Background and aims: The expression of osteopontin (OPN), a protein postulated to play a role in tumorigenesis, is induced by the tumor promoter, 12-O-tetradecanoylphorbol-13-acetate (TPA) in vivo and in the in vitro initiation-promotion skin carcinogenesis model (JB6 cells). Although TPA-induced OPN expression in JB6 cells has been suggested to involve protein kinase C (PKC), the PKC isoforms and the downstream pathway mediating OPN expression have not been extensively studied.
Methods: Using the JB6 cell model, we determined the involvement of PKC isoforms, mitogen-activated protein kinase kinase (MAPK kinase/MEK) and MAPK in TPA-induced OPN expression using inhibitors specific to PKC isoforms and MEK and performing Northern blot analyses. Western blot analyses of cells treated with specific inhibitors were also performed to determine whether PKC isoforms or MEK were involved in activation of MAPK.
Key results: TPA increased the steady-state level of OPN mRNA as early as 2-4h and this expression persisted for at least 4 days. TPA induction of OPN expression in JB6 cells is mediated through PKC epsilon and PKC delta, which also mediated the phosphorylation of MAPK. Additionally, inhibition of MEK activity, which activates MAPK, attenuated TPA-induced OPN expression. These findings suggest that activation of MAPK is important in mediating OPN expression.
Conclusion: TPA-induced steady-state OPN mRNA expression in mouse JB6 cells involves the activation of MAPK mediated through PKC epsilon and/or PKC delta.