Primary liver cancers, which are generally hypervascular in nature, depend highly on blood supply. So far there are few reports on apoptosis of liver cancer cells upon deprivation of serum-derived survival factors. The aim of our study is to clarify molecular mechanisms by which liver cancer cells survive with the aid of serum. In HepG2 liver cancer cells, serum deprivation induced time-dependent increase in the number of apoptotic cells, which was detected by fragmentation of genomic DNA and fluorescent nuclear staining. The activity of extracellular signal-regulated kinase (ERK) did not decrease considerably after serum deprivation, although it increased after serum stimulation. However, we found that the MEK1 inhibitor PD98059, but not the p38 kinase inhibitor SB203580, potently induced apoptosis of the liver cancer cells in the presence of serum, indicating that the MEK-ERK signaling pathway is required for serum-dependent survival of HepG2 cells. In agreement with this notion, transient expression of active MEK1 prevented apoptosis in serum-deprived condition. We also found that the protective effect of serum against apoptosis was totally abrogated by LY294002 or wortmannin, which are the inhibitors of phosphatidylinositol (PI) 3-kinase. The activity of Akt, the target of PI 3-kinase, decreased gradually after deprivation of serum, whereas it was rapidly reactivated upon serum stimulation. These data indicate that survival of HepG2 liver cancer cells depends upon serum and that both the MEK1-ERK- and the PI 3-kinase-Akt- pathways are required for survival signaling to the nucleus.
Copyright 2001 Wiley-Liss, Inc.