mTORC1 activation downregulates FGFR3 and PTH/PTHrP receptor in articular chondrocytes to initiate osteoarthritis.

OBJECTIVE: Articular chondrocyte activation, involving aberrant proliferation and prehypertrophic differentiation, is essential for osteoarthritis (OA) initiation and progression. Disruption of mechanistic target of rapamycin complex 1 (mTORC1) promotes chondrocyte autophagy and survival, and decreases the severity of experimental OA. However, the role of cartilage mTORC1 activation in OA initiation is unknown. In this study, we elucidated the specific role of mTORC1 activation in OA initiation, and identify the underlying mechanisms. METHOD: Expression of mTORC1 in articular cartilage of OA patients and OA mice was assessed by immunostaining. Cartilage-specific tuberous sclerosis complex 1 (Tsc1, mTORC1 upstream inhibitor) knockout (TSC1CKO) and inducible Tsc1 KO (TSC1CKOER) mice were generated. The functional effects of mTORC1 in OA initiation and development on its downstream targets were examined by immunostaining, western blotting and qPCR. RESULTS: Articular chondrocyte mTORC1 was activated in early-stage OA and in aged mice. TSC1CKO mice exhibited spontaneous OA, and TSC1CKOER mice (from 2 months) exhibited accelerated age-related and DMM-induced OA phenotypes, with aberrant chondrocyte proliferation and hypertrophic differentiation. This was associated with hyperactivation of mTORC1 and dramatic downregulation of FGFR3 and PPR, two receptors critical for preventing chondrocyte proliferation and differentiation. Rapamycin treatment reversed these phenotypes in KO mice. Furthermore, in vitro rescue experiments demonstrated that p73 and ERK1/2 may mediate the negative regulation of FGFR3 and PPR by mTORC1. CONCLUSION: mTORC1 activation stimulates articular chondrocyte proliferation and differentiation to initiate OA, in part by downregulating FGFR3 and PPR.

Sequential exposure to fibroblast growth factors (FGF) 2, 9 and 18 enhances hMSC chondrogenic differentiation.

OBJECTIVE: To test the effects of sequential exposure to FGF2, 9 and 18 on human Mesenchymal Stem Cells (hMSC) differentiation during in vitro chondrogenesis. DESIGN: Control and FGF2-expanded hMSC were cultured in aggregates in the presence of rhFGF9, rhFGF18 or rhFGFR3-specific signaling FGF variants, starting at different times during the chondroinductive program. Quantitative real time polymerase chain reaction (qRT-PCR) and immunocytochemistry were performed at different stages. The aggregate cultures were switched to a hypertrophy-inducing medium along with rhFGFs and neutralizing antibodies against FGFR1 and FGFR3. Histological/immunohistochemical/biochemical analyses were performed. RESULTS: FGF2-exposed hMSC during expansion up-regulated Sox9 suggesting an early activation of the chondrogenic machinery. FGF2, FGF9 and 18 modulated the expression profile of FGFR1 and FGFR3 in hMSC during expansion and chondrogenesis. In combination with transforming growth factor-beta (TGF-ß), FGF9 and FGF18 inhibited chondrogenesis when added at the beginning of the program (≤ d7), while exhibiting an anabolic effect when added later (≥d14), an effect mediated by FGFR3. Finally, FGFR3 signaling induced by either FGF9 or FGF18 delayed the appearance of spontaneous and induced hypertrophy-related changes. CONCLUSIONS: The stage of hMSC-dependent chondrogenesis at which the growth factors are added impacts the progression of the differentiation program: increased cell proliferation and priming (FGF2); stimulated early chondrogenic differentiation (TGF-ß, FGF9/FGF18) by shifting the chondrogenic program earlier; augmented extracellular matrix (ECM) production (FGF9/FGF18); and delayed terminal hypertrophy (FGF9/FGF18). Collectively, these factors could be used to optimize pre-implantation conditions of hMSC when used to engineer cartilage grafts.

Role of FGFR3 in urothelial cell carcinoma: biomarker and potential therapeutic target.

Although non-invasive bladder tumours (pTa) are the most common group of bladder tumours at presentation, there has until recently been relatively little information on their molecular biology. Thus it was of great interest when mutations in the FGF receptor 3 (FGFR3) were identified in bladder tumours and it became apparent that these were most common in tumours of low grade and stage. Since the initial description of activating mutations of FGFR3, there have been numerous studies confirming the frequency and spectrum of these mutations in bladder cancers of all grades and stages. Mutation screening techniques have evolved and improved. FGFR3 mutation has been assessed as a predictive biomarker in tumour tissues and as a diagnostic biomarker in urine. Efforts have been made to understand the function of FGFR3 in urothelial and other cells. Although our understanding of FGFR3 function is incomplete, it is already apparent that this may represent an important therapeutic target not only in non-invasive bladder cancer but also in a significant number of invasive tumours. This review summarises the current state of knowledge of this interesting receptor in urothelial carcinoma (UC).