The serine protease inhibitor trappin-2 is present in cartilage and synovial fluid in osteoarthritis.
OBJECTIVE
Trappins are small serine protease inhibitors bound to extracellular matrix (ECM) through the actions of transglutaminase (TGase) enzymes. Trappin-2 is present in many tissues and is upregulated at sites of injury. In osteoarthritis (OA), serine proteases contribute to articular cartilage destruction, and TGase activity is increased. Yet little is known about matrix-bound serine protease inhibitors or TGase substrates in articular cartilage. Our purpose was to determine if trappin-2 was present in OA cartilage and synovial fluid (SF).


METHODS
OA knee articular cartilage and SF were assayed for trappin-2 protein by Western blotting, ELISA, and immunohistochemistry. Trappin-2 mRNA was detected with RT-PCR. The ECM components bound to trappin-2 were identified by 2-D gel electrophoresis and peptide fingerprinting.


RESULTS
Trappin-2 was detectable in OA articular cartilage extracts, cultured chondrocytes, conditioned media, and SF by Western blotting. OA cartilage protein extracts contained significantly higher quantities of trappin-2 than normal cartilage protein extracts (22.98 +/- 1.28 ng/mg wet weight vs 14.97 +/- 1.92 ng/mg wet weight; p < 0.01). RT-PCR confirmed the presence of trappin-2 mRNA in OA chondrocytes. Immunohistochemical studies of OA cartilage revealed trappin-2 protein in chondrocytes. Peptide mapping of trappin-2 binding partners showed that fibromodulin was bound to trappin-2 in cartilage.


CONCLUSIONS
We confirmed the presence of trappin-2 in OA cartilage and SF. Elevated levels of TGase activity in OA cartilage may increase levels of this serine protease inhibitor in response to injury.
Jaovisidha
Kanyakorn
K
Division of Rheumatology, Department of Medicine, Medical College of Wisconsin, Milwaukee, 53295, USA.
Etim
Ann
A
Yamakawa
Koji
K
Masuda
Ikuko
I
Gohr
Claudia M
CM
Wakim
Bassam T
BT
Boonapatcharoen
Nimaradee
N
Ninomiya
James
J
Rosenthal
Ann K
AK
D000293:Adolescent; D000328:Adult; D002358:Cartilage, Articular / Q000378:metabolism* / Q000473:pathology; D002478:Cells, Cultured; D019902:Chondrocytes / Q000378:metabolism / Q000473:pathology; D017077:Culture Media, Conditioned / Q000737:chemistry; D053492:Elafin; D006801:Humans; D007124:Immunoenzyme Techniques; D019272:Leukocyte Elastase / Q000037:antagonists & inhibitors*; D010003:Osteoarthritis / Q000378:metabolism* / Q000473:pathology; D011498:Protein Precursors / Q000235:genetics / Q000378:metabolism*; D012333:RNA, Messenger / Q000378:metabolism; D020133:Reverse Transcriptase Polymerase Chain Reaction; D013582:Synovial Fluid / Q000166:cytology / Q000378:metabolism*
D017077:Culture Media, Conditioned; D053492:Elafin; C506806:PI3 protein, human; D011498:Protein Precursors; D012333:RNA, Messenger; D019272:Leukocyte Elastase