POLYAMINES MAY FAVOR HYPERTROPHY AND TERMINAL DIFFERENTIATION OF OSTEOARTHRITIC CHONDROCYTES

Purpose: Polyamines are naturally occurring, positively charged polycations able to control several cellular processes in different cell types, by interacting with negatively charged compounds and structures within the living cell. Several pieces of evidence from the literature point at a role of polyamines in promoting chondrocyte differentiation. Hence, we investigated the effects of exogenously added spermine or spermidine in chondrocyte maturation recapitulated in 3D micromass cultures, to tease out the effects on gene and protein expression of key chondrogenesis regulatory transcription factors, markers and effectors, as well as their posttranscriptional regulation. Methods: Micromasses were seeded in control medium or in the presence of either 5 µM spermine or spermidine. We evaluated the effects on molecular markers of chondrocyte differentiation at the level of gene (real time PCR) and protein (western blot and immunohistochemistry) expression as well as the effects on extracellular matrix deposition and remodeling, and caspase activation and mineralization. The subcellular localization of RUNX-2 in stimulated samples was investigated with confocal microscopy. Results: Both spermine and spermidine were able to increase the rate and the extent of chondrogenesis, with enhanced collagen 2 deposition and remodeling with downstream generation of collagen 2 bioactive peptides. These were able to promote nuclear localization of RUNX-2, the pivotal transcription factor in chondrocyte hypertrophy and osteoblast generation. Indeed, samples stimulated with polyamines showed an enhanced mineralization, along with increased caspase activity, indicating increased chondrocyte terminal differentiation. Conclusions: Polyamine pathway can represent a potential target to control and correct chondrocyte inappropriate maturation in osteoarthritis.