Aim: Osteosarcomas represent the most common primary malignant bone tumors. However, their pathogenesis is unclear. In vitro and in vivo studies have demonstrated the participation of the JNK–c-Jun signal transduction cascade and oncoproteins c-Jun and c-Fos in osteoblast proliferation and differentiation. JNKs activate c-Jun, which forms the AP-1 transcription factor as a homo/heterodimeric complex. Alpha-NAC is an osteo-blast-specific AP-1 coactivator that potentiates c-Jun/c-Jun, but not c-Jun/c-Fos transcriptional activity. We addressed the possibility that upregulation of the JNK–c-Jun pathway, as well as expression/activation of c-Fos and á-NAC, are implicated in osteosarcoma pathogenesis.

Materials and method: We assessed immunohistochemically the protein levels of the two major JNK isoforms (JNK1,2), their phosphorylated/activated species, p-JNK, their substrate, c-Jun, its phosphorylated/activated form, pc-Jun, its partner, c-Fos, and á-NAC, in 71 human osteo-sarcomas (56 high and 15 low grade).

Results: Positive immunostaining for JNK1, JNK2, p-JNK, c-Jun, pc-Jun, c-Fos and á-NAC was observed in 86%, 93%, 94%, 99%, 97%, 99% and 97.5% of the samples, respectively, but not in normal bone. Cellular levels of all proteins were significantly correlated to each other (p< 0.001). Moreover, significantly higher expression levels of all proteins were detected in high-grade osteosarcomas, compared to low-grade ones (p< 0.001).

Discussion: Our findings provide novel evidence that the JNK–AP-1 pathway is involved in osteoblast malignant transformation and osteosarcoma development and progression. Furthermore, the expression profile of α-NAC suggests that the active AP-1 population in human osteosarcomas is most likely comprised of c-Jun/c-Jun homodimers. Evaluation of c-Jun expression and JNK-dependent activation may facilitate an improved prediction of tumors’ clinical behaviour and potentially be exploited in designing patient-tailored treatment regimens.