Inhibition of autophagy enhances adenosine-induced apoptosis in human hepatoblastoma HepG2 cells

In cancer research, autophagy acts as a double-edged sword: It increases cell viability or induces cell apoptosis depending upon the cell context and functional status. Recent studies have shown that adenosine (Ado) has cytotoxic effects in many tumors. However, the role of autophagy in Ado-induced apoptosis is still poorly understood. In the present study, Ado-induced apoptotic death and autophagy in hepatoblastoma HepG2 cells was investigated and the relationship between autophagy and apoptosis was identified. In the present study, it was demonstrated that Ado inhibited HepG2 cell growth in a time- and concentration-dependent manner and activated endoplasmic reticulum (ER) stress, as indicated by G0/G1 cell cycle arrest, the increased mRNA and protein levels of GRP78/BiP, PERK, ATF4, CHOP, cleaved caspase-3, cytochrome c and the loss of mitochondrial membrane potential (m). Ado also induced autophagic flux, revealed by the increased expression of the autophagy marker microtubule-associated protein 1 light chain 3-II (LC3-II), Beclin-1, autophagosomes, and the degradation of p62, as revealed by western blot analysis and macrophage-derived chemokine (MDC) staining. Blocking autophagy using LY294002 notably entrenched Ado-induced growth inhibition and cell apoptosis, as demonstrated with the increased expression of cytochrome c and p62, and the decreased expression of LC3-II. Conversely, the autophagy inducer rapamycin alleviated Ado-induced apoptosis and markedly increased the m. Moreover, knockdown of AMPK with si-AMPK partially abolished Ado-induced ULK1 activation and mTOR inhibition, and thus reinforced CHOP expression and Ado-induced apoptosis. These results indicated that Ado-induced ER stress resulted in apoptosis and autophagy concurrently. The AMPK/mTOR/ULK1 signaling pathway played a protective role in the apoptotic procession. Inhibition of autophagy may effectively enhance the anticancer effect of Ado in human hepatoblastoma HepG2 cells.