ANTIHYPERTROPHIC EFFECT OF PGE2 ON CHONDROCYTES SUGGESTS ADJUSTMENTS OF OA ANALGESIA

Abstract

The signaling molecule prostaglandin E2 (PGE2), synthesized by cyclooxygenase-2 (COX-2), is immunoregulatory and reported to be essential for skeletal stem cell function. Nonsteroidal anti-inflammatory drugs (NSAIDs) are widely used in osteoarthritis (OA) analgesia, but cohort studies suggested that long-term use may accelerate pathology. Interestingly, OA chondrocytes secrete high amounts of PGE2. Mesenchymal stromal cell (MSC) chondrogenesis is an in vitro OA model that phenocopies PGE2 secretion along with a hypertrophic OA-like cell morphology. Our aim was to investigate cause and effects of PGE2 secretion in MSC-based cartilage neogenesis and hypertrophy and identify molecular mechanisms responsible for adverse effects in OA analgesia.Human bone marrow-derived MSCs were cultured in chondrogenic medium with TGFβ (10ng/mL) and treated with PGE2 (1µM), celecoxib (COX-2 inhibitor; 0.5µM), AH23848/AH6809 (PGE2 receptor antagonists; 10µM), or DMSO as a control (n=3–4). Assessment criteria were proteoglycan deposition (histology), chondrocyte/hypertrophy marker expression (qPCR), and ALP activity. PGE2 secretion was measured (ELISA) after TGFβ withdrawal (from day 21, n=2) or WNT inhibition (2µM IWP-2 from day 14; n=3).Strong decrease in PGE2 secretion upon TGFβ deprivation or WNT inhibition identified both pathways as PGE2 drivers. Homogeneous proteoglycan deposition and COL2A1 expression analysis showed that MSC chondrogenesis was not compromised by any treatment. Importantly, hypertrophy markers (COL10A1, ALPL, SPP1, IBSP) were significantly reduced by PGE2 treatment, but increased by all inhibitors. Additionally, PGE2 significantly decreased ALP activity (2.9-fold), whereas the inhibitors caused a significant increase (1.3-fold, 1.7-fold, 1.8-fold). This identified PGE2 as an important inhibitor of chondrocyte hypertrophy.Although TGFβ and WNT are known pro-arthritic signaling pathways, they appear to induce a PGE2-mediated antihypertrophic effect that can counteract pathological cell changes in chondrocytes. Hampering this rescue mechanism via COX inhibition using NSAIDs thus risks acceleration of OA progression, indicating the need of OA analgesia adjustment.