Cycloheximide Protects HepG2 Cells from Serum Withdrawal-Induced Apoptosis by Decreasing p53 and Phosphorylated p53 Levels

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Abstract
Cycloheximide (CHX), an inhibitor of protein synthesis, has been reported to prevent cell death in a wide variety of cell types and produced by different apoptotic stimuli. However, the mechanisms by which CHX protects cells from apoptosis are still unclear. In this study, we investigated whether p53 plays a role in the protection by CHX against serum withdrawal-induced apoptosis. Deprivation of serum from the culture medium causes apoptosis in HepG2 cells, and CHX dramatically protects cells from death. p53, p21, and Bax protein levels were elevated, and cell cycle arrest was produced after serum withdrawal. CHX abolished this elevation of p53, p21, and Bax as well as the cell cycle arrest induced by serum deprivation. The p53 inhibitor pifithrin-α protects HepG2 cells against apoptosis induced by serum withdrawal. HepG2 cells expressing a dominant negative form of mutant p53 and Hep3B cells lacking p53 were resistant to serum withdrawal-induced apoptosis. Lowering of p53 by small interfering RNA protects HepG2 cells from serum withdrawal-induced apoptosis. p53 phosphorylation was induced by serum withdrawal and other chemotherapeutic reagents such as actinomycin D, doxorubicin, and etoposide. CHX decreases the levels of phosphorylated p53 (pp53) even in the presence of a proteasome inhibitor, which maintains the total p53 levels, whereas it does not affect the dephosphorylation of pp53. These results suggest the possibility that kinases that phosphorylate p53 might be affected by CHX administration. In summary, CHX protects HepG2 cells from serum withdrawal-induced apoptosis through inhibiting the synthesis of p53 and the phosphorylation of p53.