To define the roles of transforming growth factor alpha (TGFalpha) in cartilage degradation.
Primary rat articular chondrocytes and articular osteochondral explants were cultured with TGFalpha to assess the effects of TGFalpha on chondrocyte physiology and phenotype.
TGFalpha altered chondrocyte morphology through reorganization of the actin cytoskeleton and formation of stress fibers. Expression of anabolic genes, including aggrecan, type II collagen, and cartilage link protein, was reduced in response to TGFalpha. Proliferation of chondrocytes and formation of articular chondrocyte clusters was stimulated by TGFalpha. Expression of matrix metalloproteinase 13 and cathepsin C was increased by TGFalpha. We demonstrated the down-regulation of Sox9 messenger RNA and protein levels by TGFalpha. This was associated with reduced levels of phosphorylated and total SOX9 in cartilage explants upon TGFalpha treatment. In contrast, another growth factor identified in our microarrays, Kitl, had no effects on the chondrocyte parameters tested. To examine correlations between the increased levels of TGFalpha in experimental knee osteoarthritis (OA) with the levels of TGFalpha in humans with knee OA, a microarray analysis of mRNA from 13 normal and 12 late-stage OA cartilage samples was performed. Seven OA samples showed TGFA mRNA levels similar to those in the normal controls, but expression was markedly increased in the other 5 OA samples. These data confirm that TGFA transcript levels are increased in a subset of patients with OA.
This study adds TGFalpha to the list of dysregulated cytokines present in degrading cartilage in OA. Since TGFalpha inhibits articular chondrocyte anabolic capacity, increases catabolic factors, and contributes to the development of chondrocyte clusters, TGFalpha may be a potential target for therapeutic strategies in the treatment of OA.
"Pharmacological inhibition of EGFR can reduce the severity of collagen-induced RA in mice, and the inhibition of EGFR by adenovirus can reduce joint damage in arthritic mice [19,20]. Increased TGFα and EGFR signaling have also been shown in rat OA [21,22]. "
[Show abstract][Hide abstract] ABSTRACT: Ligament and meniscus damage can cause joint disease. Arthritic joints contain increased amounts of EGFR protein, and polymorphisms in EGFR are associated with arthritis risk. The role of endogenous EGFR regulation during joint disease due to ligament and meniscus trauma is unknown. Mitogen-inducible gene 6 (MIG-6) can reduce EGFR phosphorylation and downstream signaling. We examined the effect of EGFR modulation by MIG-6 on joint disease development after ligament and meniscus injury.
Knee ligament transection and meniscus removal was performed surgically on mice homozygous for a global inactivating mutation in MIG-6 (Mig-6-/-) and in WT animals.
Two weeks after surgery, Mig-6-/- mice had bone erosion as well as greater fibrous tissue area and serum RANKL concentration. Four weeks after surgery, Mig-6-/- mice had less cartilage and increased proliferating cells relative to contralateral control and WT knees; increased apoptotic cells and growth outside of the articulating region occurred in Mig-6-/- mice. Tibia trabecular BMD and the number of trabeculae were lower in surgery knees relative to respective control knees for both groups. BMD, as well as trabecular thickness and number, were lower in surgery knees from Mig-6-/- mice relative to WT surgery knees. Phospho-EGFR staining in surgery knees decreased for WT mice and increased for Mig-6-/- mice. Fewer inflammatory cells were present in WT knees.
Mig-6-/- mice have rapid and increased joint damage after ligament and meniscus trauma. Mig-6 modification could lessen degenerative disease development after this type of injury.
"The nature of the endogenous ligand-receptor interactions mediating the EGFR responses we have observed in Mig6-deficient articular cartilage is unknown. For example, while the EGFR ligands transforming growth factor alpha (TGF-α), and EGF are expressed by articular chondrocytes [32,63], studies typically implicate their functions in catabolic effects of EGFR signaling associated with osteoarthritic damage [29-32], rather than the anabolic effects we have observed here. As distinct EGFR signal outputs may be generated by differential ligand activation , it is possible that anabolic EGFR activities could be mediated by ligands other than EGF or TGF-α; alternately, anabolic vs. catabolic EGFR activities in articular cartilage could be related to differences in the timing or level of EGFR activation achieved in in vitro studies vs. our in vivo studies. "
[Show abstract][Hide abstract] ABSTRACT: Introduction
Signals from the epidermal growth factor receptor (EGFR) have typically been considered to provide catabolic activities in articular cartilage, and accordingly have been suggested to have a causal role in osteoarthritis progression. The aim of this study was to determine in vivo roles for endogenous EGFR signal activation in articular cartilage.
Transgenic mice with conditional, limb-targeted deletion of the endogenous intracellular EGFR inhibitor Mig-6 were generated using CreLoxP (Mig-6-flox; Prx1Cre) recombination. Histology, histochemical staining and immunohistochemistry were used to confirm activation of EGFR signaling in the articular cartilage and joints, and to analyze phenotypic consequences of Mig-6 loss on articular cartilage morphology, proliferation, expression of progenitor cell markers, presence of chondrocyte hypertrophy and degradation of articular cartilage matrix.
The articular cartilage of Mig-6-conditional knockout (Mig-6-cko) mice was dramatically and significantly thicker than normal articular cartilage at 6 and 12 weeks of age. Mig-6-cko articular cartilage contained a population of chondrocytes in which EGFR signaling was activated, and which were three to four times more proliferative than normal Mig-6-flox articular chondrocytes. These cells expressed high levels of the master chondrogenic regulatory factor Sox9, as well as high levels of putative progenitor cell markers including superficial zone protein (SZP), growth and differentiation factor-5 (GDF-5) and Notch1. Expression levels were also high for activated β-catenin and the transforming growth factor beta (TGF-β) mediators phospho-Smad2/3 (pSmad2/3). Anabolic effects of EGFR activation in articular cartilage were followed by catabolic events, including matrix degradation, as determined by accumulation of aggrecan cleavage fragments, and onset of hypertrophy as determined by type × collagen expression. By 16 weeks of age, the articular cartilage of Mig-6-cko knees was no longer thickened and was degenerating.
These results demonstrate unexpected anabolic effects of EGFR signal activation in articular cartilage, and suggest the hypothesis that these effects may promote the expansion and/or activity of an endogenous EGFR-responsive cell population within the articular cartilage.
"Among the shared Brachyury targets are the Ets-family transcription factor ETV1, which is translocated in certain sarcomas , CIT (citron Rho-interacting kinase; ), GABRA2 (GABA receptor alpha2), the zinc-finger protein ZDHHC17, which has been previously found to be upregulated in leukemic cells over-expressing Gfi-1B , PIGK (phosphatidylinositol glycan, class K), which was found to be downregulated in colorectal cancer due to a polymorphism in its 3′-UTR , TNFRSF19 (tumor necrosis factor receptor superfamily, member 19, a.k.a. TROY), previously found to be over-expressed in glial tumors , VEPH1, encoding a PH-domain protein expressed in the developing central nervous system , PTN (pleiotrophin), which is expressed in the canine notochord and NP , SCRG1, encoding an ECM-localized protein involved in chondrogenesis , and the cell proliferation activator TGF-alpha, which has been shown to stimulate proliferation of chondrocytes . Interestingly, with the exception of PTN, none of the other genes was previously reported as a possible notochord marker, which likely reflects the fact that cancerous hallmarks are chordoma-specific targets of Brachyury. "
[Show abstract][Hide abstract] ABSTRACT: Chordoma is a rare, but often malignant, bone cancer that preferentially affects the axial skeleton and the skull base. These tumors are both sporadic and hereditary and appear to occur more frequently after the fourth decade of life; however, modern technologies have increased the detection of pediatric chordomas. Chordomas originate from remnants of the notochord, the main embryonic axial structure that precedes the backbone, and share with notochord cells both histological features and the expression of characteristic genes. One such gene is Brachyury, which encodes for a sequence-specific transcription factor. Known for decades as a main regulator of notochord formation, Brachyury has recently gained interest as a biomarker and causative agent of chordoma, and therefore as a promising therapeutic target. Here, we review the main characteristics of chordoma, the molecular markers, and the clinical approaches currently available for the early detection and possible treatment of this cancer. In particular, we report on the current knowledge of the role of Brachyury and of its possible mechanisms of action in both notochord formation and chordoma etiogenesis.
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