Background/Purpose: We have recently reported that adenosine A2A receptor (A2AR) knockout mice develop spontaneous osteoarthritis (OA) and intra-articular injection of adenosine prevents OA progression in a rat model of post-traumatic OA. Changes in chondrocyte gene expression in A2AR KO mice are evident in neonatal mice before OA is apparent. A2AR stimulation has previously been shown to regulate expression of NURR-1, a member of the NR4a family of nuclear receptors. There are 3 members of the NR4a family of nuclear receptors Nurr77, Nurr1 and NOR1 which act as transcriptional regulators to diminish binding of NFkB to its regulatory sites and thereby diminish inflammation. We probed overall expression differences in neonatal chondrocytes from A2AR KO and WT mice and, based on these results, examined the effect of A2AR on NR4a family expression in chondrocytes in vitro and in vivo.

Methods: Knee cartilage from human OA patients, WT and A2ARKO mice, and from OA mice treated with an intrarticular knee injection (10 µl) of empty liposome, liposome containing adenosine or CGS21680 (selective A2AR agonist) were examined immunohistologically for expression of Nurr77, Nurr1 and NOR1. mRNA was isolated from chondrocytes from WT and A2ARKO newborn mice and subject to RNAseq analysis. Human chondrocytic cell line TC28a2 was used to study the effect of A2AR agonists on expression of NR4A receptors.

Results:

RNAseq analysis revealed alteration in expression of 648 genes, of which 319 were downregulated and 329 upregulated in A2ARKO vs WT chondrocytes. Pathway analysis revealed, among other changes, upregulation of pro-inflammatory and downregulation of extracellular matrix pathways in A2ARKO mice. All 3 of the NR4a genes were downregulated (p<0.001) and a number of pro-inflammatory genes were upregulated (e.g. Nfkb1, Rela and MMP-13 (p<0.001)). By immunohistochemistry NOR1, but not NURR77 or NURR1, is highly expressed in knee chondrocytes in healthy human cartilage and NURR1 is upregulated in human OA cartilage. In OA cartilage of A2ARKO mice there is diminished chondrocyte expression of Nur-77 and increased NURR-1 expression. When studied in a murine obesity model of OA intra-articular injection of liposomal suspensions of CGS21680 promoted NURR-1 expression in OA chondrocytes. When studied in vitro in the TC28a2 cell line Nur-77 is present in nuclei and the A2AR agonist stimulated increased nuclear Nur-77, Nurr-1 and Nor-1.

Conclusion: A2AR maintain homeostasis in chondrocytes including maintenance of expression of the anti-inflammatory nuclear receptor Nur-77 in chondrocytes. Diminished Nur-77 expression in OA cartilage likely contributes to OA progression and replacement by Nurr-1 during OA progression is likely insufficient to diminish OA progression but may play a role in A2AR-mediated cartilage repair.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/adenosine-a2a-receptors-maintain-chondrocyte-and-cartilage-homeostasis-by-maintaining-expression-of-anti-inflammatory-regulators-nur-77-and-suppressing-expression-of-pro-inflammatory-mediators/