C-Fos Regulation by the MAPK and PKC Pathways in Intervertebral Disc Cells

BACKGROUND The gene encoding c-fos is an important factor in the pathogenesis of joint disease in patients with osteoarthritis. However, it is unknown whether the signal mechanism of c-fos acts in intervertebral disc (IVD) cells. We investigated whether c-fos is activated in relation to mitogen-activated protein kinases (MAPKs) and the protein kinase C (PKC) pathway in nucleus pulposus (NP) cells. METHODOLOGY/RESULTS Reverse transcription-polymerase chain reaction and western blotting analyses were used to measure the expression of c-fos in rat IVD cells. Transfections were performed to determine the effects of c-fos on target gene activity. The effect of c-fos protein expression was examined in transfection experiments and in a 3- (4,5-dimethylthiazol-2-yl) -2,5-diphenyltetrazolium bromide cell viability assay. Phorbol 12-myristate 13-acetate (PMA), the most commonly used phorbol ester, binds to and activates protein kinase C (PKC), causing a wide range of effects in cells and tissues. PMA induced the expression of c-fos gene transcription and protein expression, and led to activation of the MAPK pathways in NP cells. The c-fos promoter was suppressed completely in the presence of the MAPK inhibitor PD98059, an inhibitor of the MEK/ERK kinase cascade, but not in the presence of SKF86002, SB202190, or SP600125. The effects of the PKC pathway on the transcriptional activity of the c-fos were evaluated. PKCγ and PKCδ suppressed the promoter activity of c-fos. Treatment with c-fos inhibited aggrecan and Col2 promoter activities and the expression of these genes in NP cells. CONCLUSIONS This study demonstrated, for the first time, that the MAPK and PKC pathways had opposing effects on the regulation of c-fos in NP cells. Thus, the expression of c-fos can be suppressed in the extracellular matrix of NP cells.