The melanocortin system in articular chondrocytes: melanocortin receptors, pro-opiomelanocortin, precursor proteases, and a regulatory effect of alpha-melanocyte-stimulating hormone on proinflammatory cytokines and extracellular matrix components.
OBJECTIVE
The pro-opiomelanocortin (POMC)-derived neuropeptide alpha-melanocyte-stimulating hormone (alpha-MSH) mediates its effects via melanocortin (MC) receptors. This study was carried out to investigate the expression patterns of the MC system and the effects of alpha-MSH in human articular chondrocytes.


METHODS
Articular chondrocytes established from human osteoarthritic joint cartilage were analyzed by reverse transcription-polymerase chain reaction (RT-PCR) and Western blotting for the expression of MC receptors, POMC, and prohormone convertases (PCs). MC-1 receptor (MC-1R) expression in articular cartilage was further studied by immunohistochemistry. Ca(2+) and cAMP assays were used to monitor alpha-MSH signaling, while studies of alpha-MSH function were performed in cultures with chondrocyte micromass pellets stimulated with alpha-MSH. Expression of cytokines and extracellular matrix (ECM) components was determined by real-time RT-PCR, Western immunoblotting, and enzyme-linked immunosorbent assays.


RESULTS
MC-1R expression was detected in articular chondrocytes in vitro and in articular cartilage in situ. In addition, expression of transcripts for MC-2R, MC-5R, POMC, and PCs was detected in articular chondrocytes. Stimulation with alpha-MSH increased the levels of intracellular cAMP, but not Ca(2+), in chondrocytes. Both messenger RNA and protein expression of various proinflammatory cytokines, collagens, matrix metalloproteinases (MMPs), and SOX9 was modulated by alpha-MSH.


CONCLUSIONS
Human articular chondrocytes are target cells for alpha-MSH. The effects of alpha-MSH on expression of cytokines and MMPs suggest that this neuropeptide plays a role in inflammatory and degenerative processes in cartilage. It is conceivable that inflammatory reactions can be mitigated by the induction of endogenous MCs or administration of alpha-MSH to the affected joints. The induction pattern of regulatory and structural ECM components such as collagens as well as SOX9 and anabolic and catabolic cytokines points to a function of alpha-MSH as a trophic factor in skeletal development during endochondral ossification rather than as a factor in homeostasis of permanent cartilage.
Department of Orthopaedic Surgery and Centre for Medical Biotechnology, University of Regensburg, Regensburg, Germany. susanne.graessel@klinik.uni-regensburg.de
Susanne
S
Grässel
Alfred
A
Opolka
Sven
S
Anders
Rainer H
RH
Straub
Joachim
J
Grifka
Thomas A
TA
Luger
Markus
M
Böhm
D000368:Aged; D002118:Calcium / Q000378:metabolism; D002358:Cartilage, Articular / Q000378:metabolism / Q000473:pathology; D002478:Cells, Cultured; D019902:Chondrocytes / Q000378:metabolism* / Q000473:pathology; D003094:Collagen / Q000378:metabolism; D000242:Cyclic AMP / Q000378:metabolism; D016207:Cytokines / Q000378:metabolism*; D016326:Extracellular Matrix Proteins / Q000378:metabolism*; D006801:Humans; D020782:Matrix Metalloproteinases / Q000378:metabolism; D053486:Melanocortins / Q000378:metabolism*; D008875:Middle Aged; D020370:Osteoarthritis, Knee / Q000378:metabolism / Q000473:pathology; D010447:Peptide Hydrolases / Q000378:metabolism*; D011333:Pro-Opiomelanocortin / Q000378:metabolism*; D044101:Receptors, Melanocortin / Q000378:metabolism*; D055758:SOX9 Transcription Factor / Q000378:metabolism; D000521:alpha-MSH / Q000378:metabolism*
D016207:Cytokines; D016326:Extracellular Matrix Proteins; D053486:Melanocortins; D044101:Receptors, Melanocortin; D055758:SOX9 Transcription Factor; C527932:SOX9 protein, human; D000521:alpha-MSH; D011333:Pro-Opiomelanocortin; D003094:Collagen; D000242:Cyclic AMP; D010447:Peptide Hydrolases; D020782:Matrix Metalloproteinases; D002118:Calcium