Identification of a specific haptoglobin C‐terminal fragment in arthritic synovial fluid and its effect on interleukin‐6 expression

Abstract

SummaryHaptoglobin (Hp), a major acute-phase plasma protein, has been found in arthritic synovial fluid (SF). However, the function and structural modifications of Hp in arthritic SF are unknown. To investigate in vivo generation of modified Hp associated with inflammatory disease, we examined a new Hp isoform in SF from patients with rheumatoid arthritis (RA). Specific Hp fragments of 28 000 and 15 000 molecular weight were identified in SF of patients with RA, and the two polypeptides were presumed to be fragments of the Hp b-chain (43 000 MW) produced by cleavage with plasmin. The 15 000 MW fragment, which is a C-terminal region of Hp, was observed at higher frequency and levels in RA than in osteoarthritis. Plasmin activity was also higher in SF of RA patients. A recombinant 15 000 MW Hp fragment up-regulated interlukin-6 expression in monocytic cells. These findings indicate that the C-terminal Hp fragment is generated by plasmin in local inflammatory environments and acts as an inflammatory mediator. They further suggest that a specific Hp fragment might be applied as a novel biomarker for the diagnosis and prognosis of inflammatory diseases such as RA.

A novel inflammatory biomarker, GlycA, associates with disease activity in rheumatoid arthritis and cardio-metabolic risk in BMI-matched controls

“…While haptoglobin is typically anti-inflammatory, glycosylation site alterations have been identified in RA and other diseases such as cancer; the ability of glycosylation to alter protein function and immunogenicity suggests that glycosylation alterations may serve pathogenic roles [ 28 – 30 ]. In RA synovial fluid, a specific haptoglobin isoform upregulates monocyte IL-6 production [ 31 ]. Also, synthesis of haptoglobin is primarily hepatic; however, it is also produced by activated neutrophils and taken up peripherally by monocytes [ 32 , 33 ].…”

“…Also, synthesis of haptoglobin is primarily hepatic; however, it is also produced by activated neutrophils and taken up peripherally by monocytes [ 32 , 33 ]. Thus, the source, balance, and functions of haptoglobin and other acute phase proteins in RA are different from those in healthy controls and likely contribute to different GlycA associations [ 31 ]. Although we did not assess the individual acute phase protein contributions to GlycA in this sample, we suggest that GlycA is a comprehensive measure of pathogenic inflammation in RA.…”

Biomarkers of systemic inflammation in farmers with musculoskeletal disorders; a plasma proteomic study

“…Accordingly, haptoglobin was found to be a monocyte chemoattractant whose chemotactic potential was mediated, at least in part, by its interaction with chemokine (C-C motif) receptor 2 [ 20 ]. Recently, Park and co-workers demonstrated that the C-terminal haptoglobin fragment is generated by plasmin in local inflammatory environments, suggesting that this fragment might be applied as a novel biomarker for the diagnosis and prognosis of inflammatory joint diseases such as rheumatoid arthritis [ 21 ]. Hemopexin belongs to the acute phase reactants, the synthesis of which is induced after inflammation.…”

Single chain precursor prohaptoglobin promotes angiogenesis by upregulating expression of vascular endothelial growth factor (VEGF) and VEGF receptor2

“…These conflicting findings may be due to the diverse functions of Hp isoforms produced in various cellular environments. In fact, we recently identified a specific Hp fragment in the synovial fluid of patients with rheumatoid arthritis that appears to be produced by plasmin and induces IL-6 expression [24]. Because of different processing and post-translational modifications, such as glycosylation and protease cleavage, diverse Hp isoforms may be generated in vivo.…”

“…COS‐7 CM and serum proteins were separated on a 10% SDS–polyacrylamide gel, and Hp isoforms were identified using anti‐human Hp antibody (Sigma–Aldrich) and polyclonal antibodies against human Hp α‐chain and Hp β‐chain, which were prepared from rabbit sera after immunization against the corresponding recombinant protein [24]. The immunoreactive proteins were detected using an enhanced chemiluminescence detection kit (Amersham Biosciences, Buckinghamshire, UK) and luminescent image analysis LAS‐3000 system (Fujifilm, Tokyo, Japan).…”