. Published by Elsevier Ltd. All rights reserved.

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

“…23 Overexpression of T19N-Cdc42 also results in enhanced mRNA expression of collagen type II (Col2A1) and diminished expression of matrix metalloproteinase-3 (MMP-3), and treatment of T19N-Cdc42 expressing chondrocytes with IGF-I leads to a further increase in Col2A1 and MMP-3 gene expression. 22 Together, these results suggest that keeping the activation status of Cdc42 at a diminished level aids in preserving the normal chondrocyte phenotype. Previous studies demonstrated that IGF-I-induced GTP-Cdc42 repression is at the level of increased GAP activity.…”

“…Previous studies demonstrated that IGF-I-induced GTP-Cdc42 repression is at the level of increased GAP activity. 22 Although the precise GAP has not yet been identified, and therefore neither have potential upstream regulators of GAP activity, the idea that an IGF-Iregulated GAP might be exploited to reverse or slow loss of chondrocyte phenotype and preserve IGF-I responsiveness is attractive. There are over 70 Rho-GAP domain containing proteins, constituting the 31st largest protein family in the human genome.…”

“…Time and concentration of IGF-I treatment were chosen based on previous studies investigating Cdc42 activation. 22 After treatment, assays were performed as described below.…”

“…[16][17][18][19][20][21] We have previously shown that in articular chondrocytes, IGF-I downregulates the small GTPase protein Cdc42 (cell-division-cycle 42), thereby decreasing the GTP-bound, active pool of Cdc42. 22 Combined with gain and loss-of-function studies of Cdc42 in chondrocytes, the theory of active, GTP-Cdc42 functioning as a dedifferentiation factor was proposed. 22 This concept is supported by studies in which expression of dominant negative mutants of Cdc42 in articular chondrocytes attenuates the induction of c-Jun transcription activity by shear stress.…”

“…22 Combined with gain and loss-of-function studies of Cdc42 in chondrocytes, the theory of active, GTP-Cdc42 functioning as a dedifferentiation factor was proposed. 22 This concept is supported by studies in which expression of dominant negative mutants of Cdc42 in articular chondrocytes attenuates the induction of c-Jun transcription activity by shear stress.…”