SIP24 is an acute phase
iron binding
lipocalin physiologically expressed in vivo in developing cartilage by prehypertrophic/hypertrophic chondrocytes. Taking advantage of the chondrocytic cell line MC615 and using SIP24 as a marker we investigated the pathways active in cartilage differentiation and
inflammation. MC615 cells were cultured as: (i) proliferating prechondrogenic cells expressing
type I collagen (ii) differentiated hyperconfluent cells expressing Sox9 and
type II collagen. In proliferating cells the pathway PKC/ERK1, ERK2 was activated and SIP24 was not expressed while in differentiated cells the pathway p38/
NF-kappaB was activated and SIP24 was expressed. Proliferating cells treated with inflammatory agents expressed a large amount of SIP24 and showed activation of p38/
NF-kappaB pathway and inhibition of PKC/ERK1, ERK2 pathway indicating that in
inflammation and differentiation the same factors are activated (p38, NF-kappaB) or inactivated (PKC, ERKs). Treatment of proliferating cells with the p38 specific inhibitor
SB203580 inhibited the
inflammation induced activation of p38 and the synthesis of SIP24. PMA treatment induced activation of PKC, inactivation of p38 and suppression of SIP24 synthesis, suggesting that PKC activation inhibits p38 activation. In differentiated hyperconfluent cells the same factors (p38/
NF-kappaB/SIP24) are constitutively activated: treatment with inflammatory agents does not increase synthesis of SIP24 while treatment with
SB203580 and with PMA does not repress activation of p38 nor synthesis of SIP24. We propose that the SIP24 stress related
protein is expressed via p38 activation/
NF-kappaB recruitment both in chondrocyte differentiation and
inflammation and that a signaling pathway active in the
acute phase response is physiologically activated in differentiation.