Induction of an incomplete autophagic response by cancer-preventive geranylgeranoic acid (GGA) in a human hepatoma-derived cell line

GGA (geranylgeranoic acid) is a natural polyprenoic acid, derivatives of which has been shown to prevent second primary hepatoma. GGA induces mitochondria-mediated PCD (programmed cell death), which may be relevant to cancer prevention. To gain further insights into GGA-induced PCD, autophagy processes were examined in human hepatoma-derived HuH-7 cells. Treatment of HuH-7/GFP (green fluorescent protein)-LC3 cells with GGA induced green fluorescent puncta in the cytoplasm within 30 min and their massive accumulation at 24 h. After 15 min of GGA treatment, a burst of mitochondrial superoxide production occurred and LC3β-I was appreciably converted into LC3β-II. GGA-induced early stages of autophagy were unequivocally confirmed by electron-microscopic observation of early/initial autophagic vacuoles. On the other hand, LC3β-II as well as p62/SQSTM1 (sequestosome 1) continuously accumulated and co-localized in the cytoplasmic puncta after GGA treatment. Furthermore, GGA treatment of HuH-7/mRFP (monomeric red fluorescent protein)-GFP-LC3 cells showed yellow fluorescent puncta, whereas glucose deprivation of the cells gave red fluorescent puncta. These results strongly suggest that GGA induces the initial phase of autophagy, but blocks the maturation process of autolysosomes or late stages of autophagy, insomuch that GGA provides substantial accumulation of autophagosomes under serum-starvation conditions in human hepatoma cells.