Background/Purpose: Recent studies indicate that fibroblast-like synoviocyte (FLS) glucose metabolism is altered in rheumatoid arthritis (RA). Hexokinases (HKs) catalyze the first step in glucose metabolism. HK2, which constitutes the principal HK inducible isoform, is mostly expressed in the lining of RA synovial tissue but not in OA tissue. Fibroblast-like synoviocytes (FLS) are a key component of RA hypertrophic and invasive synovium, and display unique aggressive features, including increased migration and invasion. We hypothesize that HK2 expression in FLS contributes to the synovial lining hypertrophy and plays a critical role in the arthritic joint bone and cartilage damage.

Methods: HK2 expression was determined in joints from wild-type (WT) and arthritic mice by immunohistochemistry (IHC). HK2 expression by qPCR in CD45^negPDPN^pos subpopulation obtained after collagenase digestion and magnetic beads isolation was also performed. To study the effect of HK2 overexpression, we injected 10¹⁰ particles of adenovirus (ad) carrying HK2 (ad-HK2) or GFP (ad-GFP) into the right or left knee respectively in WT mice. After 14 days, we collected the knees and H&E and safranin O staining was performed. Double color IHC was performed to confirm HK2 overexpression in FLS and to determine FLS activation with alpha-smooth muscle active (a-sma) staining. We also used HK2^F/F mice, harboring Col1a1-Cre, in order to delete HK2 in non-hematopoietic cells including FLS. Serum transfer K/BxN arthritis was induced injecting intraperitoneally 150ml of serum. Clinical scores were assessed daily and histopathological studies were performed on joints harvested on day 12.

Results: HK2 was highly expressed by IHC in the synovial lining of arthritic joints from mice after serum transfer K/BxN arthritis. After CD45^neg enrichment, PDPN^pos cells had higher expression of HK2 in the arthritic joint (1.04 ± 0.4 in normal joints vs 1.8 ± 0.3 in arthritic joints; p=0.03). Over-expression of HK2 after intra-articular injection of ad-HK2 in normal knees dramatically increased synovial lining thickness (p=0.03). Double color IHC confirmed HK2 expression in FLS after ad-HK2 injection and showed elevated a-sma expression in the hypertrophic lining. Safranin O staining revealed a higher proteoglycan loss in the cartilage from ad-HK2 injected knees compared with control knees. Finally, deletion of HK2 in Col1a1 expressing cells significantly decreased arthritis severity (clinical score at day 12 was 9 ± 2.5 in WT mice vs. 6.1 ± 3.2 in HK2^F/FCol1a1-Cre; p=0.02). Histopathological studies at day 12 showed significantly less synovial hypertrophy (p=0.04), infiltration (p=0.05); bone erosion (p=0.05) and cartilage damage (p=0.03).

Conclusion: HK2 expression is elevated in the synovial intimal lining of arthritic mice. Over-expression of HK2 in the synovial lining promotes hypertrophy of healthy synovium as well as FLS activation and cartilage damage. HK2 deletion in FLS ameliorates disease severity of arthritis. Taken together, the data suggest that HK2 is involved in FLS activation and synovial hypertrophy and could play a role in RA. HK2 inhibitors could serve as a new therapeutic option for the treatment of RA.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/role-of-hexokinase-2-in-synovial-lining-hypertrophy-in-murine-arthritis/