Activation of Erk1/Erk2 and transiently increased p53 levels together may account for p21 expression associated with phorbol ester-induced transient growth inhibition in HepG2 cells.

In HepG2 cells grown in the presence of serum, enhanced Raf-activation correlated with transient growth inhibition. The activation of Raf was increased either by the phorbol ester-induced activation of protein kinase C (PKC) or by the addition of the PKC inhibitor bisindolylmaleimide I (BIM). Either of these treatments increased the cellular levels of p21 by an Erk1/Erk2 MAP kinase cascade-dependent way, since this increase was prevented by the MEK-inhibitor PD98059. Nevertheless, the growth inhibition correlated with the transient increase of p53 levels as well. Either the activation of PKC with phorbol ester or the addition of BIM to cells growing in serum induced a rapid but transient increase of p53 levels, which preceded growth inhibition. This increase of p53 levels was probably due to the transient stabilisation of p53 and did not require the activation of Erk1/Erk2.