A plasmid containing the CCAAT/enhancer-binding protein (C/EBP alpha)
gene transcriptionally controlled by the metallothionein promoter was
constructed, The gene was transfected into the human hepatocellular ca
rcinoma cell lines Hep3B and HepG2. When cultured in vitro in the pres
ence of 100 mu M ZnSO4, C/EBP alpha expression caused reversible growt
h arrest, In soft agar clonogenic assays, C/EBP alpha expression decre
ased both the colony size and the total number of colonies compared wi
th zinc-free controls, C/EBP alpha expressing cells s.c. implanted in
CD-1 nu/nu mice were essentially nontumorigenic, whereas C/EBP alpha t
umor cells implanted into immunodeficicnt SCID mice demonstrated a sig
nificantly delayed time of tumor appearance compared with cells transf
ected with a vector control plasmid. These studies suggest that the ex
pression of endogenous genes normally associated with a quiescent, dif
ferentiated state, such as C/EBP alpha, can result in impaired prolife
rative activity and suppressed tumorigenicity of hepatoma cell lines.