Zhang H, Wang H, Zeng C, Yan B, Ouyang J, Liu X, Sun Q, Zhao C, Fang H, Pan J, Xie D, Yang J, Zhang T, Bai X, Cai D
Academy of Orthopedics, Guangdong Province, Department of Orthopedics, The Third Affiliated Hospital, Southern Medical University, Guangzhou, China.
State Key Laboratory of Organ Failure Research, Department of Cell Biology, School of Basic Medical Sciences, Southern Medical University, Guangzhou, China; Key Laboratory of Tropical Diseases and Translational Medicine of the Ministry of Education, Hainan Medical College, Haikou, China.
Osteoarthritis Cartilage. 2017 Jun;25(6):952-963. doi: 10.1016/j.joca.2016.12.024. Epub 2016 Dec 31.
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.
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 (TSC1CKO) 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.
Articular chondrocyte mTORC1 was activated in early-stage OA and in aged mice. TSC1CKO mice exhibited spontaneous OA, and TSC1CKO 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.
mTORC1 activation stimulates articular chondrocyte proliferation and differentiation to initiate OA, in part by downregulating FGFR3 and PPR.
关节软骨细胞激活，包括异常增殖和前肥大分化，是骨关节炎（OA）起始和进展的关键因素。雷帕霉素复合物1（mTORC1）机制靶点的破坏可促进软骨细胞自噬和存活，并减轻实验性OA的严重程度。然而，软骨mTORC1激活在OA起始中的作用尚不清楚。在本研究中，我们阐明了mTORC1激活在OA起始中的具体作用，并确定了潜在机制。
通过免疫染色评估OA患者和OA小鼠关节软骨中mTORC1的表达。构建软骨特异性结节性硬化复合物1（Tsc1，mTORC1上游抑制剂）敲除（TSC1CKO）和诱导型Tsc1敲除（TSC1CKO）小鼠。通过免疫染色、蛋白质免疫印迹和定量聚合酶链反应检测mTORC1在OA起始和发展中对其下游靶点的功能影响。
在早期OA和老年小鼠中，关节软骨细胞mTORC1被激活。TSC1CKO小鼠表现出自发性OA，而TSC1CKO小鼠（2个月起）表现出加速的年龄相关性和半月板横向切除（DMM）诱导的OA表型，伴有软骨细胞异常增殖和肥大分化。这与mTORC1的过度激活以及成纤维细胞生长因子受体3（FGFR3）和脯氨酸丰富蛋白（PPR）的显著下调有关，这两种受体对防止软骨细胞增殖和分化至关重要。雷帕霉素治疗可逆转敲除小鼠的这些表型。此外，体外拯救实验表明，p73和细胞外信号调节激酶1/2（ERK1/2）可能介导mTORC1对FGFR3和PPR的负调控。
mTORC1激活刺激关节软骨细胞增殖和分化以起始OA，部分是通过下调FGFR3和PPR实现的。
