Blaney Davidson E N, van Caam A P M, Vitters E L, Bennink M B, Thijssen E, van den Berg W B, Koenders M I, van Lent P L E M, van de Loo F A J, van der Kraan P M
Radboud University Medical Centre, Experimental Rheumatology, Geert Grooteplein 26-28, 6500 HB Nijmegen, The Netherlands.
Osteoarthritis Cartilage. 2015 Mar;23(3):478-86. doi: 10.1016/j.joca.2014.12.005. Epub 2014 Dec 18.
Pain is the main problem for patients with osteoarthritis (OA). Pain is linked to inflammation, but in OA a subset of patients suffers from pain without inflammation, indicating an alternative source of pain. Nerve Growth Factor (NGF) inhibition is very efficient in blocking pain during OA, but the source of NGF is unclear. We hypothesize that damaged cartilage in OA releases Transforming Growth Factor-β (TGF-β), which in turn stimulates chondrocytes to produce NGF.
Murine and human chondrocyte cell lines, primary bovine and human chondrocytes, and cartilage explants from bovine metacarpal joints and human OA joints were stimulated with TGF-β1 and/or Interleukin-1 (IL-1)β. We analyzed NGF expression on mRNA level with QPCR and stained human OA cartilage for NGF immunohistochemically. Cultures were additionally pre-incubated with inhibitors for TAK1, Smad2/3 or Smad1/5/8 signaling to identify the TGF-β pathway inducing NGF.
NGF expression was consistently induced in higher levels by TGF-β than IL-1 in all of our experiments: murine, bovine and human origin, in cell lines, primary chondrocytes and explants cultures. TAK1 inhibition consistently reduced TGF-β-induced NGF whereas it fully blocked IL-1β-induced NGF expression. In contrast, ALK5-Smad2/3 inhibition fully blocked TGF-β-induced NGF expression. Despite the large variation in basal NGF in human OA samples (mRNA and histology), TGF-β exposure led to a consistent high level of NGF induction.
We show for the first time that TGF-β induces NGF expression in chondrocytes, in a ALK5-Smad2/3 dependent manner. This reveals a potential alternative non-inflammatory source of pain in OA.
疼痛是骨关节炎（OA）患者的主要问题。疼痛与炎症相关，但在OA中，一部分患者存在无炎症的疼痛，提示存在其他疼痛来源。神经生长因子（NGF）抑制在OA期间阻断疼痛方面非常有效，但NGF的来源尚不清楚。我们假设OA中受损的软骨释放转化生长因子-β（TGF-β），进而刺激软骨细胞产生NGF。
用TGF-β1和/或白细胞介素-1（IL-1）β刺激小鼠和人软骨细胞系、原代牛和人软骨细胞以及来自牛掌指关节和人OA关节的软骨外植体。我们用定量聚合酶链反应（QPCR）分析NGF在mRNA水平的表达，并对人OA软骨进行NGF免疫组织化学染色。培养物还预先用TAK1、Smad2/3或Smad1/5/8信号通路的抑制剂孵育，以确定诱导NGF的TGF-β途径。
在我们所有的实验中，包括小鼠、牛和人来源的细胞系、原代软骨细胞和外植体培养物，TGF-β诱导的NGF表达水平始终高于IL-1。TAK1抑制持续降低TGF-β诱导的NGF，而它完全阻断IL-1β诱导的NGF表达。相反，ALK5-Smad2/3抑制完全阻断TGF-β诱导的NGF表达。尽管人OA样本中基础NGF存在很大差异（mRNA和组织学），但TGF-β暴露导致NGF诱导水平持续升高。
我们首次表明，TGF-β以ALK5-Smad2/3依赖的方式诱导软骨细胞中NGF的表达。这揭示了OA中潜在的替代性非炎症性疼痛来源。
