Induction of hepatoma cell apoptosis by c-myc requires zinc and occurs in the absence of DNA fragmentation

Since c-myc expression is increased during apoptosis in toxin-induced liver injury in vivo, the role of c-myc in liver cell apoptosis was investigated. The human hepatoma cell line HuH-7, which constitutively expresses c-myc, was stably transfected with sense and antisense c-myc expression vectors under the control of the zinc-inducible metallothionein promoter. None of the three cell types (wild-type, sense c-myc, or antisense c-myc) underwent apoptosis when cultured in serum-free medium (SFM). With the addition of SFM plus 37.5 mu M zinc, wild-type and sense c-myc-expressing cells underwent rapid cell death, whereas antisense c-myc-expressing cells exhibited increased survival. This cell death had the light, fluorescent, and electron microscopic appearance of apoptosis, but did not result in DNA fragmentation. This apoptosis could be terminated by the addition of medium containing 2% fetal calf serum or the overexpression of bcl-2 but not by supplementation with specific growth factors. Altering c-myc expression did not affect cellular metallothionein mRNA levels or the rate of cell death from copper or cadmium. The requirement for zinc and absence of DNA fragmentation in c-myc-induced hepatoma cell apoptosis under serum-free conditions provides further evidence of the complex regulation of apoptosis in different cell types.