Dephosphorylation of ribosomal protein P0 in response to troglitazone-induced cytotoxicity

Troglitazone (TRO)-induced cytotoxicity was investigated in HepG2 cells. The cells were exposed to TRO as well as rosiglitazone (RSG) at concentrations of 0, 25, 50 and 75 μM for 48 h. Total proteins were separated by two-dimensional electrophoresis and visualized by silver staining. We focused on a protein spot at an approximate molecular weight of 35 kDa and isoelectric point (pI) of 5.7, which appeared only with the cytotoxic concentrations (50 and 75 μM) of TRO, but not with the low concentration (25 μM) of TRO or any concentrations of RSG. This protein spot was subjected to amino acid sequence analysis and identified as ribosomal protein P0 (P0). Interestingly, without any significant induction of its protein and mRNA, P0 was dephosphorylated depending on the concentration- and time-dependent manner of TRO-induced cytotoxicity. Pretreatment with a general caspase inhibitor, Z-VAD.fmk, prevented cleavage of caspase-3 but demonstrated a slight improvement of cytotoxicity induced by TRO. Thus, these effects could not prevent the dephosphorylation of P0. Our results strongly suggest that a post-translational modification, dephosphorylation, of P0 is associated with TRO-induced cytotoxicity.