A p53-dependent G1 checkpoint function is not required for induction of apoptosis by acute choline deficiency in immortalized rat hepatocytes in culture

Apoptosis is an intrinsic cell program that causes unwanted or damaged cells to commit suicide. It is not well appreciated that nutrients can modulate this significant process. Choline is a nutrient that is crucial for the normal function of all cells, and its absence induces apoptosis. We examined whether p53 or p21(Waf/Cip1) were required for induction of apoptosis by choline deficiency in CWSV-1 rat hepatocytes immortalized with SV40 large T antigen. When grown in a defined, serum-free, choline-sufficient (70 mu M) medium, CWSV-1 cells failed to undergo apoptosis in response to gamma-irradiation, a well-known p53-dependent trigger of apoptosis. However, primary hepatocytes with an intact p53 gene expressed typical morphologic features of apoptosis and underwent G1 cell cycle arrest in response to gamma-irradiation. CWSV-1 cells continued to proliferate following gamma-irradiation, which is consistent with the loss of the G1 checkpoint response. These cells also failed to undergo apoptosis in response to cisplatin, whereas the p53-dependent primary hepatocytes underwent apoptosis in response to this drug. When maintained for 48 hours in choline-deficient (CD) medium (5 mu M choline), CWSV-1 cells exhibited terminal dUTP nucleotide DNA end-labeling assay (TUNEL)-positivity, a DNA ladder typical of internucleosomal DNA fragmentation, and classical cellular features of apoptosis. CD also induced apoptosis in Hep3B hepatocytes, a p53 deletion-mutant. Thus, it is reasonable to suggest that choline deficiency is capable of overcoming a block in the p53 pathway, activating an alternative apoptosis signaling pathway. (J. Nutr. Biochem. 9: 476-481, 1998). (C) Elsevier Science Inc. 1998.