Abstract
Osteoarthritis (OA) is hallmarked by a progressive degradation of articular cartilage. Besides risk factors including trauma, obesity or genetic predisposition, inflammation has a major impact on the development of this chronic disease. During the course of inflammation, cytokines such as tumor necrosis factor-alpha(TNF-α) and interleukin (IL)-1β are secreted by activated chondrocytes as well as synovial cells and stimulate the production of other inflammatory cytokines and matrix degrading enzymes. The mTORC1 inhibitor rapamycin is a clinical approved immunosuppressant and several studies also verified its chondroprotective effects in OA. However, the effect of blocking the mechanistic target of rapamycin complex (mTORC)1 on the inflammatory status within OA is not well studied. Therefore, we aimed to investigate if inhibition of mTORC1 by rapamycin can preserve and sustain chondrocytes in an inflammatory environment. Patient-derived chondrocytes were cultured in media supplemented with or without the mTORC1 inhibitor rapamycin. To establish an inflammatory environment, either TNF-α or IL-1β was added to the media (=OA-model). The chondroprotective and anti-inflammatory effects of rapamycin were evaluated using sulfated glycosaminoglycan (sGAG) release assay, Caspase 3/7 activity assay, lactate dehydrogenase (LDH) assay and quantitative real time polymerase chain reaction (PCR). Blocking mTORC1 by rapamycin reduced the release and therefore degradation of sGAGs, which are components of the extracellular matrix secreted by chondrocytes. Furthermore, blocking mTORC1 in OA chondrocytes resulted in an enhanced expression of the main chondrogenic markers. Rapamycin was able to protect chondrocytes from cell death in an OA-model shown by reduced Caspase 3/7 activity and diminished LDH release. Furthermore, inhibition of mTORC1 preserved the chondrogenic phenotype of OA chondrocytes, but also reduced inflammatory processes within the OA-model. This study highlights that blocking mTORC1 is a new and promising approach for treating OA. Low side effects make rapamycin an attractive implementation to existing therapeutic strategies. We showed that rapamycin’s chondroprotective property might be due to an interference with IL-1β triggered inflammatory processes.
Highlights
The chronic degenerative joint disease Osteoarthritis (OA) is the single most common cause of disability in older adults [1]
We aimed to investigate whether blocking mechanistic target of rapamycin complex 1 (mTORC1) by rapamycin influences maintenance of the chondrogenic phenotype of OA chondrocytes including extracellular matrix (ECM) production and chondrogenic marker expression
Patient-derived OA chondrocytes were formed into three-dimensional aggregates (=pellets) and were cultured in chondrogenic culture media supplemented with or without the mTORC1 inhibitor rapamycin for 14 days
Summary
The chronic degenerative joint disease Osteoarthritis (OA) is the single most common cause of disability in older adults [1] It is characterized by disruption of the articular cartilage, degradation of the extracellular matrix (ECM) secreted by chondrocytes, remodeling of the subchondral bone, reduced cellularity within cartilage and swelling of the joint [2,3]. The two most prominent inflammatory cytokines involved in OA are tumor necrosis factor-α (TNF-α) and interleukin-1β (IL-1β) Within the joints, they promote catabolic and destructive processes [8]. Activated chondrocytes produce matrix metalloproteinases (MMPs) with MMP-1, MMP-3, MMP-13 and a disintegrin and metalloproteinase (ADAM) with thrombospondin-1 domains (ADAMTS)-4 being the most prominent Activation of these enzymes results in a progressive degradation of collagen type II, gylcosaminoglycans and proteoglycans (aggrecans) resulting in the onset of OA [17,18,19]. They are present in the pathophysiology of OA, but the impact of inflammatory cytokines is more dominant than the one of anti-inflammatory ones
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