The small GTPase Rho mediates articular chondrocyte phenotype and morphology in response to interleukin-1alpha and insulin-like growth factor-I.

Small GTPases regulate the cytoskeleton and numerous other cellular functions. In this study, the role of Rho GTPase was examined in articular chondrocytes. Chondrocytes grown in monolayer were treated with interleukin-1alpha (IL-1alpha), insulin-like growth factor-I (IGF-I), C3 Transferase, Y27632, or transfected with Rho wild type or two constitutively active mutants of Rho (Q63L and G14V). Quantitative PCR was used to determine changes in matrix metalloproteinase-13 (MMP-13), collagen types IIB (COL2A1) and type I (COL1A1), aggrecan (AGG), and SOX-9 gene expression. Affinity assays were performed to measure endogenous GTP-bound Rho, and confocal microscopy was used to assess changes in organization of the actin cytoskeleton. IL-1alpha and RhoG14V increased cytoplasmic actin stress fiber formation, which was blocked by C3 Transferase, and Y27632. IL-1alpha treatment also increased Rho activity. Conversely, IGF-I lead to formation of a cortical rim of actin and decreased Rho activity. Inhibition of Rho signaling with C3 Transferase significantly decreased Rho activity and returned IL-1alpha-induced Rho activity to a level not different from control. C3 Transferase treatment also increased mRNA expression of AGG, COL2A1, and SOX-9, and decreased expression of MMP-13. Expression of RhoQ63L or RhoG14V resulted in increased MMP-13 expression; however, inhibition of Rho with Y27632 was unable to inhibit IL-1alpha-induced MMP-13 expression. Together, these results indicate a role for increased Rho activity in mediation of chondrocyte catabolic signaling pathways.