at Col2a1 peaked on day 6 of micromass culture, Lrp6 e pression decreased beginning on day 6 and Lrp5 e pression was constant during chondrocyte differen tiation. The basal levels of Lrp5 and Lrp6 mRNA were very low in mouse articular chondrocytes. In pathogenic primary culture chondrocytes treated with IL 1B, however, Lrp5 e pression was drama tically increased in a dose dependent manner and a time dependent manner, whereas Lrp6 e pression was constant. Consistent with our previous observations, IL 1B treatment increased the levels of Mmp13 while abrogating Col2a1 e pression. Our qRT PCR analysis revealed that IL 1B treatment triggered an appro imately tenfold increase of Lrp5 e pression, but had no effect on Lrp6 e pression.
IL 1B treatment of chondrocytes triggered the activation of nuclear factor ��B and various mitogen activated protein kinase subtypes, including ERK, p38 kinase and JNK. Inhibition of ERK or p38 kinase had no effect on LRP5 e pression, but the blockade of JNK or NF ��B signaling markedly inhi bited the IL 1B induced increase in LRP5 e pression. These data indicate that LRP5 is increased during GSK-3 IL 1B induced chondrocyte dedifferentiation and that this upregulation of LRP5 is mediated via the JNK and NF ��B signaling pathways. LRP5 e pression is elevated in human and mouse osteoarthritic cartilage Because Lrp5 e pression was distinctly regulated during IL 1B induced chondrocyte dedifferentiation, we e amined whether LRP5 plays a role in OA cartilage destruction in vivo. We initially e amined LRP5 levels in OA affected human cartilage obtained from individuals who had under gone arthroplasty.
The degree of cartilage damage in the human OA samples was ICRS grade 4 as confirmed by Alcian blue staining. In these samples, LRP5 was significantly e pressed in OA affected human cartilage but barely detectable in normal cartilage. This upregulation of Lrp5 mRNA in human OA cartilage was confirmed by RT PCR and qRT PCR analyses. We also found that the protein and mRNA levels of LRP5 were increased in cartilage from STR ort mice compared with that from control CBA CaCrl mice. We also observed increased LRP5 e pression in mouse OA cartilage following collagenase injection and DMM surgery. Thus, LRP5 e pression was significantly elevated in all human and mouse OA cartilage samples e amined in the present study.
Catabolism promoting gene regulation by LRP5 in dedifferentiated chondrocytes Because the above described results suggest that LRP5 may negatively regulate cartilage maintenance, we investi gated the effects of LRP5 on catabolic and anabolic gene e pression levels in chondrocytes. Ectopic e pression of LRP5 significantly suppressed type II collagen e pression at the transcript and protein levels but had no effect on the e pression levels of catabolic genes such as Mmp3, Mmp13, Adamts4, Adamts5 and Ptgs2. Our qRT PCR analysis clearly revealed that type II collagen e pression was dose dependently decreased by LRP5 overe pression. Double