Fibromodulin Suppresses Nuclear Factor-κB Activity by Inducing the Delayed Degradation of IKBA via a JNK-dependent Pathway Coupled to Fibroblast Apoptosis

Fibulin-5 (FBLN5) belongs to the Fibulin family of secreted extracellular matrix proteins, and our laboratory first established FBLN5 as a novel target for TGF-beta in fibroblasts and endothelial cells. To better understand the pathophysiology of FBLN5, we carried out microarray analysis to identify fibroblast genes whose expressions were regulated by FBLN5 and TGF-beta. In doing so, we identified fibromodulin (Fmod) as a novel target gene of FBLN5, and we validated the differential expression of Fmod and 12 other FBLN5-regulated genes by semi-quantitative real time PCR. Fmod belongs to the small leucine-rich family of proteoglycans, which are important constituents of mammalian extracellular matrices. Interestingly, parental 3T3-L1 fibroblasts displayed high levels of nuclear factor-kappa B (NF-kappa B) activity, although those engineered to express Fmod constitutively exhibited significantly reduced NF-kappa B activity, suggesting that Fmod functions to inhibit NF-kappa B signaling. By monitoring alterations in the activation of NF-kappa B and the degradation of its inhibitor, I kappa B alpha, we demonstrate for the first time that Fmod contributes to the constitutive degradation of I kappa B alpha protein in 3T3-L1 fibroblasts. Mechanistically, we observed Fmod to delay the degradation of I kappa B alpha by promoting the following: (i) activation of c-Jun N-terminal kinase; (ii) inhibition of calpain and casein kinase 2 activity; and (iii) induction of fibroblast apoptosis. Taken together, our study identified a novel function for Fmod in directing extracellular signaling, particularly the regulation of NF-kappa B activity and cell survival.