Increased expression of the histamine H ₄ receptor subtype in hypertrophic differentiation of chondrogenic ATDC5 cells

Histamine has been regarded as an inflammatory mediator of arthritic disorders. We have previously reported that the expression of histamine H ₄ receptor (H ₄R) mRNA in synovial tissues was significantly higher in patients with osteoarthritis (OA) compared to those with rheumatoid arthritis. Chondrocyte hypertrophy and endochondral ossification are essential processes in pathologic disorders such as osteophyte formation during OA progression. In the present study, we examined the expression of H ₄R during differentiation into hypertrophic chondrocytes in the ATDC5 cells, a widely used in vitro model of chondrogenic differentiation. Quantitative reverse transcription polymerase chain reaction showed that the levels of histidine decarboxylase and H ₄R mRNA on ATDC5 cells were increased in a time-dependent manner during the culture period. By contrast, the expressions of H1R and H2R were not increased from day 7 onwards. The mRNA expression of the hypertrophic chondrocyte marker type X collagen (COL X) was increased markedly from 14 to 21. Immunocytochemical analysis indicated that H ₄R staining was strongly immunoreactive on the plasma membrane of ATDC5 cells. Flow cytometry showed increased expression of H ₄R and COL X protein in ATDC5 chondrocytes. Furthermore, the majority of the COL X-positive cells expressed H ₄R throughout the culture period. In summary, we showed for the first time that H ₄R is expressed in ATDC5 chondrocytes. Moreover, we found that most hypertrophic chondrocytes express H ₄R, suggesting that this receptor might be associated with the differentiation of chondrocytes into hypertrophic cells, which are abnormally observed in joint lesions in OA.