The effects of cisplatin (cisPt) on the extra cellular signal-regulated kinase (ERK) and the protein kinase B (PKB/Akt), known to play important roles in promoting cell survival and in down regulating apoptosis, were investigated in thyroid cell lines. The cytotoxic effect of cisPt was highest in normal PC-Cl3 cells, intermediate in dedifferentiated PC-E1A and PC-raf cells and lowest in fully transformed and tumorigenic PC-E1Araf cells. CisPt provoked ERK phosphorylation; such phosphorylation was unaltered by Gö6976, a conventional PKC inhibitor, whilst blocked by low doses (0.1 microM) or high doses (10 microM) of GF109203X, an inhibitor of all PKC isozymes, in PC-Cl3 and in PC-E1Araf cells, respectively. In PC-E1Araf, but not in PC-Cl3 cells, the cisPt-provoked ERK phosphorylation was also blocked by a myristoylated PKC-zeta pseudo substrate peptide (PS-zeta). The cytotoxic effects of cisPt increased when cells were pre-incubated with the mitogen-activated protein kinase (MEK) inhibitor PD98059. CisPt provoked the phosphorylation of PKB/Akt and this effect was blocked by LY294002, a PI3K inhibitor. In PC-Cl3 cells pre-incubated with LY294002 the effects of cisPt on ERK phosphorylation and cell mortality resulted unaffected; conversely, LY294002 reduced the ERK phosphorylation and increased cisPt cytotoxity of in PC-E1Araf cells. Furthermore, in PC-E1Araf cells pre-incubated with LY294002 and PS-zeta ERK phosphorylation was abolished and cisPt cytotoxicity was highest. Altogether results highlight a role for PKCs in the upstream regulation of ERK pathway facing the cell response to cisPt treatments. Understanding the mechanisms by which cells process cisPt provides important insights for designing more efficient platinum-based drugs.